Wavelet analysis is a highly mathematical subject. The level of abstraction often makes it difficult for the scientist to extract from the literature the concepts and tools necessary in doing their day-to-day work. This informal talk presents many wavelet applications. By my presenting wavelets with a focus on the applications, I hope that some of the underlying concepts might be more easily understood.
Abstract I will talk about how coordinates of major solar flares and big sunspot groups are orgnized. This talk is based on analysis results of major solar flares observed from 1955 to now (4 consecutive solar cycles) and analysis results of sunspots observed during the last 12 solar cycles. There are preferred locations on the surface of the Sun in rigidly-rotating coordinate systems where major flares and big sunspot groups occur more frequently than elsewhere. These locations, called "hot spots," can last many solar cycles. Hot spots are different from active longitudes whose concept was proposed in the 1960s without any rigorous quantiative proof in that hot spots are generally limited to one hemisphere. Northern hemisphere hot spots, in general, do not have its counter parts in the southern hemisphere, having the same rotation period and the same longitude interval(s), and vice versa. I will discuss the implications of these intriguing phenomena.
Abstract I will describe an algorithm which simultaneously models the observed solar oscillation $\ell - \nu$ power spectrum in both dimensions. The algorithm determines both the line profile parameters (including asymmetry) and the spatial response function for the observations using fewer free-parameters than are required by current ?1-dimensional? algorithms. I will also describe how the measured response function data may be used to: (1) reveal the true shape of the spectral line profiles and (2) improve the modeling of the high-frequency ($\nu ? 5 mHz$) region of the spectrum.
Implication of long-term frequency variation
MDI In Context: Joint Observations with Coronal Instruments
Much of the value of SoHO has come from the ability to combine data
from widely disparate instruments on board. Three examples of this instrument
synergy are the March 1996 south pole observations; the whole-sun month
campaign SoHO ran in late August of 1996; and recent attempts to detect
coronal low-frequency Alfven waves using MDI as a phase reference. Intercomparisons
of coronal instruments and MDI will be discussed in light of these examples.
Refreshments: Ginzton Courtyard, 10:00 am
Astrophysical Solution of the Solar Neutrino Deficit Problem
P>
Dynamical Study of the Solar Photosphere.
Abstract: This study addresses mainly the convective structures visible
in the solar photosphere, especially the "mesogranulation" phenomenon.
Recent results of observations obtained at SacPeak are presented. Further,
the principals of a dynamical study of structures in the solar atmosphere
based on the investigation of the phase relations between velocity and
intensity fluctuations are introduced. These are subject of a proposal
for observations with SOI/MDI which are currently in progress.
"Accurate measurement of solar oscillation mode parameters"
Abstract:
Solar acoustic power spectra provide useful information on the properties
of the solar oscillations: i.e. mode frequencies, widths and amplitudes.
To ensure a correct theoretical interpretation of these parameters it is
necessary to accurately model the power spectrum over as large a domain
of the spectrum as possible. We have developed a 2-D fitting method which
allows us to measure precisely the mode parameters as well as the amplitude
of the spatial leaks in m-averaged spectra. I will present the main characteristics
of our 2-D fitting method and the first results. ----------------------------------------------------------------
Tea, coffee and doughnuts will be served at 3:15 pm.
"Implications of Localized Acoustic Absorption for Heliotomography
of Sunspots"
P>
----------------------------------------------------------------
Tea, coffee and doughnuts will be served at 3:45 pm.
"First Results from GOLF and Some Constraints on Solar Modelling"
"Recent Joys with Simulations of Turbulent Convection"
"Einstein-Brillouin-Keller Quantization of Stellar Waves"
----------------------------------------------------------------
Tea, coffee and doughnuts will be served at 3:45 pm.
----------------------------------------------------------------
Tea, coffee and doughnuts will be served.
"Predicting the Solar Cycle"
"The Waiting-time Distribution of Solar Flare Hard X-ray Bursts"
----------------------------------------------------------------
Tea, coffee and doughnuts will be served at 3:45 pm.
Wednesday, August 12, 4:00 pm, HEPL Conference Room
"A model of the double magnetic cycle, and solar longitudinal
activity inferred from SOHO MDI data
"Numerical Simulations of Spicules Driven by Weakly-Damped
Alfven Waves"
Tom Berger,
Bart De Pontieu, Karel Schrijver, Alan Title
Lockheed Martin Solar and Astrophysics Laboratory
"Results
of the TRACE EUV Moss Investigations"
Douglas Biesecker (SM?A Corporation)
"Destruction
of comets in the solar corona:
Kreutz sungrazing comet fragments
observed with SOHO"
Jesper Schou (Stanford)
``Stability of CCDs in Space-Based
Photometry''
Rich Wolfson (Middlebury College)
"Energetics
of Coronal Mass Ejections"
Doug Braun (Solar Physics Research Corporation and High Altitude Observatory)
``Helioseismic Holography: Recent Discoveries from SOI-MDI''
Abstract.
A technique called ``Helioseismic Holography'' has been successfully
applied in recent years to data from the Solar Oscillations
Investigation-Michelson Doppler Imager (SOI-MDI) on board the Solar
and Heliospheric Observatory (SOHO). This procedure applies
helioseismic observations at the solar surface in time-reverse to a
model of the solar interior, coherently regressing the surface
disturbances downward to render acoustic images of subsurface
structure.
I will describe the development and application of this technique
in the exploration of the acoustic properties of a wide variety of
solar
phenomena. Notable discoveries include the ``acoustic moat''
(3 mHz
suppression surrounding sunspots), ``acoustic glories'' (high frequency
halos of emission around complex active regions), acoustic condensations
up to 20,000 km beneath active regions, seismic images of a solar
flare, and the episodic nature of acoustic emission.
Wednesday, March 10, 1999, 4 pm, HEPL Conference Room:
Peter Sturrock (Stanford)
"Analysis
of the GALLEX solar neutrino data"
Markus J. Aschwanden (Lockheed-Martin
ATC)
``3D-Studies
of Solar Loops and Flares''
Abstract. Employing the method of "dynamic stereoscopy" using
the
solar rotation we obtain 3D models of coronal active region loops.
The obtained 3D parametrization allows us a much more
stringent test of radiation equilibria and scaling laws
(e.g. Rosner-Tucker-Vaiana law), which ultimately may
constrain the heating function in coronal loops.
While previous flare models were mostly restricted to
2D concepts, we explore 3D models of sheared quadrupolar
reconnection. Fitting such 3D models to Yohkoh HXT+SXT
images of interacting flare loops (as observed by Hanaoka and
Nishio et al. ) we obtain information on the pre-reconnection
geometry, changes in magnetic shear, estimates of the amount
of released magnetic energy, and trajectories of flare-accelerated
particles (which can be compared with time-of-flight measurements).
James Rickett and Jon Claerbout (SEP, Stanford)
"Acoustic daylight imaging via spectral factorization:
Helioseismology and reservoir monitoring"
Abstract:
The acoustic time history of the sun's surface is a stochastic
(t,x,y)-cube of information. Helioseismologists cross-correlate these
noise traces to produce impulse response seismograms, providing the
proof of concept for a long-standing geophysical conjecture. We pack
the
(x,y)-mesh of time series into a single super-long one-dimensional
time
series. We apply Kolmogoroff spectral factorization to the super-trace,
unpack, and find the multidimensional acoustic impulse response of
the
sun. State-of-the-art seismic exploration recording equipment offers
tens of thousands of channels, and permanent recording installations
are
becoming economically realistic. Helioseismology, therefore, provides
a
conceptual prototype for using natural noises for continuous reservoir
monitoring.
Louis Strous (Stanford-Lockheed
Institute for Solar Research)
PROPERTIES OF THE DYNAMIC SMALL-SCALE STRUCTURE
IN AN EMERGING ACTIVE REGION
The magnetic flux emergence in growing active region NOAA 5617, when
it is
about 8 hours old, shows an intricate fine structure. The small-scale
emergence events are characterized by a coincident upflow and transient
darkenings (of about 2 Mm and 10 minutes) in the continuum and line-center
intensity, followed by the appearance of one, or in some cases two,
new
bright grains flanking the line-center darkening, which coincide with
magnetic flux concentrations and downflows. The footpoints move apart
at
on average 1.4 km/s. Flux emergence happens recurrently in a
a number of
locations widely distributed over the active region. A preferred
orientation which fits Hale's polarity law is displayed by the emergence
events and is maintained during subsequent footpoint motion and in
the
filaments in the H-alpha arch filament system. Another feature
of the
growing active region is slightly curved alignments of faculae of one
magnetic polarity that also follow the preferred orientation.
We adapt
the model for flux emergence to accomodate the observed dynamic fine
structure. Essential new features are: (1) the emerging bundle
of flux
tubes is frayed in two systems; in vertical stacks, arranged in slightly
curved, nearly parallel planes, and (2) each flux tube emerges in a
``sea
serpent'' fashion.
Wednesday, May 12, 1999, 4 pm, HEPL Conference Room:
Gerhard Haerendel (Max-Planck-Institut für extraterrestrische Physik)
"The generation of chromospheric plages by collisional
damping of
high-frequency Alfven waves"
Shi Tsan Wu
Center for Space Plasma and Aeronomic Research
University of Alabama
"An MHD solar interplanetary model for the prediction of
geoeffective solar wind parameters at 1 AU"
Yu-qin Lou
Department of Astronomy and Astrophysics
The University of Chicago, Chicago, IL 60637
"Photospheric Rossby Waves and Periodicities in Solar Flare Activities"
Abstract
The Gamma-Ray Spectrometer aboard the Solar Maximum Mission (SMM)
first discovered a 154-day periodicity in solar flare rates.
Subsequently, periodicities in various solar flare activities and
sunspot areas during the solar maximum have been extensively
monitored using different diagnostics and at many electromagnetic
wavelengths. Notable periods are roughly 154, 128, 102, 78 and 51
days during maxima of different solar cycles from various data sets.
The origin of such long-time periodicities particularly prominent
during solar maxima remains a mystery for nearly two decades.
The solar photosphere is thin. For slow and large-scale dynamics, the
shallow magnetofluid approximation can be invoked when the Rossby
number is small. The physical properties of equatorially trapped
Kelvin waves, Poincare waves, Rossby waves and mixed Rossby-Poincar\'e
waves are presented. In particular, for m=12, 10, 8, 6 and 4 with a
small n, estimates of the equatorially trapped Rossby wave periods
and the low-frequency branch of the equatorially trapped mixed
Rossby-Poincar\'e waves give periods as 151-155, 126-127, 101-102,
76-78 and 51-54 days, respectively, in good agreement with
observationally inferred periodicities.
The physical connections among the large-scale photospheric wave
dynamics, Rossby radius of deformation, sunspot zones, subsurface
magnetic fields, magnetic flux emergence, active regions and solar
flare activities are discussed. The scenario of a dynamic feedback
cycle is advanced for excitations of these Rossby-type waves.
-----------------------------------------------------------------------
Wednesday, June 9, 1999, 4pm, HEPL Conference Room:
Rakesh Nigam (Stanford)
"The source of solar acoustic oscillations"
Solar acoustic oscillations are believed
to be excited by turbulent convection.
In this study the role of line asymmetry and phase difference
between velocity and intensity helioseismic spectra
for understanding the excitation of solar oscillations is discussed.
Since the oscillation modes are excited by
granulation, we argue that the granulation signal (noise)
is partially correlated with the oscillations.
This explains the reversal of asymmetry between velocity and
intensity and also the unusual phase difference between them.
We studied different types of excitation sources
at various depths and found that monopole and quadrupole
acoustic sources when placed in the superadiabatic layer
(at a depth of 75 km below the photosphere)
match the MDI observations.
Wednesday, April 19, 2000, 4 pm, HEPL Conference Room:
Karel Schrijver (LMSAL)
"A construction kit for the Sun and other cool stars:
Simulations of the photospheric activity and outer-atmospheric
radiative losses of cool stars based on solar characteristics"
Abstract.
Stars other than the Sun can, at present, be studied only as
point sources. The Sun, in contrast, can be studied in great
detail, but it shows us the magnetic workings of only a single
star. This talk presents numerical simulations of model stars
of widely different levels of magnetic activity, based on the
solar recipes for emergence, dispersal, and disappearance of
magnetic field. These simulations are used to simulated stellar
observations. The Sun is found to be typical of its class:
the solar properties are compatible with the disk-integrated
properties of other stars with convective envelopes that exhibit
magnetic activity. This conclusion requires three new processes
to be included in a long-familiar description of the surface field:
the emergence and evolution of small and ephemeral regions, the
collision and fragmentation of concentrations of flux, and the
magnetoconvective coupling that slows the motion of large
concentrations. The model allows us to draw conclusions on
ephemeral-region populations in other stars, on rotation-activity
relationships, and on radiative losses from the outer atmospheres
(from chromosphere to corona) of all cool stars: the solar-stellar
connection is a strong component of our quest to understand what
makes the Sun tick. With this model, the solar outer atmosphere can
be modeled in a laboratory setting, in which the experimenter has
control over all parameters.
Wednesday, September 20, 4 pm, Ginzton Lab Room 31 (***note location***):
Prof. Ashot Chilingaryan
Cosmic Ray Division of Yerevan Physics Institute
Armenia
"Investigating Solar Activity and Space Plasma Conditions
at the Aragats High Altitude Cosmic Ray Stations in Armenia"
Abstract
Solar Energetic Phenomena (SEP)-- pose a serious threat to the performance
of working space systems such as satellites, space stations and so
on,
causing anomalies in their operation. Predictions of solar activity
are
important for various technologies, including operation of low-Earth
orbiting satellites, electric power transmission grids, high-frequency
radio
communications and radar.
The Aragats Space Environment Center (ASEC) initiated in 1999, will
provide
real-time monitoring and forecasting of the extreme geomagnetic and
radiation events.
The project consists of the two interconnected parts, the first dealing
with
the high energy Galactic Cosmic Rays (CR) and the second - with Solar
Cosmic
Rays and Solar modulations of Galactic CR. Despite the fact that research
in
these fields in the last decade is conducted separately numerous ties
and
correlations exist both from the experimental point of view and from
the
standpoint of data interpretation and modeling of the very complicated
physical processes.
The experimental facilities at the stations on Mr. Aragats are and
will be
used for both data gathering and data interpretation and modeling of
the
physical processes. A huge amount of the scintillation detectors from
our
Extensive Air Shower (EAS) installations continuously monitor background
radiation for calibration purposes. The neutron monitors measuring
the CR
variations will be used for the detection of the EAS neutrons to form
the
multidimensional EAS signature along with the soft and muon component.
The Local Area Networks, connecting installations located at the altitudes
of 2000m and 3200 m with computing and the space weather forecasting
center
in Yerevan, along with the precise synchronization of all apparatus
triggers
will allow us to correlate data from all the monitors, revealing the
complicated space-time structure of the investigated phenomena.
Multiparameter, multidetector investigation of Cosmic rays will help
us
understand the origin and acceleration mechanisms of both Solar and
Galactic
CR, using Solar flares and Coronal mass ejections with originated shock
waves as a model of far more energetic Supernovae explosions and stochastic
acceleration and acceleration by the very strong magnetic fields of
neutron
stars.
Thursday, September 21, 4 pm, HEPL Annex B Conference Room:
Martin Woodard (BBSO)
"Recent Results on Solar Oscillations and Irradiance"
Monday, November 20, 2000, 4 pm, HEPL Conference Room:
Craig DeForest (SWRI)
MUSINGS ON MAGNETOGRAMS
For the last few months, I've been working on a project to quantify
the energy input to isolated coronal structures (notably polar plumes)
from the underlying magnetic field, using a novel resistance-free
quasi-stationary MHD model (still in development). MDI is currently
the only magnetograph that provides stable, ongoing average-field
measurements on the appropriate time scales of hours to days.
The
most significant observational challenge for the project is tracking
magnetic flux concentrations. The difficulty is that flux
concentrations are not isolated entities; rather, they're weak
groupings of smaller objects (called "flux elements" at Lockheed and
"stenflos" at St. Andrews). To understand flux element behavior
better, I have recently written a simple numerical code to model field
evolution in the granulation flow. The code is nearly physics-free,
yet produces alarmingly realistic simulated MDI data. I will
ramble
for a microcentury about the more interesting intellectual byways
associated with this line of research: the field-line analogy; polar
magnetography; Voronoi tesselation; flux motion statistics; and the
relationship between surface motions and coronal morphology.
Search for Variability of the Solar Neutrino Flux
Peter A. Sturrock
Center for Space Science and Astrophysics, Varian 302G, Stanford
University, Stanford, CA 94305, USA
ABSTRACT
It is generally believed that the solar neutrino flux is constant,
in
which case the deficit can be understood on the basis of the MSW
effect, which converts electron neutrinos into mu or tau neutrinos
as
they propagate through the dense solar interior. A variable flux
would call into question some of our current assumptions. A
stochastic variation might be due to variability in the nuclear
burning process; a periodic variation tied to the solar cycle or to
solar rotation would point towards an influence of the Sun's internal
magnetic field on neutrinos, and indicate that electron neutrinos
have nonzero magnetic moment.
Time-series spectra formed from flux measurements made by the
chlorine and gallium radiochemical experiments show peaks close to
the rotation rates of the radiative zone and the convection zone. On
the other hand, we find no evidence of periodic variation on the time
scale of the solar cycle.
We have also analyzed the histogram of neutrino measurements. A
histogram should be unimodal if the data are derived from a
stationary process, but need not be unimodal if the data are derived
from a variable process. Our analysis indicates that the histogram
formed from neutrino flux measurements by the gallium experiments is
bimodal. This fact indicates that the solar neutrino flux varies on
a
time scale of weeks, providing supporting evidence for rotational
modulation of the solar neutrino flux.
Three-dimensional numerical model of the evolution
and eruption of solar arcades using exponential
propagation methods
Mayya Tokman (California Institute of Technology)
ABSTRACT:
The three-dimensional structure and the eruption mechanism
of magnetic solar arcades constitute major questions
in the study of the dynamics of coronal mass ejections (CME).
In this talk we present a new numerical magnetohydrodynamic
(MHD) model which gives an insight into the three-dimensional
topology of the magnetic field of plasma configurations in the
solar corona. We discuss the dynamics of the system and point
out similarities between the results of our simulations and
observations. The stiffness of the resistive MHD equations
constitutes a major difficulty for numerical simulations.
Calculations in our model are performed using a novel exponential
propagation method which allows efficient integration of the
equations with time steps far exceeding the CFL bound that
constrains explicit schemes.
OBSERVATIONAL ASSOCIATIONS BETWEEN THE SOLAR INTERIOR, CORONA, AND SOLAR
WIND
Richard Woo
Jet Propulsion Laboratory, California Institute of Technology
Pasadena, CA 91109
Associating observations of the Sun, solar corona, and solar wind provides
clues for identifying and understanding the physical processes by which
magnetic fields generated within the Sun make their way through the
different layers of the solar atmosphere, shaping the solar wind flow
and
determining solar activity. Such associations have been difficult
because
of the paucity and diversity of observations.
Recent advances with radio occultation, ultraviolet and white-light
measurements have filled crucial gaps in our knowledge of the distribution
of density and solar wind flow in the solar corona. They have
also led to
new observational associations between the Sun and solar wind, and
changes
in our understanding of the origin and evolution of the solar wind.
The
purpose of this talk is to present an overview of these latest results.
Monday, March 5, 2001, 4pm, HEPL Conference Room:
On the Reconstruction of 3D Coronal Fields and Associations with Flares
Yihua Yan (Beijing Astronomical Observatory)
A boundary integral equation to describe a force-free magnetic field
with
finite energy content in the open space above the solar surface is
introduced. This is a new representation for a 3-D non-linear force-free
field in terms of the boundary field and its normal gradient at the
boundary. The method has been applied to a number of problems and here
we
present the results for the reconstruction of the non-linear force-free
magnetic field of NOAA 9077 before the X5.7/3B (10:24 UT) flare on
14
July 2000. The presence of a magnetic rope is, for the first time,
revealed from the extrapolation of the 3-D magnetic field structure.
This
magnetic rope is located above the magnetic neutral lines in space
of the
filament. The calculated field lines of the rope rotate around its
axis
for more than 3 turns. Overlying the rope are multi-layer magnetic
arcades
with different orientations. These arcades are in agreement with TRACE
observations. Such magnetic field structure provides a favorable model
for
the interpretation of the energetic flare processes as revealed by
H$\alpha$, EUV and radio observations.
Analyzing the Solar Cycle
R.N. Bracewell (Stanford University)
Abstract.
After Galileo saw sunspots in his telescope in 1612 further
discoveries followed, such as the 27-day rotation period of the
sun and the restriction of the sunspots to certain zones of
latitude. In 1843 the 11-year cycle was discovered, with the
implication that the sun contains an oscillator. It is still
not known where or what the oscillator is; many propose that
it is just below the surface, others contemplate the core (if
there is one), while a source above the surface has not been
ruled out to everyone's satisfaction. So it is reasonable to
look for further properties of the oscillator that might be
helpful, for example by applying Fourier analysis of the
sunspot-number series, a daily data set extending from 1700 to
the present. (You may wonder why the cyclicity was not
discovered until 1843.)
The Fourier spectrum proves to be very complicated, but it can
be simplified somewhat by manipulating the data. Linearity is
an important factor in spectral analysis; if a nonlinear
transformation, such as rectification or hard limiting, is
applied to a signal, the spectrum gets messy.
Stanford University
Center for Space Science and Astrophysics
Helioseismology Seminars
Monday, April 30, 2001, 4pm, HEPL Conference
Room:
Peter Sturrock (Stanford)
"The reconstruction of nonlinear force-free fields from
vector-magnetograph data"
Abstract:
One would like to be able to determine the structure of the coronal
magnetic from observations made in the photosphere or chromosphere.
If one assumes that the magnetic field is current free, or if one
assumes that it is a linear force-free field (with a constant ratio
of current density to field strength), the problem is not difficult.
However, a current-free field is uninteresting, and a linear
force-free field is unphysical.
The next step is to try to construct nonlinear force-free fields,
that do not have a constant ratio of current density to field
strength. This is still rests on big assumptions, that gravity and
gas pressure are unimportant, and that there are no current-sheets.
I shall review a number of methods that have been proposed for
reconstructing nonlinear force-free fields.
Thursday, July 5, 2001, 4pm, HEPL Conference Room:
Professor Zuo Xiao (Peking University)
Director of Chinese Space Science Union
"Solar proton event and its effect on particle radiation
environment
inside a satellite cabin."
Abstract.
A polar orbited earth resource satellite was launched on October
14,1999. There is an instrument on board of the satellite to detect
the
energetic particle fluxes inside the satellite cabin so that the
energetic particle radiation environment in the cabin can be monitored
continuously. Some preliminary results showed that the particle flux
inside satellite is closely correlated with that outside. Almost all
the
flux events occur over polar regions and South Atlantic geomagnetic
anomaly region and both the occurrence of flux events and values of
fluxes at southern polar region are much greater than that at northern
region. The results during the strong solar proton event of July
14-16,2000 was analyzed and compared with quiet days.
___________________________________________________________________________
Wednesday, July 11, 2001, 4pm, HEPL Conference Room:
Richard W. Nightingal (Lockheed Martin Solar & Astrophysics
Laboratory)
"Rotating Sunspot Observations by TRACE With Twisting EUV
Coronal Fans"
Abstract.
In an on-going study, several sunspots, in apparent rotation, have
been
identified in TRACE photospheric white light (WL) images. In many cases
the rotation can also be seen in the corresponding UV (1600 A) and/or
EUV (171, 195 A) images. Twisting EUV coronal fans, where loops appear
to cross over one another, have been observed above rotating sunspots
on
August 15-18, 1999 for AR8667 and on August 8-10, 2000 for AR9114,
both
of which had an inverted S shaped region visible in Yohkoh SXT data,
and
on May 20, 2000. Several rotating sunspots were also seen in the active
region of the July 14, 2000 Bastille Day event, where an inverted S
shaped region was also visible. More recent rotations observed only
in
WL and UV occurred on December 11 and 22, 2000. Several more apparently
rotating sunspots have also been identified, but not analyzed, in the
TRACE and Mees data sets. Movies and plots of some of these rotations
will be shown, as well as flow maps and magnetic field data from MDI
on
SOHO for the August 2000 event. Analysis of the rotational rates of
the
sunspots will be given. I will report on our attempt to determine the
vertical electric current flowing through the 8 August 2000 sunspot
utilizing the Mees vector magnetograph data in order to better
understand the apparent rotation "driver". These observations display
the coupling of the magnetic field from the photosphere into the corona.
_________________________________________________________________________
Thursday, July 12, 2001, 4pm, HEPL Conference Room:
Alexander Ruzmaikin (JPL)
"On Multi-Angle Observations of Solar Activity"
Abstract.
Opportunities that can be opened with simultaneous observations of
the Sun from different longitudes will be discussed. With these
observations we can follow solar active regions from their formation
deep within the convection zone and below, through their development
on the photosphere, leading to CMEs and acceleration of hazardous
high-energy particles.
----------------------------------------------------------
Tuesday, August 14, 2001, 4pm, HEPL Conference Room:
David Landy (NSO)
"Fitting Nigam Asymmetrical Line Profiles to GONG Power Spectra"
Abstract.
Until recently, the global power spectra of helioseismic data were
typically fit to the symmetric Lorentzian line profiles which would
result, in the limit, from damped, stochastically excited point sources.
Actual asymmetries in the power spectrum profiles were noticed as early
as
1993 (Duvall) and were identified as interference effects between the
oscillation cavity & the excitation source. In 1998, Nigam,
et al
postulated a background noise term correlated with the solar eigenmodes,
which predicted the observed reversal in asymmetry between velocity
and
intensity modes, and, simultaneously, they provided a low parameter,
easy
to calculate, approximation to the line shape. After a brief
review of
the GONG processing pipeline and previous profile fitting routine (called
"Peakfind"), I will present the results of a first attempt to fit the
GONG
velocity power spectra with the Nigam Profile from l=0 to l=150.
The fits
presented more or less conform to previously published fits of MDI
and
GOLF data at low l & intermediate frequency, and appear to greatly
reduce
the frequency differences between lorentzian fits to intensity and
velocity data in this range. I will discuss the significant problems
for
Peakfind's fitting method's at high l values and high frequencies,
and
review possible changes which could be made to GONG peakbagging
methods to
fix these problems, including the simultaneous fitting of velocity
and
intensity spectra, the use of phase information, and the use of a more
realistic background.
Hiromoto Shibahashi
(University of Tokyo)
"The Neutrino Problem,
Extra-solar Planetary Systems
& Helioseismology"
Monday, October 29, 2001, 10 am, HEPL Conference Room:
S.T. Wu
Center for Space Plasma, Aeronomy, and Astrophysics Research
University of Alabama in Huntsville
"MHD Simulation Models for Coronal Dynamics"
Michael Wheatland
The University of Sydney
"The local Poisson hypothesis for solar flares"
Abstract:
The question of whether flares occur as a Poisson process has important
consequences for flare physics. Recently Lepreti et al. (2001, ApJ
555
L133-L136) presented evidence for local departure from Poisson statistics
in the Geostationary Operational Environmental Satellite (GOES) X-ray
flare
catalog. The authors attributed particular significance to the appearance
of a power law in the tail of the distribution of waiting times (times
between events) for the GOES events. Here it is argued that the departure
from Poisson statistics arises from a selection effect inherent in
the
soft X-ray observations; namely that the slow decay of enhanced flux
following a large flare makes detection of subsequent flares less likely.
It is also shown that the power-law tail of the GOES waiting-time
distribution varies with the solar cycle. This counts against any intrinsic
significance to the appearance of a power law, or to the value of its
index.
-----------------------------------------------------------------------
Friday, November 16, 2001, 4pm, HEPL Conference Room:
Sebastien Couvidat
CEA/DSM/DAPNIA/SAp (Service d'Astrophysique)
"The contribution of helioseismology to the solar neutrino puzzle
through the building of seismic models"
Abstract.
The long-standing solar neutrino puzzle benefits now from high event
rate
neutrino detection experiments on Earth. The results of these experiments,
like
SuperKamiokande and SNO, need to be compared with the neutrino fluxes
predicted
by the solar models.
Thanks to the quality of the helioseismic data of both the GOLF and
MDI
instruments aboard the SOHO satellite, we have access to an accurate
solar sound
speed profile. By using this profile to derive solar models closer
to the Real
Sun, we can compute more precise emitted neutrino fluxes.
These fluxes are compared with the results recently released by SNO.
We also derive different physical quantities related to the possible
solutions
to the neutrino puzzle (such as the MSW and RSFP theories).
------------------------------------------------------------------------
Wednesday, November 28, 2001, 10 am, HEPL Conference
Room:
Martin Woodard
Big Bear Solar Observatory
"Seismic Imaging of Solar Convection"
Abstract.
A new seismic technique is being developed
to image inhomogeneous structure,
such as subsurface convective cells, in the Sun.
In the direct imaging approach, a physical model
of the solar interior is inferred directly from correlations
in the observed seismic wave field.
A preliminary map of supergranular convection, inferred
from SOHO/MDI helioseismology images, agrees reasonably
well with simultaneous surface Doppler maps of supergranulation.
Further development of the method is expected to
improve the technique considerably.
----------------------------------------------------------------
Wednesday, December 19, 2001, 3:00 pm, HEPL Conference Room:
Tom Metcalf (Lockheed Martin ATC, Solar & Astrophysics Lab.)
"Observations of the Magnetic Free Energy in an Active Region"
Abstract.
I will discuss observations of the magnetic free energy in solar active
regions
using vector magnetic field data using the Na-I D-line at 5896 A.
The
chromospheric magnetic field derived from these observations is force-free
and
hence can be used with the magnetic virial theorem to derive the total
free
energy in an active region. The method is applied to AR8299 where
an
intriguing
dip in the free energy is observed around the time of a CME.
------------------------------------------------------------------------
Thursday, December 20, 2001, 4:00 pm, HEPL Conference Room:
Richard Wachter (World Radiation Center, Davos)
"What can we do to reveal g-modes from MDI velocity data?"
Abstract.
We are looking for spatial masks on MDI velocity data that are optimized
for
revealing low l g-modes with frequencies above 50 mikroHz. These masks
take
into account the horizontal component of the g-mode velocity eigenfunctions
as
well as the time dependent mode projection properties due to the changing
solar
B angle, and the varying noise level across the solar disk. A model
of the
solar noise is used to include the disk to center variation of the
solar noise
in the filter. Analysis of the signal to noise ratio for artificially
included
modes is used to evaluate the usefulness of different filters.
--------------------------------------------------------------------
Monday, February 4, 2002, 4 pm, HEPL Conference Room:
Jonathan Graham (HAO)
"Inference of Solar Magnetic Field Parameters from Data
with Limited Wavelength Sampling"
We investigate the diagnostic potential of polarimetric measurements
with
filtergraph instruments. Numerical simulations are used to explore
the
possibility of inferring the magnetic field vector, its filling factor,
and
the thermodynamics of model atmospheres when only a few wavelength
measurements are available. The results indicate that two wavelength
measurements are insufficient to reliably determine the magnetic parameters,
regardless of whether magnetograph techniques or least-squares fitting
inversions are used. However, as few as four measurements analyzed
with the
inversion technique provide enough information to retrieve the intrinsic
magnetic field. Application to the 676.8 nm Ni line is investigated
both with
simulated solar profiles and ASP data.
Friday, July 12, 2002, 2pm, HEPL Conference Room:
Dean-Yi Chou (Tsing Hua University, Hsinchu, Taiwan)
"Solar-Cycle Variations of p-Mode Travel Time"
Abstract
We study the solar-cycle variations of solar p-mode travel time for
different wave packets to probe the magnetic fields at the base of
the solar convection zone. We select the wave packets which return
to the same spatial point after traveling around the Sun with integral
number of bounces. The change in one-bounce travel time at solar
maximum
relative to minimum is approximately the same for all wave packets
studied except a wave packet whose lower turning point is located at
the base of the convection zone. This particular wave packet
has an
additional decrease in travel time at solar maximum relative to other
wave packets. The magnitude of the additional decrease in travel
time
for this particular wave packet increases with solar activity.
If this additional decrease is interpreted as the effect of the magnetic
fields at the base of the convection zone, the field strength is estimated
to be about 0.4 - 0.7 million gauss at solar maximum if the filling
factor
is unity. We also discuss the problem of this interpretation.
-------------------------------------------------------------------------
Tuesday, July 16, 2002, 4pm, HEPL Conference Room:
Giora Shaviv (Technion, Haifa, Israel)
"The controversy about the screening of nuclear reactions
in the Sun"
Abstract: We review the debate about how to calculate the effect
of the plasma on the rate of nuclear reactions in the dense solar plasma.
We show how the classical Salpeter expression comes about and why it
is
not relevant. We explain why people thought that there is a dynamics
effect
and how they were proven wrong using the wrong reasons. (But they are
still wrong). We then define the screening from first principles and
show
how to calculate it (hopefully correctly).
We then show the connection between the screening and relaxation
processes in the gas and show how to calculate the screening using
the Langevin equation and the Rosenbluth potential. In doing so
we explain how thermodynamic arguments can sometimes mislead.
----------------------------------------------------------------------
Thursday, August 1, 2002, 4pm, HEPL Conference Room:
Markus J. Aschwanden (LMSAL)
"The Nanoflare Myth"
There is already a "flare myth", a de-emphasis of local flare processes
in juxtaposition to the more global coronal mass ejections, a term
coined by Jack Gosling. In analogy, I like to raise the heresy of a
"nanoflare myth", de-emphasizing the importance of nanoflares
in the heating budget of the solar corona. This conclusion is strongly
supported by the latest results of nanoflare statistics obtained from
TRACE and Yohkoh. Recent improvements in the analysis technique
include (1) the determination of the fractal dimension of nanoflares,
which is important to retrieve the correct geometric scaling, as well
as (2) a temperature synthesis in EUV and soft X-ray wavelengths,
which yield an unbiased and comprehensive statistical distribution.
From this latest analysis we conclude that the frequency distribution
of nanoflares has a powerlaw slope of ~1.5-1.6, significantly below
the critical limit of 2 that is required for a divergence of energy
at
small scales. Much steeper slopes up to 2.6 have been published
earlier, biased by incomplete temperature coverage and inappropriate
geometric models. The new results appeared in the article:
Aschwanden & Parnell 2002, ApJ 572, 1048-1071. June 20 issue.
-----------------------------------------------------------------------
Thursday, August 8, 2002, 4pm, HEPL Conference Room:
Bill Abbett (UC Berkeley)
"Coupled Models for the Emergence of Active Region Magnetic Flux
into the Solar Corona"
Abstract:
Active regions are areas of strong, bipolar magnetic field
that represent the largest observable concentrations of
magnetic flux on the Sun. Most important measures of solar
activity, including solar flares, UV and X-ray radiation, and
many (though not all) coronal mass ejections are associated
with active regions. Global properties of active regions are
governed, at least in part, by photospheric and sub-surface
magnetic fields and flows, and some of the most intense episodes
of activity may well be related to the dynamic process of magnetic
flux emergence from the high-beta convection zone into the low-beta
environment of the solar corona.
I will present a summary of our efforts to model the flux
emergence process by creating a simple interface between two
3-D MHD codes: ANMHD, a code designed to efficiently model the
low acoustic Mach number plasma of the solar convection zone, and
ZeusAMR, a fully-compressible 3-D adaptive mesh MHD code that
we use to model the solar atmosphere from the photosphere to the
low corona.
-----------------------------------------------
Thursday, August 15, 2002, 4pm, HEPL Conference Room:
Hiromoto Shibahashi (Department of Astronomy, University of Tokyo)
"Constraint on particle physics from helioseismology"
Abstract:
Helioseismology used to be used to see what is the cause of the solar
neutrino
problem. Recent progress in experiments of neutrino detection showed,
however,
that neutrino oscillation accounts for this long-lived issue.
This fact does not mean at all that helioseismology is not useful for
elementary particle physics. Rather, combination of helioseismology
and recent
experiments of neutrino detection enables to give stringent constraint
on some
aspects of fundamental particle physics. I will demonstrate some examples;
---limits on the neutrino parameters, the axion parameter, and the
pp-nuclear
cross section.
--------------------------------------------------------------------
Wednesday, August 21, 2002, 4pm, HEPL Conference Room:
Takashi Sekii (National Astronomocal Observatory, Japan)
"Auto-correlation analysis of MDI high-frequency data"
_______________________________________________________________
Thursday, September 5, 2002, 4pm, HEPL Conference Room:
Fredrik Boberg (Lund Observatory, Sweden)
"Solar wind variations related to fluctuations of the North
Atlantic Oscillation"
Abstract:
Heat radiation from the Sun is the primary factor for the Earth's
climate condition. This phenomenon is not sufficient to explain
the observed global temperature fluctuations on Earth during the
last century. The solar wind interacts with the Earth's
magnetosphere causing geomagnetic activity. The solar wind also
modulates the current flow in the global electric circuit causing
changes in tropospheric temperature and wind dynamics.
I will present the results of a study on a possible connection
between solar wind variations and fluctuations of the North
Atlantic Oscillation (NAO). The NAO is a hemispheric meridional
oscillation in atmospheric mass between the Arctic and the
subtropical Atlantic. Phases with positive NAO index give
stronger winter storms crossing the Atlantic resulting in
above-normal temperatures in Northern Europe. Negative phases
give fewer and weaker winter storms bringing cold air to northern
Europe.
----------------------------------------------------------------
Thursday, September 12, 2002, 4pm, HEPL Conference Room:
Karel Schrijver (LMSAL)
"Photospheric and heliospheric magnetic fields."
The magnetic field in the heliosphere originates from a
variety of sources on the surface of the Sun, including mature,
decaying, and decayed active regions, as well as sunspots. The
emergence of new active regions together with the dispersal of flux
from older active regions causes the coronal magnetic field topology
to continually evolve, allowing previously closed-field regions to
open into the heliosphere and previously open-field regions to close.
Such evolution of the coronal field, together with the rotation of
the
Sun, drive space weather through the continually changing conditions
of the solar wind and the magnetic field embedded within it.
We
combine observations and numerical simulations by assimilating
SOHO/MDI magnetograms into a surface flux transport model, in order
to
investigate the origins of the heliospheric field on the solar surface
through the rising phase of the current activity cycle. We find
that
around cycle maximum, the interplanetary magnetic field (IMF) is
typically rooted in a dozen disjoint regions on the solar
surface. Whereas active regions are sometimes ignored as a source for
the IMF, the fraction of the IMF that connects directly to magnetic
plage is found to reach up to 30-50% at cycle maximum, with even
direct connections between sunspots and the heliosphere. We further
compare this data assimilation model with a pure simulation model,
in
which the properties of the emergent active regions were chosen at
random from parent distribution functions measured for the sun.
The
two models show remarkable agreement in the temporal behavior of the
sector structure of the IMF, in the magnitude and time-behavior of
the
heliospheric field, and even in such global properties as the tilt
angle of the Sun's large scale dipole. We thus conclude that
no
additional flux-emergence patterns or field-dispersal properties are
required of the solar dynamo beyond those that are included in the
model in order to understand the large-scale solar and heliospheric
fields.
---------------------------------------------------------------------
Tuesday, September 24, 2002, 4 pm, HEPL Conference Room:
Michael Dragowsky (Los Alamos National Laboratory)
"Calibrating the Sudbury Neutrino Observatory"
Abstract: The Sudbury Neutrino Observatory (SNO) is
a heavy water Cherenkov detector used primarily
to measure solar neutrinos. The SNO collaboration
announced in April 2002 the first total flux measurement
for 8B solar neutrinos. This finding resolves
the Solar Neutrino Problem in favor of neutrino flavor
transformation rather than Solar Model deficiencies.
The measurement is intended to be the first in a
series of progressively more precise 8B solar neutrino
flux measurements.
The program to characterize the dominant systematic
uncertainties in this heavy water Cherenkov detector
will be described. This program is organized around
optical, gamma-ray and neutron calibration sources.
Data from these sources is used to constrain Monte Carlo
predicted probability density functions used to assign
neutrino events into electron or non-electron types.
Implications for future neutrino physics experiments
will be presented.
-----------------------------------------------------------------
Wednesday, September 25, 2002, 4 pm, HEPL Conference Room:
Keiji Hayashi (Nagoya University)
"On MHD approaches to the solar corona, solar wind and space weather prediction"
Abstract.
I would like to present the various MHD simulations applicable
for the studies on the solar wind, solar corona and space weather prediction.
At first, my current study, MHD tomography analysis, will be presented.
This analysis method can reconstruct the global solar wind structures
consistent with MHD equations from the IPS (interplanetary radio scintillation)
observational data. This is a powerful analysis method, but it can
merely treat
the super-Alfvenic flow region (usually from 50 Rs to 1 AU) because
it is difficult for an analysis method to treat the phenomena occurring
outside
of the domain of observation.
Next, the MHD simulation of the solar corona will be presented.
This simulation uses the observed photospheric magnetic field as
one of the boundary conditions and produces the open and closed
magnetic field regions near the Sun. To reproduce the trans-Alfvenic
flow
at the solar coronal hole, the nearly-isothermal polytropic gas and/or
the momentum/energy addition must be assumed in the simulation.
Comparing with the photographs of the solar corona and other observational
data,
the adequateness of the model can be examined. While I still do not
found
the best model to produce the global solar corona, some model cases
will be presented.
As summary, I would like to present the some results of the MHD simulation
possibly applicable to the space weather predictions. Utilizing the
global
solar wind structure derived by the IPS analysis and/or the coronal
heating model,
we can carry out the time-dependent MHD simulation of the interplanetary
disturbances propagating in the background "quiet" solar wind close
to reality.
The numerical interplanetary disturbance can be initiated by giving
the numerical perturbation at the site of the observed flares, fearlessly
at the possible sites in advance.
-----------------------------------------------------------------
Thursday, September 26, 2002, 4 pm, HEPL Conference Room:
B. Bala (NJIT)
"Study of Statistical Properties of Microflares"
Abstract.
The results of a study of the temporal, morphological, and magnetic
properties of microflares using the
TRACE data in the wavelengths of 1600 and 171 A and the BBSO magnetograms
will be presented. For
this purpose we selected the active region NOAA 9393 observed on March
27, 2001. About 70
microflares are found using the UV 1600 A filtergrams over a time span
of 7 hrs. The following results
have been obtained:
(i) based on their lifetime, we classified these microflares as impulsive
and persistent events. By our
definition, impulsive events are short lived ones with a lifetime of
about a few minutes and the
persistent events are those lived on average for half an hour or longer.
Most of the microflares (82%)
were found to be impulsive events.
(ii) From the morphological point of view, the microflares were found
to occur in a variety of sizes and
shapes. This was found to be true for both impulsive and persistent
events. In other words, the impulsive
and persistent events exhibited similar morphological properties.
(iii) On average, the maximum intensity exhibited by persistent events
were higher, by more than a
factor of 2, than the impulsive ones, suggesting that the persistent
events are much more energetic than
the impulsive events.
(iv) In terms of their distribution in the magnetograms, we find that
majority (67%) of the persistent
events are located at or close to the magnetic neutral lines, while
for the impulsive events, about a half
are located in the positive polarity and the rest on magnetic neutral
lines as well as in negative polarity
region.
(v) It is also found that these microflares are not scattered over the
entire active region but tend to cluster
around three major spots seen in UV 1600 A and at the base of large-scale
coronal structures as can be
seen in 171 A images. The present analysis gives some indication that
the impulsive and persistent
microflares differ in terms of their magnetic and energetic properties.
And the relationship between
microflares and coronal heating is briefly discussed.
Wednesday, October 9, 2002, 4pm, HEPL Conference Room:
Kristof Petrovay (Eotvos University, Budapest)
THE SOLAR TACHOCLINE: FAST OR SLOW?
It is generally accepted that turbulence and/or magnetic fields play
a dominant
role in the dynamics of the solar tachocline; the amplitude of these
effects is
however not known. The essential question is whether the turbulent
diffusuvity
exceeds the limiting value of ~10^9 cm^2/s. If not, the time scales
associated
with the turbulent transfer and with the supposed steady internal primordial
magnetic field will be long compared to the solar cycle. This is the
case of
the slow tachocline, studied rather extensively in recent years.
In the alternative case of a fast tachocline, the dynamo-generated oscillatory
magnetic field will pervade the tachocline and dominate its dynamics.
In the
talk I will present results of our ongoing project to model the fast
tachocline,
emphasizing the importance of helioseismic evidence in constraining
the
theoretical possibilities. Finally, I will discuss the available evidence
on
turbulence below the convective zone, as well as other possible methods
to
decide between a fast or a slow tachocline.
P
Wednesday, November 20, 2002, 4pm, HEPL Conference Room:
Taeil Bai (Stanford University)
"Spatial and Temporal Organizations of Solar Flare Activity: Hot
Spots and Periodicities."
Wednesday, January 15, 2003, 2pm, HEPL Conference Room:
"Simulations of Solar Active Region Magnetoconvection"
Neal Hurlburt (Lockheed Martin Solar and Astrophysics Laboratory)
Abstract.
Vigorous fluid motions associated with the observed patterns of
supergranulation, mesogranulation, and granulation on the sun are likely
to play a large role in the continual emergence, evolution, and
redistribution of magnetic field within solar active regions. To
investigate such non-linear dynamics, we have constructed numerical
simulations of fully compressible magnetized fluids within cylindrical
and spherical segments nominally located near the top of the solar
convection zone. We present recent results of several idealized active
region simulations, investigating the analogs of both plage, active
regions, pores and sunspots by varying the amount of magnetic flux
that
permeates the layer. Simplified field-line extrapolations into the
volume above the computational domain are then used to assess how the
corona might respond to the structure and evolution of magnetic field
emerging through the solar photosphere.
-------------------------------------------
Wednesday, February 5, 2003, 4pm, HEPL Conference Room:
Paul Boerner (Stanford University)
"Results from the MSSTA III"
The Multi-Spectral Solar Telescope Array was launched on April 30th,
2002
on a Terrier-boosted Black Brant rocket from White Sands Missile Range.
It
used an array of multilayer-coated Ritchey-Chretien telescopes to image
the solar corona at 150, 171, 180, 195 and 211 Angstroms. The resolution
of these images appears to be limited, by the grain of the x-ray-sensitive
film used to record the observations, to several arc-seconds. High-resolution
(~1 arc-second) chromospheric images at 1216 and 1550 Angstroms were
also
obtained. All telescope mirrors and filters, and the photographic
emulsions used, were calibrated prior to flight. I'll present the results
of the mission and report on the status of the data reduction. I'll
also describe early results of correlation analysis of the MSSTA data
and
comparison with supporting observations made by TRACE and SOHO.
--------------------------------------------------------------
Wednesday, February 12, 2003, 4pm, HEPL Conference Room:
Tom Berger (LMSAL)
"First Results from the 1-meter Swedish Solar Telescope on La Palma"
The Swedish Solar Telescope (SST) is the 1-meter diameter follow-on
to the
0.5-meter Swedish Vacuum Solar Telescope (SVST) on La Palma. The SST,
commissioned in July 2002, is a novel vacuum turret refractor employing
a
singlet primary lens and a Schupmann chromatic correction system. A
19-element adaptive optics (AO) mirror in concert with a fast tip-tilt
mirror is used for image stabilization and wavefront correction. The
telescope has now achieved 0.1-arcsec diffraction limited imaging in
the
430~nm G-band spectral region on many occasions. We present the results
of
sunspot studies conducted in July and September of 2002 using the SST
in
several wavelength regions. New results on the structure of penumbral
filaments as well as the structure and dynamics of sunspot light-bridges
are
shown. Penumbral filaments are shown to have dark central cores on
the order
of 0.1-arcsec (70 km) in width. These cores are not predicted or explained
by any theories of penumbral structure. Light-bridges are shown to
have
300--400 km-wide dark central lanes surrounded on either side by convective
granule-like structures 0.2-arcsec (150 km) in minimum observed width.
In
one larger bridge, the granules are seen to flow along the length of
the
bridge at an average speed of 900 m/sec. A c-class flare, one ribbon
of
which occurs along the light-bridge, gives evidence that the flow
accumulates magnetic field shear stress over the bridge.
-----------------------------
Monday, March 10, 2003, 4 pm, HEPL Conference Room:
Friedrich Busse (University of Bayreuth, Germany)
"Convection in Rotating Spherical Fluid Shells and Its Dynamo Action"
Abstract.
The present understanding of convection in rotating spherical fluid
shells is reviewed, and some recent results on coherent structures
and magnetic field generation are considered in detail. The
constraints exerted by the Coriolis force lead to unusual properties
not found in nonrotating systems, such as vacillations, localized
convection, and chaotic relaxation oscillations. The central role
played by the differential rotation generated by the Reynolds stress
of convection in the case of Prandtl numbers of the order unity or
less is emphasized. Magnetic fields generated through the dynamo
process offer new degrees of freedom. Through the braking of the
differential rotation the Lorentz force contributes to an enhanced
heat transport. Of particular interest are torsional oscillations
and reverses of magnetic polarity.
--------------------------------------------------------------
Wednesday, May 7, 2003, 4pm, HEPL Conference Room:
Ted Tarbell (LMSAL)
"Time Profiles of Magnetic Reconnection Measured from Flare Ribbons"
I use observations of flare ribbons with very high
temporal and spatial resolution to measure time profiles
of magnetic reconnection. TRACE images in the 1600 band
show flare ribbons in the low chromosphere. Several M and
X-class events with time resolution as high as a few
seconds throughout the buildup, impulsive and decay phases
have been analyzed. MDI high resolution magnetograms
provide photospheric fields for estimation of reconnection
rates in Mx/s. Another M-class event is studied with La
Palma observations of very high spatial resolution but
lower cadence; H-alpha images are used to locate the flare
ribbons in this case. Both types of data show examples in
weak plage or network where the flare ribbons do not
overlie significant photospheric magnetic fields. In
regions of stronger fields, careful spatial coalignment of flare
ribbons with photospheric fields can provide information
about the time profile of reconnection in the corona.
Wednesday, May 14, 2003, 2 pm, HEPL Conference Room:
Sylvain Turcotte (LLNL)
"Djehuty, a code for modeling whole stars in 3D"
Stellar models of stars in 1D have been used very successfully for decades
and are still the main tools to study stars theoretically.
There are however many hydrodynamical processes in stars that cannot
be
studied in 1D. Djehuty is a tool that has been developped at LLNL to
study
some of these processes, such as convection for example, in single
stars
and close binary systems.
I will present the code, its capabilities and limitations. I will also
show some examples of the first calculations and discuss our prospects
for future work.
Wednesday, August 13, 2003, 4 pm, HEPL Conference Room:
Hiromoto Shibahashi (University of Tokyo)
"Hydrogen profile around the tachocline layer of the sun"
Sound speed inversions of the Sun show that the profile of the relative
difference between the Sun and the standard solar model has a sharp
peak
around r/R = 0.65, which is the location of the tachocline layer found
by rotation inversions. It has been suggested that this sharp peak
would
be due to the difference, between the Sun and the model, in hydrogen
abundance in the tachocline layer possibly caused by the weak mixing
process.
I quantitatively discuss the hydrogen abundance in the tachocline layer
based on the seismic solar model, which is constructed using the sound
speed and density profiles as well as the depth of the convection zone
obtained by helioseismology. One of the important characteristics of
the seismic solar model is that it gives us hydrogen profile as a part
of solution. It is found that hydrogen abundance of the seismic solar
model
decreases more mildly than that of the standard solar models constructed
by incorporating the diffusion process. This feature hardly depends
on
the profile of the heavy elements as well as uncertainties in the opacity
and the equation of state.
Monday, August 18, 2003, 4 pm, HEPL Conference Room:
Wolfgang Finsterle (Maui Scientific Research Center,
University of New Mexico)
"Seismic probing of the Sun's lower atmosphere"
First results from a new experiment to measure sound waves in the solar
atmosphere show unexpected behaviour of waves between the photosphere
and the low chromosphere. In sunspot and plage regions upward travelling
waves are detected well below the acoustic cut-off frequency. On the
other
hand, at high frequencies (above nu~6 mHz) evanescent-like waves appear
around magnetic regions while most of the quiet Sun shows upward travelling
waves (pseudo modes). There is also evidence for intermittent downward
wave propagation in areas with strong magnetic fields such as sunspots.
The power of the travelling waves can change substantially within
hours, i.e. on time scales comparable to short-term events in the
chromosphere, such as flares or CME's.
Possible implications of these preliminary results on our current
understanding of the Sun will be discussed.
--------------------------------------------------------------
Thursday, August 21, 2003, 4 pm, HEPL Conference Room:
Dean-Yi Chou (National Tsing Hua University, Taiwain)
"Solar Cycle Variations of p-Mode Frequencies"
Abstract:
Observations show that the solar p-mode frequencies change with
the solar cycle. The horizontal-phase-velocity dependence of
the relative frequency change, scaled by mode mass, provides depth
information on the perturbation in the solar interior. We find
that the smoothed scaled relative frequency change varies along
the solar cycle for horizontal phase velocities higher than
a critical value, which corresponds to a depth near the base of
the convection zone. This phenomenon suggests that the physical
conditions in a region near the base of the convection zone change
with the solar cycle.
--------------------------------------------------------------
Friday, August, 22, 2003, 4 pm, HEPL Annex B Conference Room
Takashi Sekii (National Astronomical Observatory of Japan)
TBA
--------------------------------------------------------------
Tuesday, August 26, 2003, 4 pm, HEPL Conference Room:
Douglas Gough (University of Cambridge)
"On Helioseismic Ring Analyses"
--------------------------------------------------------------
Thursday, August 28, 2003, 4 pm, HEPL Conference Room
Shi-Tsan Wu (University of Alabama in Huntsville)
"Acceleration of Fast CME Due to Magnetic Buoyancy Force"
The analysis of LASCO/SOHO, Skylab and Solar Maximum Mission (SMM)
observations show that there are many CMEs initiated with streamer
and flux-
rope magnetic topology (Dere et al. 1999; St. Cyr et al., 1999; Plunkett
et
al., 2000). Two types of CMEs have been distinguished with different
kinematic characteristics (MacQueen and Fisher, 1983; Andrews and Howard,
2001). These are fast CMEs with high initial speeds (i.e. constant
speed)
and slow CMEs with low initial speeds but gradual acceleration (i.e.
accelerated CMEs).
Efforts have been made to probe the underlying physics responsible for
these dual characteristics. Low and Zhang (2002) proposed that
fast and
slow CMEs result from initial topology of the magnetic field characterized
by normal and inverse quiescent prominences, respectively. Liu
et al. have
successfully performed a numerical MHD simulation for this scenario.
In
this presentation, we explore other possible processes using a 2.5D,
time-
dependent streamer and flux-rope MHD model (Wu and Guo, 1997) to
investigate the dual kinematic properties of the CMEs by specifying
the
different initiation processes with a particular magnetic topology
(i.e.
inverse quiescent prominence magnetic topology). Two typical
initiation
processes are tested; (1) injection of the magnetic flux into the flux-rope
causes additional Lorentz force to destabilize the streamer launching
a CME
(Wu et al., 1997) resulting in a category slow CME and (2) draining
the
plasma from the flux-rope together with flux injection leads to a balloon
instability due to the magnetic buoyancy force which results in a impulsive
eruption and launches a fast CME.
----------------------------------------------------------------------
Monday, November 24, 2003, 4pm, HEPL Conference Room
P.A. Sturrock (Stanford)
"SEARCH FOR VARIABILITY OF THE SOLAR NEUTRINO FLUX"
ABSTRACT
We review evidence for variability of the solar neutrino flux derived
from analysis of data from radiochemical experiments. We then discuss
data in 5-day bins recently released by the Super-Kamiodande
Consortium. We have analyzed the data by a likelihood procedure that
has certain advantages over the Lomb-Scargle procedure used by the
consortium. There are three notable peaks in the spectrum. We suggest
that two of these are due to a single r-mode oscillation. The other
peak may be attributed to solar rotation.
-------------------------------------------------------------------
===============================================================
Monday, December 1, 2003, 4pm, HEPL Conference Room
S. P. Rajaguru (Imperial College, London)
"On the physical content of time-distance correlations
of solar acoustic oscillations."
The stochastic nature of solar acoustic oscillations tends to preserve
the
time-symmetry of cross-covariance signal in an average sense, extending
the
validity of source- receiver concept of time-distance correlation.
This
enables us to detect any ordered flow patterns or any localised sound
speed
changes, by measuring the travel times of acoustic waves travelling
in
opposite directions as first shown by Duvall et al (1993). On the other
hand, the stochasticity itself is due to such nature of physical processes
that excites and damps the oscillation modes. While the modal power
spectral
analyses have several quantifiable properties related to such processes,
the
time-distance analysis so far has not exploited such information that
may be
potentially available in the cross-correlation signals. Here, we discuss
the
"noise" properties of travel times measured in time-distance analyses
and
try to differentiate those purely arising from measurement process
itself
and those attributable to physical processes. By comparing noise in
travel
times from GONG and MDI data, we also identify those attributable to
instrument and the earth's atmospheric contribution. We also
discuss the
possible diagnostic capability of time-distance measurements to probe
the
sources of oscillations and in general the nature of convective turbulence.
----------------------------------------------------------------------------
Monday, December 15, 2003, 4pm, HEPL Conference Room
Paul Boerner (Stanford)
"Coronal diagnostics derived from multilayer images"
Spectacular images and movies of the million-degree solar corona have
become available from a variety of spacecraft in recent years, and
have
lead to a deeper understanding of the solar atmosphere. However, the
information about the thermodynamic state of the coronal plasma provided
by images in two or three different bandpasses is generally ambiguous,
and
thus the ability of such observations to constrain models of coronal
structures and provide insight into the physics governing the corona
is
limited. Spectral observations provide more complete diagnostic
information, but lack the spatial resolution and field of view of
multilayer images. We discuss techniques that use multiple narrowband
images (i.e. from four to ten separate bandpasses) to reconstruct the
differential emission measure in a line of sight through the corona.
We
then present the results of efforts to use data taken with the
Multi-Spectral Solar Telescope Array (which produced a set of solar
images in 5 EUV and 2 FUV bandpasses) to construct spatially-resolved
DEMs
that can be used in modeling. We evaluate the limitations of this
technique, primarily its extreme sensitivity to noise, and examine
the
prospects for improving the results with additional data.
--------------------------------------------------------------
Thursday, February 5, 2004, 4pm, HEPL Conference Room
Wolfgang Finsterle (University of New Mexico)
"New Results from South Pole"
Abstract
We used multi-height dopplergrams to measure the sound speed in the
solar
atmosphere and to determine the propagation behaviour of waves below
and
above the acoustic cut-off frequency. In quiet Sun areas, our findings
agree with a simple dispersion relation for acoustic waves
(omega^2=k^2c^2+omega_ac^2). In active regions, the wave field model
needs
at least another component in order to match with the data. Wave reflection
and/or conversion to MHD and Alfven waves could provide the additional
component. We speculate whether it is possible to use these effects
to
derive a magnetic proxy for the chromosphere.
Wednesday, March 31, 2004, 4pm, HEPL Conference Room:
Cheng Fang (Department of Astronomy, Nanjing University)
MAGNETIC RECONNECTION IN THE SOLAR LOWER
ATMOSPHERE: EVIDENCES AND SIMULATIONS
Abstract
Accumulating observational evidences indicate that magnetic reconnection
is a fundamental process in the solar lower atmosphere, which is
responsible for many localized activities and dynamic phenomena in
the
layer. Both the qualitative theoretical consideration and quantitative
numerical simulations demonstrate the applicability of the reconnection
to
the lower atmosphere. After reviewing the evidences in observations
and
the results of some numerical simulations, we present in detail our
numerical simulation of the magnetic reconnection, which can account
for
Ellerman Bombs and Type II white light flares in many observational
aspects, such as the lifetime, weak temperature rise in the upper
chromosphere and in the photosphere, and strong heating in the lower
chromosphere, and so on.
----------------------------------------------------------------------
Thursday, April 8, 2004, 4pm, HEPL Conference Room:
Jiong Qiu (BBSO)
"Magnetic Reconnection and Mass Acceleration in Solar Eruptive Events"
ABSTRACT: I will discuss the observational relationship among magnetic
reconnection, flare emission, and the rising motion of the erupting
flux ropes derived from several well-observed solar eruptive events.
The role of magnetic reconnection in the process of coronal mass ejections
(CMEs) remains an issue in heated debate. Very recently, some observations
suggested that fast acceleration of CMEs takes place in the
early stage of flux rope eruption, which coincides with the impulsive
phase of the flare. We furthered such studies by comparing the rate
of magnetic reconnection with the evolution of flare emission and
erupting flux ropes. By measuring the magnetic flux swept through by
flare
ribbons as they separate in the lower atmosphere, we can infer the
magnetic reconnection rate in terms of the reconnection electric field
$E_c$ inside the reconnecting current sheet (RCS) and also the
total voltage drop along the RCS, or the rate of magnetic flux convected
into the diffusion region. The study verified that the reconnection
rate
in the low corona is temporally correlated with the flare non-thermal
lightcurves, both reaching their peak values during the rising phase
of
the soft X-ray emission. More importantly, the observations reveal
a
temporal correlation between the magnetic reconnection rate and the
acceleration of erupting flux ropes observed as erupting filaments
in the low corona and CMEs at 2-30 solar radii. These results clearly
indicate that magnetic reconnection is the physical link between the
evolution of flares and CMEs.
Wednesday, August 25, 2004, 4pm, HEPL Conference Room
Sandrine Lefebvre (UCLA)
"Solar radius measurements at
Mount Wilson"
Variations of the solar radius are not only important for solar
physics but they also play a fundamental role in the research of
terrestrial climate. In fact, changes in the apparent size of the Sun
could account for a significant fraction of the total irradiance
variations, and solar irradiance is known to be a primary force in
driving atmospheric circulation. While the MDI instrument aboard SOHO
is
likely to provide the most accurate constraint on possible solar radius
variations, the radius measurements obtained from ground base
observations represent a unique resource due to their long temporal
coverage. This paper presents more than 30 years of solar radius
measurements obtained at the Mount Wilson 150-foot solar tower. Our
results are compared to recent MDI observations and to the data obtained
with the heliometer at the Pic du Midi observatory.
Friday, November 5, 2004, 3 pm, HEPL Conference Room
Takeru Suzuki (Department of Physics, Kyoto University)
"What determines the solar wind speed?"
Abstract:
Firstly, I will talk about the roles MHD waves play in coronal heating
and
solar wind acceleration in open field regions based on theoretical
and
computational studies. Then, I will focus on the dependence of the
terminal speed of the solar wind (or equivalently, the wind speed near
Earth at around ~ 1 AU) on various wave injections and geometries of
flux
tubes. In comparison with recent interplanetary scintillation (IPS)
observations by the Nagoya STE group in Japan (Kojima et al., 2004),
I
show that a combination of the field strength and the expansion of
the
flux tubes is an important factor in determining the solar wind speed.
----------------------------------------------------------------
Monday, November 22, 2004, 4pm, HEPL Conference Room:
Andreas Lagg (Max-Plank Institut fur Sonnensystemforschung, Lindau)
"Stokes Polarimetry in He I 10830:
Magnetic Field Topology of an Emerging Flux
Region"
Abstract:
Magnetic field measurements in the upper chromosphere are essential
for
understanding the coupling between the photosphere and the corona.
Unfortunately it is very difficult to retrieve the magnetic field vector
in
this region by direct observations. Here I present a tool based on
the analysis
of Stokes profiles of the He I 10830 triplet which allows for the determination
of the full magnetic vector using a combined Zeeman and Hanle diagnostics.
I
apply this technique to an emerging flux region observed with the Tenerife
Infrared Polarimeter (TIP). Magnetic loops, a current sheet and regions
of fast
downflows were identified.
Tuesday, November 23, 3pm (please, note time change), HEPL Conf. Room
"The Inconstant Sun"
Prof. S.M. Chitre
Department of physics
University of Mumbai, India
The interior of the Sun is clearly not directly accessible to
observations; nonetheless, it is possible to infer the physical
conditions inside the Sun with the help of structure equations
governing its equlibrium and using the powerful observational
tools provided by accurately measured solar oscillation
frequencies and neutrino fluxes. It turns out from the inversion
of helioseismic data that the internal constitution of the Sun
is adequately described by the standard solar model.
The Global Oscillations Network Group and Michelson Doppler Imager
have generated helioseismic data for more than half a solar cycle.
With the availability of these valuable data it is now possible to
study the changes that take place withing the Sun as the solar
cycle progresses and, in particular, to examine the implications of
the temporal variations in the solar rotation rate.
---------------------------------------------------------------
Thursday, December 9, 2004, 4pm, HEPL Conference Room
Jean-Pierre Rozelot (Observatoire de la Cote d'Azur)
"A story of the Sun oblateness:
from Princeton, 1966 to Pic du Midi, 2004"
Abstract:
In this talk we will first introduce the concept of oblateness
in slowly, non rigid rotating stars. Then, we will extend to stars
having a differential rotation. For the Sun, we will emphasize
the fact that the differential rotation, anchored not only on the
surface, but also deeper, contributes to distort the free surface
of the body. Then, we will review measurements of solar oblateness
since the measurements made by Brans and Dicke in Princeton in 1996.
It will be shown how inaccurate measurements show themselves useful.
We will extend the subject to our own measurements made from 1996 to
2004
by means of the scanning heliometer at the Pic du Midi Observatory.
We will compare these measurements with those obtained at Mount Wilson.
We will describe solar asphericities whose departures from sphericity
should certainly not exceed a few milliseconds of arc. These low values
still allow to constrain helioseismic models. Incidentally, we
will give the latest values of the solar gravitational moments
as deduced from models of rotation using for the first time
a radial gradient of rotation. A few words on related alternative
theories on general relativity will also be said. Lastly, we
will conclude by showing the contribution of some space experiments
or balloon fights, like SDS.
Wednesday, March 30, 2005, 4pm, HEPL Conference Room
Axel Brandenburg (Nordita)
"The case for a distributed solar dynamo shaped by near-surface shear"
Arguments for and against the widely accepted picture
of a solar dynamo being seated in the tachocline are reviewed
and alternative ideas concerning dynamos operating in the
bulk of the convection zone, or perhaps even in the near-surface
shear layer, are discussed. Based on the angular velocities
of magnetic tracers it is argued that the observations are compatible
with a distributed dynamo that may be strongly shaped by
the near-surface shear layer. Direct simulations of dynamo action
in a slab with turbulence and shear are presented to discuss
filling factor and tilt angles of bipolar regions in such a model.
Friday, December 9, 2005, 4:00pm, HEPL Conference Room:
Kazunari Shibata (Kyoto University)
"Magnetic Reconnection in Solar Plasmas"
Monday, February 6, 2006, 4pm, HEPL Conference Room:
Giora Shaviv (Technion)
"The state of Be7 in the Sun"
Abstract. The problem of the ionization of the heavy elements
including Be7 was last visited by Iben, Kalata and Schwartz
in 1967. We revisit the problem using modern methods for
calculating ionization in dense media and get different results.
The implication to the solar neutrino problem are discussed.
Tuesday, March 21, 2006, 4pm, HEPL Conference Room:
Martin Woodard (NWRA/CoRA)
"The helioseismic correlation signature
of moderate-scale convection"
Abstract:
Flows in the solar convection zone introduce correlation between distinct
horizontal-wave-vector and frequency components of wave motion in the
Sun
and helioseismic correlation (covariance) data can be inverted directly
to map subsurface flow.
The success of the inversions depends on having an accurate forward
model
to relate correlation data to subsurface flow.
I am using a model of randomly-excited p- and f-mode oscillations
in a plane-parallel, but otherwise realistic, solar envelope, to
model the data dependence on supergranular- and mesogranular-scale
convection.
Since the calculation uses the Born approximation, appropriate to weak
flows, the data are linearly related to the subsurface velocity by
a sensitivity kernel.
I will illustrate the use of correlation data with some examples and
describe some inversion results using SOHO/MDI high-resolution Doppler
data. The correlation data also contain information about the
sensitivity kernel and I will describe a method of estimating the
kernel from the observations. The measured and theoretical kernels
compare reasonably well -- a direct indication of the soundness
of the forward model. I will also discuss shortcomings of the model
and how they might be overcome.
Monday, May 22, 2006, 4 pm, HEPL Conference Room:
Dominique Fluri (Institute of Astronomy, ETH, Zurich)
"Dynamo modes and active longitudes on cool stars"
Abstract:
Magnetic activity similar to that of the Sun is observed on a variety
of
cool stars with external convection envelopes. On the Sun we can investigate
activity phenomena in great detail, while cool, active stars cover
a large
range of global stellar parameters and provide key constraints for
stellar
and solar dynamo theory.
In the first part of the talk we review the properties of magnetic activity
on cool stars with an emphasis on star spots. Time-series observations
over
decades reveal stellar cycles similar to the 11-year sunspot cycle.
Of particular
interest is the observation that starspots tend to appear preferably
at
certain longitudes, so called active longitudes, indicating the presence
of
non-axisymmetric dynamo modes. In the second part of the talk
we have a look at the configuration of dynamo modes that is necessary
to
explain the observed phenomena, and we introduce a method that allows
us to
determine empirically the magnetic field configuration on cool, active
stars.
----------------------------------------------------------------------
Wednesday, May 24, 2006, 4pm, HEPL Conference Room:
S.M. Chitre (University of Mumbai)
"Seismic View of the Changing Sun"
Abstract.
With the accumulation of helioseismic data from GONG and MDI
projects over the past decade, it has now become possible to investigate
changes taking place inside the Sun as the solar cycle progresses.
The temporal variations with the activity cycle in the f- and p-mode
oscillation frequencies, rotational kinetic energy and magnetic
energy in the solar interior can be studied with a view to
understand the underlying mechanism responsible for driving
the solar dynamo and causing the total irradiance variations.
Monday, June 12, 2006, 4pm, HEPL Conference Room:
Bernhard Kliem (Astrophysical Institute Potsdam) B
"Flux Rope Instabilities at the Onset of CMEs"
Images of erupting prominences typically suggest a magnetic topology
of a
single line tied flux rope. The majority of prominence eruptions and
CMEs
begins with an approximately exponential rise, suggesting that an
instability of a flux rope may occur at the onset of the eruptions.
I
will present numerical simulations of two relevant instabilities, the
well-known helical kink instability and the torus (expansion)
instability, which has received less attention so far. An elementary
analytical description of the torus instability will also be presented.
The kink instability appears to occur in fast eruptions that develop
a
clearly helical shape while rising. Very good quantitative agreement
is
obtained between simulations of kink-unstable flux ropes and several
well-observed events, which range from a failed filament eruption to
one
of the fastest CMEs on record. However, the kink instability is expected
to saturate relatively quickly, and a significant fraction of the
eruptions does not seem to be strongly twisted or helical. Here the
torus
instability is relevant. It can trigger the eruption of non-kinking
flux
ropes, provided they have risen sufficiently to nearly semicircular
shape, and it governs the medium to large-scale expansion of flux ropes.
The torus instability is driven by the hoop force and requires the
external poloidal field to decrease sufficiently rapidly with height
h
above the rope, at least as, roughly, h^{-3/2}. It provides a uniform
description of fast and slow CMEs. Its relation to a catastrophic loss
of
equilibrium requires further study. I will conclude with a brief
discussion of the relationship between ideal flux rope instability
and
magnetic reconnection in the developing vertical current sheet.
---------------------------------------------------
Wednesday, June 21, 2006, 4pm, HEPL Conference Room:
Sylvaine Turck-ChiŰze (SAp/DAPNIA/CEA)
"On gravity modes with GOLF instrument and perspectives
with GOLFNG and the DynaMICS project"
Tuesday, July 10, 2007, 4pm, P&A Conference Room 232:
Timur Rashba (Max-Planck-Institut fur Physik)
Probing the internal solar magnetic field through g-modes and neutrinos
I will discuss the effect of radiative zone magnetic fields on g-mode
frequencies. It will be shown that a 1% g-mode frequency shift with
respect to the Solar Seismic Model (SSeM) prediction, currently hinted
in
the GOLF data, can be obtained for magnetic fields as low as 300 kG,
for
the radial order mode n=-20. On the other hand, a similar shift for
the
low order g-mode candidate (l=2, n=-3) can not result from central
magnetic fields, unless these exceed 8 MG. I will also comment that
future
solar neutrino observations may provide us some information about magnetic
fields.
Tuesday, July 24, 2007, 4pm, P&A Conference Room 232:
Joao Pulido (Department of Physics, Instituto Superior TÝcnico, Lisbon, Portugal)
'Solar neutrinos: oscillations and magnetic moment'
Abstract
The solar neutrino deficit observed by experiment was explained with
resort to previously unsuspected properties of the neutrino which
were inconsistent with conventional views. Several hypotheses were
proposed and among them the oscillation one was confirmed after three
decades of experimental observations. An important question must now
be addressed, since there is a large amount and variety of data: is
the flux constant or is it variable in time? If it is variable, new
properties of the neutrino will be necessary to introduce.
Tuesday, July 31, 2007, 4pm, P&A Conference Room 232:
Douglas Gough (University of Cambridge)
The gamma Equulei phenomenon: shock waves in rapidly oscillating Ap-star atmosphere
Urmila Mitra-Kraev (University of Sheffield)
Measuring the Sun's meridional flow with SOHO/MDI
The aim is to measure the radial profile of the solar meridional flow
throughout the entire convection zone.
The meridional circulation is an important ingredient in many
theoretical models. In mean-field models of angular momentum
transport, the meridional circulation is responsible for how
differential rotation is established and maintained, while in
flux-transport models of the solar dynamo it is needed to transport
the magnetic field from the top to the bottom of the convection zone.
Since the detection of the surface meridional flow in 1979 by Tom
Duvall, many helioseismic measurements revealed a poleward flow in
the
near surface layers of the convection zone of around 10-20
m/s. Assuming mass conservation and that the meridional circulation
cell is contained within the convecton zone, Peter Giles (1999) has
inferred from time-distance measurements a return flow below 0.8 solar
radii. Other efforts to measure a return flow have so far not given
conclusive results.
Here, we apply a novel approach of measuring the radial meridional
flow profile by applying a 2D Fourier transform on latitudinal
averaged data and establishing the shift in the p-mode ridges caused
by the North-South flow. This techniques allows us in principe to
measure the average meridional flow profile throughout the convection
zone.
Monday. December 17, 11:00 am,Physics & Astrophysics Conference Room 232:
Claus Fr?hlich (PMOD/WRC, Davos)
What are the mechanisms for long-term changes of TSI?
The main issues are:
o TSI is reaching
a much lower minimum
than the last ones. Comparison with ACRIM II and
III and with TIM/SORCE confirm the observed
downward trend within about 10-50ppm/decade form
ACRIM and TIM respectively. This has to compared
to the difference between the 1996 and 2007
minima of 165 ppm. The normal magnetically
related indices like Mg-II and F10.7 do not show
a similar decrease. On the other hand the IMF
(from the OMMNI tape) shows a similar decrease
and so would the open field from the Sun. Does
MDI show an indication of a lower magnetic Sun
during the present minimum than the last one.
What is the possible influence of the much lower
polar field since its reversal? So, all of this
could still be magnetic - we may just not have
the real understanding of the activity cycle.
o Frequency changes
of the low order p
modes show a good correlation with TSI until
about the beginning of 2003. Then the frequency
changes increase for an year or so whereas TSI
continues to decrease.
Wednesday, January 30, 2008, 4:00 pm, Physics & Astrophysics Conference Room 232:
"New insights into the non-equilibrium state of coronal loops from TRACE and STEREO"
Markus J. Aschwanden (LMSAL)
Abstract:
Attempts have been made to understand the coronal heating problem
by applying equilibrium solutions to coronal loops, where the spatial
heating function balances the energy losses by thermal conduction
and radiation. Hydrodynamic simulations, however, demonstrate that
loops are underdense during the heating phase and overdense during
the cooling phase. This overdensity of coronal loops can now accurately
be measured from triple-filter analysis of TRACE data and from
stereoscopic 3D reconstruction of loop geometries with STEREO/EUVI.
The observational results clearly demonstrate that virtually all loops
observed
in EUV are in the non-equilibrium cooling phase, after they have been
heated to
soft X-ray temperatures of T~1-10 MK before. We will discuss the non-
equilibrium scaling laws that apply to coronal loops, as well as to
solar and
stellar flares.
We conclude with a new outlook about the consequences for solving the
coronal heating problem.
Wednesday, March 19, 2008, 4pm, Physics & Astrophysics Conf.Room 232:
Alberto Sainz Dalda (IAC, Spain)
"Sunspot and magnetic flux decay: what are we missing?"
Abstract: The sunspots structure, evolution and the role that
they play
in the evolution of the solar magnetic field are still open questions.
We present a wide vision about these issues from several points of
view.
We firstly show the last observational evidences about a relation
between the sunspot penumbra, the moat, the
MMFs and their role in the decay of sunspots. Later, we explain why
these important aspects are not enough for understanding the decay
(and
dissapearance) of the magnetic flux of the Active Region. We show how
we
have attacked this question using MDI and TRACE data.
Thursday, March 20, 2008, 4pm, Physics & Astrophysics Conf.Room 232:
Elena Khomenko (IAC, Spain)
"NUMERICAL SIMULATIONS OF MAGNETO-ACOUSTIC WAVE PROPAGATION
IN MAGNETIC STRUCTURES OF DIFFERENT SIZE"
Abstract:
In this talk, we present results of simulations of
magneto-acoustic waves from the photosphere to the chromosphere in
magnetic structures with different size. We compare the behaviour
of a thick flux tube with a size similar to a small sunspot with
the behaviour of a small-scale thin flux tube with a size typical
for the solar network elements. Both, thin and thick flux tubes in
our modeling posses an internal structure and have gradients of
the Alfven and Acoustic speeds in vertical and horizontal
directions. The waves are generated by a localized driver at the
photospheric level. In the case of the sunspot, the fast
(magnetic) mode in the region cS < vA does not reach the
chromosphere and reflects back to the photosphere at a somewhat
higher layer than the cS = vA line. This behavior is due to
wave refraction, caused, primarily, by the vertical and horizontal
gradients of the Alfven speed. The slow (acoustic) mode
continues up to the chromosphere along the magnetic field lines
with increasing amplitude. In the case of a network flux tube, the
photospheric driver moves the magnetic field lines in horizontal
direction exciting the slow (magnetic) mode. After the mode
transformation at heights above the temperature minimum, the
acoustic mode is produced. It follows straight up to the
chromosphere forming shocks. The direction of propagation of these
shocks is along the flux tube boundary. The main period observed in
the
chromosphere depends on the radiative losses of oscillations.
The channeling of five-minute waves into the chromosphere is observed
in our simulations if the radiative relaxation time is sufficiently
small, as expected in small-scale magnetic structures as flux tubes.
Wednesday, April 16, 2008, 4:00 pm, Physics & Astrophysics Conf.Room 232:
Buoyant plumes in solar prominences: Hinode/SOT observations of a new phenomenon on the Sun
Thomas Berger
Lockheed Martin Solar and Astrophysics Laboratory
We report new findings from multi-hour movies of solar quiescent
prominences (QPs) observed with the recently launched Solar Optical
Telescope (SOT) on the Hinode satellite. SOT is capable of observing
prominences for up to 6 hours with a spatial resolution of 0.22
arcseconds (160 km) and a temporal resolution of 15 seconds. The
image quality is uniform throughout all time series. SOT observations
verify previous ground-based observations of filamentary downflows
as
well as large-scale vortex flows in QP sheets. SOT observations also
verify the existence of large-scale transverse ``body oscillations''
in QPs, with periods of 20--40 minutes and amplitudes of 5--10 Mm.
We
measure the upward propagation speed of several waves to be ~10 km/
sec, consistent with the sound speed of a 10,000 K plasma. Most
surprisingly, SOT movies show that buoyant starting plumes occur
episodically in most QPs observed to date. The plumes are visible as
dark turbulent upflows plowing through the prominence plasma with
relatively constant ascent velocities of ~20 km/sec. The plumes are
200--700 km in width and sometimes exhibit "mushroom head" vortex
formation at the leading edge. The plumes rise to heights of ~10--20
Mm above the prominence base and are clearly visible in both Ca II
396.8 nm and H-alpha 656.3 nm images. The plumes are never seen in
active region prominences or in QPs with horizontal thread
structuring. We discuss possible mechanisms for the plume formation
as well as the structural and dynamic differences between active
region and quiescent prominences in the SOT database.
Wednesday, April 30, 2008, 4:00 pm, Physics & Astrophysics Conf.Room 232:
Vertical Temperature Structures of the Solar Corona derived with Hinode/XRT
Kano, Ryouhei (NINS/National Astronomical Observatories of Japan)
Abstract:
We obtain temperature structures in faint coronal features above and
near the solar limb with the X-ray Telescope (XRT) on board the Hinode
satellite by accurately correcting the scattered X-rays from
surrounding bright regions with occulted images during the solar
eclipses. Our analysis yields a polar coronal hole temperature of
about 1.0 MK and emission measure in the range of 10^25.5 -- 10^26.0
cm^{-5}. In addition, our methods allow us to measure the temperature
and emission measure of two distinct quiet-Sun structures: radial
(plume-like) structures near the boundary of the coronal-hole and
diffuse quiet Sun regions at mid-latitudes. The radial structures
appear to have increasing temperature with height during the first
100
Mm and constant temperatures above 100 Mm. For the diffuse quiet-Sun
region the temperatures are the highest just above the limb and appear
to decrease with height. These differences may be due to different
magnetic configurations.
------------------------------------------------------------------------
Wednesday, May 7, 2008, 4:00 pm, Physics & Astrophysics Conf.Room
232:
Numerical simulations of MHD waves in coronal loops
Marcin Gruszecki (Maria Curie Skladowska University of Lublin)
Abstract:
Magnetic loops are the main building blocks of the solar corona, that
are able to sustain various kinds of waves and oscillations. The waves
may play an important role in the heating of the loops, as was first
suggested by Ionson (1978), and also can be used as a diagnostic tool
of
loop parameters (e.g., Nakariakov and Ofman 2001).
The hot coronal loops were detected to oscillate mainly in a slow
magnetosonic mode (Wang et al. 2002). Cool loops primarily sustain
fast
magnetosonic kink oscillations which are observed in two polarizations:
horizontal (Aschwanden et al. 1999, Nakariakov et al. 1999, Van
Doorsselaere et al. 2007) and vertical (Wang & Solanki 2004). The
detection and identification of magnetohydrodynamic (MHD) waves and
oscillations has made MHD coronal seismology a viable diagnostic tool
for the determination of unknown parameters of the corona (Uchida 1970,
Roberts et al. 1984).
In my work I consider to the observationally determined signatures of
the vertical oscillations (Wang & Solanki 2004). I modelled the
oscillation by taking into account a simple two dimensional geometry
of
a coronal loop arcade in which loop oscillations are excited by
localized pulses in the lower atmosphere.
Any suggestions for speakers should be sent to:
akosovichev@solar.Stanford.EDU
.
-Alexander Kosovichev (415) 723-7667
To be added to the CSSA email list, please contact:
todd@quake.Stanford.EDU
.
-Todd Hoeksema (415) 723-1506