HS_seminars

Tom Berger, Bart De Pontieu, Karel Schrijver, Alan Title
Lockheed Martin Solar and Astrophysics Laboratory

"Destruction of comets in the solar corona:
Kreutz sungrazing comet fragments observed with SOHO"

Rich Wolfson (Middlebury College)
"Energetics of Coronal Mass Ejections"

Doug Braun (Solar Physics Research Corporation and High Altitude Observatory)

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.

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).

"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.

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.

"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

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.
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Wednesday, June 9, 1999, 4pm, HEPL Conference Room:

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.

"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.

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"

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.

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.

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

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.

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.

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.

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

"The reconstruction of nonlinear force-free fields from
vector-magnetograph data"

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.

"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.
___________________________________________________________________________

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.
_________________________________________________________________________

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.

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Tuesday, August 14, 2001, 4pm, HEPL Conference Room:

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.

S.T. Wu
Center for Space Plasma, Aeronomy, and Astrophysics Research
University of Alabama in Huntsville

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.

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Friday, November 16, 2001, 4pm, HEPL Conference Room:

"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).
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Wednesday, November 28, 2001, 10 am, HEPL Conference Room:

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.
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Wednesday, December 19, 2001, 3:00 pm, HEPL Conference Room:

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.

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Thursday, December 20, 2001, 4:00 pm, HEPL Conference Room:

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.

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Monday, February 4, 2002, 4 pm, HEPL Conference Room:

"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.

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.

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Tuesday, July 16, 2002, 4pm, HEPL Conference Room:

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.

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Thursday, August 1, 2002, 4pm, HEPL Conference Room:

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.

"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.
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Thursday, August 15, 2002, 4pm, HEPL Conference Room:

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.

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Wednesday, August 21, 2002, 4pm, HEPL Conference Room:

"Solar wind variations related to fluctuations of the North
Atlantic Oscillation"

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.

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Thursday, September 12, 2002, 4pm, HEPL Conference Room:

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:

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:

"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:

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.

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.

"Spatial and Temporal Organizations of Solar Flare Activity: Hot
Spots and Periodicities."

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:

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:

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:

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:

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.

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.

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.

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:

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

TBA
--------------------------------------------------------------
Tuesday, August 26, 2003, 4 pm, HEPL Conference Room:

--------------------------------------------------------------
Thursday, August 28, 2003, 4 pm, HEPL Conference Room

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

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

"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

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.

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.

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:

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.

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.

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.

"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.

                 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

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.

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.

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.

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.

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.
----------------------------------------------------------------------

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.

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:

"On gravity modes with GOLF instrument and perspectives
with GOLFNG and the DynaMICS project"

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.

Joao Pulido (Department of Physics, Instituto Superior Tщcnico, Lisbon, Portugal)

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.

The gamma Equulei phenomenon: shock waves in rapidly oscillating Ap-star atmosphere

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.

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"

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.

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.

"NUMERICAL SIMULATIONS OF MAGNETO-ACOUSTIC WAVE PROPAGATION
IN MAGNETIC STRUCTURES OF DIFFERENT SIZE"

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.

Buoyant plumes in solar prominences: Hinode/SOT observations of a new phenomenon on the Sun

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.

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:

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.

Combined Analysis of Solar Neutrino and Solar Irradiance Data:
Indications of an Inhomogeneous, Fluctuating, Slowly Rotating Core,
and Suggestions of an Inner Tachocline, an Inner Dynamo, and a
Second Solar Cycle

Abstract. A search for any particular feature in any single solar
neutrino dataset is unlikely to establish variability of the solar
neutrino flux since the count rates are very low. It helps to combine
datasets, and in this talk I examine data from both the Homestake and
GALLEX experiments. These show evidence of modulation with a
frequency of 11.85 yr-1, which could be indicative of rotational
modulation originating in the solar core. I find that precisely the
same frequency is prominent in power spectrum analyses of the ACRIM
irradiance data for the Homestake and GALLEX time intervals. These
results suggest that the solar core is inhomogeneous and rotates with
sidereal frequency 12.85 yr-1. I find, by Monte Carlo calculations,
that the probability that the neutrino data would match the
irradiance data in this way by chance is only a few parts in 10,000.
This rotation rate is significantly lower than that of the inner
radiative zone (13.97 yr-1) as recently inferred from analysis of
Super-Kamiokande data, suggesting that there may be a second, inner
tachocline separating the core from the radiative zone. This opens up
the possibility that there may be an inner dynamo that could produce
a strong internal magnetic field and a second solar cycle. Analysis
of ACRIM data points to the existence of such a cycle with frequency
0.92 yr-1.

The moving magnetic features (hereafter MMFs) activity around the sunspots
is a well-known phenomenon, but several questions are still opened. They
have been related with the Evershed flow, penumbral filaments,
moat velocity flow and decay of ARs, but their behaviour is not well understood.
We don't know if they reach the chromosphere or they are a photospheric
shallow structure. We present several observations apparently related with the MMF
activity taken with SoHO/MDI, TRACE and Hinode observatories.
Besides, we show the results of simultaneous spectropolarimetric observation
at the photosphere and chromosphere on an MMF and ER observed by THEMIS telescope.

Measurements of Absorption, Emissivity Reduction and Local Suppression
of Solar Acoustic Waves in Sunspots

Abstract:
Absorption, emissivity reduction, and local suppression of solar acoustic
waves may contribute to the observed acoustic power reduction in magnetic
regions. Determining the contribution from these three mechanisms in
sunspots is one of important problems in local helioseismology. We propose
a model for the energy budget of acoustic waves, and use the property
that the waves emitted along the wave path between two points have no
correlation with the signal at the starting point to separate the effects
of these three mechanisms. Applying the cross-correlation technique to
the waves filtered with the direction and phase-velocity filters, we
measure the fraction of the contribution from each mechanism to the power
deficit in the leading sunspot of NOAA 9057. The power in the sunspot
and the secondary image computed from the measured coefficients of
absorption, emissivity reduction, and local suppression is consistent
with the power directly measured from acoustic power maps.

"Tying The Solar Wind To Its Photospheric Origins: Tracking The Footpoint Of Earth-Directed Solar Wind"

Abstract:
We present a new method of visualizing the solar photospheric magnetic
field based on the ``Magnetic Range of Influence'' (MRoI). The MRoI is
a simple realization of the magnetic environment in the photosphere,
reflecting the distance required to balance the integrated magnetic
field contained in any magnetogram pixel. It provides a new
perspective on where sub-terrestrial field lines in a Potential Field
Source Surface (PFSS) model connect to the photosphere, and thus the
source of Earth-directed solar wind (within the limitations of PFSS
models), something that is not usually obvious from a regular synoptic
magnetogram. In each of a number of sample solar rotations, at
different phases of the solar cycle, the PFSS footpoint either jumps
between isolated areas of high MRoI or moves slowly within one such
area.

We make predictions of interplanetary magnetic field direction, solar
wind composition and speed based on the footpoint position as it
crosses neutral lines and moves into and out of coronal holes, and
compare these to the observed quantities in situ.

Knowing when the jumps will occur has great relevance to the
generation of synoptic maps and other derived data products from SDO
-- i.e., on what timescales it is necessary to update them?

Abstract:
Hinode enables the highest resolution imaging of prominences
as yet seen with a temporal uniformity that allows long-hours
of diffraction-limited movies and with a capability of photospheric
vector magnetic field measurements. I investigate fine
structures of active region prominences and their formation
and evolution processes with Hinode Solar Optical Telescope
(SOT) observations. Here I'd like to introduce my research
activities which are in my Ph.D. thesis and also on-going studies.
The topics are as follows:

(1) Detection of Alfvenic waves in an AR prominence
(2) Discovery of an emerging helical flux rope under an AR prominence
(3) Ongoing studies about QS prominence associated with
activation of overlaid cavity

Tuesday, August 25, 2009, 4:00 pm, Physics & Astrophysics Conf.Room 102/103

Gerhard Haerendel (Max Planck Institute for Extraterrestrial Physics)

"Chromospheric Evaporation via Alfvén Waves"

Thursday, December 10, 2009, 4:00 pm, Physics & Astrophysics Conf.Room 232

Oscar Olmedo (George Mason University)

"Stability of a Partial Torus Flux Rope"

Flux ropes are now generally accepted to be the magnetic
configuration of coronal mass ejections (CMEs), which are formed
prior or during solar eruptions. Recent simulations and observations
have revealed that the magnetic fields overlying flux ropes play an
important role in their eruption or confinement due to the torus
instability (TI). A scenario where it is assumed that a flux rope has
formed prior to an eruption is studied. Various profiles of overlying
magnetic fields are investigated and stability criteria for each are
determined. The equations that describe the motion of the flux rope
are solved numerically and a time dependent flux injection is used to
drive the flux rope. The function of flux injection seems to
be its influence on the kinematical evolution of the CME, as flux is
injected the flux rope rises and will erupt as soon as it has
reached a height associated with the a critical index n that
quantifies the overlying fields. This kind
of eruption scenario may be associated with catastrophe.

Friday, December 11, 2009, 10:30 am, Physics & Astrophysics Conf.Room 232

Henrik Lundstedt (Swedish Institute of Space Physics)

"Solar storms, cycles and topology"

Space weather is a practical concern and predicting severe events is a key task of the International Space Environment Service, ISES. Long term prediction of complex dynamical systems is difficult, but important.

Two systems of Lorenz-type equations modeling solar magnetic activity are studied: Firstly a low order dynamic system in which the toroidal and poloidal fields are represented by x and y coordinates respectively, and the hydrodynamical information is given by the z coordinate. Secondly a complex generalization of the three ordinary differential equations studied by Lorenz.

By studying the Poincarй map we give numerical evidence that the flow has an attractor with fractal structure.

The period is defined as the time needed for a point on a hyperplane to return to the hyperplane again. The periods are distributed in an interval. For large values of the Dynamo number there is a long tail toward long periods and other interesting comet-like features.

These general relations found for periods can further be physically interpreted with improved helioseismic estimates of the parameters used by the dynamical systems. The Solar Dynamic Observatory is expected to offer such improved measurements.

Monday, January 11, 2010, 4:00 pm, Physics & Astrophysics Conf.Room 232

Peter Sturrock (Stanford)

"Solar-Neutrino, Solar-Irradiance, and Nuclear-Decay-Rate Variations: Is There a Common Thread? "

Power-spectrum analyses of solar-neutrino data and ACRIM total solar irradiance measurements reveal a common periodicity that looks as if it is due to rotation, but is too low to be compatible with either the convection zone or the radiative zone. Inspection of nuclear decay-rate data acquired at Brookhaven National Laboratory and at the Physikalisch-Techische Bundesanstalt in Germany shows unmistakable evidence of an annual periodicity. Power-spectrum analysis of BNL data also reveals a periodicity compatible with that found in neutrino and irradiance data. What are these data trying to tell us?

Monday, September 27, 2010, 4:00 pm, Physics & Astrophysics Conf.Room 232

EIT Waves: A new window for coronal seismology

P. F. Chen
Department of Astronomy, Nanjing University, Nanjing 210093, China

Waves provide a unique approach to diagnose the physical properties of the
propagation media, which led to the great success of helioseismology in
the past two decades. On the contrary, the coronal seismology advances
relatively slower, with the main progress coming from coronal loop
oscillations, which treating the trapped waves. In the corona, a globally
propagating wave, namely EIT wave, has been discovered, and it may be
promising for the global coronal seismology. However, the key problem is
that there are hot debates on the mode of EIT waves. They were initially
proposed to be fast-mode MHD waves, which are, however, incompatible with
many observational features of EIT waves. In this talk, after reviewing
the drawbacks of the fast-mode wave model, I will concentrate on how the
EIT wave properties can be accounted for in the framework of our fieldline
stretching model. According to this model, EIT waves are the apparent
propagation of density enhancements produced by the successive stretching
of the magnetic field lines overlying the flux rope. The application of
EIT waves to the coronal seismology is proposed.

Gustavo Guerrero

NORDITA

Abstract:

One of the existing hypothesis on sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong vertical shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and rises through the convection zone up to the surface. However, several physical constrains are required to be fulfilled for this model to be feasible. In this seminar I will present the results of numerical simulations of thermal convection including a narrow radial shear layer. The model tries to mimic, within the numerical limitations, the conditions in the solar convection zone and the tachocline. The excitation of dynamo action as well as the buoyant properties of the generated magnetic field are explored under different conditions.

To be added to the CSSA email list, please contact:
todd@quake.Stanford.EDU . -Todd Hoeksema (415) 723-1506