Applications of SOI-MDI Magnetic Images - 1
Possible Changes of Large-scale Photospheric magnetic Fields and the 6 January 1997 CME

Xuepu Zhao, J. Todd Hoeksema and Philip H. Scherrer
Stanford University

(Last modified on April 8, 1997. Please send comments and suggestions to Xuepu Zhao)

1. Introduction

The LASCO halo coronal mass ejection (CME) observed late on 6 January 1997 is well known for its heliospheric and terrestrial effects, unusual during solar minimum. By assuming a constant propagation velocity of 450 km/s, the arrival of its interplanetary counterpart at the Earth on January 10 has been successfully predicted (LASCO press release). The SHINE Report (Webb, 1997) indicated that the Jan. 6 CME was associated with a disappearing filament (DSF) observed between 13:01 and 14:53 UT Jan. 6 and centered at S24 W01. From the DSF and the polarity distribution of the photospheric magnetic field around the DSF measured by SOI-MDI at the time the quiet filament disappeared, the associated southward interplanetary magnetic field (IMF) event can be inferred (Zhao and Hoeksema, 1997) to be a strong southward IMF event with long duration. The Jan. 6 Halo CME and its interplanetary counterpart are thus expected to be deleterious and geoeffective. The predictability of such deleterious CMEs is very useful. However, the nature of Halo CMEs is little understood.

It is generally believed (e.g., Low, 1996, and references therein) that CMEs are driven by the free magnetic energy that is stored in the pre-eruption coronal magnetic field. Two physical processes have been suggested that may increase the free magnetic energy in the corona. One is the continuous injection of fresh magnetic fluxes with non-zero magnetic helicity into the low corona through the photosphere from the convection zone. The other is the photospheric movement, whether turbulent transport or differential shearing, of footpoints of the closed coronal field lines. This is the basis for the popular notion that magnetic energy can build up on a time scale long compared to the typical coronal Alfven transit time (which is of the order of minutes.) Thus on the large scales, a magnetic structure may evolve quasi-statically and then erupt.

Observations show that episodic CMEs often originate in long-lived large-scale coronal streamers (Hundhausen, 1995). The WSO computed source surface field map for Carrington rotation 1918 suggests that there might be a coronal streamer above 2.5 solar radii located 15 degrees south on the 6 January solar disk. The MLSO/HAO white light daily images on 2 and 13 January 1997 (no observations between 27 December 1996 and 1 January 1997) also imply the existence of such a coronal helmet streamer. These suggest that the 6 January Halo CME originates in the coronal helmet streamer.

A few dynamical processes have been proposed to highlight how a quasi-stable energized magnetic structure becomes unstable, rapidly releasing the stored free magnetic energy. It has been suggested that a configuration with strong arcade fields can erupt when the arcades evolves to become globally unstable (Moore and Roumeliotis, 1992); are subject to a loss of equilibrium (Low, 1981; Linker and Mikic, 1995); or are destabilized by interactions with external structures (Rust, 1976; McAllister et al., 1996) or with newly emerging fields (Rust, Nakagawa, and Neupert, 1975; Feynman and Martin, 1995).

From the energy point of view, no agents in the photospheric magnetic field could drive the mass ejection at a speed high enough to keep pace with the rapid motions, typically at the Alfvenic speed, in the corona (Low, 1996). However, changes of the photospheric field configuration may be associated with the coronal field configuration at the time when the coronal field becomes globally unstable, loses equilibrium or is destabilized, leading to the launch of CMEs.

The unprecedented high-cadence SOI-MDI synoptic observations of the photospheric magnetic field that began in May 1996 make it possible to monitor changes of the photospheric field on the time scale of 96 minutes with 2" pixels. During some campaigns the field can be measured each minute in 0.6" pixels. This poster first displays the 15 MDI full disk magnetic images observed on January 6, 1997, showing no significant changes visible in the high resolution images, then shows how to emphasize the large scale structure by lowering the spatial resolution of the MDI images, and indicates what is the change of the large-scale photospheric field at the onset time of the Jan. 6 CME. Three more samples are examined to confirm this finding. Finally we discusses the onset time of CMEs.

We note that the calibration and analysis of the MDI magnetograms presented here is preliminary and subject to change.

2. Small scale field variation around the 6 January 1997 CME

The MDI GIF movie of the January 6 - 7, 1997 event is a collection of 9 1-minute MDI magnetic images observed between 9:19 and 9:22 UT on Jan. 6 and between 10:24 and 10:28 UT on Jan. 7. It shows concentrations of the photospheric magnetic field on the spatial scale of 1000 km.

Figure 1 displays 15 96-minute full disk MDI magnetic images observed on January 6, 1997. The numbers on top of each panel give the observational time and the maximum and minimum values of the observed line-of-sight field. Except for slight changes of the maximum and minimum values from one image to the next, it is hard to see any significant change in the images over the spatial scale of 3000 km. Because of the extreme low solar activity in solar minimum phase, it is also not easy to clearly identify the polarity inversion zone on the either side of the coronal `inversion line' (Martin, 1990). It is certain from the MDI magnetic observations that no active-region type of magnetic fluxes emerged on 6 January. Thus the CME was not initiated by destabilization through interactions with newly emerging fields and structures.

We have shown that the Jan. 6 CME probably originates in a coronal streamer including a filament. The photospheric field structures that are associated with such large scale coronal structures may be found in large scale photospheric magnetic fields.

3. large scale field variation around the 6 January 1997 CME

The change of the large-scale photospheric magnetic field on time scales shorter than a couple of hours may be detected from the large-scale photospheric field included in the MDI full disk magnetic images. By lowering the spatial resolution of MDI magnetic images, the larger scale field structures may be seen, as shown in Figure 2 . Figure 2 displays the image of 01:06_11:12 (see Figure 1) with spatial resolutions of 3000 km, 6000 km, 12000 km, 24,000 km, 48,000 km and 96,000 km, corresponding to panels from top left to bottom right, respectively. The numbers on each panels are the number of pixals.

Figure 3 shows how to exhibit structures of the largest scale (0.14 solar radii) photospheric field in Figure 2. Panel a is the same as the bottom right panel in Figure 2. Panels b and c are obtained, respectively, by square root of the data in panel a and by smoothing the panel b. Panel d is a plot of contours with filled colors using the data of panel a. It shows that the method to obtain panel d is the easiest way to exhibit the large scale structure of the photospheric magnetic field from the MDI full disk magnetic images.

Figure 4 displays 15 plots of contours with filled colors obtained from Figure 1. The numbers on the top of each panel give the maximum and minimum values of the averaged line-of-sight field. It shows dramatic changes occurred in the plots of 11:12 UT and 12:48 UT, just before the onset time (14:02 UT) of the Jan. 6 CME (see The SHINE Report).

4. Large scale field variations around other CMEs between 5 and 7 January 1997

Figure 5 shows the 15 color plots of filled contours obtained from the 15 MDI full disk images on Jan. 5, 1997. Some configuration changes occurred at 00:00 UT, 09:36 UT and 22.24 UT. However,there is only one probable CME was detected in white light observations of the LASCO coronagraphs ( LASCO CME List). It is a multiple concentric loops with V shape, observed on east limb at 05:21 UT (see the discussion in Section 5).

Figure 6 shows the 14 color plots of filled contours obtained from the 14 MDI full disk images on Jan. 7, 1997. Dramatic changes of large scale structure occurred at 01:36 and 17:36. Two probable CMEs were detected in the LASCO CME List. One looks like a paint slanted front, observed on east limb at 03:50 UT, the other is a rapid expansion of previously slow streamer blowout, observed on north-east limb at 17:45 UT. The two dramatic changes of large scale photospheric field configuration appeared before the two CMEs.

5. Summary and discussions

We suggest that the 6 January 1997 Halo CME originates in a coronal helmet streamer located about 15 degrees to the south on the January 6 solar disk. The DSF observed around midday is located within the helmet.

By lowering the spatial resolution of the 96-minute full disk SOI-MDI magnetic images, we have examined the variation of the large scale photospheric magnetic field on the time scale of a couple of hours. In addition to continuous slight evolution from one to next, dramatic changes in the large scale field configuration are seen that last a few hours. Between 5 and 7 January 1997, 3 of 4 CMES in the LASCO CME List are preceeded by dramatic changes in the large scale photospheric field configuration and accompanied by significant field strength increases in large areas of positive polarity.

The CME that had no such changes accompanying it has multiple concentric loops with V shapes, and was observed on the east limb at 05:21 UT 5 January 1997. The concave outward V or U shape is interpreted as evidence of disconnected magnetic field lines (Illing and Hundhausen, 1986) and always occur late in CMEs (Chris St. Cyr, private communication, 1997). Thus the dramatic configuration change with field strength decreasing at 00:00 5 January 1997 might possibly be associated with the CME.

Not all dramatic changes in large scale field configuration have CMEs accompanied. Further study with more samples is needed. Further analysis and calibration of these preliminary MDI magnetic images also needs to be completed.

The determination of the onset time for CMEs is a key point for understanding the relation of CMEs to the dramatic changes in large scale photospheric magnetic fields. The changes may be associated with the cause or effect of CMEs depending on whether the onset time is later or earlier than the dramatic changes.

The plasma and magnetic field structures within energized quasi-stable coronal streamers, such as the dark cavities and bright prominences, are believed to be held by the closed magnetic field lines. The closed line that is first opened up must be the outermost closed line of a streamer. Many models of the opening of the coronal field have focused on the rise of the axial magnetic fields or on disturbances of the overlying arcades. For the former, the onset location of CMEs should be low in the corona and the onset time is consistent with the filament disappearing. For the later, the onset location should be high up, around cusp points, and the onset time must be near the time when the outermost closed field line opens up. The SXR dimming observed before the formation of a SXR loop arcade may be close to the onset time of the CME.

The traditional method used to estimate the onset time of CMEs implicitly assumes that the onset location is near the coronal base. However, the onset time of CMEs inferred using the method is sometimes earlier than the near-surface activities. The ascending velocity of the inner filament is often less than that of overlying bright shell. Some estimation of buoyancy indicates that the axial field can be buoyant, but too weak to overcome the containing arcades.

It will be useful to develop techniques to determine the onset location and time of CMEs on the basis of the second model of opening of coronal field.


MDI research is supported by NASA contract NAG5-3077 at Stanford University.


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