Using SOHO/MDI magnetograms taken every 96 minutes, we determined that no significant amount of magnetic flux emerged for several hours anywhere near the disappearing filament associated with the 6 January 1997 CME. However there was a significant change in the global-scale distribution of magnetic polarity just before and after the onset time of the CME.
The 7 April 1997 CME was associated with an eruption in an active region observed by EIT, unlike the 6 January CME that was associated with a disappearing quiet filament. Both CMEs were halo-shaped and therefore almost certainly directed toward the Earth and their sources were not too far from the center of the solar disk. In this study we examine the differences between adjacent MDI magnetic images and the field strength distributions near the time of the 7 April CME.
The timeline of observations for the flare and CME is shown in Table 1. It shows that the CME is Earth-directed and the flare is associated with the CME, and located near the onset region of the CME. The onset time of the CME is near 14:00, though we can't accurately determine it.
Flares and CMEs are believed to be fuled by the free magnetic energy. Most prediction scheme of solar flare are builded on the complexity of photospheric magnetic field configuration in active regions. The high-cadence MDI observations of the photospheric magnetic field provide an unique oppotunity to examine the possible relationship between the CME-flare and photospheric magnetic field.
Figure 1a and Figure 1b display the location the CME-associated flare in EIT 195 A and the MDI magnetic images around the onset time of the flare. The images are 1024x1024 pixels with 2 arcsecond per pixel. The 14:27 image represent the photospheric field after onset of the CME as well as the flare. The 11:15 and 12:51 images most probably represents the photospheric field before the events.
Figure 1a and 1b show that no new significant actve region occurred and the field configuration changes slightly on April 7. It seems that not much is happening on the solar surface, though MDI observations of the FINE scale field show that all of the flux concentrations in the network outside of active regions are constantly changing, with essentially 100% of the flux being replaced on a time scale of a few days, and the small elements are not simply the result of repeated emergence and submergence of the same flux, but are manifestations of presumably locally generated field that interact on the scale of supergranules.
Figure 2 and Figure 3 show the distribution of line-of-sight field strength for 11:15, 12:51, and 14:27 "active region". No newly emerging magnetic flux was found, though some changes occurred in the strength distribution.
To determine is there any new magnetic structure occurred in the period of the time (3 hours), we examine the difference of adjacent images. Figure 4 shows that the shift due to Sun's rotation is about 6 pixels. Figure 5a and Figure 5b display the six difference images with three sets of maximum and minimum strengthes at the bottom of each panel, further showing no large newly emeging flux on April 7.
The SOHO-MDI observations show that the Sun is constantly and furiously changing on the small scale. This changing may have major implications for our understanding of the structure and dynamics seen on larger scales. We'll examine the large-scale magnetic pattern using the IDL smooth code. Figure 6 displays the smoothed images obtained using REBIN, CONGRID, and SMOOTH codes. Figure 7 shows how to exhibit the larger scale patterns from MDI full-disk images.
Figure 8a, Figure 8b,, and Figure 8c show the difference images obtained by lowering the spatial resolution.
What cause the changes? Whether or not the changes relate to CMEs? and How if it is the case?
Figure 9a, Figure 9b, and Figure 9c show the noise level for the MDI magnetic field in active and quiet regions.
Figure 10a, Figure 10b, and Figure 10c show,respectively, the distribution of all components, strong and weak components of the photospheric field in the source region.
Figure 11a, Figure 11b, and Figure 11c show,respectively, the differece images of all components, strong and weak components of the photospheric field in the source region.
The same as the case of the Jan. 6, 1997 CME, there is no obvious change pattern of the photospheric magnetic field observed that is associated with the appropriate CMEs.
There appears a temporary change in the large-scale weak field at the time of the CME.
Although the photospheric magnetic field is composed of small magnetic structures, these structures are organized into large-scale patter, as observed by the WSO low-resolution high-sensitivity magnetograph. Large-scale pattern on the solar surface are manifest by actual physical connections in the corona covering immense distances.
The study of the cause of the change found in the work and its implications for the onset of CME are in progress.
