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H. Wang (Big Bear Solar Observatory)
In this talk, I will first briefly review the studies on the evolution of photospheric magnetic fields associated with CMEs and flares. Then I will present some very recent results on the study of six events, five of them are Halo CMEs, and all are associated with X-class flares.
We found significant changes in the photospheric magnetic fields associated with all of the events. Based on the analyses of the line-of-sight magnetograms, all six events had an increase of the magnetic flux of the leading polarity on the order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive, as the "change-over" time (time to change from pre-CME/flare to post-CME/flare state) ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops to trigger CMEs/flares, (2) the changing of magnetic field line direction and (3) the expansion of the sunspot which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference. The flux of leading polarity tends to increase while the flux of following polarity tends to slightly decrease.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.