With an extensive analysis,we study the temporal evolution of magnetic flux during three successive M-class flares in two adjacent active regions：NOAA 10039 and 10044.The primary data are full disk longitudinal magnetograms observed by SOHO/MDI.All three flares are observed to be accompanied by magnetic flux changes.The changes occurred immediately or within 1 ～ 10 minutes after the starting time of the flares,indicating that the changes are obvious consequences of the solar flares.Although changes in many points are intrinsic in magnetic flux,for some sites,it is caused by a rapid expansion motion of magnetic flux.For the second flare,the associated change is more gradual compared with the ＇step-function＇ reported in literature.Furthermore,we use the data observed by the Imaging Vector Magnetograph（IVM） at Mees Solar Observatory to check possible line profile changes during the flares.The results from the IVM data confirm the flux changes obtained from the MDI data.A series of line profiles were obtained from the IVM＇s observations and analyzed for flux change sites.We find that the fluctuations in the width,depth and central wavelength of the lines are less than 5.0 even at the flare＇s core.No line profile change is observed during or after the flare.We conclude that the magnetic field changes associated with the three solar flares are not caused by flare emission.
With the aim of studying the relationship between the relative motions of the loop-top （LT） source and footpoints （FPs） during the rising phase of solar flares, we give a detailed analysis of the X7.1 class flare that occurred on 2005 January 20. The flare was clearly observed by RHESSI, showing a distinct X-ray flaring loop with a bright LT source and two well-defined hard X-ray （HXR） FPs. In particular, we correct the projection effect for the positions of the FPs and magnetic polarity inversion line. We find that： （1） The LT source showed an obvious U-shaped trajectory. The source of the higher energy LT shows a faster downward/upward speed. （2） The evolution of FPs was temporally correlated with that of the LT source. The converging/separating motion of FPs corresponds to the downward/upward motion of the LT source. （3） The initial flare shear of this event is found to be nearly 50 degrees, and it has a fluctuating decrease throughout the contraction phase as well as the expansion phase. （4） Four peaks of the time profile of the unshearing rate are found to be temporally correlated with peaks in the HXR emission flux. This flare supports the overall contraction pic- ture of flares： a descending motion of the LT source, in addition to converging and unshearing motion of FPs. All results indicate that the magnetic field was very highly sheared before the onset of the flare.
Tuan-Hui ZhouJun-Feng WangDong LiQi-Wu SongVictor MelnikovHai-Sheng Ji
