Magnetic field changes associated with three successive M-class solar flares on 2002 July 26

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.