Study On The Magnetic Field Change During Solar Flares

Solar flares are due to sudden release of free magnetic energy in the solar atmo-sphere. It is involved with many complicated physical processes, including fast mag-netic reconnection, energy transportation, plasma instability, acceleration and propa-gation of energetic particles, radiation and dynamics of the flaring atmosphere, mass motions and ejections, and so on. Solar flares are the main source of disaetrous space weather.In this thesis, we mainly use ground and space observations to study the magnetic features and the changes during the solar flares. The main contents are as follows:In Section 1, we make a brief introduction to the research history, common fea-tures and classification of solar flares. We also make a necessary description of the flare models and the energy release and transportation mechanisms during solar flares. In addition, a detailed description of previous studies on magnetic filed changes during solar flares is provided.In Section 2, by using the full disk longitudinal magnetograms observed by SOHO MDI and the Imaging Vector Magnetograph at Mees Solar Observatory, we report on abrupt magnetic flux changes associated with three successive M-class flares on July 26,2002 in the active regions NOAA 10039 and 10044. We divide the active regions into a number of small boxes and determine the detailed timing of the magnetic flux evolution in the course of the three flares. In addition, for the first time, we analyze the line profiles of a flaring region, where abrupt flux change occurs.In Section 3, by using a nonlinear force-free model of Low (1982) and an extrap-olation of linear force- free magnetic field to an observed magnetogram of a complex solar active region, we quantitatively show that, at any given point, a less sheared mag-netic loop tends to have a smaller size. From observational point of view, the results support that the relaxation of sheared magnetic loops in the solar corona, which is the result of the release of magnetic energy, should accompanied by the contraction of flare loops.In Section 4, we analyze in detail the X2.6 flare that occurred on 2005 January 15 in the active region NOAA 10720 using multiwavelength observations. There are several interesting features of the flare that reveal possible two-stage magnetic recon-nection similar to that in the physical picture of the tether-cutting model, where the magnetic fields of two separate loop systems reconnect at the flare core region, and subsequently a large flux rope forms, erupts, and cuts through the overlying arcade fields. The observed manifestations include:(1) remote Ha brightenings appear min-utes before the main phase of the flare; (2) separation of the flare ribbons has a slow and a fast phase, and the flare hard X-ray emission appears in the later fast phase; (3) rapid transverse field enhancement near the magnetic polarity inversion line (PIL) is found to be associated with the flare; and (4) the non-linear force-free extrapolation demon-strates the proposed magnetic connectivity before the flare. We conclude that the flare occurrence fits the tether-cutting reconnection picture in a special way, in which there are three flare ribbons outlining the sigmoid configuration. One of the magnetic elbows of the sigmoid connects the remote brightening region and the following sunspot, while the other connects the leading and following spots in thisδsunspot. We also discuss this event in the context of what was predicted by Hudson, Fisher,& Welsch (2008), where the Lorentz force near the flaring PIL drops after the flare and consequently the magnetic field lines there turn to be more horizontal as we observed.We also give a summary and prospect of future studies in Section 5.