OBSERVATIONS OF THE MAGNETIC RECONNECTION SIGNATURE OF AN M2 FLARE ON 2000 MARCH 23

摘要

Multiwavelength observations of an M 2.0 flare event on 2000 March 23 in the NOAA active region 8910 provide us a good chance to study the detailed structure and dynamics of the magnetic reconnection region. In the process of the flare, extreme-ultraviolet (EUV) loops displayed two types of sideward motions upon a loop-top hard X-ray source with average velocities of 75 and 25.6 km s–1, respectively. Meanwhile, a part of the loops disappeared and new post-flare loops formed. We consider these two motions to be the observational evidence of reconnection inflow, and find an X-shaped structure upon the post-flare loops during the period of the second motion. Two separations of the flare ribbons are associated with these two sideward motions, with average velocities of 3.3 and 1.3 km s–1, respectively. The sideward motions of the EUV loops and the separations of the flare ribbons are temporally consistent with two peaks of the X-ray flux. This indicates that there are two types of magnetic reconnection in the process of the flare. Using the observation of photospheric magnetic field, the velocities of the sideward motions, and the separations, we deduce the corresponding coronal magnetic field strength to be about 13.2-15.2 G, and estimate the reconnection rates to be 0.05 and 0.02 for these two magnetic reconnection processes, respectively. Besides the sideward motions of EUV loops and the separations of flare ribbons, we also observe motions of bright points upward and downward along the EUV loops with velocities ranging from 45.4 to 556.7 km s–1, which are thought to be the plasmoids accelerated in the current sheet and ejected upward and downward when magnetic reconnection occurs and energy releases. A cloud of bright material flowing outward from the loop-top hard X-ray source with an average velocity of 51 km s–1 in the process of the flare may be accelerated by the tension force of the newly reconnected magnetic field lines. All the observations can be explained by schematic diagrams of magnetic reconnection.