Seminar Title	Magnetic Field and Energy Evolution in a Major Active Region Observed by SDO/HMI
Speaker:	Dr. Xundong Sun
Affiliation:	（Stanford University)
When:	Tuesday morning, Sep. 25th，9:30 a.m
Where:	Room 517, Office Block, 2 West Beijing Road (PMO, CAS)
	Welcome to Attend
	( PMO Academic Committee & Academic Circulating committee)

　

Abstract　 　 
Full-disk, high-cadence observations from the the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamic Observatory (SDO) now allow us to study the evolving magnetic field in solar active regions with great detail. Using a vector magnetogram series from SDO/HMI and a non-linear force-free extrapolation, we monitor the field and energy evolution in NOAA AR 11158, where the first X-class flare of Solar Cycle 24 took place. Several findings are as follows. 1) Fast flux emergence and shearing created a quadrupolar system. A substantial amount of electric current and free energy were concentrated near a low-lying sigmoidal filament channel. The inferred magnetic energy showed a sudden decrease during the X-class flare, which was enough to power the explosion. 2) The horizontal magnetic field in the AR core underwent fast, irreversible increase during the flare. As a result, the coronal field displayed a downward collapse motion, consistent with the "coronal implosion" conjecture. 3) A small bipole that emerged on the northeastern side of AR led to a series of "homologous eruptions"; all ejecta followed a highly inclined trajectory. The phenomenon can be explained by the asymmetric structure with anisotropic magnetic pressure. A persisting coronal null point was found in the model and the observation, which provides further evidence for our interpretation.