Time……… Friday, Oct 16th 2015, 3:00 -- 4:15 pmWhere……. Room 415, North Physics Building Speaker…. Xudong Sun, Stanford University Title………. Recent Results from HMI/SDO: The Polar Field Reversal and The CME-less X-Class Flares
Abstract: The Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamic Observatory (SDO) has been observing the full Sun magnetic field with high cadence, high duty cycle, and moderate resolution for the past five years, accumulating millions of records. In this talk, I discuss two recent studies using this excellent data archive. The first study focuses on the evolution of Cycle 24, characterizing the polar field reversal and the surface flux transport process (Sun+, 2015, ApJ, 798, 114). We find that the north and south polar field reversed polarity in 2012 November and 2014 March, respectively. The north has since stopped growing, whereas the south has rapidly recovered to the pre-SDO values. The strong asymmetry provides new insights to the prediction of Cycle 25. We find an interesting anti-corrlation between the mid-latitude magnetic field and the local near-surface meridional flow speed from helioseismology measurement, which may be important to solar cycle modulation. In the second study, we explore the magnetic causes and consequences of the CME-less X-class flares (Sun+, 2015, ApJL, 804, L28). Active region (AR) 12192 is the largest sunspot group in 24 years and the most flare-productive AR of Cycle 24, but surprisingly generated no CMEs. We find that its core region exhibits weaker non-potentiality, stronger overlying field, and smaller flare-related field changes compared to other major flare-CME-productive ARs. The unexpected behavior suggests that AR eruptiveness is limited by some relative measure of magnetic non-potentiality over the restriction of background field, and that confined flares may leave weaker photospheric and coronal imprints compared to their eruptive counterparts.