Article

Response of the Photospheric Magnetic Field to the X2.2 Flare on 2011 February 15

The Astrophysical Journal Letters (Impact Factor: 5.6). 12/2011; 745(2). DOI: 10.1088/2041-8205/745/2/L17
Source: arXiv

ABSTRACT It is well known that the long-term evolution of the photospheric magnetic
field plays an important role in building up free energy to power solar
eruptions. Observations, despite being controversial, have also revealed a
rapid and permanent variation of the photospheric magnetic field in response to
the coronal magnetic field restructuring during the eruption. The Helioseismic
and Magnetic Imager instrument (HMI) on board the newly launched Solar Dynamics
Observatory (SDO) produces seeing-free full-disk vector magnetograms at
consistently high resolution and high cadence, which finally makes possible an
unambiguous and comprehensive study of this important back-reaction process. In
this study, we present a near disk-center, GOES -class X2.2 flare, which
occurred in NOAA AR 11158 on 2011 February 15. Using the magnetic field
measurements made by HMI, we obtained the first solid evidence of a rapid (in
about 30 minutes) and irreversible enhancement in the horizontal magnetic field
at the flaring magnetic polarity inversion line (PIL) by a magnitude of ~30%.
It is also shown that the photospheric field becomes more sheared and more
inclined. This field evolution is unequivocally associated with the flare
occurrence in this sigmoidal active region, with the enhancement area located
in between the two chromospheric flare ribbons and the initial conjugate hard
X-ray footpoints. These results strongly corroborate our previous conjecture
that the photospheric magnetic field near the PIL must become more horizontal
after eruptions, which could be related to the newly formed low-lying fields
resulted from the tether-cutting reconnection.

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