Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal Active Region Eruptions |
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| Authors: | Lirong Tian David Alexander |
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| Institution: | (1) Department of Physics and Astronomy, Rice University, Houston, TX, 77251-1892, U.S.A. |
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| Abstract: | We study active region NOAA 9684 (N06L285) which produced an X1.0/3B flare on November 4, 2001 associated with a fast CME
(1810 km s−1) and the largest proton event (31 700 pfu) in cycle 23. SOHO/MDI continuum image data show that a large leading sunspot rotated
counter-clockwise around its umbral center for at least 4 days prior to the flare. Moreover, it is found from SOHO/MDI 96
m line-of-sight magnetograms that the systematic tilt angle of the bipolar active region, a proxy for writhe of magnetic fluxtubes,
changed from a positive value to a negative one. This signifies a counter-clockwise rotation of the spot-group as a whole.
Using vector magnetograms from Huairou Solar Observing Station (HSOS), we find that the twist of the active region magnetic
fields is dominantly left handed (αbest = −0.03), and that the vertical current and current helicity are predominantly negative, and mostly distributed within the
positive rotating sunspot. The active region exhibits a narrow inverse S-shaped Hα filament and soft X-ray sigmoid distributed along the magnetic neutral line. The portion of the filament which is most closely
associated with the rotating sunspot disappeared on November 4, and the corresponding portion of the sigmoid was observed
to erupt, producing the flare and initiating the fast CME and proton event. These results imply that the sunspot rotation
is a primary driver of helicity production and injection into the corona. We suggest that the observed active region dynamics
and subsequent filament and sigmoid eruption are driven by a kink instability which occurred due to a large amount of the
helicity injection. |
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