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A. C. Sterling (CPI, NRL, and ISAS), H. S. Hudson (SPRC and ISAS), B. J. Thompson (NASA/Goddard), D. M. Zarro (SAC and NASA/Goddard)
A subset of the solar-disk counterparts to halo coronal mass ejections (CMEs) display an evolution in soft X-rays (SXR) characterized by a preflare ``S''-shaped structure, dubbed a ``sigmoid,'' evolving into a postflare cusp or arcade. We examine the morphological properties of the evolution of sigmoids into cusps and arcades for four such regions associated with SXR flares, using the Soft X-ray Telescope (SXT) on Yohkoh and the 195~Å\ Fe~{\sc xii}\ channel of the EUV Imaging Telescope (EIT) on SOHO. There is, at most, only a weak counterpart to the SXR sigmoid in the preflare EUV images, indicating that the preflare sigmoid has a temperature >1.5~MK\@. During the time of the flare itself, however, an EUV sigmoid brightens near the location of the SXR preflare sigmoid. Initially the SXR sigmoid lies along a magnetic neutral line. As the SXR flare progresses new field lines appear with orientation normal to the neutral line and with footpoints rooted in opposite polarity regions; these footpoints are different from those of the preflare sigmoid. The cusp structures in SXRs develop from these newly-ignited field lines. In EIT images the EUV sigmoid broadens out as the flare progresses, forming an arcade which resides beneath the SXR cusp. In many respects, our findings are consistent with a standard picture where the origin of the flare and CME is due to the eruption of a filament-like feature, and the stretching of overlying preflare fields produces the cusp. We do not, however, observe these preflare overlying fields prior to flare onset.
This work was supported by the NRL Naval basic research program and NASA.
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