YohkohSXT andSOHOEIT Observations of Sigmoid‐to‐Arcade Evolution of Structures Associated with Halo Coronal Mass Ejections

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Abstract
A subset of the solar-disk counterparts to halo coronal mass ejections (CMEs) displays an evolution in soft X-rays (SXR) that is characterized by a preflare S-shaped structure, dubbed a "sigmoid," which evolves 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 the Yohkoh satellite and the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) satellite. Most of our EIT observations are with the 1.5 MK 195 Å Fe XII channel. At most, there is only a weak counterpart to the SXR sigmoid in the preflare 195 Å EUV images, indicating that the preflare sigmoid has a temperature greater than 1.5 MK. While more identifiable than in the 195 Å channel, a clear preflare sigmoid is also not observed in the 2.0 MK EIT 284 Å Fe XV channel. During the time of the flare, 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 regions of opposite polarity; 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 as the flare progresses, forming an arcade beneath the SXR cusp. Our findings are consistent with a standard picture in which the origin of the flare and CME is caused by the eruption of a filament-like feature, with the stretching of field lines producing a cusp. We infer that the cusp-producing fields may be overlying the sigmoid fields in the preflare phase, but we do not directly observe such preflare overlying fields.