Yohkoh SXT and SOHO EIT observations of sigmoid-to-arcade evolution of structures associated with halo coronal mass ejections

A subset of the solar-disk counterparts to halo coronal mass ejections (CME s) displays an evolution in soft X-rays (SXR) that is characterized by a pr eflare S-shaped structure, dubbed a "sigmoid," which evolves into a postfla re cusp or arcade. We examine the morphological properties of the evolution of sigmoids into cusps and arcades for four such regions associated with S XR flares, using the Soft X-Ray Telescope (SXT) on the Yohkoh satellite and the EW Imaging Telescope (EIT) on the Solar and Heliospheric Observatory ( SOHO) satellite. Most of our EIT observations are with the 1.5 MIG. 195 Ang strom Fe XII channel. At most, there is only a weak counterpart to the SXR sigmoid in the preflare 195 Angstrom EUV images, indicating that the prefla re sigmoid has a temperature greater than 1.5 MK. While mon identifiable th an in the 195 Angstrom channel, a clear preflare sigmoid is also not observ ed in the 2.0 ME: EIT 284 Angstrom Fe XV channel. During the time of the fl are, however, an EUV sigmoid brightens near the location of the SXR preflar e sigmoid. Initially the SXR sigmoid lies along a magnetic neutral line. As the SXR flare progresses, new held lines appear with orientation normal to the neutral line and with footpoints rooted in regions of opposite polarit y; these footpoints are different from those of the preflare sigmoid. The c usp structures in SXRs develop from these newly ignited held lines. In EIT images, the EW sigmoid broadens as the flare progresses, forming an arcade beneath the SXR cusp. Our findings are consistent with a standard picture i n which the origin of the hare and CME is caused by the eruption of a filam ent-like feature, with the stretching of held lines producing a cusp. We in fer that the cusp-producing fields may be overlying the sigmoid fields in t he preflare phase, but we do not directly observe such preflare overlying f ields.