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S. M. Hill, V. J. Pizzo, C. Balch, W. Neupert (NOAA/SEC)
Soft X-ray (SXR) observations have indicated the presence of a diffuse high temperature source above the position of incipient flare arcade loops, which subsequently become visible in the extreme ultraviolet (EUV). It has been postulated that the diffuse appearance of the hot source may be due to a combination of instrument response and emission measure structure. We have studied several CME-associated post-flare loop arcades occurring near the solar limb in late 2001, using observations made in the EUV by SOHO/EIT and in the SXR band by both Yohkoh/SXT and the GOES-12 SXI instrument. The observations are of moderate cadence and spatial resolution, but are unusual in that they include full EUV and SXR event coverage from start to finish. In addition, the SXI filter observations in the 6-60Å band bridge the thermal ranges accessible to EIT and SXT. The events show a consistent progression: 1) An impulsive flare phase in SXR, sometimes accompanied by an SXR and/or EUV transient, 2) formation of a diffuse SXR emission region above the limb, 3) formation of EUV loops below the SXR emission, 4) slow apparent rise of the SXR source and the EUV loops accompanied by transient brightenings of the EUV loop-tops, and 5) fading of the SXR source and ceasing of the EUV loop-top brightenings. SXR filter ratios indicate the peak temperature occurs above the peak SXR emission centroid, which is consistent with previous observations. By subtracting a 'high-temperature' SXR image from a 'low-temperature' SXR image, we are able to clearly observe loops evident in the EUV 195Å channel. The fact that we can observe the diffuse high-temperature emission and the resolved lower temperature loops with the same broadband SXR instrument suggests that the diffuse appearance of the hot material is physical in nature, as opposed to a byproduct of the instrument response.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.