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Session 5 - SOHO Corona III.
Oral session, Friday, June 27
Ballroom A, Chair: John Mariska
Transient soft X-ray brightenings occur frequently in solar active regions, with typical durations of 2-10 minutes. They have been observed with the Yohkoh Soft X-ray Telescope (SXT) and appear to be associated primarily with interactions of multiple loops that brighten initially near their footpoints (Shimuzu et al. 1994, Ap.J., 422, 906). Suggested mechanisms for the production of soft X-ray emission include: conduction-driven chromospheric evaporation; Alfvenic outflows from reconnection of colliding field lines; and expulsion of untwisting loop material in emerging flux tubes (Uchida and Shibata 1988, Solar Phys., 116, 291).
To further study the dynamics of transient soft X-ray brightenings and their relationship to the lower atmospheric magnetic field, we have conducted a coordinated SOHO/Yohkoh campaign to observe soft X-ray brightenings in a small active region at disk center on 1996 June 6. The region was observed simultaneously by Yohkoh SXT, the Coronal Diagnostic Spectrometer (CDS) on SOHO, and the Imaging Vector Magnetograph (IVM) at Mees Observatory, Hawaii. In particular, the CDS instrument obtained Mg X (609Åline spectra with 2-3 arcsec spatial resolution in a 2\times2 arcmin field with approximately 8 minute cadence. The Mg X line is formed in the low corona at approximately 10^6 K. Individual Mg X spectra were obtained with 5 second exposures per slit position. The SXT and CDS observations show evidence of soft X-ray brightness variations on timescales of 5-10 minutes. The CDS Mg X spectra show a mixture of red and blue Doppler shifts (< 100 km s^-1) that are spatially associated with loop footpoints indicated by the IVM. Based on the observed temporal and spatial variations of the implied plasma upflows and downflows, we investigate the validity of different proposed models of transient soft X-ray brightenings.
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