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Session 86 - Coronal Activity.
Oral session, Thursday, June 13
Soft X-ray images of the Sun's corona, such as those from the Yohkoh SXT, show that the sites of strongest persistent (non-flare) coronal heating are located within the strong (>100 gauss) magnetic fields in sunspot regions and are limited to only certain places within these stong-field domains, covering only a fraction of the total area. We have examined the structure of the magnetic field at these sites in 5 active regions by superposing Yohkoh SXT coronal X-ray images on MSFC vector magnetograms. We find: nearly all of the enhanced (outstandingly bright) coronal features that persist for tens of minutes are rooted near polarity neutral lines in the photospheric magnetic flux; (2) in most cases the core magnetic field closely straddling the neutral line at the root of the strong heating is strongly sheared; (3) the enhanced coronal X-ray brightness in the low-lying core fields shows spatial substructure that fluctuates on time scales of minutes, in the manner of microflaring; and (4) large parts of extensive enhanced coronal features often last for no more than a few hours. From these results, it appears that most enhanced coronal heating in active regions is a consequence of some process that (1) acts only in the presence of a photospheric polarity neutral line, (2) is episodic on times of about an hour, (3) usually gives stronger coronal heating in the presence of stronger magnetic shear, but is not required to act by the presence of magnetic shear, and (4) is often accompanied by microflaring in the core field. We point out that magnetic flux cancellation (driven by photospheric flows at the neutral line) is a process that plausibly meets all these requirements. The flux cancellation might directly drive microflaring, or trigger microflaring in the sheared core field, or both. The microflaring might directly produce the enhanced coronal heating in the core fields as well as generate MHD waves that propagate up into the enhanced extended coronal loops to provide the strong coronal heating in these.
Program listing for Thursday