HIGH-VELOCITY PLASMA IN THE TRANSITION REGION OF AU-MICROSCOPII - EVIDENCE FOR MAGNETIC RECONNECTION AND SATURATED HEATING DURING QUIESCENT AND FLARING CONDITIONS

We analyze high-resolution HST spectra of the dM0e flare star AU Mic, including the profiles of the C IV 1548.2 angstrom, 1550.8 angstrom and Si IV 1393.8 angstrom, 1402.8 angstrom lines obtained with the G160M grating of the Goddard High-Resolution Spectrograph. The quiescent profiles of the C IV and Si IV lines are broad, and not simple Gaussians in shape. Flux in the C IV and Si IV lines, for example, can be measured reliably out to about +/- 200 km s-1 from line center. Each of the C IV and Si IV profiles can be fitted accurately by two Gaussians (one narrow and the other broad) centered on nearly the same wavelength, with the narrower component accounting for roughly 60% of the total integrated flux. The narrow components have similar line widths to those observed in solar active and quiet regions. The broad Gaussian components of the AU Mic line profiles are reminiscent of the broad C IV profiles observed in solar transition region explosive events, which are thought to be associated with emerging magnetic flux regions where field reconnection occurs. The broad component surface flux of the C IV multiplet is 6.3 x 10(5) ergs cm-2 s-1, which is 3 times larger than the saturated level found previously by Vilhu (1987), who assumed that the C IV emission is distributed uniformly over the entire stellar surface. We believe that 6.3 x 10(5) erp cm-2 s-1 is a more realistic estimate of the C IV flux corresponding to the maximum steady state magnetic heating rate. The width of the broad component and the very high line surface fluxes provide important new evidence for magnetic reconnection and magnetic heating processes occurring over a significant fraction of the stellar surface, driven by the emergence of large quantities of magnetic flux from below. For AU Mic, we estimate that the broad component, which we believe results directly from the magnetic reconnection process, emanates from 12% of the stellar surface. We also discuss the very broad Si iv line profiles observed during a weak flare. At the beginning of the flare the profiles appear to be redshifted, but during the peak of the flare they are slightly blueshifted.