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Session 37 - First Results from the Solar and Heliospheric Observatory (SOHO).
Display session, Tuesday, June 11
Coronal magnetic loops may display a wide variety of flow configurations, depending upon their geometries and the plasma conditions at their footpoints, as well as upon the distribution of nonthermal heating along them. Existing models of steady state loop flows ("siphon" flows, such as may exist in the closed-field regions of large, long-lived helmet streamers) show that the bulk plasma streaming velo- city at the top of a loop can easily reach values of order the acoustic speed, i.e., in excess of 100 km/s, and may be even higher in the supersonic (descending) leg of the loop.
In this paper we consider some of the observational con- sequences of such siphon flows. Using a simple polytropic model to describe the variation of plasma parameters along a typical coronal loop, we have calculated the expected frequency-dependent emissivities of the resonance-scattered H I Lyman-alpha and O VI emission lines as functions of position on the loop and of the loop orientation relative to the viewing direction. Comparison with similar calculations for a static plasma with the same temperature and density provides the degree of Doppler dimming to be expected at various points along the loop. This Doppler dimming, and the related Doppler shift, comprise important diagnostic signatures of loop flows which are potentially observable with the UVCS instrument on SOHO. Surprisingly, Doppler dimming, which is ordinarily regarded as an indicator of radial outflow from the Sun, does not vanish entirely for the horizontal flow at the top of a loop. For a hot (T 2 MK), nearly isothermal loop extending less than, say, 1 solar radius above the limb, the Doppler dimming there can achieve values of more than 10 per cent at the loop's summit.
Program listing for Tuesday