Ultraviolet Properties of Halo Coronal Mass Ejections: Doppler Shifts, Angles, Shocks, and Bulk Morphology

Abstract

We present UV spectral information for 22 halo or partial halo CMEsobserved by UVCS. The CME fronts show broad line profiles, while theline intensities are comparable to the background corona. The Dopplershifts of the front material are generally small, showing that themotion of gas in the fronts is mostly transverse to the line of sight.This indicates that, at least in halo CMEs, the fronts generallycorrespond to coronal plasma swept up by a shock or compression wave,rather than plasma carried outward by magnetic loops. This favors an icecream cone (or a spherical shell) model, as opposed to an expandingarcade of loops. We use the line widths to discriminate between shockheating and bulk expansion. Of 14 cases where we detected the CME front,the line broadening in 7 cases can be attributed to shock heating, whilein 3 cases it is the line-of-sight component of the CME expansion. Forthe CME cores we determine the angles between the motion and the planeof the sky, along with the actual heliocentric distances, in order toprovide quantitative estimates of projection effects.