Broadening of Resonantly Scattered Ultraviolet Emission Lines by Coronal Hole Outflows

Abstract

Measurements by the Ultraviolet Coronagraph Spectrometer on board the Solar and Heliospheric Observatory have revealed that the widths of several ultraviolet emission lines are unexpectedly large in solar coronal holes. Here we investigate the line-broadening mechanisms for resonantly scattered emissions, which constitute the major part of the coronal H I Lyα and O VI λλ1032, 1037 lines. In particular, the ability of bulk outflows to cause local emissivities to be Doppler-shifted differently along the line of sight is systematically examined. If the bulk motion is outward from the Sun, the geometry of superradial expansion and anisotropic temperature distributions (T⊥ > T||) are favorable for such Doppler broadening. In the case that T⊥/T|| > 2, the broadening can be far more effective, exceeding the threshold that exists in the isotropic case. Quantitative estimates made using a model superradially outflowing coronal hole suggest that the resonantly scattered component may have large line widths, comparable to those observed, without assuming excessively high line-of-sight kinetic temperatures.