Measurement of Hydrogen Velocity Distributions in the Extended Solar Corona

H I Lyα spectral line profiles have been measured in polar regions of the solar corona at projected heliocentric heights of 1.8-3.5 R_☉. Observations were made with the Ultraviolet Coronal Spectrometer on the Spartan 201 satellite from 16:52 to 04:04 UT on 1993 April 11-12 (south pole) and from 12:28 to 22:09 UT on 1993 April 12 (north pole). In general, the coronal profiles cannot be accurately curve-fitted with a single-Gaussian function. The fits with two Gaussians yield most probable velocities of 158 and 322 km s^-1 (south) and 98 and 266 km s^-1 (north). These parameters vary by less than 10% (1 σ) over the observed heights.

The observations are consistent with a line-of-sight model that attributes the narrow component to background streamers and to sites within polar coronal holes or a surrounding diffuse corona, and the broad component to the polar coronal holes and/or plumes. This interpretation suggests that there are regions within the observed coronal holes that have hydrogen and proton kinetic temperatures of (4-6) × 10⁶ K, which is 4-10 times higher than the expected electron temperatures at the same heights. However, other models with, for example, local non-Maxwellian velocity distributions are also consistent with the observations.