Empirically Determined Anisotropic Velocity Distributions and Outflows of O 5+ Ions in a Coronal Streamer at Solar Minimum

Abstract

Empirical constraints on the O5+ velocity distributions and outflow speeds in a solar minimum equatorial streamer between 2.6 and 5.1 R-circle dot are determined using a spectral synthesis code that includes O VI Doppler dimming. These constraints follow directly from UV spectra taken on 1996 October 12 with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) satellite and three-dimensional electron densities derived from tomography applied to a time series of polarized white-light images taken with the Large Angle and Spectrometric Coronagraph (LASCO) also on SOHO. Four conclusions result from this work: (1) our analysis shows O5+ velocity distribution anisotropy in the streamer legs and stalk and gives strong evidence that the microscopic velocity distribution (which excludes wave motions that equally affect all charged particles) is anisotropic, where the most probable speed perpendicular to the magnetic field direction exceeds that in the parallel direction; (2) there is preferential heating of the O5+ ions over the protons in the streamer stalk and legs; (3) there is no evidence for preferential O5+ heating in the core; and (4) the outflow velocity of the O5+ ions is determined at heights above 4.6 R-circle dot. All results have a confidence level of at least 70%.