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[16.01D] Empirically Determined Anisotropic Velocity Distributions and Outflows of O{}⁵⁺\ Ions in a Coronal Streamer at Solar Minimum

R.A. Frazin, S.R. Cranmer, J.L. Kohl (Harvard-Smithsonian Center for Astrophysics)

Empirical constraints on the O{}⁵⁺\ velocity distributions and outflow speeds in a solar minimum equatorial streamer between 2.6 and 5.1 R_\odot\ are determined using a spectral synthesis code that includes O VI Doppler dimming. These constraints follow directly from UV spectra taken on 12 October 1996 with the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO satellite and 3D electron densities derived from tomography applied to a time series of polarized white-light images taken with the Large Angle Spectrometric Coronagraph Experiment (LASCO) on SOHO. Four conclusions result from this work: 1) Our analysis shows O{}⁵⁺\ velocity distribution anisotropy in the streamer legs and stalk and that the microscopic velocity distribution (which excludes wave motions that equally affect all charged particles) is also anisotropic, where the most probable speed perpendicular to the magnetic field direction exceeds that in the parallel direction. 2) There is no evidence of anisotropy in the streamer core. 3) There is preferential heating of the O{}⁵⁺\ ions over the protons in the streamer stalk and legs, but not in the core. 4) The outflow velocity of the O{}⁵⁺\ ions is determined at heights above 4.6 R_\odot. All results have a confidence level of at least 70%.

The evidence for microscopic anisotropy in the O{}⁵⁺\ velocity distributions and preferential heating of the O{}⁵⁺\ ions over the protons presented here is reminiscent of that provided for coronal holes by Cranmer et al. (1999). One particularly favorable candidate mechanism to explain these phenomena is \emph{ion cyclotron resonance}, in which high frequency Alfvén waves are absorbed by the heavy ions. Cranmer et al. discuss the relevance of this process to an empirical model of a polar coronal hole. Our data suggest that the dominant processes that heat the heavy ions in coronal holes may also be important in streamers.

Reference:

Cranmer, S.R., et al. 1999, ApJ, 511, 481

The author(s) of this abstract have provided an email address for comments about the abstract: rfrazin@cfa.harvard.edu

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.