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Session 4 - SOHO Corona II.
Oral session, Friday, June 27
Ballroom A, Chair: Therese Kucera

[4.02] Thermal Coupling of Protons and Neutral Hydrogen in the Fast Solar Wind

L. Allen, S. R. Habbal (Harvard-Smithsonian Center for Astrophysics)

Motivated by the recent Spartan and UVCS observations [Kohl et al. 1996] of hot protons with temperatures exceeding 4 \times 10^6 K below 3.5 R_s in coronal holes, as inferred from the measured broadening of the Lyman \alpha spectral line profile, we studied the thermal coupling of neutral hydrogen to protons in the presence of Alfvén waves in the solar wind. The approach used is adopted from Olsen et al. [1994] in which the neutral hydrogen atoms are treated as test particles in a background electron-proton solar wind. The model computations show that an anisotropy in the neutral hydrogen temperature in the directions parallel and perpendicular to the magnetic field develops in the inner corona well below 5 R_s for background solar wind solutions consistent with observational constraints of the high speed wind. In particular, we find that the neutral hydrogen temperature parallel to the magnetic field direction remains strongly coupled to the proton temperature, T_p, while the perpendicular neutral hydrogen temperature exceeds this by \sim 10^6 K for a wide range of proton flow speeds, densities and temperatures for a spectrum of Alfvén waves. The neutral hydrogen effective temperature, T_H(eff)^\perp, incorporating both random thermal motion and wave motion of the particles, is found to be independent of frequency and significantly less than the proton effective temperature, T_p(eff), in the inner corona. Thus, without additional information about the waves, which would allow T_H^\perp and T_p to be extracted from the models, T_H(eff)^\perp provides an upper limit on T_p and a lower limit on T_p(eff). However, with increasing proton temperature, the anisotropy in the inner corona decreases, with a temperature difference of < 8 \times 10^5 K between the protons and neutrals below 3 R_s when the latter reach 6 \times 10^6 K.

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