NEUTRAL HYDROGEN IN THE SOLAR-WIND ACCELERATION REGION

Observation of solar Ly alpha radiation scattered by coronal neutral h ydrogen atoms can be used to investigate the acceleration region of th e solar wind. In this paper we focus on the use of these observations to study Alfven waves, which can accelerate the solar wind plasma to f low speeds observed in high-speed streams if their amplitude at the co ronal base is 20 km s(-1) or larger. The wave amplitude is then larger than the proton thermal speed in the outer corona, so that the mean p roton speed (averaged over a wave period) is significantly larger than the proton thermal speed. For low-frequency waves the hydrogen atoms follow the proton motion in the waves, while for higher frequencies th e protons move relative to the neutrals. Nevertheless, in the higher f requency case, the rates for charge exchange and recombination are hig h enough to broaden the velocity distribution function of neutral hydr ogen. Both the wave motion of the hydrogen atoms in low-frequency Alfv en waves and the ''heating'' by higher frequency waves lead to a broad ening of the scattered solar Ly alpha line. For coronal base amplitude s of 20 km s(-1), the line broadening increases with heliocentric dist ance beyond 4-5 R..