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Astron. Astrophys. 335, 303-308 (1998)

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1. Introduction

Recent UVCS/SOHO data (Kohl et al. 1997, Cranmer et al. 1997) show, for the heavy ions exemplified by [FORMULA], very high temperatures reaching [FORMULA] K in the inner corona with large temperature anisotropy (T). This is in general agreement with the picture suggested by McKenzie et al. (1995, 1997) where plasma heating is taken to be due to dissipation of high frequency waves close to the solar surface. The present work embodies ideas stretching back almost three decades. The idea that the corona could be very hot ([FORMULA] K) was first hinted at by Holzer and Axford (1970) in order to account for high solar wind speeds. Ryan and Axford (1975) explored the consequences of minor ions in the solar wind being heated proportional to their mass. Leer and Axford (1972) extended the two fluid (electron-proton) work of Sturrock and Hartle (1966) to allow for different perpendicular and parallel proton temperatures (T). This body of work taken together with Bame et al. (1975) and the HELIOS (Marsch et al. 1982) and ISEE observations (Schmidt et al., 1980) to the effect that T, T, u lends support to the idea that the ion cyclotron resonant processes would be a natural candidate to account for these observed properties (Axford, 1980).

In the present paper we extend the one-ion treatment of McKenzie et al. (1997) by considering two ion components, the minor component being either the [FORMULA] or [FORMULA] (in the latter case the main component is taken to be the ([FORMULA]) mixture). In parallel to the early work (Yeh 1970, Weber 1973) our treatment exploits the critical structure of the phase space of the stationary bi-ion flow equations using the generalized concept of sonic points in a bi-ion flow including differential streaming (McKenzie et al. 1993). In order to avoid solving the coupled energy and momentum equations we use the proton temperature profiles of McKenzie et al. (1997) and assume the minor component is preferentially heated proportional to its mass (with an extra factor of K). The acceleration due to waves and the Coulomb friction are taken into account. The case with isotropic temperatures and adiabatic evolution of the wave force, based on McKenzie et al. (1995), was considered in Czechowski et al. (1997).

The published work on the multi-ion solar wind acceleration problem, apart from early isothermal models (Yeh 1970, Weber 1973) or the calculations assuming simple temperature profiles (Leer, Holzer & Shoub 1992), include also, starting from Burgi (1992), the more complete calculations in which the energy transport equations are solved simultaneously with the flow equations, with some assumptions made for the source terms. However, none of those cover the case that we consider, namely the anisotropic temperature profiles and a complete expression for the wave force. The extensive study of Hansteen et al. (1997), while encompassing also the chromosphere, is restricted to the case of isotropic temperatures. Isenberg (1984) included both the wave force and anisotropic temperatures as well as a specific model of the resonant interaction, but his calculations were limited to the region outside 10 [FORMULA], beyond the solar wind critical points. We do not intend to review here the whole field of solar wind acceleration theories but we want to draw attention to interesting recent work by Hu et al. (1997) and Tu and Marsch (1997).

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998