ION CYCLOTRON WAVES, INSTABILITIES AND SOLAR WIND HEATING XING LI and SHADIA R. HABBAL Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MA 02138, U.S.A. (Received 16 September 1999; accepted 5 November 1999) Abstract. The effect of alpha particles on the dispersion relation of ion cyclotron waves and its influence on the heating of the solar wind plasma are investigated. The presence of alpha particles can dramatically change the dispersion relation of ion cyclotron waves, and significantly influence the way that ion cyclotron waves heat the solar wind plasma. We find that a spectrum of ion cy- clotron waves affects the thermal anisotropy of the solar wind protons and other ions differently in interplanetary space: When alpha particles have a speed u α > 0.5v A , and both protons and alpha particles have a thermal anisotropy T ⊥ /T ‖ > 1, ion cyclotron waves heat protons in the direction perpendicular to the magnetic field, cool them in the parallel direction, and exert the opposite effect on alpha particles. 1. Introduction Four decades after Parker (1958) predicted the existence of the solar wind which was verified a few years later by spacecraft in situ measurements (Snyder and Neugebauer, 1964), the solar wind heating and acceleration mechanism continues to elude us and excite our imagination. In situ measurements have shown that the solar wind minor ions not only flow faster than the major ions – protons, but they are preferentially heated as well (e.g., Marsch et al., 1982a, b; Bochsler, Geiss, and Joos, 1985; von Steiger et al., 1995). The interest in ion-cyclotron resonance (e.g., Hollweg and Turner, 1978; Isen- berg, 1984a; Marsch, Goertz, and Richter, 1982) has been recently renewed (Cran- mer, Field, and Kohl, 1999; Hollweg, 1999; Hu and Habbal, 1999; Li et al., 1999a, b) by the ultraviolet spectral line observations from UVCS on SOHO indicating larger ion than proton temperatures and implying large temperature anisotropies for protons and minor ions in the inner corona (e.g., Kohl et al., 1998; Li et al., 1998; Cranmer et al., 1999). UVCS observations have shown that oxygen ions O +5 have extraordinary perpendicular temperatures (Kohl et al., 1997). By recognizing the importance of the pumping by the second C II line in the interpretation of the UVCS observations, Li et al. (1998) concluded that not only do the oxygen ions flow at a speed around 400 km s −1 at 3 solar radii and faster than protons, but a large temperature anisotropy exists at the same distance as well (see also Dodero et al., 1998). Solar Physics 190: 485–497, 1999. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.