New plasma instabilities in the solar wind

Abstract

We discuss the instability of transverse electromagnetic waves propagating parallel to the average magnetic field in an electron-proton plasma, and we examine the question of whether the instabilities may occur in the solar wind. Our discussion is based on Stix' well-known dispersion relation, but we depart from most previous analyses by assuming that the resonant protons lie near the peak of their distribution function. We find three instabilities, two of which are new. The first new instability is driven by anisotropic electrons in the same manner as is the firehose, and we believe that it represents an extension of the firehose instability to large wave numbers. The growth rates are large over a broad frequency range around Ωp. We suggest that the excitation of this instability results in a partial transfer of thermal energy from the electrons to the protons and, if it occurs in the solar wind, may therefore heat the solar-wind protons and reduce their anisotropy. The second, new instability is driven by anisotropic protons with T⊥p⪞3T∥p; it may occur in local regions of the solar wind. The third instability is not new; it represents a continuation of the unstable whistler mode near Ωp to those cases where there are many resonant protons. In contrast to earlier papers, in which it is assumed that the number of resonant protons is small, our work demonstrates that this instability can in fact be effective in destroying proton anisotropies of the type observed in the solar wind.