Microinstabilities — Instabilities Due to Velocity Space Nonequilibrium

Abstract

In this chapter, we treat instabilities that originate from a non-Maxwellian type velocity distribution. Except for special cases (Section 2.5), we assume a uniform plasma. Because plasmas in space are collision free, they either maintain their original distribution functions or the distribution functions change slowly due to their movement through the background magnetic field. Hence the velocity distribution function is almost never Maxwellian but rather is determined by the past history of the plasma. The velocity distribution can differ from a Maxwellian in basically two different ways. One occurs when the distribution function has more than two humps: for example, when a group of streaming particles is intermixed with the background plasma or when electrons and protons have different average velocities. Such cases are treated in Section 2.2. The other way is by having an anisotropic distribution. The force due to the magnetic field acts only in the direction perpendicular to the field, causing particles with collision frequency much smaller than the cyclotron frequency to move in an anisotropic way. This creates an anisotropie velocity distribution with respect to the direction of the ambient magnetic field. We discuss instabilities associated with such a distribution in Section 2.3. Section 2.4 is devoted to instabilities associated with anisotropic velocity distributions which occur in the hydromagnetic frequency range (frequencies much smaller than the proton cyclotron frequency). In the last section of this chapter, Section 2.5, we treat instabilities in partially ionized plasmas as they relate to various ionospheric phenomena. These instabilities usually originate from the electron-ion two stream effect or from a combination of the two stream and a density gradient. We will see that the existence of a density gradient modifies the two stream instability in a very interesting way in a collision dominated plasma.