Heat Conductivity, Plasma Instabilities, and Radio-Star - Lations in the Solar Wind

Abstract

The motion of a typical solar-wind electron is periodic because of the combination of magnetic mirror and electrostatic reflections. The periodic motion reduces the electron heat conductivity from the Spitzer formula and causes magnetoacoustic waves with an inward-directed phase velocity to become unstable. A nonlinear theory of this instability quantitatively accounts for radio-star scintillation data and an increased rate of energy exchange between electrons and ions above the Spitzer value. Subject headings: interplanetary medium — plasmas — solar wind — instabilities Although energy transport plays a crucial role in hydrodynamic theories of the solar wind, the relative importance of the various processes which contribute to energy transport remains unclear (Montgomery 1972). For example, recent studies (Cuperman and Harten 1971 ; Wolff, Brandt, and Southwick 1971) show that if the electron thermal conductivity is suppressed below the value of the Spitzer formula, then hydrodynamic calculations yield good agreement with solar-wind parameters both in the corona and near the Earth. On the other hand, Hartle and Barnes (1970) and Barnes, Hartle, and Bredekamp (1971) find that hydromagnetic wave heating is an essential element of solar-wind energetics. The purpose of this paper is to argue that, for regions where the solar wind con-stitutes an almost collisionless plasma, the Spitzer conductivity is inapplicable and a smaller value based on the ideas of neoclassical transport developed in the controlled-fusion program (Galeev and Sagdeev 1967; Sagdeev and Galeev 1972) should be used. The term " neoclassical " means that the collisional processes causing diffusion are Coulomb collisions, but that the particle trajectories between collisions are bounded, periodic orbits and not simple straight lines. The same considerations also show that, in the absence of collisions, the electron velocity distribution takes on the form assumed by Forslund (1970) which leads to magnetoacoustic plasma instabilities. These plasma instabilities increase the energy exchange rate between electrons and ions, but do not affect the neoclassical thermal conductivity. The density fluctuations associated with unstable magnetoacoustic waves will cause radio-star scintillations. Let us start by supposing that the electrons in the solar wind are collisionless and that the magnetic field and solar-wind flow are purely radial. Then, because the flow speed of the solar wind is substantially below the electron thermal velocity, an electro-static potential is built up which reflects most of the electrons (Dessler 1969; Eviatar and Schulz 1970; Schulz and Eviatar 1972). The electrostatic potential O always achieves a value which makes the escaping electron flux, composed of those few suprathermal electrons energetic enough to surmount the electrostatic barrier, equal to the escaping ion flux. The radial motion of a typical electron then becomes bounded and periodic: an outward-going electron undergoes reflection off the electrostatic barrier while an inward-going electron reflects at a magnetic mirror point because the magnetic field increases rapidly inward.