Waves and Instabilities

Abstract

It is widely recognized that even though the solar wind is essentially collisionless, to a good approximation it can be treated as a fluid. To understand this behavior, it is first necessary to understand the microscopic processes that control the macroscopic properties of the plasma. In a collisionless plasma such as the solar wind, it is now widely recognized that waves play a role similar to collisions in an ordinary fluid. As the plasma flows outward from the sun, dynamical changes cause the velocity distribution function to deviate from an equilibrium thermal distribution. In the absence of collisions these deviations continue to grow until the velocity-space gradients parallel and perpendicular to the magnetic field, ∂f/∂v _|| and ∂f/∂v _⊥ become so large that plasma instabilities start to occur. These instabilities lead to the growth of waves. As the waves grow to large amplitudes, wave-particle interactions eventually act to eliminate the velocity- space gradients that cause the instability. Waves and instabilities thereby play a crucial role in preventing large deviations from thermal equilibrium. For a review of the types of plasma instabilities and wave-particle interactions that can occur in a plasma see, for example, Hasegawa [9.34], or Melrose [9.47].