Heating and cooling of protons by turbulence‐driven ion cyclotron waves in the fast solar wind

Abstract

The effects of parallel propagating nondispersive ion cyclotron waves on the solar wind plasma are investigated in an attempt to reproduce the observed proton temperature anisotropy, namely, T p ⊥ ≫ T p ‖ in the inner corona and T p ⊥ < T p ‖ at 1 AU. Low‐frequency Alfvén waves are assumed to carry most of the energy needed to accelerate and heat the fast solar wind. The model calculations presented here assume that nonlinear cascade processes, at the Kolmogorov and Kraichnan dissipation rates, transport energy from low‐frequency Alfvén waves to the ion cyclotron resonant range. The energy is then picked up by the plasma through the resonant cyclotrou interaction. While the resonant interaction determines how the heat is distributed between the parallel and perpendicular degrees of freedom, the level of turbulence determines the net dissipation. Ion cyclotron waves are found to produce a significant temperature anisotropy starting in the inner corona, and to limit the growth of the temperature anisotropy in interplanetary space. In addition, this mechanism heats or cools protons in the direction parallel to the magnetic field. While cooling in the parallel direction is dominant, heating in the parallel direction occurs when T p ⊥ ≫ T p ‖ . The waves provide the mechanism for the extraction of energy from the parallel direction to feed into the perpendicular direction. In our models, both Kolmogorov and Kraichnan dissipation rates yield T p ⊥ ≫ T p ‖ in the corona, in agreement with inferences from recent ultraviolet coronal measurements, and predict temperatures at 1 AU which match in situ observations. The models also reproduce the inferred rapid acceleration of the fast solar wind in the inner corona and flow speeds and particle fluxes measured at 1 AU. Since this mechanism does not provide direct energy to the electrons, and the electron‐proton coupling is not sufficient to heat the electrons to temperatures at or above 10 6 K, this model yields electron temperatures which are much cooler than those currently inferred from observations.