Ion Heating in Inhomogeneous Expanding Solar Wind Plasma: The Role of Parallel and Oblique Ion-Cyclotron Waves

TL;DR

This study uses a hybrid simulation to show that small-scale inhomogeneities in the solar wind enhance ion heating and generate oblique waves, affecting ion temperature anisotropies observed in the heliosphere.

Contribution

It demonstrates that plasma inhomogeneities significantly influence ion heating and wave spectra in the solar wind, extending previous models by including inhomogeneous backgrounds and expansion effects.

Findings

Inhomogeneities lead to increased ion heating.

Oblique waves are generated in inhomogeneous plasma.

Ion temperature anisotropy matches observations.

Abstract

Remote sensing observations of coronal holes show that heavy ions are hotter than protons and their temperature is anisotropic. In-situ observations of fast solar wind streams provide direct evidence for turbulent Alfv\'en wave spectrum, left-hand polarized ion-cyclotron waves, and He$^{++}$ -- proton drift in the solar wind plasma, which can produce temperature anisotropies by resonant absorption and perpendicular heating of the ions. Furthermore, the solar wind is expected to be inhomogeneous on decreasing scales approaching the Sun. We study the heating of solar wind ions in inhomogeneous plasma with a 2.5D hybrid code. We include the expansion of the solar wind in an inhomogeneous plasma background, combined with the effects of a turbulent wave spectrum of Alfv\'enic fluctuations and initial ion-proton drifts. We study the influence of these effects on the perpendicular ion heating and cooling and on the spectrum of the magnetic fluctuations in the inhomogeneous background wind. We find that inhomogeneities in the plasma lead to enhanced heating compared to the homogenous solar wind, and the generation of significant power of oblique waves in the solar wind plasma. The cooling effect due to the expansion is not significant for super-Alfv\'enic drifts, and is diminished further when we include an inhomogenous background density. We reproduce the ion temperature anisotropy seen in observations and previous models, which is present regardless of the perpendicular cooling due to solar wind expansion. We conclude that small scale inhomogeneities in the inner heliosphere can significantly affect resonant wave ion heating.