Magnetic pumping as a source of particle heating and power-law distributions in the solar wind

TL;DR

This paper proposes a magnetic pumping model to explain particle heating and power-law distributions in the solar wind, emphasizing large-scale turbulence effects over microscopic dissipation.

Contribution

It introduces a novel magnetic pumping mechanism that energizes particles directly from large-scale fluctuations, including pressure anisotropy effects, differing from previous turbulence dissipation models.

Findings

Magnetic pumping can generate power-law particle distributions.

The model predicts significant plasma heating from large-scale turbulence.

Numerical simulations support the analytical predictions.

Abstract

Based on the rate of expansion of the solar wind, the plasma should cool rapidly as a function of distance to the Sun. Observations show this is not the case. In this work, a magnetic pumping model is developed as a possible explanation for the heating and the generation of power-law distribution functions observed in the solar wind plasma. Most previous studies in this area focus on the role that the dissipation of turbulent energy on microscopic kinetic scales plays in the overall heating of the plasma. However, with magnetic pumping particles are energized by the largest scale turbulent fluctuations, thus bypassing the energy cascade. In contrast to other models, we include the pressure anisotropy term, providing a channel for the large scale fluctuations to heat the plasma directly. In this work a complete set of coupled differential equations describing the evolution, and energization, of the distribution function are derived, as well as an approximate closed form solution. Numerical simulations using the VPIC kinetic code are applied to verify the model's analytical predictions. The results of the model for realistic solar wind scenario are computed, where thermal streaming of particles are important for generating a phase shift between the magnetic perturbations and the pressure anisotropy. In turn, averaged over a pump cycle, the phase shift permits mechanical work to be converted directly to heat in the plasma. The results of this scenario show that magnetic pumping may account for a significant portion of the solar wind energization.