Perpendicular Ion Heating by Cyclotron Resonant Dissipation of Turbulently Generated Kinetic Alfv\'en Waves in the Solar Wind

TL;DR

This paper demonstrates that kinetic Alfvén waves can cause perpendicular ion heating in the solar wind through cyclotron resonance, providing a potential explanation for observed ion temperature anisotropies.

Contribution

It introduces a novel mechanism where KAWs induce perpendicular ion heating via cyclotron resonance, challenging previous assumptions about dissipation processes.

Findings

Protons are strongly heated perpendicularly within minutes at 20 Rs.

Alpha particles also gain perpendicular thermal speeds similar to observations.

KAWs can produce significant perpendicular heating through cyclotron resonance.

Abstract

Solar wind ions are observed to be heated in the directions perpendicular to the large-scale magnetic field, with preferential heating given to heavy ions. In the solar corona, this heating may be responsible for the generation of the wind itself. It is likely that this heating results from the dissipation of plasma turbulence, but the specific kinetic mechanism that produces these preferential effects is not known. Solar wind turbulence at proton scales is often characterized in terms of highly oblique kinetic Alfv\'en waves (KAWs), which have been thought to dissipate through the Landau resonance and yield parallel heating. We show that the quasilinear resonant cyclotron interaction between KAWs and solar wind ions can actually produce perpendicular ion heating. We present an illustrative calculation of a steady, critically balanced spectrum of KAWs acting on homogeneous ion distributions with a plasma \b{eta} = 0.1, representative of turbulent conditions in the fast solar wind near 20 Rs. The KAWs are described here by a two-fluid dispersion relation. We find that thermal protons are strongly heated in the perpendicular direction within a typical quasilinear time of several thousand gyroperiods, which corresponds to only a few minutes at 20 Rs. Alpha particles in the same fluctuation field are heated to similar perpendicular thermal speeds, equivalent to the greater than mass proportional perpendicular temperatures that are commonly observed. We discuss improvements to this simple model that may determine whether this mechanism can be responsible for the observed coronal and solar wind ion heating.