On the evaluation of solar wind's heating rates

TL;DR

This paper critiques traditional methods for calculating solar wind heating rates, proposing an invariant-based approach that confirms perpendicular proton heating but finds no evidence of parallel heating deviations between 0.3 and 1 AU.

Contribution

It introduces an invariant-based method for evaluating solar wind heating, avoiding arbitrary model assumptions and providing clearer insights into proton heating mechanisms.

Findings

Confirmed perpendicular proton heating in solar wind.

No significant change in parallel adiabatic invariant observed.

Method reduces dependence on arbitrary model functions.

Abstract

Solar wind heating rates have often been calculated by fitting plasma and magnetic field data with a set of model functions. In this letter, we show that the rates obtained by such an approach strongly depend on the rather arbitrary choice one makes for these model functions. An alternative approach, consisting in monitoring the radial evolution of the adiabatic invariants, based on locally and consistently measured plasma and magnetic field parameters, is free from such a flaw. We apply this technique to a recently released Helios proton dataset, and confirm the existence of a clear perpendicular heating of solar wind's protons. On the other hand, no significant change in the parallel adiabatic invariant is visible in the data. We conclude that to date, and in the distance range of 0.3 to 1 AU, no clear observation of a deviation of solar wind's protons from parallel adiabaticity has ever been made.