Empirical Constraints on Proton and Electron Heating in the Fast Solar Wind

TL;DR

This study uses measurements of proton and electron temperatures in the fast solar wind to empirically determine how heat is partitioned between particles, providing insights into collisionless plasma processes.

Contribution

It presents the first empirical estimates of proton and electron heating rates in the fast solar wind, highlighting the increasing proton heating fraction with distance from the Sun.

Findings

Protons receive about 60% of plasma heating near 0.3 AU.

Proton heating fraction increases to about 80% by Jupiter's orbit.

Empirical heat partitioning aligns with linear Vlasov wave damping models.

Abstract

We analyze measured proton and electron temperatures in the high-speed solar wind in order to calculate the separate rates of heat deposition for protons and electrons. When comparing with other regions of the heliosphere, the fast solar wind has the lowest density and the least frequent Coulomb collisions. This makes the fast wind an optimal testing ground for studies of collisionless kinetic processes associated with the dissipation of plasma turbulence. Data from the Helios and Ulysses plasma instruments were collected to determine mean radial trends in the temperatures and the electron heat conduction flux between 0.29 and 5.4 AU. The derived heating rates apply specifically for these mean plasma properties and not for the full range of measured values around the mean. We found that the protons receive about 60% of the total plasma heating in the inner heliosphere, and that this fraction increases to approximately 80% by the orbit of Jupiter. A major factor affecting the uncertainty in this fraction is the uncertainty in the measured radial gradient of the electron heat conduction flux. The empirically derived partitioning of heat between protons and electrons is in rough agreement with theoretical predictions from a model of linear Vlasov wave damping. For a modeled power spectrum consisting only of Alfvenic fluctuations, the best agreement was found for a distribution of wavenumber vectors that evolves toward isotropy as distance increases.