A zone of preferential ion heating extends tens of solar radii from Sun

TL;DR

This study empirically determines that preferential ion heating in the solar corona extends from the transition region to 20-40 solar radii, suggesting the physical mechanism is active over a large solar region and will be observable by Parker Solar Probe.

Contribution

The paper introduces an empirical method combining solar wind data and energy modeling to identify the outer boundary of preferential ion heating in the solar corona.

Findings

Preferential ion heating extends up to 20-40 solar radii from the Sun.

The super-mass-proportional alpha-to-proton temperature ratio is about 5.2-5.3.

Existing spacecraft likely haven't directly observed the heating mechanism.

Abstract

The extreme temperatures and non-thermal nature of the solar corona and solar wind arise from an unidentified physical mechanism that preferentially heats certain ion species relative to others. Spectroscopic indicators of unequal temperatures commence within a fraction of a solar radius above the surface of the Sun, but the outer reach of this mechanism has yet to be determined. Here we present an empirical procedure for combining interplanetary solar wind measurements and a modeled energy equation including Coulomb relaxation to solve for the typical outer boundary of this zone of preferential heating. Applied to two decades of observations by the Wind spacecraft, our results are consistent with preferential heating being active in a zone extending from the transition region in the lower corona to an outer boundary 20-40 solar radii from the Sun, producing a steady state super-mass-proportional $\alpha$-to-proton temperature ratio of $5.2-5.3$. Preferential ion heating continues far beyond the transition region and is important for the evolution of both the outer corona and the solar wind. The outer boundary of this zone is well below the orbits of spacecraft at 1 AU and even closer missions such as Helios and MESSENGER, meaning it is likely that no existing mission has directly observed intense preferential heating, just residual signatures. We predict that {Parker Solar Probe} will be the first spacecraft with a perihelia sufficiently close to the Sun to pass through the outer boundary, enter the zone of preferential heating, and directly observe the physical mechanism in action.