# Strong Preferential Ion Heating is Limited to within the Solar Alfven Surface

## TL;DR

This study shows that strong preferential ion heating in the solar wind is confined within the Alfvén surface, with the boundary moving in sync with the Alfvén point over the solar cycle, suggesting heating occurs predominantly below this surface.

## Contribution

It identifies the outer boundary of preferential ion heating as closely tracking the Alfvén surface and highlights the need for in situ measurements below this surface to understand heating mechanisms.

## Key findings

- The boundary of preferential ion heating moves with the Alfvén point over the solar cycle.
- Strong ion heating is mainly active below the Alfvén surface.
- Future Parker Solar Probe measurements are expected to observe this heating directly.

## Abstract

The decay of the solar wind helium to hydrogen temperature ratio due to Coulomb thermalization can be used to measure how far from the Sun strong preferential ion heating occurs. Previous work has shown that a zone of preferential ion heating, resulting in mass-proportional temperatures, extends about $20-40 R_\odot$ from the Sun on average. Here we look at the motion of the outer boundary of this zone with time and compare it to other physically meaningful distances. We report that the boundary moves in lockstep with the Alfv\'en point over the solar cycle, contracting and expanding with solar activity with a correlation coefficient of better than 0.95 and with an RMS difference of $4.23 R_\odot$. Strong preferential ion heating apparently is predominatly active below the Alfv\'en point. To definitively identify the underlying preferential heating mechanisms, it will be necessary to make in situ measurements of the local plasma conditions below the Alfv\'en surface. We predict Parker Solar Probe (PSP) will be the first spacecraft to directly observe this heating in action, but only a couple of years after launch as activity increases, the zone expands, and PSP's perihelion drops.