# Radial evolution of stochastic heating in low-$\beta$ solar wind

## TL;DR

This study investigates how stochastic ion heating driven by wave turbulence evolves with distance from the Sun, using Helios data to quantify energy dissipation in the inner heliosphere.

## Contribution

It provides the first observational constraints on the radial evolution of stochastic heating in the solar wind between 0.29 and 0.98 au.

## Key findings

- Stochastic heating likely occurs throughout the inner heliosphere.
- Radial dependence of stochastic heating is steeper than previous empirical models.
- Fast solar wind streams show an even steeper radial decline in heating.

## Abstract

We consider the radial evolution of perpendicular ion heating due to the violation of magnetic moment conservation caused by Alfven and kinetic Alfven wave turbulence. This process, referred to as stochastic heating, can be quantified by the ratio between the average velocity fluctuations at the ion gyroradius and the perpendicular ion thermal speed $\epsilon \equiv \delta v / v_{t\perp}$. Using 17 years of Helios observations, we constrain how much energy could be dissipated by this mechanism between 0.29 and 0.98 au. We find that stochastic heating likely operates throughout the entire inner heliosphere, but that its radial dependence is steeper than that of empirically derived dissipation rates, being $r^{-2.5}$ compared with $r^{-2.08}$. This difference is significantly increased in fast solar wind streams to $r^{-3.1}$ compared with $r^{-1.8}$.