Proton heating estimates from near-Earth observations of coronal mass ejections in solar cycle 24

TL;DR

This study compares the energy dissipation in CME propagation models with actual near-Earth observations, revealing the need to adjust the polytropic index for accurate proton heating estimates and improving space weather predictions.

Contribution

It quantifies the energy dissipation in CME models using in-situ data, proposing a revised polytropic index to better match observed proton heating.

Findings

Turbulent velocity fluctuations provide less power than needed at $eta=5/3$

A polytropic index of 1.35 best matches observed proton heating

Results improve CME propagation models and Earth arrival time estimates

Abstract

As solar coronal mass ejections (CMEs) propagate through the heliosphere, they expend energy in heating protons to compensate for the cooling that occurs due to expansion. CME propagation models usually treat energy dissipation implicitly via a polytropic index ($\delta$). Here we calculate the power dissipation implied by a given $\delta$ and compare it with the power available in the turbulent velocity fluctuations. We make this comparison using near-Earth {\em in-situ} observations of 27 of the most geoeffective CMEs ($D_{\rm st} < -75$ nT) in solar cycle 24. For $\delta = 5/3$, the power in the turbulent velocity fluctuations is $\approx 54$\% smaller than what would be required to maintain the proton temperature at the observed values. If the power in the turbulent cascade is assumed to be fully expended in local proton heating, the most probable value for $\delta$ is 1.35. Our results contribute to a better understanding of CME energetics, and thereby to improved CME propagation models and estimates of Earth arrival times.