Solar energetic electron events measured by MESSENGER and Solar Orbiter. Peak intensity and energy spectrum radial dependences: statistical analysis

TL;DR

This study analyzes 61 solar energetic electron events measured by MESSENGER and Solar Orbiter, revealing how electron peak intensities and energy spectra vary with distance from the Sun, with implications for understanding particle acceleration and propagation.

Contribution

It provides the first comprehensive statistical analysis of radial dependencies of solar energetic electron intensities and spectra between 0.3 and 1 au, combining data from MESSENGER and Solar Orbiter.

Findings

Electron peak intensities generally decrease with radial distance, following approximately R^{-3}.

The electron spectral index near 0.3 au is harder than near 1 au in most events.

Most events are associated with CME-driven shocks and display relativistic electron enhancements.

Abstract

Context/Aims: We present a list of 61 solar energetic electron (SEE) events measured by the MESSENGER mission and the radial dependences of the electron peak intensity and the peak-intensity energy spectrum. The analysis comprises the period from 2010 to 2015, when MESSENGER heliocentric distance varied between 0.31 and 0.47 au. We also show the radial dependencies for a shorter list of 12 SEE events measured in February and March 2022 by spacecraft near 1 au and by Solar Orbiter around its first close perihelion at 0.32 au. Results: Due to the elevated background intensity level of the particle instrument on board MESSENGER, the SEE events measured by this mission are necessarily large and intense; most of them accompanied by a CME-driven shock, being widespread in heliolongitude, and displaying relativistic ($\sim$1 MeV) electron intensity enhancements. The two main conclusions derived from the analysis of the large SEE events measured by MESSENGER, which are generally supported by Solar Orbiter's data results, are: (1) There is a wide variability in the radial dependence of the electron peak intensity between $\sim$0.3 au and $\sim$1 au, but the peak intensities of the energetic electrons decrease with radial distance from the Sun in 27 out of 28 events. On average and within the uncertainties, we find a radial dependence consistent with $R^{-3}$. (2) The electron spectral index found in the energy range around 200 keV ($\delta$200) of the backward-scattered population near 0.3 au measured by MESSENGER is harder in 19 out of 20 (15 out of 18) events by a median factor of $\sim$20% ($\sim$10%) when comparing to the anti-sunward propagating beam (backward-scattered population) near 1 au.