Solar energetic electron events measured by MESSENGER and Solar Orbiter

Peak intensity and energy spectrum radial dependences: Statistical analysis^⋆

¹ Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n, 28801 Alcalá de Henares, Madrid, Spain
e-mail: l.rodriguezgarcia@edu.uah.es
² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Physics and Astronomy Department, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁶ The Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD 20723, USA
⁷ Institut fuer Experimentelle und Angewandte Physik, University of Kiel, Leibnizstrasse 11, 24118 Kiel, Germany

Received: 20 July 2022
Accepted: 19 November 2022

Abstract

Context. We present a list of 61 solar energetic electron (SEE) events measured by the MESSENGER mission and the radial dependences of some parameters associated with these SEE events. The analysis covers the period from 2010 to 2015, when the heliocentric distance of MESSENGER varied between 0.31 and 0.47 au. We also show the radial dependences for a shorter list of 12 SEE events measured in February and March 2022 by spacecraft near 1 au and by Solar Orbiter at about its first close perihelion at 0.32 au.

Aims. We study the radial dependences of the electron peak intensity and the energy spectrum of the electron intensity at the time of the SEE event peak intensity, taking advantage of multi-spacecraft measurements.

Methods. We compiled the list of SEE events measured by MESSENGER and Solar Orbiter using hourly averages to find the prompt component of the near-relativistic (∼70–110 keV) electron peak intensities and to calculate the peak-intensity energy spectra. We also obtained the peak intensities and energy spectra for the same events as measured by the STEREO-A, -B, ACE, or Wind spacecraft when one of these spacecraft was in close nominal magnetic connection with MESSENGER or Solar Orbiter to derive the radial dependences of these SEE parameters.

Results. (1) Because the background intensity level of the particle instrument on board MESSENGER is high, the SEE events measured by this mission are necessarily large and intense; most of them are accompanied by a shock driven by a coronal mass ejection and are widely spread in heliolongitude. The SEE events display relativistic (∼1 MeV) electron intensity enhancements. For this SEE sample, we found that (2) the SEE peak intensity shows a radial dependence that can be expressed as R^α, where the median value of the α index is α_Med = −3.3±1.4 for a subsample of 28 events for which the nominal magnetic footpoints of the near 0.3 au and 1 au spacecraft were close in heliographic longitude. (3) The mean spectral index δ of a subset of 42 events for which the energy spectrum could be analysed is ⟨δ⟩= − 1.9 ± 0.3, which is harder than the value found in previous studies using data from spacecraft near 1 au. SEE events observed by Solar Orbiter also display harder energy spectra than previous studies using data obtained near 1 au.

Conclusions. There is a wide variability in the radial dependence of the electron peak intensities, but on average and within uncertainties, the ∝R⁻³ dependence found in previous observational and modelling studies is confirmed. The electron spectral index found in the energy range around ∼200 keV (δ200) of the backward-scattered population near 0.3 au measured by MESSENGER is harder by a median factor of ∼20% and ∼10% when comparing to the near 1 au anti-sunward propagating beam and the backward-scattered population, respectively.