Multi-spacecraft observations of near-relativistic electron events at different radial distances

¹ Institute of Experimental and Applied Physics, Kiel University, Christian-Albrechts-Platz 4, 24118 Kiel, Germany
e-mail: kollhoff@physik.uni-kiel.de
² Department of Physics and Astronomy, 20014 University of Turku, Finland
³ Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n, 28801 Alcalá de Henares, Madrid, Spain

Received: 20 January 2023
Accepted: 29 May 2023

Abstract

Aims. We study the radial evolution of near-relativistic solar energetic electron (SEE) events observed by at least two spacecraft at different heliocentric distances and with small separation angles between their magnetic footpoints at the Sun.

Methods. We identified SEE events for which Solar Orbiter and either Wind or STEREO-A had a small longitudinal separation (< 15°) between their nominal magnetic footpoints. For the approximation of the footpoint separation, we followed a ballistic back-mapping approach using in situ solar wind speed measurements. For all the SEE events that satisfied our selection criteria, we determined the onset times, rise times, peak fluxes, and peak values of the first-order anisotropy for electrons in the energy range from ∼50 − 85 keV. We compared the event parameters observed at different spacecraft and derived exponential indices α_p for each parameter p, assuming an R^α-dependence on the heliocentric distance R.

Results. In our sample of SEE events, we find strong event-to-event variations in the radial dependence of all derived parameters. For the majority of events, the peak flux decreases with increasing radial distance. For the first-order anisotropy and the rise time no clear radial dependence was found. The derived onset delays observed between two spacecraft were found to be too long to be explained by ideal Parker spirals in multiple events.

Conclusions. The rudimentary methods presented in this study lead to event parameters with large uncertainties. The absence of a clear radial dependence on the first-order anisotropy and the rise time as well as the ambiguous onset timing of the SEE events found in this study could be the result of general limitations in the methods we used. Further studies, including analyses of the directional fluxes and transport simulations that take the individual instrument responses into account, would allow a better interpretation of the radial evolution of SEE events.