Multi-spacecraft observations of the decay phase of solar energetic particle events

¹ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, United Kingdom
² Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³ Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁴ California Institute of Technology, Pasadena, CA 91125, USA
⁵ Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, Leibnizstraße 11, 24118 Kiel, Germany

^⋆ Corresponding author; rahyndman@uclan.ac.uk

Received: 15 November 2024
Accepted: 19 December 2024

Abstract

Context. The parameters of solar energetic particle (SEP) event profiles such as the onset time and peak time have been researched extensively to obtain information on the acceleration and transport of SEPs. The corotation of particle-filled magnetic flux tubes with the Sun is generally thought to play a minor role in determining intensity profiles. However recent simulations have suggested that corotation affects the SEP decay phases and depends on the location of the observer with respect to the active region associated with the event.

Aims. We aim to determine whether signatures of corotation are present in observations of the decay phases of SEP events, and we study the dependence of the parameters of the decay phase on the properties of the flares and coronal mass ejections associated with the events.

Methods. We analysed multi-spacecraft observations of SEP intensity profiles from 11 events between 2020 and 2022 using data from Solar Orbiter, PSP, STEREO-A, and SOHO. We determined the decay-time constant, τ, in three energy channels; electrons ∼ 1 MeV, protons ∼ 25 MeV, and protons ∼ 60 MeV. We studied the dependence of τ on the longitudinal separation, Δϕ, between the source of the active region and the spacecraft magnetic footpoint on the Sun.

Results. Individual events show a tendency for the decay-time constant to decrease with increasing Δϕ. This agrees with test particle simulations. The magnitude of the event as measured through the intensity of the associated flare and SEP peak flux affects the measured τ values and likely is the cause of the observed large inter-event variability together with the varying solar wind and the conditions in the interplanetary magnetic field.

Conclusions. We conclude that corotation affects decay phase of an SEP event and should be included in future simulations and interpretations of these events.