Research Article

Improved modelling of SEP event onset within the WSA–Enlil–SEPMOD framework

¹ Predictive Science Inc., San Diego, CA 92121, USA
² Space Sciences Laboratory, University of California–Berkeley, Berkeley, CA 94720, USA
³ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁵ University of Houston, Houston, TX 77204, USA
⁶ KBR, Inc., Houston, TX 77002, USA
⁷ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁸ Department of Mathematics/Centre for Mathematical Plasma Astrophysics, KU Leuven, B-3001 Leuven, Belgium
⁹ Laboratório de Instrumentação e Física Experimental de Partículas, 1649-003 Lisbon, Portugal
¹⁰ Instituto Superior Técnico, University of Lisbon, 1049-001 Lisbon, Portugal
¹¹ European Space Research and Technology Centre, European Space Agency, 2201 AZ Noordwijk, The Netherlands
¹² Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, D-38106 Braunschweig, Germany
¹³ Space Research Institute, Austrian Academy of Sciences, A-8042 Graz, Austria

^* Corresponding author: epalmerio@predsci.com

Received: 31 August 2023
Accepted: 9 January 2024

Abstract

Multi-spacecraft observations of solar energetic particle (SEP) events not only enable a deeper understanding and development of particle acceleration and transport theories but also provide important constraints for model validation efforts. However, because of computational limitations, a given physics-based SEP model is usually best suited to capture a particular phase of an SEP event, rather than its whole development from onset through decay. For example, magnetohydrodynamic (MHD) models of the heliosphere often incorporate solar transients only at the outer boundary of their so-called coronal domain – usually set at a heliocentric distance of 20–30 R_⊙. This means that particle acceleration at coronal mass ejection (CME)-driven shocks is also computed from this boundary onwards, leading to simulated SEP event onsets that can be many hours later than observed, since shock waves can form much lower in the solar corona. In this work, we aim to improve the modelled onset of SEP events by inserting a “fixed source” of particle injection at the outer boundary of the coronal domain of the coupled WSA–Enlil 3D MHD model of the heliosphere. The SEP model that we employ for this effort is Solar Energetic Particle MODel (SEPMOD), a physics-based test-particle code based on a field line tracer and adiabatic invariant conservation. We apply our initial tests and results of SEPMOD’s fixed-source option to the 2021 October 9 SEP event, which was detected at five well-separated locations in the inner heliosphere – Parker Solar Probe, STEREO-A, Solar Orbiter, BepiColombo, and near-Earth spacecraft.