Solar energetic particles injected inside and outside a magnetic cloud

The widespread solar energetic particle event on 2022 January 20

¹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n, 28801 Alcalá de Henares, Madrid, Spain
³ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
⁴ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Physics and Astronomy Department, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁶ Predictive Science Inc., San Diego, CA 92121, USA
⁷ The Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD 20723, USA
⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), D-14482 Potsdam, Germany
⁹ Mullard Space Science Laboratory, University College London, Dorking RH5 6NT, UK
¹⁰ Deep Space Exploration Laboratory/School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
¹¹ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
¹² Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
¹³ California Institute of Technology, Pasadena, CA 91125, USA
¹⁴ Southwest Research Institute, San Antonio, TX 78238, USA
¹⁵ National Observatory of Athens/IAASARS, I. Metaxa & Vas. Pavlou, GR-15236 Penteli, Greece

^⋆ Corresponding author; laura.rodriguezgarcia@esa.int

Received: 6 September 2024
Accepted: 5 December 2024

Abstract

Context. On 2022 January 20, the Energetic Particle Detector (EPD) on board Solar Orbiter measured a solar energetic particle (SEP) event showing unusual first arriving particles from the anti-Sun direction. Near-Earth spacecraft separated by 17° in longitude to the west of Solar Orbiter measured classic anti-sunward-directed fluxes. STEREO-A and MAVEN, separated by 18° to the east and by 143° to the west of Solar Orbiter, respectively, also observed the event, suggesting that particles spread over at least 160° in the heliosphere.

Aims. The aim of the present study is to investigate how SEPs are accelerated and transported towards Solar Orbiter and near-Earth spacecraft, as well as to examine the influence of a magnetic cloud (MC) present in the heliosphere at the time of the event onset on the propagation of energetic particles.

Methods. We analysed remote-sensing data, including flare, coronal mass ejection (CME), and radio emission to identify the parent solar source of the event. We investigated energetic particles, solar wind plasma, and magnetic field data from multiple spacecraft.

Results. Solar Orbiter was embedded in a MC erupting on 16 January from the same active region as that related to the SEP event on 20 January. The SEP event is related to a M5.5 flare and a fast CME-driven shock of ∼1433 km s⁻¹, which accelerated and injected particles within and outside the MC. Taken together, the hard SEP spectra, the presence of a Type II radio burst, and the co-temporal Type III radio burst being observed from 80 MHz that appears to emanate from the Type II burst, suggest that the shock is likely the main accelerator of the particles.

Conclusions. Our detailed analysis of the SEP event strongly suggests that the energetic particles are mainly accelerated by a CME-driven shock and are injected into and outside of a previous MC present in the heliosphere at the time of the particle onset. The sunward-propagating SEPs measured by Solar Orbiter are produced by the injection of particles along the longer (western) leg of the MC still connected to the Sun at the time of the release of the particles. The determined electron propagation path length inside the MC is around 30% longer than the estimated length of the loop leg of the MC itself (based on the graduated cylindrical shell model), which is consistent with the low number of field line rotations.