The first gradual solar energetic particle event with an enhanced ³He abundance on Solar Orbiter

¹ Southwest Research Institute, San Antonio, TX 78238, USA
e-mail: radoslav.bucik@swri.org
² Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA
³ Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain
⁴ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore, MD 21250, USA
⁶ Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Kiel, Germany
⁷ NASA Johnson Space Center, Houston, TX 77058, USA
⁸ George Mason University, Fairfax, VA 22030, USA
⁹ California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ Lockheed Martin Advanced Technology Center, Palo Alto, CA 94304, USA
¹¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 21 September 2022
Accepted: 27 October 2022

Abstract

The origin of ³He abundance enhancements in gradual solar energetic particle (SEP) events remains largely unexplained. Two mechanisms have been suggested: the reacceleration of remnant flare material by coronal mass ejection (CME)-driven shocks in interplanetary space, and concomitant activity in the corona. We explore the first gradual SEP event with enhanced ³He abundance that was observed by Solar Orbiter. The event started on 2020 November 24 and was associated with a relatively fast halo CME. During the event, the spacecraft was at 0.9 au from the Sun. The event-averaged ³He/⁴He abundance ratio is 24 times higher than the coronal or solar wind value, and the timing of the ³He intensity was similar to that of other species. We inspected available imaging, radio observations, and the spacecraft magnetic connection to the CME source. The most probable cause of the enhanced ³He abundance apparently are residual ³He ions remaining from a preceding long period of ³He-rich SEPs on 2020 November 17–23.