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

• Published in: Astronomy and astrophysics (Berlin) 2023-01, Vol.669, p.A13
• Main Authors: Bučík, R., Mason, G. M., Gómez-Herrero, R., Krupar, V., Lario, D., Starkey, M. J., Ho, G. C., Rodríguez-Pacheco, J., Wimmer-Schweingruber, R. F., Espinosa Lara, F., Tadesse, T., Balmaceda, L., Cohen, C. M. S., Dayeh, M. A., Desai, M. I., Kühl, P., Nitta, N. V., Wiedenbeck, M. E., Xu, Z. G.
• Format: Article
• Language: English
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202245037

The origin of 3 He 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 3 He 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 3 He/ 4 He abundance ratio is 24 times higher than the coronal or solar wind value, and the timing of the 3 He 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 3 He abundance apparently are residual 3 He ions remaining from a preceding long period of 3 He-rich SEPs on 2020 November 17–23.