3He and Fe Spectral Properties in 3He-rich Solar Energetic Particle Events

Abstract

We have surveyed ^3 He-rich events on the Solar Orbiter mission from 2020 April to 2024 April, selecting isolated injections whose rollover ^3 He spectral shape is presumed to represent the initial acceleration state, unprocessed by subsequent activity such as coronal mass ejections or jets. A main goal has been to find relationships between the spectra of ^3 He and heavy ions C–Fe, in order to explore a common acceleration mechanism in spite of the fact that these events show ^3 He enrichments of up to ∼10 ^4 , while the heavy-ion enrichment is rarely larger than ∼10. Selecting 34 ^3 He injections, we find that heavy ions are always present, and arrive at the same time as the ^3 He signaling a common origin. Concentrating on Fe since it is a minor ion but with higher abundance than many others, we find its spectral shape and intensity is similar to ^3 He. In ∼two-thirds of the cases, if the ^3 He spectrum is shifted to lower energy by a factor 3.0 ± 1.3, it nearly coincides with the Fe spectrum, illustrating their close connection. Several plasma wave turbulence models have calculated spectra that also show the ion rollovers around 1 MeV nucleon ^−1 . The unique mass-to-charge ratio of ^3 He allows it to interact more efficiently with the turbulence, thereby gaining several times more energy per nucleon than the other heavy ions. In the spectral rollover region this can lead to the observed enormous enhancements of ^3 He. The acceleration appears to be associated with magnetic reconnection in emerging flux regions on the Sun.

Keywords

• Solar flares
• Solar energetic particles
• Solar particle emission
• Solar abundances
• Solar magnetic fields
• Solar magnetic reconnection