Solar Energetic Particle Events with Extremely High ³He Content Observed by Solar Orbiter

Abstract We performed a survey of 3 He-rich events using observations from the Suprathermal Ion Spectrograph on the Solar Orbiter mission from 2020 to 2024. We identified 57 3 He-rich events, each showing clear 3 He enhancements above the background and 3 He/ 4 He abundance ratios greater than 1 in at least one energy channel in the range from 193.2 keV nucleon −1 to 8.74 MeV nucleon −1 . The analysis of the events’ abundance ratios shows characteristics of impulsive solar energetic particle events with 3 He/ 4 He ∼ 3.1 and Fe/O ∼ 1.3. We observe an energy dependence in the 3 He/ 4 He ratio, which increases with energy, while the Fe/O ratios are almost constant with energy. We also found that six events in our list have extremely high 3 He content (e.g., 3 He/C–Fe ratios in the range 25–62). These events show curved 3 He and heavy nuclei spectra that roll over at higher energies and flatten toward low energy. Their spectral forms can be reasonably fit with simple exponentials with a 3 He rollover energy in the range of a few hundred keV per nucleon and a C–Fe rollover energy ∼2–3 times lower. The differences in spectral rollover energy lead to a strong energy dependence of the ratio 3 He/C–Fe, rising sharply with energy. We conclude that the features of these events with extremely high 3 He content have a reasonable likelihood of depicting the initial acceleration process, which is driven by magnetic reconnection in flaring loops near the solar surface.