Origin of Unusual Composition of 3He-Rich Solar Energetic Particles

We examine 3He-rich solar energetic particles (SEPs) detected on 2023 October 24-25 by Solar Orbiter at 0.47 au. The measurements revealed heavy-ion enhancement not increasing smoothly with mass. C, and especially N, Si, and S, stand out in the enhancement pattern with large abundances. Except for 3He, heavy ion spectra can only be measured below 0.5 MeV/nucleon. At 0.386 MeV/nucleon, the event showed a huge 3He/4He ratio of 75.2±33.9, larger than ever previously observed. Solar Dynamics Observatory extreme ultraviolet data showed a mini filament eruption at the solar source of 3He-rich SEPs that triggered a straight tiny jet. Located at the boundary of a low-latitude coronal hole, the jet base is a bright, small-scale region with a supergranulation scale size. The emission measure provides relatively cold source temperatures of 1.5 to 1.7 MK between the filament eruption and nonthermal type III radio burst onset. The analysis suggests that the emission measure distribution of temperature in the solar source could be a factor that affects the preferential selection of heavy ions for heating or acceleration, thus shaping the observed enhancement pattern. Including previously reported similar events indicates that the cool material of the filament in the source is a common feature of events with heavy-ion enhancement not ordered by mass. Surprisingly, sources with weak magnetic fields showed extreme 3He enrichment in these events. Moreover, the energy attained by heavy ions seems to be influenced by the size and form of jets.