Linking solar flare observations to a series of impulsive solar energetic particle events measured with Solar Orbiter at 0.5 au. A case study to explore linkage capabilities, limitations, and potentials

Impulsive solar energetic particle (SEP) events are often associated with solar flares, but a detailed quantitative understanding of the underlying particle acceleration and injection process into interplanetary space is still lacking. We aim to identify the solar sources of a series of four impulsive SEP events measured with Solar Orbiter at a close solar distance of 0.51,au between March 5-6, 2022. By linking and analyzing remote sensing and in situ observations of these events, we aim to provide new constraints for the acceleration of flare-associated SEPs. We analyzed the energetic electron, proton, and heavy ion data measured with the Energetic Particle Detector (EPD) suite on Solar Orbiter. We estimated the injection times of the SEPs from the Sun and compared these times to the occurrence of solar flares, related extreme ultraviolet (EUV) brightenings, and type-III radio bursts. The flares and their source region were observed with remote sensing instruments from Solar Orbiter and the Solar Dynamics Observatory (SDO), with the two spacecraft radially aligned on March 5-6, 2022. We find that all four SEP events in the observed series can be plausibly linked to the same activated filament in the flaring active region AR12961. Although the combined flare and filament activity did not trigger a coronal mass ejection (CME), the produced energetic particles were able to escape into interplanetary space, possibly along open fan loops in the vicinity of the flare sites. All four linked SEP events show typical in situ SEP composition of impulsive flare-associated events with enhanced 3He/^4He and Fe/O ratios. Most interestingly, we find that the two SEP events associated with larger flares and (failed) filament eruptions show about five times stronger ^3He/^4He enhancements compared to the other two events. Since all four SEP events originate from the same repeatedly flaring filament and the heliospheric particle transport conditions are generally comparable, the differences in the measured SEP spectra among the events are likely related to the details of reconnection at the flare site or the associated (failed) filament eruptions. In particular, the precisely measured differences in 3He / ^4He ratios could be used to test model mechanisms for preferential ^3He acceleration in solar flares, such as particle beam-driven ion-cyclotron waves.