The 17 April 2021 widespread solar energetic particle event


Context. A complex and long-lasting solar eruption on 17 April 2021 produced a widespread Solar Energetic Particle event (SEP) that was observed by five longitudinally well-separated observers in the inner heliosphere covering distances to the Sun from 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and close-to-Earth spacecraft. The event was the second widespread SEP event of solar cycle 25 and produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare showing multiple hard X-ray peaks over a duration of one hour. The event was further accompanied by a medium fast Coronal Mass Ejection (CME) with a speed of 880 km s −1 driving a shock, an EUV wave as well as long-lasting and complex radio burst activity showing four distinct type III burst groups over a period of 40 minutes. Aims. We aim at understanding the reason for the wide SEP spread as well as identifying the underlying source regions of the electron and proton event. Methods. A comprehensive multi-spacecraft analysis of remote-sensing observations and in-situ measurements of the energetic particles and interplanetary context is applied to attribute the SEP observations at the different locations to the various potential source regions at the Sun. An ENLIL simulation is used to characterize the complex interplanetary state and its role for the energetic particle transport. The magnetic connection between the spacecraft and the Sun is determined using ballistic backmapping in combination with potential field source surface extrapolations in the lower corona. In combination with a reconstruction of the coronal shock front we then determine the times when the shock establishes magnetic connections with the different observers. Radio observations are used to characterize the directivity of the four main injection episodes, which are then employed in a 2D SEP transport simulation to test the importance of these different injection episodes. Results. A comprehensive timing analysis of the inferred solar injection times of the SEPs observed at the different spacecraft suggests different source processes being important for the electron and the proton event. Comparison with characteristics and timing of the potential sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source of the electron event. Conclusions. Different to earlier studies on widespread SEP events, we find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110◦ covered by distinct SEP injections, which is also supported by our SEP transport modeling.

Astronomy and Astrophysics