Parker Solar Probe Observations of the 4 April 2019 Solar Energetic Particle Event Near Perihelion

A solar energetic particle (SEP) event was detected by the ISOIS instrument suite on Parker Solar Probe (PSP) on 4 April 2019 when the spacecraft was almost at its second perihelion of 0.17 AU, providing an opportunity to study solar particle acceleration and transport unprecedentedly close to the source. The event was small, with peak 1 MeV proton intensities at PSP of ~0.3 protons/(cm² sr s MeV), and was not observed above background levels in either STEREO or ACE particle detectors at 1 AU. It was strongly anisotropic, with intensities flowing outward from the Sun ~30 times greater than those flowing inward. Curiously, although the event was clearly observed in the ISOIS EPI-Hi instrument at proton energies of a few MeV, it was much less pronounced at energies of a few hundred keV in EPI-Lo. Conversely, an event two days earlier on 2 April was observed in EPI-Lo, with almost no detectable signature in EPI-Hi except for a significant (though statistically limited) enhancement in heavy ions up to Fe at energies of several MeV/nucleon. Based on STEREO/EUVI observations of small brightness surges (several of which were also associated with type III emission seen by both STEREO/WAVES and PSP/FIELDS), the source of both these events appears to be an active region that was later numbered Region 2738 when it rotated over the east limb and became visible from Earth several days later. This region was far (nearly 90 degrees) to the east of the nominal PSP magnetic footpoint, raising the question of how these particles traveled to the observer.

We present EPI-Hi and EPI-Lo observations of the time profiles, spectra, composition, and anisotropy of this event, compare it with the earlier 2 April event, and examine the associated solar activity and interplanetary environment to assess the implications for particle acceleration and transport.

This work was supported by NASA under contract NNN06AA01C.