3He-rich Solar Energetic Particles Observations at Parker Solar Probe

Report from ISSI Team #425 Origins of 3He-Rich Solar Energetic Particles led by R. Bucik and J.F. Drake 

Left: Mass histogram for 3He-rich SEP event on 2019 April 20 shows small but clear 3He peak. Right: Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).

3He-rich solar energetic particles (SEPs) are one of the most peculiar and least explored particle populations in the heliosphere with a tremendously enhanced abundance of the 3He nuclide and ultra-heavy elements (e.g., Pb) by a factor up to 104 above the solar corona or solar wind. One reason for the current lack of understanding of 3He-rich SEPs is the small size of these events. Recently launched Parker Solar Probe (PSP) is able to approach the solar sources of 3He rich SEPs at distances (~0.05 au; 1 au ~ 1.5⨉108 km) that have never been reached before. On 2019 April 20-21, the IS⊙IS energetic particle suite on PSP made its first observations of 3He-rich SEPs. 3He-rich SEPs were observed at energies near 1 MeV/nuc in association with energetic protons, heavy ions, and electrons. At the time of 3He-rich SEP observations, the spacecraft was near 0.46 au. The event was also detected by ULEIS and EPAM on Advanced Composition Explorer (ACE) spacecraft, located near Earth, at 0.99 au from the Sun. The average intensity at ~ 1 MeV/nuc was a factor ~4 greater at PSP than at ACE, which might be attributable to a 1/r2 dependence of the fluence, where r is a distance from the Sun. At that time, PSP and ACE were both magnetically connected to a location near the west limb of the Sun. Remote sensing measurements showed the presence of a type III radio burst and also a helical unwinding jet from this region of the Sun. This activity, which is commonly associated with 3He-rich SEP acceleration on the Sun, originated from the active region number AR 12738. We also searched for smaller 3He-rich SEP events that are not observable near the Earth but might have been detectable closer to the Sun because of the expected strong radial dependence of the intensities of SEP events impulsively released from localized sources. Although no such events were detected during the first two orbits of PSP, this search will be continuing as PSP moves progressively closer to the Sun, and as solar activity increases. These observations should enable IS⊙IS to make significant progress in understanding small 3He-rich SEP events.


Animation of the Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).



Wiedenbeck M.E., Bucik R., Mason G.M., Ho G.C., Leske R.A. et al., 3He-rich Solar Energetic Particle Observations at Parker Solar Probe and Near Earth, Astrophys. J. Suppl. Ser. 246, 42, 2020.