Parker Solar Probe observations of He/H abundance variations in SEP events inside 0.5 au^★

¹ California Institute of Technology, Pasadena, CA 91125, USA
e-mail: cohen@srl.caltech.edu
² NASA/Goddard Space Flight Center, Greenbeelt, MD 20771, USA
³ University of Texas at San Antonio, San Antonio, TX 78249, USA
⁴ University of Arizona, Tucson, AZ 85721, USA
⁵ Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
⁶ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁷ University of Delaware, Newark, DE 19716, USA
⁸ Department of Physics, George Washington University, Washington, DC 20052, USA
⁹ University of New Hampshire, Durham, NH 03824, USA
¹⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹¹ Physics Department, University of California at Berkeley, Berkeley, CA 94720, USA
¹² Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA

Received: 30 August 2020
Accepted: 1 December 2020

Abstract

Aims. The Parker Solar Probe (PSP) orbit provides an opportunity to study the inner heliosphere at distances closer to the Sun than previously possible. Due to the solar minimum conditions, the initial orbits of PSP yielded only a few solar energetic particle (SEP) events for study. Recently during the fifth orbit, at distances from 0.45 to 0.3 au, the energetic particle suite on PSP, Integrated Science Investigation of the Sun (IS⊙IS), observed a series of six SEP events, adding to the limited number of SEP events studied inside of 0.5 au. Variations in the H and He spectra and the He/H abundance ratio are examined and discussed in relation to the identified solar source regions and activity.

Methods. IS⊙IS measures the energetic particle environment from ~20 keV to >100 MeV/nuc. Six events were selected using the ~1 MeV proton intensities, and while small, they were sufficient to calculate proton and helium spectra from ~1 to ~10 MeV/nuc. For the three larger events, the He/H ratio as a function of energy was determined. Using the timing of the associated radio bursts, solar sources were identified for each event and the eruptions were examined in extreme ultraviolet emission.

Results. The largest of the selected events has peak ~1 MeV proton intensities of 3.75 (cm² sr s MeV)⁻¹. Within uncertainties, the He and H spectra have similar power law forms with indices ranging from −2.3 to −3.3. For the three largest events, the He/H ratios are found to be relatively energy independent; however, the ratios differ substantially with values of 0.0033 ± 0.0013, 0.177 ± 0.047, and 0.016 ± 0.009. An additional compositional variation is evident in both the ³He and electron signatures. These variations are particularly interesting as the three larger events are likely a result of similar eruptions from the same active region.