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

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.

Impulsive Solar Energetic Particle Events: Extreme-Ultraviolet Waves and Jets

“…Thus, EUV waves may be related to some mechanism that modifies rounded spectra. Furthermore, Cohen et al (2021) have reported somewhat harder H and 4 He power law spectra in the two ISEP events with EUV wave than in the event with jet. Though the shock radio signature is not measured in ISEP events, it does not rule out that perhaps weak shocks that do not manifest as type II bursts can be formed in the flare site and re-accelerate 3 He-rich SEPs.…”

“…We note that the eruptions associated with the 2010 October 17 and 2012 May 14 ISEP events in the study of Nitta et al (2015) are marked by Bučík et al (2016a) and Shen et al (2018a) as EUV waves, respectively. Cohen et al (2021) identified three ISEP events at ∼0.3-0.5 au where one was associated with the jet and the other two with signatures of EUV waves.…”

“…It was primarily motivated by measurements of enhanced H abundance in ISEP events with fast narrow CMEs (e.g., Kahler et al, 2001). Recently, an excess in H has been reported for one ISEP event associated with a narrow (18 °) 450 km/s CME (Cohen et al, 2021). Bronarska et al (2018) speculated that small-width shocks associated with fast and narrow CMEs can contribute to the generation of ISEP events.…”

“…Recently, with help of improved imaging resolution from Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO) that provide a full-disk view of the Sun from different observing angles, there have been reported EUV waves in many impulsive solar energetic particle (ISEP) events (Wiedenbeck et al, 2013;Bučík et al, 2015Bučík et al, , 2016aNitta et al, 2015;Cohen et al, 2021). Though these waves had a smaller spatial scale and were fainter than those in GSEP events, their observations in ISEP events, previously associated with compact flare signatures, were surprising.…”

New Data from the ISʘIS instrument Suite on Parker Solar Probe

“…The variation of these early SEP events in composition has been remarkable. One very intriguing series of events occurred in the middle of 2020 during orbit 5 [7], which are labeled events 3, 4, and 5 on Figure 3. Even though these events occurred within a few days and apparently from the same active region, their composition varied widely.…”