Origin of Unusual Composition of 3He-Rich Solar Energetic Particles

TL;DR

The paper addresses why 3He-rich SEPs exhibit extreme $^3$He enrichment and a non-mass-ordered heavy-ion pattern. It analyzes the Oct 24–25, 2023 event observed by Solar Orbiter at $0.47$ au, leveraging SIS measurements and multi-instrument context (PFSS, type III bursts) to test a novel approach that maps the solar-source emission-measure temperature distribution onto expected heavy-ion abundances for heating/acceleration models. The results suggest that the emission-measure distribution can bias preferential heating of certain heavy ions, that mini-filament eruptions at coronal-hole boundaries are common in heavy-ion enhanced events not ordered by mass, and that weak magnetic fields and jet geometry influence the resulting particle energies. These findings connect solar-source microphysics to SEP composition, improving understanding of acceleration processes and aiding interpretation of future 3He-rich events.

Abstract

We examine 3He-rich solar energetic particles (SEPs) detected on 2023 October 24-25 by Solar Orbiter at 0.47 au. The measurements revealed that heavy-ion enhancements increase irregularly with mass, peaking at S. C, and especially N, Si, and S, stand out in the enhancement pattern with large abundances. Except for 3He, heavy ion spectra can only be measured below 0.5 MeV/nucleon. At 0.386 MeV/nucleon, the event showed a huge 3He/4He ratio of 75.2+/-33.9, larger than previously observed. Solar Dynamics Observatory extreme ultraviolet data showed a mini filament eruption at the solar source of 3He-rich SEPs that triggered a straight tiny jet. Located at the boundary of a low-latitude coronal hole, the jet base is a bright, small-scale region with a supergranulation scale size. The emission measure provides relatively cold source temperatures of 1.5 to 1.7 MK between the filament eruption and nonthermal type III radio burst onset. The analysis suggests that the emission measure distribution of temperature in the solar source could be a factor that affects the preferential selection of heavy ions for heating or acceleration, thus shaping the observed enhancement pattern. Including previously reported similar events indicates that the eruption of the mini filament is a common feature of events with heavy-ion enhancement not ordered by mass. Surprisingly, sources with weak magnetic fields showed extreme 3He enrichment in these events. Moreover, the energy attained by heavy ions seems to be influenced by the size and form of jets.

Figures (5)

• Figure 1: Left: Location of PSP, Solar Orbiter (SO), Earth, and STEREO-A (A) in Heliocentric Earth Equatorial (HEEQ) coordinates at the time of the event-associated type III radio burst onset observed on each spacecraft. The arrow indicates the SEP source flare. The Parker IMF lines were derived from measured 1-hour averaged solar wind speeds at type III radio burst onset. For PSP, where solar wind speed is not available, we averaged values measured $\sim$1.5 days before and $\sim$2 days after the type III burst. Heliocentric distance in au and solar wind speed in km s$^{-1}$ for respective spacecraft/locations are indicated. Right: Photospheric magnetic field map (grayscale), scaled to $\pm$30 G for contrast enhancement, and PFSS model of an open coronal field (red is the negative and green is the positive magnetic polarity). Shown are field lines that intersect the source surface at latitudes 0$^{\circ}$ and $\pm$7$^{\circ}$. Upward triangle, square, circle, and diamond mark PSP, Solar Orbiter, near-Earth Advanced Composition Explorer (ACE), and STEREO-A magnetic footpoints at the source surface (set at 2.5 R$_{\odot}$ from Sun center), respectively. The star marks the SEP source flare.
• Figure 2: Solar Orbiter measurements. (a) 10-min solar wind speed (black) and total pressure (blue). A pair of vertical magenta dotted lines approximately marks CIR. (b) 1-hour ion intensities at 0.23--0.32 MeV nucleon$^{-1}$. (c) Mass versus time at 0.4--10 MeV nucleon$^{-1}$. (d) 1/(ion speed) versus arrival times of 10--70 AMU ions. Slanted lines mark arrival times for particles traveling along the nominal Parker field line without scattering. The trapezoid, highlighted in magenta, marks the period where energy spectra and elemental ratios were calculated. Heliocentric distances in au at beginning of each day are displayed. The measurements in panel (b) are from both SIS telescopes and panels (c--d) from SIS-a. The vertical black dashed line indicates the onset of the type III radio burst at Solar Orbiter (19:45 UT).
• Figure 3: Intensity spectra for selected ion species measured by SIS-a at the 2023 October 24 event.
• Figure 4: SIS-a He mass histogram for the 2023 October 24 event in the energy range of 0.4--10.0 MeV nucleon$^{-1}$.
• Figure 5: Left: SIS-a 0.097, 0.137, 0.193, and 0.273 MeV nucleon$^{-1}$ abundances relative to oxygen for the 2023 October 24 event (circles). Overplotted are 0.386 MeV nucleon$^{-1}$ ion abundances from the $^3$He-rich SEP event survey by 2004ApJ...606..555M and the CIR event survey by 2008ApJ...678.1458M. Right: Abundance enhancement factor relative to oxygen for the 2023 October 24 event (at 0.193 MeV nucleon$^{-1}$) relative to coronal abundances from 1995AdSpR..15g..41R. Black squares are values from 2004ApJ...606..555M$^3$He-rich SEP survey at 0.386 MeV nucleon$^{-1}$.