3He-rich Solar Energetic Particle Events Research Articles

Longitudinal Extent of 3He-rich Solar Energetic Particle Events Near 1 au

Multispacecraft observations of 3He-rich solar energetic particle (SEP) events are scarce, but much needed in order to understand and properly constrain the source and transport of these remarkably enriched 3He SEP events. In this paper, we report 15 3He-rich SEP events that were detected by the Advanced Composition Explorer, the Solar Terrestrial Relations Observatory, and Solar Orbiter near 1 au during Solar Orbiter’s aphelion pass at the end of 2022 and early 2023. Three (five) of these events were detected simultaneously by at least two (three) spacecraft at up to ∼40° longitudinal separation, while seven events were detected by only a single spacecraft, even though an adjacent spacecraft was less than 20° apart. Using a magnetic connectivity tool, we show statistically that there is a >50% probability of detection when the spacecraft-modeled footpoints have an angular separation angle of <24° to the potential source region back at the Sun. This supports previous studies suggesting that the source of these 3He-rich SEP events is narrow in longitudinal extent. On the other hand, the magnetic connectivity due to the presence of coronal mass ejections, footpoint motion, and/or field-line meandering may also lead to difference in a detection at 1 au.

Spectral Properties and the Influence of Coronal Mass Ejections in 3He-rich Solar Energetic Particle Events

We analyze the spectral properties of 3He and 4He as well as the heavy ions (oxygen, neon, magnesium, silicon, and iron) in 80 3He-rich solar energetic particle (SEP) events observed by the Ultra-Low-Energy Ion Spectrometer on board the Advanced Composition Explorer spacecraft since its launch in 1997 until 2024. We split the spectral analysis into two criteria: events with fast and wide coronal mass ejections (CMEs; called “FW events”) and events with slow, narrow, or no observed CMEs (called “non-FW events”). Overall, we find that events with fast and wide CMEs exhibit more uniform spectra across all species, and their low-energy spectral indices are strongly correlated, suggesting a CME provides an additional reacceleration mechanism for the 3He-rich SEPs. When comparing each species’ low-energy spectral index for events with no associated fast-and-wide CME, we find a primary peak in the spectral hardness of 3He, and a secondary peak in Mg and Si. If we consider a plasma temperature of 1.0–1.3 MK, Mg and Si have a charge-to-mass ratio (Q/M) nearest to one-third (1/3), directly half that of 3He. Thus, our results support the results of Roth & Temerin, which suggest heavy ions resonate with the second harmonic of the same ion cyclotron waves energizing 3He. However, it is unclear why the Fe enhancement is not reflected in its spectral index, and we propose that additional acceleration and/or transport mechanisms are playing a role in the abundance enhancement of Fe and heavier ions.

3He and Fe Spectral Properties in 3He-rich Solar Energetic Particle Events

We have surveyed 3He-rich events on the Solar Orbiter mission from 2020 April to 2024 April, selecting isolated injections whose rollover 3He spectral shape is presumed to represent the initial acceleration state, unprocessed by subsequent activity such as coronal mass ejections or jets. A main goal has been to find relationships between the spectra of 3He and heavy ions C–Fe, in order to explore a common acceleration mechanism in spite of the fact that these events show 3He enrichments of up to ∼104, while the heavy-ion enrichment is rarely larger than ∼10. Selecting 34 3He injections, we find that heavy ions are always present, and arrive at the same time as the 3He signaling a common origin. Concentrating on Fe since it is a minor ion but with higher abundance than many others, we find its spectral shape and intensity is similar to 3He. In ∼two-thirds of the cases, if the 3He spectrum is shifted to lower energy by a factor 3.0 ± 1.3, it nearly coincides with the Fe spectrum, illustrating their close connection. Several plasma wave turbulence models have calculated spectra that also show the ion rollovers around 1 MeV nucleon−1. The unique mass-to-charge ratio of 3He allows it to interact more efficiently with the turbulence, thereby gaining several times more energy per nucleon than the other heavy ions. In the spectral rollover region this can lead to the observed enormous enhancements of 3He. The acceleration appears to be associated with magnetic reconnection in emerging flux regions on the Sun.

Heavy-ion Acceleration in 3He-rich Solar Energetic Particle Events: New Insights from Solar Orbiter

We present Solar Orbiter energetic particle observations of two 3He-rich events with features more clearly observed than in prior studies. The event of 2022 November 9 observed from 0.59 au contained hundreds of ultraheavy (UH; mass >78 amu) ions whereas previous observations at 1 au have shown only an occasional count or two. The event of 2023 April 8 observed from 0.29 au fortuitously had very low ambient activity, making it possible to observe spectra from the 3He acceleration mechanism without contamination, revealing extremely low H and 4He intensities arriving simultaneously with other ions observed in typical 3He-rich events. Taken together with previous studies, we believe these data show that 3He-rich events have a single acceleration mechanism that is responsible for the unique abundance features of 3He, heavy ions, and UH ions. Considering the acceleration model of Roth & Temerin that heats the ions over a broad range of gyrofrequencies away from those damped by H and 4He, we calculate reasonable fits to the observed abundances O–Fe. A key result is that high values of, e.g., Fe/O typical of such events is not due to preferential Fe heating, but on the contrary is due mainly to the depletion of O, which at elevated temperatures has a charge-to-mass ratio in the region of the waves damped by 4He. The model also naturally incorporates features of high-ionization states and neutron-rich isotope enhancements that have been long-standing puzzles in observations of this type of flare.

Element Abundances in Impulsive Solar Energetic-Particle Events

Impulsive solar energetic-particle (SEP) events were first distinguished as the streaming electrons that produce type III radio bursts as distinct from shock-induced type II bursts. They were then observed as the surprisingly enhanced 3He-rich SEP events, which were also found to have element enhancements rising smoothly with the mass-to-charge ratio A/Q through the elements, even up to Pb. These impulsive SEPs have been found to originate during magnetic reconnection in solar jets where open magnetic field lines allow energetic particles to escape. In contrast, impulsive solar flares are produced when similar reconnection involves closed field lines where energetic ions are trapped on closed loops and dissipate their energy as X-rays, γ-rays, and heat. Abundance enhancements that are power laws in A/Q can be used to determine Q values and hence the coronal source temperature in the events. Results show no evidence of heating, implying reconnection and ion acceleration occur early, rapidly, and at low density. Proton and He excesses that contribute their own power law may identify events with reacceleration of SEPs by shock waves driven by accompanying fast, narrow coronal mass ejections (CMEs) in many of the stronger jets.

Solar activities associated with 3He-rich solar energetic particle events observed by Solar Orbiter

A series of 3He-rich solar energetic particle (SEP) events was observed by Solar Orbiter in May 2021 at a radial distance of 0.95 AU. An isolated active region AR 12824 was likely the ultimate source of these SEP events. The period of the enhanced flux of 3He was also a period of frequent type III bursts in the decametric-hectometric range, confirming their close relationship. As in past studies, we try to find the solar activities possibly responsible for 3He-rich SEP events, using the type III bursts close to the particle injection times estimated from the velocity dispersion. But this exercise is not as straightforward as in many of the past studies since the region produced many more type III bursts and jet-like eruptions than the SEP injections. We may generalize the solar activities for the 3He-rich SEP events in question as coronal jets, but their appearances do not necessarily conform to classic jets that consist of a footpoint and a spire. Conversely, such jets often did not accompany type III bursts. The areas that produced jet-like eruptions changed within the active region from the first to the second set of 3He-rich SEP events, which may be related to the extended coronal mass ejection that launched stealthily.

Proton and Helium Heating by Cascading Turbulence in a Low-beta Plasma

How ions are energized and heated is a fundamental problem in the study of energy dissipation in magnetized plasmas. In particular, the heating of heavy ions (including 4He2+, 3He2+, and others) has been a constant concern for understanding the microphysics of impulsive solar flares. In this article, via two-dimensional hybrid-kinetic particle-in-cell simulations, we study the heating of helium ions (4He2+) by turbulence driven by cascading waves launched at large scales from the left-handed polarized helium ion cyclotron wave branch of a multi-ion plasma composed of electrons, protons, and helium ions. We find significant parallel (to the background magnetic field) heating for both helium ions and protons due to the formation of beams and plateaus in their velocity distribution functions along the background magnetic field. The heating of helium ions in the direction perpendicular to the magnetic field starts with a lower rate than that in the parallel direction, but overtakes the parallel heating after a few hundreds of the proton gyro-periods due to cyclotron resonances with mainly obliquely propagating waves induced by the cascade of injected helium ion cyclotron waves at large scales. There is, however, little evidence for proton heating in the perpendicular direction due to the absence of left-handed polarized cyclotron waves near the proton cyclotron frequency. Our results are useful for understanding the preferential heating of 3He and other heavy ions in the 3He-rich solar energetic particle events, in which helium ions play a crucial role as a species of background ions regulating the kinetic plasma behavior.

Interplanetary Ion Flux Dropouts Across Multiple 3He-Rich Events

Solar Orbiter, a joint ESA/NASA mission, is studying the Sun and inner heliosphere in greater detail than ever before. Launched in February 2020, Solar Orbiter has already completed its first three orbits, reaching perihelia of 0.5 au from the Sun in June 2020, February and August 2021. During the first 2 years in orbit, Solar Orbiter observed multiple 3He-rich Solar Energetic Particle (SEP) events inside 1 au. Even though these events were small, their spectral forms, 3He content, and association with energetic electrons and type III bursts convincingly identifies them as 3He-rich SEP events with properties similar to those previously observed at 1 au, and promising new insights as Solar Orbiter moves much closer to the Sun in 2022. In May 2021, we observed six 3He-rich SEP events in close succession within 48 h when Solar Orbiter was at 0.95 au. These events were likely released from the same active region at the Sun, and the particles arrived at Solar Orbiter in two batches with various abundances and intensities, showing strong anisotropies throughout. Multiple ion flux dropouts were also observed with these six 3He-rich SEP events. The fact that we observed so many ion injections in such a short period of time indicates the 3He enrichment and acceleration mechanism can produce SEP from the same region very efficiently and with varying enrichment levels and intensities. In addition, we report for the first-time dropout features that spanned multiple ion events simultaneously. This implies the field line random walk that we observe at 1 au still maintains magnetic connections to a small region back at the Sun up to the entire duration of these events (∼48 h).

Quiet-time low energy ion spectra observed on Solar Orbiter during solar minimum

Context. The Solar Orbiter spacecraft cruised in the inner heliosphere during Feb. 2020 – Jan. 2021, moving between ∼0.5–1.0 au radial distance. The Energetic Particle Detector suite operated continuously during this period. Aims. The Suprathermal Ion Spectrograph and High Energy Telescope observations made during intervals in between transient intensity increases were used to determine the low energy ion spectra and composition during quiet times. Methods. Energetic particle spectra and major ion components, including 3He, were measured over the range ∼0.1–100 MeV nucleon−1. The radial dependence of 4.4 MeV nucleon−1 4He and O was measured. A short interval of extremely low intensities (“super-quiet”) was also studied. Results. Spectra measured during the quiet period showed transitions, including galactic cosmic rays (&gt; 50 MeV nucleon−1), anomalous cosmic rays (a few to ∼50 MeV nucleon−1), and a steeply rising “turn-up” spectrum below a few MeV nucleon−1 whose composition resembled impulsive, 3He-rich solar energetic particle events. The radial dependence had large uncertainties but was consistent with a small gradient. During the super-quiet interval, the higher energy components remained similar to the quiet period, while the approximately flat low energy 4He spectrum extended downward, reaching ∼300 keV nucleon−1 before transitioning to a steeply rising spectrum.

Isotopic Fractionation in 3He-rich SEP Events

A well established characteristic of 3He-rich solar energetic particle events is a pattern of abundance enhancements relative to normal solar system values that increase approximately monotonically with atomic number (or mass), leading to enhancements that can exceed 100× for the heaviest elements relative to oxygen. In a 2018 paper, Mason and Klecker (M&K) suggested that the heavy element enhancements could be due to characteristics of the stopping powers of low-energy nuclei penetrating thin layers of H gas or plasma. We present results of a new calculation of enhancements in a H gas and show them to be in reasonable agreement with the M&K results. Studies of heavy-element isotopic composition in 3He-rich events using data from ACE/SIS have also shown that heavier isotopes of an element typically exhibit significant abundance enhancements relative to lighter isotopes of the same element. We applied the new calculation to investigate whether the observed isotopic fractionation can be explained using the M&K H-gas model with the same parameters that account for the heavy-element abundances and found that the measured isotopic enhancements are significantly greater than the predicted values. A search of a larger parameter space was also not able to reconcile the elemental and isotopic enhancement values. However, this search did reveal trends in the dependence on the model parameters that merit further investigation.

Correction to: 3He-Rich Solar Energetic Particle Events with no Measurable 4He Intensity Increases

Correction to: Solar Phys (2019) 294:33 The original article has been corrected due to typesetting mistakes made in the last paragraph of section 2. Observations.

3He-Rich Solar Energetic Particle Events with No Measurable 4He Intensity Increases

We investigated 3He-rich solar energetic particle (SEP) events in the current solar cycle starting in 2009 through 2017. Both “impulsive” (flare-related) 3He-rich and CME-related “gradual” events are included. In the former solar cycle, we found the number of observed 3He-rich events correlated with solar activity. The same correlation is seen again in Cycle 24. Because of the comparatively weak activity, both the occurrence of 3He-rich events and their intensities are significantly less than those from Cycle 23. Interestingly, we found in several of the 3He-rich events that there is no measurable 4He intensity increase above the instrument background. Previously, we found that there is a limit on the number of 3He ions that can be released from the Sun in an impulsive SEP event, while there is no such limit on the 4He ions (Ho, Roelof, and Mason in Astrophys. J., 621, L862, 2005). In this paper, we examine several of these 3He-rich events in detail and discuss the lack of observable 4He intensity increases and the implications for the enhancement and acceleration mechanism of this special type of SEP events.

Magnetic Structures at the Boundary of the Closed Corona: Interpretation of S-Web Arcs

Abstract The topology of coronal magnetic fields near the open-closed magnetic flux boundary is important to the the process of interchange reconnection, whereby plasma is exchanged between open and closed flux domains. Maps of the magnetic squashing factor in coronal field models reveal the presence of the Separatrix-Web (S-Web), a network of separatrix surfaces and quasi-separatrix layers, along which interchange reconnection is highly likely. Under certain configurations, interchange reconnection within the S-Web could potentially release coronal material from the closed magnetic field regions to high-latitude regions far from the heliospheric current sheet, where it is observed as slow solar wind. It has also been suggested that transport along the S-Web may be a possible cause for the observed large longitudinal spreads of some impulsive, 3He-rich solar energetic particle events. Here, we demonstrate that certain features of the S-Web reveal structural aspects of the underlying magnetic field, specifically regarding the arcing bands of highly squashed magnetic flux observed at the outer boundary of global magnetic field models. In order for these S-Web arcs to terminate or intersect away from the helmet streamer apex, there must be a null spine line that maps a finite segment of the photospheric open-closed boundary up to a singular point in the open flux domain. We propose that this association between null spine lines and arc termination points may be used to identify locations in the heliosphere that are preferential for the appearance of solar energetic particles and plasma from the closed corona, with characteristics that may inform our understanding of interchange reconnection and the acceleration of the slow solar wind.

A Possible Mechanism for Enriching Heavy Ions in 3He-rich Solar Energetic Particle Events

Abstract We investigate a mechanism to produce a seed population enriched in heavy ions, such as those observed in 3He-rich solar energetic particle events. It is shown that if an initial particle population following a power law in energy nucleon−1 passes through a small amount of material, at energies below the dE/dx Bragg peak, the greater affinity of heavier ions for electron pick-up results in their penetrating the material more easily. This results in an enhancement of heavy ions in the particle population that just barely penetrates the material. The bulk of the seed particles fall in the energy range of 10 s of keV nucleon−1. It is supposed that some further process then energizes this seed population to produce the particles observed in interplanetary space. We find a broad range of parameters that produces enhancements comparable to Fe/O ∼ 8 commonly observed.

ASSOCIATION OF 3He-RICH SOLAR ENERGETIC PARTICLES WITH LARGE-SCALE CORONAL WAVES

ABSTRACT Small, 3He-rich solar energetic particle (SEP) events have been commonly associated with extreme-ultraviolet (EUV) jets and narrow coronal mass ejections (CMEs) that are believed to be the signatures of magnetic reconnection, involving field lines open to interplanetary space. The elemental and isotopic fractionation in these events are thought to be caused by processes confined to the flare sites. In this study, we identify 32 3He-rich SEP events observed by the Advanced Composition Explorer, near the Earth, during the solar minimum period 2007–2010, and we examine their solar sources with the high resolution Solar Terrestrial Relations Observatory (STEREO) EUV images. Leading the Earth, STEREO-A has provided, for the first time, a direct view on 3He-rich flares, which are generally located on the Sun’s western hemisphere. Surprisingly, we find that about half of the 3He-rich SEP events in this survey are associated with large-scale EUV coronal waves. An examination of the wave front propagation, the source-flare distribution, and the coronal magnetic field connections suggests that the EUV waves may affect the injection of 3He-rich SEPs into interplanetary space.

EVIDENCE FOR A COMMON ACCELERATION MECHANISM FOR ENRICHMENTS OF 3He AND HEAVY IONS IN IMPULSIVE SEP EVENTS

ABSTRACT We have surveyed the period 1997–2015 for a rare type of 3He-rich solar energetic particle (SEP) event, with enormously enhanced values of the S/O ratio, that differs from the majority of 3He-rich events, which show enhancements of heavy ions increasing smoothly with mass. Sixteen events were found, most of them small but with solar source characteristics similar to other 3He-rich SEP events. A single event on 2014 May 16 had higher intensities than the others, and curved Si and S spectra that crossed the O spectrum above ∼200 keV nucleon−1. Such crossings of heavy-ion spectra have never previously been reported. The dual enhancement of Si and S suggests that element Q/M ratio is critical to the enhancement since this pair of elements uniquely has very similar Q/M ratios over a wide range of temperatures. Besides 3He, Si, and S, in this same event the C, N, and Fe spectra also showed curved shape and enhanced abundances compared to O. The spectral similarities suggest that all have been produced from the same mechanism that enhances 3He. The enhancements are large only in the high-energy portion of the spectrum, and so affect only a small fraction of the ions. The observations suggest that the accelerated plasma was initially cool (∼0.4 MK) and was then heated to a few million kelvin to generate the preferred Q/M ratio in the range C–Fe. The temperature profile may be the distinct feature of these events that produces the unusual abundance signature.

OBSERVATIONS OF EUV WAVES IN3He-RICH SOLAR ENERGETIC PARTICLE EVENTS

Small 3He-rich solar energetic particle (SEP) events with their anomalous abundances, markedly different from solar system, provide evidence for a unique acceleration mechanism that operates routinely near solar active regions. Although the events are sometimes accompanied by coronal mass ejections (CMEs) it is believed that mass and isotopic fractionation is produced directly in the flare sites on the Sun. We report on a large-scale extreme ultraviolet (EUV) coronal wave observed in association with 3He-rich SEP events. In the two examples discussed, the observed waves were triggered by minor flares and appeared concurrently with EUV jets and type III radio bursts but without CMEs. The energy spectra from one event are consistent with so-called class-1 (characterized by power laws) while the other with class-2 (characterized by rounded 3He and Fe spectra) 3He-rich SEP events, suggesting different acceleration mechanisms in the two. The observation of EUV waves suggests that large-scale disturbances, in addition to more commonly associated jets, may be responsible for the production of 3He-rich SEP events.

Case studies of multi-day3He-rich solar energetic particle periods

Context. Impulsive solar energetic particle events in the inner heliosphere show the long-lasting enrichment of 3He. Aims. We study the source regions of long-lasting 3He-rich solar energetic particle (SEP) events Methods. We located the responsible open magnetic field regions, we combined potential field source surface extrapolations (PFSS) with the Parker spiral, and compared the magnetic field of the identified source regions with in situ magnetic fields. The candidate open field regions are active region plages. The activity was examined by using extreme ultraviolet (EUV) images from the Solar Dynamics Observatory (SDO) and STEREO together with radio observations from STEREO and WIND. Results. Multi-day periods of 3He-rich SEP events are associated with ion production in single active region. Small flares or coronal jets are their responsible solar sources. We also find that the 3He enrichment may depend on the occurrence rate of coronal jets.

SOLAR SOURCES OF3He-RICH SOLAR ENERGETIC PARTICLE EVENTS IN SOLAR CYCLE 24

Using high-cadence EUV images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we investigate the solar sources of 26 ^3He-rich solar energetic particle events at ≾1 MeV nucleon^(−1) that were well-observed by the Advanced Composition Explorer during solar cycle 24. Identification of the solar sources is based on the association of ^3He-rich events with type III radio bursts and electron events as observed by Wind. The source locations are further verified in EUV images from the Solar and Terrestrial Relations Observatory, which provides information on solar activities in the regions not visible from the Earth. Based on AIA observations, ^3He-rich events are not only associated with coronal jets as emphasized in solar cycle 23 studies, but also with more spatially extended eruptions. The properties of the ^3He-rich events do not appear to be strongly correlated with those of the source regions. As in the previous studies, the magnetic connection between the source region and the observer is not always reproduced adequately by the simple potential field source surface model combined with the Parker spiral. Instead, we find a broad longitudinal distribution of the source regions extending well beyond the west limb, with the longitude deviating significantly from that expected from the observed solar wind speed.

Coronal Jet Observed by Hinode as the Source of a 3 He-rich Solar Energetic Particle Event

We study the solar source of the ^3He-rich solar energetic particle (SEP) event observed on 2006 November 18. The SEP event showed a clear velocity dispersion at energies below 1 MeV nucleon^(−1), indicating its solar origin. We associate the SEP event with a coronal jet in an active region at heliographic longitude of W50°, as observed in soft X-rays. This jet was the only noticeable activity in full-disk X-ray images around the estimated release time of the ions. It was temporally correlated with a series of type III radio bursts detected in metric and longer wavelength ranges and was followed by a nonrelativistic electron event. The jet may be explained in terms of the model of an expanding loop reconnecting with a large-scale magnetic field, which is open to interplanetary space for the particles to be observed at 1 AU. The open field lines appear to be anchored at the boundary between the umbra and penumbra of the leading sunspot, where a brightening is observed in both soft and hard X-rays during the jet activity. Other flares in the same region possibly associated with 3He-rich SEP events were not accompanied by a jet, indicative of different origins of this type of SEP event.