Gradual Solar Energetic Particle Research Articles

Fe/O Variations Relative to Source Longitude and Heliospheric Current Sheet in Large Solar Energetic Particle Events

The Fe/O enhancements exhibit significant variations in gradual solar energetic particle (SEP) events. Several causes have been suggested including transport effects in the interplanetary space and flare contribution. In this study, we investigate the relationship between the integrated Fe/O ratios of 27 gradual SEP events, locations of associated solar flares, and positions along the heliospheric current sheet (HCS) between 2010 and 2014. We employ synchronic potential field source surface (PFSS) extrapolations at 2.5R ⊙, derived in near real-time using Artificial Intelligence (AI)-generated far side and Helioseismic and Magnetic Imager (HMI) magnetograms, referred to as AIHMI-PFSS extrapolations. We examine low-energy (∼0.5 MeV/nucleon) Fe and O ion measurements obtained from Suprathermal Ion Telescope on Solar Terrestrial Relations Observatories and Ultra Low Energy Isotope Spectrometer on Advanced Composition Explorer. We found a moderate anticorrelation between the Fe/O ratios and the absolute longitudinal separation angles from the source regions to the spacecraft magnetic footpoints. Furthermore, we investigate the variations in Fe/O ratios with respect to the separation angle, grouped by the same and opposite polarity sectors of the SEP source regions. We found that the mean and median Fe/O values are higher in the same polarity group compared to the opposite polarity group, with the largest contrast at separation angles between 25° and 50°, where the values are approximately 3 times larger. The results imply that the enhanced Fe/O ratios in the examined gradual SEP events are likely associated with direct source regions, while the HCS affects particle transport.

Sigmoid Eruption Associated with the X9.3 Flare from AR 12673 Drives the Gradual Solar Energetic Particle Event on 2017 September 6

Large gradual solar energetic particle (SEP) events can pose a radiation risk to crewed spaceflight and a significant threat to near-Earth satellites; however, the origin of the SEP seed particle population, and how these particles are released, accelerated and transported into the heliosphere are not well understood. We analyze NOAA active region (AR) 12673, which was the source responsible for multiple large gradual SEP events during 2017 September, and found that almost immediately after each significant eruptive event associated with SEPs an enhanced Si/S abundance ratio was measured by Wind, consistent with the previous work by Brooks et al. The EUV Imaging Spectrometer (EIS) onboard Hinode took data roughly 8 hr before the second SEP event on 2017 September 6, which allowed the regions of enhanced Si/S abundance ratio in the AR to be determined. We have shown that the AR contains plasma with elemental abundance values detected in situ by Wind. In particular, the plasma originates from the core of the AR, similar to Brooks et al., but in the moss (footpoints) associated with hot sigmoidal AR loops. The sigmoid, which contains highly fractionated plasma, erupts and propagates toward an Earth-connected magnetic null point, providing a direct channel for the highly fractionated plasma to escape and be detected in the near-Earth environment.

Cannibals in PARADISE: The Effect of Merging Interplanetary Shocks on Solar Energetic Particle Events

Gradual solar energetic particle (SEP) events are associated with shocks driven by coronal mass ejections (CMEs). The merging of two CMEs (so-called cannibalistic CMEs) and the interaction of their associated shocks has been linked to some of the most powerful solar storms ever recorded. Multiple studies have focused on the observational aspects of these SEP events, yet only a handful have focused on modeling similar CME–CME interactions in the heliosphere using advanced magnetohydrodynamic (MHD) models. This work presents, to our knowledge, the first modeling results of a fully time-dependent 3D simulation that captures both the interaction of two CMEs and its effect on the acceleration and transport of SEPs. This is achieved by using an MHD model for the solar wind and CME propagation together with an integrated SEP model. We perform different simulations and compare the behavior of the energetic protons in three different solar wind environments, where a combination of two SEP-accelerating CMEs are modeled. We find that particle acceleration is significantly affected by the presence of both CMEs in the simulation. Initially, less efficient acceleration results in lower-energy particles. However, as the CMEs converge and their shocks eventually merge, particle acceleration is significantly enhanced through multiple acceleration processes between CME-driven shocks, resulting in higher particle intensities and energy levels.

Solar–Stellar Connection: X-Ray Flares to Energetic (E > 10 MeV) Particle Events

Energetic particle environments are an important factor for the viability of life on exoplanets surrounding flare stars. In the heliosphere, large gradual solar energetic (E > 10 MeV) particle (SEP) events are produced by shocks from fast coronal mass ejections (CMEs). Extensive observations of solar X-ray flares, CMEs, and SEP events can provide guidance for flare star models of stellar energetic particle (StEP) events, for which stellar flares, but only rarely the associated CMEs, are observed. Comparing an extensive list of peak fluxes, timescales, and peak temperatures of 585 ≥ M3.0 solar X-ray flares with the occurrence of associated SEP events of peak flux Ip > 1.4 proton flux units, enhanced with proxy decametric–hectometric type II radio bursts, we determine guidelines for StEP event outcomes, given only stellar X-ray flare inputs. Longer timescales and lower peak temperatures of X-ray flares with a given peak X-ray flux Fp are more favorable for occurrence of associated SEP/StEP events, which, however, are only a minority of all solar flare outcomes. Most solar flares do not result in SEP events, invalidating scaling laws between stellar flares, CMEs, and StEP events. We discuss recent observations and models of the flare–CME relationship and suggest that StEP intensities Ip may often be overestimated.

Turbulent Origins of Particle Intensity Dropout in Gradual Solar Energetic Particle Events During Solar Cycle 23

Dropout is a low-energy particle phenomenon in which the particle intensity drops sharply and then rises rapidly during an impulsive or gradual solar energetic particle (SEP) event. We investigated dropouts in gradual SEP events during solar cycle 23, in which we identified 77 dropout periods with an average duration of approximately 1 hr. During most of the dropout periods, we observe large angles between the mean magnetic field and the solar wind velocity, implying that the slab turbulent component dominates. We therefore explore the origin of particle intensity dropout in slab turbulent environments. At a wave frequency of 1 Hz in the spacecraft frame, we observed a significant positive correlation between the turbulent power spectral density (PSD) and its spectral index, both in the ion dissipation range. As the input PSD decreases, the correlation can amplify the reduction factor for pitch-angle scatterings, quickly suppressing particle scattering through the 90° pitch angle. Hence, particle dropout may occur due to lack of spatial diffusion of particles.

Solar Cycle Variation of 0.3–1.29 MeV Nucleon−1 Heavy Ion Composition During Quiet Times Near 1 au in Solar Cycles 23 and 24

We report on the annual variation of quiet-time suprathermal ion composition for C through Fe using Advanced Composition Explorer (ACE)/Ultra-Low Energy Isotope Spectrometer data over the energy range 0.3–1.28 MeV nuc−1 from 1998 through 2019, covering solar cycle 23's rising phase through Solar Cycle 24's declining phase. Our findings are: (1) quiet-time suprathermal abundances resemble CIR-associated particles during solar minima; (2) quiet-time suprathermals are M/Q fractionated in a manner that is consistent with M/Q fractionation in large gradual solar energetic particle events (GSEP) during solar maxima; and (3) variability within the quiet-time suprathermal pool increases as a function of M/Q and is consistent with the analogous variability in GSEP events. From these observations, we infer that quiet-time suprathermal ions are remnants of CIRs in solar minima and GSEP events in solar maxima. Coincident with these results, we also unexpectedly show that S behaves like a low FIP ion in the suprathermal regime, and is therefore drawn from low FIP solar sources.

The Effect of the Ambient Solar Wind Medium on a CME-driven Shock and the Associated Gradual Solar Energetic Particle Event

We present simulation results of a gradual solar energetic particle (SEP) event detected on 2021 October 9 by multiple spacecraft, including BepiColombo (Bepi) and near-Earth spacecraft such as the Advanced Composition Explorer (ACE). A peculiarity of this event is that the presence of a high-speed stream (HSS) affected the low-energy ion component (≲5 MeV) of the gradual SEP event at both Bepi and ACE, despite the HSS having only a modest solar wind speed increase. Using the EUHFORIA (European Heliospheric FORecasting Information Asset) magnetohydrodynamic model, we replicate the solar wind during the event and the coronal mass ejection (CME) that generated it. We then combine these results with the energetic particle transport model PARADISE (PArticle Radiation Asset Directed at Interplanetary Space Exploration). We find that the structure of the CME-driven shock was affected by the nonuniform solar wind, especially near the HSS, resulting in a shock wave front with strong variations in its properties such as its compression ratio and obliquity. By scaling the emission of energetic particles from the shock to the solar wind compression at the shock, an excellent match between the PARADISE simulation and in situ measurements of ≲5 MeV ions is obtained. Our modeling shows that the intricate intensity variations observed at both ACE and Bepi were influenced by the nonuniform emission of energetic particles from the deformed shock wave and demonstrates the influence of even modest background solar wind structures on the development of SEP events.

Solar Energetic Particle Events with Short and Long Onset Times

Gradual solar energetic particle (SEP) events, usually attributed to shock waves driven by coronal mass ejections (CMEs), show a wide variety of temporal behaviors. For example, TO, the >10 MeV proton onset time with respect to the launch of the CME, has a distribution of at least an order of magnitude, even when the source region is not far from the so-called well-connected longitudes. It is important to understand what controls TO, especially in the context of space weather prediction. Here we study two SEP events from the western hemisphere that are different in TO on the basis of >10 MeV proton data from the Geostationary Operations Environmental Satellite, despite being similar in the CME speed and longitude of the source regions. We try to find the reasons for different TO, or proton release times, in how the CME-driven shock develops and the Alfvén Mach number of the shock wave reaches some threshold by combining the CME height-time profiles with radio dynamic spectra. We also discuss how CME–CME interactions and active region properties may affect the proton release times.

On the Seed Population of Solar Energetic Particles in the Inner Heliosphere

AbstractParticles measured in large gradual solar energetic particle events are believed to be predominantly accelerated at shocks driven by coronal mass ejections (CMEs). Ion charge state and composition analyses suggest that the origin of the seed particle population for the mechanisms of particle acceleration at CME‐driven shocks is not the bulk solar wind thermal material, but rather a suprathermal population present in the solar wind. This suprathermal population could result from remnant material accelerated in prior solar flares and/or preceding CME‐driven shocks. In this work, we examine the distribution of this suprathermal particle population in the inner heliosphere by combining a magnetohydrodynamic simulation of the solar wind and a Monte Carlo simulation of particle acceleration and transport. Assuming that the seed particles are uniformly distributed near the Sun by solar flares of various magnitudes, we study the longitudinal distribution of the seed population at multiple heliocentric distances. We consider a nonuniform background solar wind, consisting of fast and slow streams that lead to compression and rarefaction regions within the solar wind. Our simulations show that the seed population at a particular location (e.g., 1 au) is strongly modulated by the underlying solar wind configuration. Corotating interaction regions and merged interactions regions can strongly alter the energy spectra of the seed particle populations. In addition, cross‐field diffusion plays an important role in mitigating strong variations of the seed population in both space and energy.

Impact of corotation on gradual solar energetic particle event intensity profiles

Context. The corotation of particle-filled magnetic flux tubes is generally thought to have a minor influence on the time-intensity profiles of gradual solar energetic particle (SEP) events. For this reason, many SEP models solve the focussed transport equation within the corotating frame, thus neglecting corotation effects. Aims. We use simulations to study the effects of corotation on gradual SEP intensity profiles at a range of observer longitudinal positions relative to the solar source. We study how corotation affects the duration and decay time constant of SEP events as well as the variation in the peak intensity with the observer’s position. Methods. We used a 3D full-orbit test particle code with time-extended SEP injection via a shock-like source. In contrast to the case of focussed transport models, the test particle approach enables us to easily switch corotation on and off. While shock acceleration and downstream features are not modelled directly, our methodology allows us to study how corotation and the time-varying observer-shock magnetic connection influence the intensity profiles detected at several observers. Results. We find that corotation has a strong effect on the SEP intensity profiles for a monoenergetic population of 5 MeV protons, having a dominant influence during the decay phase. Simulations that include corotation display dramatically shortened durations for western events, compared to ones where it is not included. When corotation effects are taken into account, for both eastern and western events, the decay time constant is reduced and its dependence on the value of the scattering mean free path becomes negligible. Corotation reduces the SEP peak intensity for western events and enhances it for eastern ones, thus making the east-west asymmetry in peak intensity stronger, compared to the non-corotation case. Modelling SEP intensity profiles without carefully accounting for the effects of corotation leads to artificially extended decay phases during western events, leading to profiles with a similar shape regardless of the observer’s longitudinal position.

The first gradual solar energetic particle event with an enhanced 3He abundance on Solar Orbiter

The origin of 3He abundance enhancements in gradual solar energetic particle (SEP) events remains largely unexplained. Two mechanisms have been suggested: the reacceleration of remnant flare material by coronal mass ejection (CME)-driven shocks in interplanetary space, and concomitant activity in the corona. We explore the first gradual SEP event with enhanced 3He abundance that was observed by Solar Orbiter. The event started on 2020 November 24 and was associated with a relatively fast halo CME. During the event, the spacecraft was at 0.9 au from the Sun. The event-averaged 3He/4He abundance ratio is 24 times higher than the coronal or solar wind value, and the timing of the 3He intensity was similar to that of other species. We inspected available imaging, radio observations, and the spacecraft magnetic connection to the CME source. The most probable cause of the enhanced 3He abundance apparently are residual 3He ions remaining from a preceding long period of 3He-rich SEPs on 2020 November 17–23.

Statistical Study and Live Catalog of Multispacecraft 3He-rich Time Periods over Solar Cycles 23, 24, and 25

Using ion measurements from Ultra-Low-Energy Isotope Spectrometer observations on board Advanced Composition Explorer and Solar Isotope Spectrometer observations on board the Solar Terrestrial Observatory (STEREO)-A and STEREO-B spacecraft, we have identified 854 3He-rich time periods between 1997 September and 2021 March. We include all event types with observed 3He enhancements such as corotating interaction regions, gradual solar energetic particle (SEP) events, interplanetary shocks, and impulsive SEP events. We employ two different mass separation techniques to obtain 3He, 4He, Fe, and O fluences for each event, and we determine the 3He/4He and Fe/O abundance ratios between 0.32–0.45 and 0.64–1.28 MeV nucleon−1. We find a clear correlation in the 3He/4He and Fe/O abundance ratios between both energy ranges. We find two distinct trends in the 3He/4He versus Fe/O relation. For low 3He/4He values, there is a positive linear correlation between 3He/4He and Fe/O. However, at 3He/4He ∼ 0.3, Fe/O appears to reach a limit and the correlation weakens significantly. We provide a live catalog of 3He-rich time periods that includes the robust determination of the onset and end times of the 3He enhancements in SEP-associated periods for different types of events observed by multiple spacecraft. This catalog is available for public use. New releases will follow after major additions such as adding new periods from new missions (e.g., Parker Solar Probe and Solar Orbiter), identifying event types (impulsive SEP events, etc.), or adding new parameters such as remote observations detailing characteristics of the events’ active regions.

Energy Spectra Variations of Energetic Ions Associated With a Stream Interaction Region

AbstractUnderstanding the evolution of energetic particles in the heliosphere is one of the most outstanding topics in heliophysics studies. The spectra of energetic particles are pivotal to investigate their origin, acceleration and transport processes. Using STEREO A data, we investigate an intensity enhancement of energetic ions associated with a stream interaction region (SIR) during the decay phase of a gradual solar energetic particle (SEP) event. We find the SIR has fine structures include the stream interface (SI) and a forward‐reverse shock pair, and the energy spectra of energetic ion intensities show complex variations around them. Furthermore, we compare the ions' energy spectra in different regions of the SIR. The results show that this SIR is capable of accelerating protons to about 35 MeV nuc−1, which is about two times of the upper limit of the energy as identified by previous observations, but they are still of the same order. We suggest this may be explained by the fact that the SIR could further accelerate the remnant lower energy ions from the gradual SEP event to higher energies. Moreover, the energetic ions in the SIR event have two populations that are accelerated by shock‐associated and non‐shock‐associated mechanisms, respectively. In addition, the ion intensity exhibits significant directional anisotropies during this SIR event, which may be a result of the combined influence of the transport effect, shock, and the intervening small flux rope.

Spatial and Temporal Variations of 2 – 10 MeV nuc−1 He/H in Gradual Solar Energetic Particle Events

Spatial and Temporal Variations of 2 – 10 MeV nuc−1 He/H in Gradual Solar Energetic Particle Events

Widespread 1–2 MeV Energetic Particles Associated with Slow and Narrow Coronal Mass Ejections: Parker Solar Probe and STEREO Measurements

Abstract Suprathermal ions in the corona are thought to serve as seed particles for large gradual solar energetic particle (SEP) events associated with fast and wide coronal mass ejections (CMEs). A better understanding of the role of suprathermal particles as seed populations for SEP events can be made by using observations close to the Sun. We study a series of SEP events observed by the Integrated Science Investigation of the Sun (IS⊙IS) suite on board the Parker Solar Probe (PSP) from 2020 May 27 to June 2, during which PSP was at heliocentric distances between ∼0.4 and ∼0.2 au. These events were also observed by the Ahead Solar TErrestrial RElations Observatory (STEREO-A) near 1 au, but the particle intensity magnitude was much lower than that at PSP. We find that the SEPs should have spread in space as their source regions were distant from the nominal magnetic footpoints of both spacecraft and the parent CMEs were slow and narrow. We study the decay phase of the SEP events in the ∼1–2 MeV proton energy range at PSP and STEREO-A, and discuss their properties in terms of both continuous injections by successive solar eruptions and the distances where the measurements were made. This study indicates that seed particles can be continuously generated by eruptions associated with slow and narrow CMEs, spread over a large part of the inner heliosphere, and remain there for tens of hours, even if minimal particle intensity enhancements were measured near 1 au.

Neutralized Solar Energetic Particles for SEP Forecasting: Feasibility Study of an Innovative Technique for Space Weather Applications

Energetic neutral atoms (ENAs) are produced by the neutralization of energetic ions formed by shock-accelerated gradual solar energetic particle events (SEP). These high-energy ENAs (HENAs) can reach the Earth earlier than the associated SEPs and thus can provide information about the SEPs at the lower corona. The HENA properties observed at Earth depend on the properties of the coronal mass ejection (CME)-driven shocks that accelerate the SEPs. Using a model of HENA production in a shock-accelerated SEP event, we semi-quantitatively investigate the energy-time spectrum of HENAs depending on the width, propagation speed, and direction of the shock, as well as the density and ion abundances of the lower corona. Compared to the baseline model parameters, the cases with a wider shock width angle or a higher coronal density would increase the HENA flux observed at the Earth, while the case with an Earth-propagating shock shows a softened HENA spectrum. The comparison of expected HENA fluxes in different cases with a flight-proven ENA instrument suggests that solar HENAs can feasibly be monitored with current technologies, which could provide a lead time of 2−3 hours for SEPs at a few MeV. We propose that monitoring of solar HENAs could provide a new method to forecast shock-driven SEP events that are capable of significant space weather impacts on the near-Earth environment.

Statistical Survey of Reservoir Phenomenon in Energetic Proton Events Observed by Multiple Spacecraft

Abstract In this work, reservoir phenomenon in the decay phase of gradual solar energetic particle (SEP) events are investigated with two Helios and the IMP 8 spacecraft from 1976 January to 1980 March and two STEREO and the SOHO spacecraft from 2010 January to 2014 September. Using these data, 62 reservoir events of solar energetic protons were identified, and the effects of perpendicular diffusion and magnetic mirroring on the formation of the reservoir phenomenon have been studied. We find that the reservoir events could be observed in almost all longitudes in the ecliptic at 1 au; thus, the perpendicular diffusion in interplanetary space is an important mechanism to explain the uniform distribution of SEPs. Furthermore, in the 1976 April 30 event, the effects of magnetic mirroring associated with an interplanetary coronal mass ejection were observed during the reservoir phenomenon. Therefore, the effects of magnetic mirroring could also help to form the reservoir phenomenon. This study could improve the understanding of the propagation of SEPs in interplanetary space.

Are Solar Energetic Particle Events and Type II Bursts Associated with Fast and Narrow Coronal Mass Ejections?

Gradual solar energetic ( $E >10~\mbox{MeV}$ ) particle (SEP) events and metric through kilometric wavelength type II radio bursts are usually associated with shocks driven by fast ( $V \geq 900~\mbox{km}\,\mbox{s}^{-1}$ ) and wide ( $W \geq 60^{\circ} $ ) coronal mass ejections (FW CMEs). This criterion was established empirically by several studies from solar cycle 23. The characteristic Alfven speed in the corona, which ranges over 500 – 1500 km s−1 at heights ${\geq}\, 2~R_{\text{o}}$ , provides the minimum $V$ requirement for a CME to drive a shock, but the general absence of SEP events or type II bursts with fast and narrow ( $W < 60^{\circ}$ ) CMEs has not been explained. We review and confirm the earlier studies with a more comprehensive comparison of SEP events and type II bursts with fast and narrow (FN) CMEs. We offer an explanation for the lack of SEP event and type II burst associations with FN CMEs in terms of recent heuristic arguments and modeling that show that the response of a magnetized plasma to the propagation of a CME depends on the CME geometry as well as on its speed. A clear distinction is made between a projectile that propagates through the medium to produce a bow shock, and a 3D piston that everywhere accumulates material to produce a broad shock and sheath. The bow shock is unfavorable for producing SEP events and type II bursts, but the $60^{\circ} $ cut-off is not explained.

Hydrogen and the Abundances of Elements in Gradual Solar Energetic-Particle Events

Despite its dominance, hydrogen has been largely ignored in studies of the abundance patterns of the chemical elements in gradual solar energetic particle (SEP) events; those neglected abundances show a surprising new pattern of behavior. Abundance enhancements of elements with 2leq Z leq 56, relative to coronal abundances, show a power-law dependence versus their average mass-to-charge ratio A/Q, which varies from event to event and with time during events. The ion charge states Q depend upon the source plasma temperature T. For most gradual SEP events, shock waves have accelerated ambient coronal material with T < 2~mbox{MK} with decreasing power laws in A/Q. In this case, the proton abundances agree rather well with the power-law fits extrapolated from elements with Z geq 6 at A/Q > 2 down to hydrogen at A/Q = 1. Thus the abundances of the elements with Z geq 6 fairly accurately predict the observed abundance of H, at a similar velocity, in most SEP events. However, for those gradual SEP events where ion enhancements follow positive powers of A/Q, especially those with T > 2~mbox{MK} where shock waves have reaccelerated residual suprathermal ions from previous impulsive SEP events, proton abundances commonly exceed the extrapolated expectation, usually by a factor of order ten. This is a new and unexpected pattern of behavior that is unique to the abundances of protons and may be related to the need for more streaming protons to produce sufficient waves for scattering and acceleration of more heavy ions at the shock.

Modeling a Single SEP Event from Multiple Vantage Points Using the iPATH Model

Abstract Using the recently extended 2D improved Particle Acceleration and Transport in the Heliosphere (iPATH) model, we model an example gradual solar energetic particle event as observed at multiple locations. Protons and ions that are energized via the diffusive shock acceleration mechanism are followed at a 2D coronal mass ejection-driven shock where the shock geometry varies across the shock front. The subsequent transport of energetic particles, including cross-field diffusion, is modeled by a Monte Carlo code that is based on a stochastic differential equation method. Time intensity profiles and particle spectra at multiple locations and different radial distances, separated in longitudes, are presented. The results shown here are relevant to the upcoming Parker Solar Probe mission.