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NASA NTRS1980

Energetic particles in solar flares

The various manifestations of energetic particles in solar flares are examined, and possible mechanisms for the acceleration of these particles are considered. Hard X-ray observations and possible mechanisms for the production of the dominant form of solar energetic particles, electrons with energies between 10 and 100 keV, are discussed, with consideration of thin-target models, thick-target models and thick-target models with reverse currents, and first-phase acceleration mechanisms for energetic electrons emitting impulsive microwave and fast-drift Type III radio bursts as well as impulsive hard X rays, which are detected themselves 20 min after the flare at 1 AU are considered. Radio evidence on the number, energy and pitch-angle distributions of energetic particles produced during solar flares is summarized, and observations at 1 AU of proton and electron energy spectra, the proton/electron ratio and energetic particle events rich in He-3 from solar flares are discussed. Finally, consideration is given to gamma-ray evidence of nuclear reactions in flares and white-light flares

Ramaty, R.↗
NASA NTRS1985

Rocket observations of energetic particles at the geomagnetic equator

Energetic particle detectors were included in the payloads of two rockets launched in Peru during the Condor campaign of 1983. These night-time flights reached altitudes of 587 and 535 km, respectively. The pitch-angle distribution is anisotropic with the maximum at 90 deg. Each payload included two solid-state detectors differing in the thickness of the aluminum coating. Comparison of the fluxes measured by the two detectors leads to the conclusion that, on both occasions, the energetic particles are predominantly helium ions. The flux is small below 200 km, increases linearly to 350 km, and then more slowly to apogee. The east-west asymmetry of flux, noted at 200 km in a previous equatorial launch, is not seen at greater altitudes, consistent with the flux profile and the large gyroradius of the ions.

Smith, L. G.↗
NASA NTRS2023

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 particles↗
NASA NTRS1990

Energetic particles from impulsive solar flares

Observations of solar energetic particles from impulsive flares are reviewed. Consideration is given to observations of electron events, He-3 rich events, and heavy-nuclei-rich events. It is found that these observations can be unified into a description of the particles from impulsive flares. The observations are compared with observations of gamma-ray line in impulsive flares and particles in flares and compared with particles in space. A model for accelerating the unique particle abundances of energetic particles is proposed.

Reames, Donald V.↗
NASA NTRS1980

On the origin of corotating energetic particle events

The paper examines energetic corotating particle events produced by acceleration at the forward and reverse shock pair which bounds a corotating interaction region by the process of successive multiple reflections. In the steady state, the energetic particle intensity at the shock depends only on the intensity of shock particles, and measurements of energetic particles show systematic variations between intensities at the forward and reverse shocks. It is concluded that the solar wind is the original particle population that is accelerated in the stationary shocks up to several MeV.

Scholer, M.↗
NASA NTRS2011

The Integrated Science Investigation of the Sun (ISIS): Energetic Particle Measurements for the Solar Probe Plus Mission

One of the major goals of NASA's Solar Probe Plus (SPP) mission is to determine the mechanisms that accelerate and transport high-energy particles from the solar atmosphere out into the heliosphere. Processes such as coronal mass ejections and solar flares, which peak roughly every 11 years around solar maximum, release huge quantities of energized matter, magnetic fields and electromagnetic radiation into space. The high-energy particles, known as solar energetic particles or SEPs, present a serious radiation threat to human explorers living and working outside low-Earth orbit and to technological assets such as communications and scientific satellites in space. This talk describes the Integrated Science Investigation of the Sun (ISIS) - Energetic Particle Instrument suite. ISIS measures key properties such as intensities, energy spectra, composition, and angular distributions of the low-energy suprathermal source populations, as well as the more hazardous, higher energy particles ejected from the Sun. By making the first-ever direct measurements of the near-Sun regions where the acceleration takes place, ISIS will provide the critical measurements that, when integrated with other SPP instruments and with solar and interplanetary observations, will lead to a revolutionary new understanding of the Sun and major drivers of solar system space weather.

McComas, D. J.↗
NASA NTRS2011

Heliospheric Observations of Energetic Particles

Heliospheric observations of energetic particles have shown that, on long time averages, a consistent v^-5 power-law index arises even in the absence of transient events. This implies an ubiquitous acceleration process present in the solar wind that is required to generate these power-law tails and maintain them against adiabatic losses and coulomb-collisions which will cool and thermalize the plasma respectively. Though the details of this acceleration process are being debated within the community, most agree that the energy required for these tails comes from fluctuations in the magnetic field which are damped as the energy is transferred to particles. Given this source for the tail, is it then reasonable to assume that the turbulent LISM should give rise to such a power-law tail as well? IBEX observations clearly show a power-law tail of index approximately -5 in energetic neutral atoms. The simplest explanation for the origins of these ENAs are that they are energetic ions which have charge-exchanged with a neutral atom. However, this would imply that energetic ions possess a v^-5 power-law distribution at keV energies at the source of these ENAs. If the source is presumed to be the LISM, it provides additional options for explaining the, so called, IBEX ribbon. This presentation will discuss some of these options as well as potential mechanisms for the generation of a power-law spectrum in the LISM.

Summerlin, Errol J.↗
NASA NTRS2019

Energetic Particles of Cosmic Accelerators II: Active Galactic Nuclei and Gamma-ray Bursts

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Our pursuit of more than a century to uncover the origins and fate of these cosmic energetic particles has given rise to some of the most interesting and challenging questions in astrophysics. Within our own galaxy, we have seen that energetic particles engage in a complex interplay with the galactic environment and even drive many of its key characteristics (for more information, see the first white paper in this series). On cosmological scales, the energetic particles supplied by the jets of active galactic nuclei (AGN) are an important source of energy for the intracluster and intergalactic media, providing a mechanism for regulating star formation and black hole growth and cultivating galaxy evolution (AGN feedback). Gamma-ray burst (GRB) afterglows encode information about their circumburst environment, which has implications for massive stellar winds during previous epochs over the stellar lifecycle. As such, GRB afterglows provide a means for studying very high-redshift galaxies since GRBs can be detected even if their host galaxy cannot. It has even been suggest that GRB could be used to measure cosmological distance scales if they could be shown to be standard candles. Though they play a key role in cultivating the cosmological environment and/or enabling our studies of it, there is still much we do not know about AGNs and GRBs, particularly the avenue in which and through which they supply radiation and energetic particles, namely their jets. Despite the enormous progress in particle-in-cell and magnetohydrodynamic simulations, we have yet to pinpoint the processes involved in jet formation and collimation and the conditions under which they can occur. For that matter, we have yet to identify the mechanism(s) through which the jet accelerates energetic particles – is it the commonly invoked diffusive shock acceleration process or is another mechanism, such as magnetic reconnection, required? Do AGNs and GRBs accelerate hadrons, and if so, do they accelerate them to ultra-high energies and are there high-energy neutrinos associated with them? MeV gamma-ray astronomy, enabled by technological advances that will be realized in the coming decade, will provide a unique and indispensable perspective on the persistent mysteries of the energetic universe. This White Paper is the second of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. Specifically, MeV astronomy will: 1. Determine whether AGNs accelerate CRs to ultra-high energies; 2. Provide the missing pieces for the physics of the GRB prompt emission; 3. Measure magnetization in cosmic accelerators and search for acceleration via reconnection.

Venters, Tonia M.↗
NASA NTRSDate not supplied

Modeling and Impact of Solar Energetic Particles in the Heliosphere and Geospace

Understanding the radiation environment due to solar energetic particles in the heliosphere and the Earth’s magnetosphere is a challenging and practically important task. Exposure to energetic particles often leads to malfunctions and unexpected failures of electronics onboard spacecraft. The most vulnerable are exploratory missions when outside of the Earth’s magnetosphere. Geomagnetic field deflects Solar Energetic Particles (SEPs) moving through geospace though some of these particles propagate to LEO and have a high penetrating capability, thus producing significant radiation hazards for human spaceflight. Solar energetic particles also have an essential effect on the composition and dynamics of the Earth’s atmosphere. Precipitating SEPs enhance the atmospheric concentration of NOx and HOx, which play a crucial role in the ozone balance in the middle atmosphere by destroying odd oxygen through catalytic reactions. Numerical modeling of the radiation environment due to SEPs in the inner heliosphere and geospace is a multifold problem. That includes simulating 1) solar wind dynamics and the interplanetary magnetic field, 2) global modeling of the Earth’s magnetosphere, and 3) modeling transport and acceleration of SEPs in the inner heliosphere and geospace. The lecture will outline the key physical processes that control the behavior of SEPs in the heliosphere and geospace, along with the contemporary numerical methods used for their modeling. It will primarily focus on describing the SEP population in geospace across different altitudes, ranging from Low Earth Orbit (LEO) through Medium Earth Orbit (MEO) and Geostationary Orbit (GEO), up to the magnetopause, while considering the realistic geomagnetic field. Additionally, the lecture will cover how the SEP population in geospace varies in response to geomagnetic activity. This includes the temporal trapping of SEPs in geospace and the reduction of the rigidity cutoff during geomagnetic storms.

solar energetic particles↗
NASA NTRS1991

Energetic particles at Uranus

The energetic particle measurements by the low-energy charged-particle and cosmic-ray instruments on the Voyager 2 spacecraft in the magnetosphere of Uranus are reviewed. Upstream events were observed outside the Uranian bow shock, probably produced by ion escape from the magnetosphere. Evidence of earthlike substorm activity was discovered within the Uranian magnetosphere. A proton injection event was observed within the orbit of Umbriel and proton events were observed in the magnetotail plasma-sheet boundary layer that are diagnostic of earthlike substorms. The magnetospheric composition is totally dominated by protons, with only a trace abundance of H(2+) and no evidence for He or heavy ions; the Uranian atmophere is argued to be the principal plasma source. Phase-space densities of medium energy protons show inward radial diffusion and are quantitatively similar to those observed at the earth, Jupiter, and Saturn. These findings and plasma wave data suggest the existence of structures analogous to the earth's plasmasphere and plasmapause.

Cheng, Andrew F.↗
NASA NTRS2013

Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-Ion Elemental Composition in Gradual Solar Energetic Particle Events

Gradual solar energetic particle (SEP) events are those in which ions are accelerated to their observed energies by interactions with a shock driven by a fast coronal mass-ejection (CME). Previous studies have shown that much of the observed event-to-event variability can be understood in terms of shock speed and evolution in the shock-normal angle. But an equally important factor, particularly for the elemental composition, is the origin of the suprathermal seed particles upon which the shock acts. To tackle this issue, we (1) use observed solar-wind speed, magnetograms, and the PFSS model to map the Sun-L1 interplanetary magnetic field (IMF) line back to its source region on the Sun at the time of the SEP observations; and (2) then look for correlation between SEP composition (as measured by Wind and ACE at approx. 2-30 MeV/nucleon) and characteristics of the identified IMF-source regions. The study is based on 24 SEP events, identified as a statistically-significant increase in approx. 20 MeV protons and occurring in 1998 and 2003-2006, when the rate of newly-emergent solar magnetic flux and CMEs was lower than in solar-maximum years and the field-line tracing is therefore more likely to be successful. We find that the gradual SEP Fe/O is correlated with the field strength at the IMF-source, with the largest enhancements occurring when the footpoint field is strong, due to the nearby presence of an active region. In these cases, other elemental ratios show a strong charge-to-mass (q/M) ordering, at least on average, similar to that found in impulsive events. These results lead us to suggest that magnetic reconnection in footpoint regions near active regions bias the heavy-ion composition of suprathermal seed ions by processes qualitatively similar to those that produce larger heavy-ion enhancements in impulsive SEP events. To address potential technical concerns about our analysis, we also discuss efforts to exclude impulsive SEP events from our event sample.

magnetic field↗
NASA NTRS2014

Integrated Science Investigation of the Sun (ISIS): Design of the Energetic Particle Investigation

The Integrated Science Investigation of the Sun (ISIS) is a complete science investigation on the Solar Probe Plus (SPP) mission, which flies to within nine solar radii of the Sun's surface. ISIS comprises a two-instrument suite to measure energetic particles over a very broad energy range, as well as coordinated management, science operations, data processing, and scientific analysis. Together, ISIS observations allow us to explore the mechanisms of energetic particles dynamics, including their: (1) Origins-defining the seed populations and physical conditions necessary for energetic particle acceleration; (2) Acceleration-determining the roles of shocks, reconnection, waves, and turbulence in accelerating energetic particles; and (3) Transport-revealing how energetic particles propagate from the corona out into the heliosphere. The two ISIS Energetic Particle Instruments measure lower (EPI-Lo) and higher (EPI-Hi) energy particles. EPI-Lo measures ions and ion composition from approx. 20 keV/nucleon-15 MeV total energy and electrons from approx.25-1000 keV. EPI-Hi measures ions from approx. 1-200 MeV/nucleon and electrons from approx. 0.5-6 MeV. EPI-Lo comprises 80 tiny apertures with fields-of-view (FOVs) that sample over nearly a complete hemisphere, while EPI-Hi combines three telescopes that together provide five large-FOV apertures. ISIS observes continuously inside of 0.25 AU with a high data collection rate and burst data (EPI-Lo) coordinated with the rest of the SPP payload; outside of 0.25 AU, ISIS runs in low-rate science mode whenever feasible to capture as complete a record as possible of the solar energetic particle environment and provide calibration and continuity for measurements closer in to the Sun. The ISIS Science Operations Center plans and executes commanding, receives and analyzes all ISIS data, and coordinates science observations and analyses with the rest of the SPP science investigations. Together, ISIS' unique observations on SPP will enable the discovery, untangling, and understanding of the important physical processes that govern energetic particles in the innermost regions of our heliosphere, for the first time. This paper summarizes the ISIS investigation at the time of the SPP mission Preliminary Design Review in January 2014.

Coronal Mass Ejections↗
NASA NTRS1993

Energetic particle environment in the early solar system - Extremely long pre-compaction meteoritic ages or an enhanced early particle flux

Individual mineral grains from meteorites have been exposed to energetic particles prior to inclusion in the host meteorite matrix, an event which occurred quite early in solar system history. Spallation-produced noble gases in such grains are observed to be orders of magnitude greater in abundance than could be produced after meteorite formation and thus provide a record of precompaction irradiation by energetic particles. If attributed to the contemporary particle environment (current galactic and solar cosmic rays) a minimum exposure time for these grains in the CM parent-body regolith of 150 Myr is obtained, with more realistic models suggesting an active parent body regolith for 300 Myr. Constraints on when meteorite compaction occurred and current models for meteorite evolution suggest that this is unreasonably long, that the time available for exposure to energetic particles on the surface of the parent-body regolith is much less than this. Pre-compaction exposure, compressed into a shorter span of time by constraints on the compaction times of carbonaceous meteorites, would require energetic particle fluxes in excess of those in the contemporary solar system and point toward an active early (T Tauri) sun as its source.

Woolum, Dorothy S.↗
NASA NTRS1986

The composition of solar energetic particles

The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.

Mcguire, R. E.↗
NASA NTRS2019

Energetic Particles of Cosmic Accelerators I: Galactic Accelerators

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Our pursuit of more than a century to uncover the origins and fate of these cosmic energetic particles has given rise to some of the most interesting and challenging questions in astrophysics. Energetic particles in our own galaxy, galactic cosmic rays (GCRs), engage in a complex interplay with the interstellar medium and magnetic fields in the galaxy, giving rise to many of its key characteristics. For instance, GCRs act in concert with galactic magnetic fields to support its disk against its own weight. GCR ionization and heating are essential ingredients in promoting and regulating the formation of stars and protostellar disks. GCR ionization also drives astrochemistry, leading to the build up of complex molecules in the interstellar medium. GCR transport throughout the galaxy generates and maintains turbulence in the interstellar medium, alters its multi-phase structure, and amplifies magnetic fields. GCRs could even launch galactic winds that enrich the circumgalactic medium and alter the structure and evolution of galactic disks. As crucial as they are for many of the varied phenomena in our galaxy, there is still much we do not understand about GCRs. While they have been linked to supernova remnants (SNRs), it remains unclear whether these objects can fully account for their entire population, particularly at the lower (approximately less than 1 GeV per nucleon) and higher (~PeV) ends of the spectrum. In fact, it is entirely possible that the SNRs that have been found to accelerate CRs merely re-accelerate them, leaving the origins of the original GCRs a mystery. The conditions for particle acceleration that make SNRs compelling source candidates are also likely to be present in sources such as protostellar jets, superbubbles, and colliding wind binaries (CWBs), but we have yet to ascertain their roles in producing GCRs. For that matter, key details of diffusive shock acceleration (DSA) have yet to be revealed, and it remains to be seen whether DSA can adequately explain particle acceleration in the cosmos. This White Paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. For the case of GCRs, MeV astronomy will: 1) Search for fresh acceleration of GCRs in SNRs; 2) Test the DSA process, particularly in SNRs and CWBs; 3) Search for signs of CR acceleration in protostellar jets and superbubbles.

Venters, Tonia M.↗