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All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Recurrent 3he-Rich Solar Energetic Particle Injections Observed By Solar Orbiter at ∼0.5 Au

We report Solar Orbiter observations of six recurrent solar energetic particle injections in 2022 March 3−6 at ∼0.5 au. All but one were associated with jets emanating from a plage near a large sunspot in active region 12 957. We saw large jets in injections with high 3He and Fe enrichments and minor jets in injections with no or lower enrichments. Furthermore, the event with the highest enrichment showed a more compact configuration of the underlying photospheric magnetic field. The higher fluences as well as harder spectra were seen in the event with a simultaneous jet and wider eruption. However, in this case, the energy buildup time in the source might be required to produce such spectra. Extreme-ultraviolet images from Solar Orbiter revealed a number of intersecting loops at the base of jets not seen from 1 au that might be a precondition for the recurrent events.

Sun: particle emission↗
NASA NTRS2020

Small Electron Events Observed by Parker Solar Probe/IS⨀IS During Encounter 2

The current understanding of the characteristics of solar and inner heliospheric electron events is inferred almost entirely from observations made by spacecraft located at 1 astronomical unit (AU). Previous observations within 1 AU of the Sun, by the Helios spacecraft at ~0.3-1 AU, indicate the presence of electron events that are not detected at 1 AU or may have merged during transport from the Sun. Parker Solar Probe's close proximity to the Sun at perihelion provides an opportunity to make the closest measurements yet of energetic electron events. We present an overview of measurements of electrons with energies between ~17 keV and ~1 MeV made by the Parker Solar Probe Integrated Science Investigation of the Sun (IS⨀IS) instrument suite during Encounter 2 (2019 March 31-April 10 with perihelion of ~ 0:17 AU), including several small electron events. We examine these events in the context of the electromagnetic and solar wind environment measured by the FIELDS and SWEAP instruments on Parker Solar Probe. We find most of these electron enhancements to be associated with type III radio emissions that reach the local plasma frequency and one enhancement that appears to be primarily associated with abrupt changes in the local magnetic field. Together, these associations suggest that these are indeed the first measurements of energetic electron events within 0.2 AU.

Sun flares↗
DOE OSTI2025

The 2024 July 16 solar event: a challenge to the coronal mass ejection origin of long-duration gamma-ray flares

We present a multi-spacecraft analysis of the 2024 July 16 long-duration gamma-ray flare (LDGRF) detected by the Large Area Telescope on the Fermi satellite. The measured > 100 MeV γ-ray emission persisted for over seven hours after the flare impulsive phase, and was characterized by photon energies exceeding 1 GeV and a remarkably hard parent-proton spectrum. In contrast, the phenomena related to the coronal mass ejection (CME)-driven shock linked to this eruption were modest, suggesting an inefficient proton acceleration unlikely to achieve energies well above the 300 MeV pion-production threshold to account for the observed γ-ray emission. Specifically, the CME was relatively slow (∼600 km/s) and the accompanying interplanetary type-II/III radio bursts were faint and short-lived, unlike those typically detected during large events. In particular, the type-II emission did not extend to kilohertz frequencies and disappeared ∼5.5 hours prior to the LDGRF end time. Furthermore, the associated solar energetic particle (SEP) event was very weak, short-duration, and limited to a few tens of MeV, even at magnetically well-connected spacecraft. These findings demonstrate that a very fast CME resulting in a high-energy SEP event is not a necessary condition for the occurrence of LDGRFs, challenging the idea that the high-energy γ-ray emission is produced by the back-precipitation of shock-accelerated ions into the solar surface. The alternative origin scenario based on local particle trapping and acceleration in large-scale coronal loops is instead favored by the observation of giant arch-like structures of hot plasma over the source region that persisted for the entire duration of this LDGRF.

Sun: UV radiation↗
NASA NTRS2020

3D Propagation of Relativistic Solar Protons through Interplanetary Space

Context. Solar Energetic Particles (SEPs) with energy in the GeV range can propagate to Earth from their acceleration region near the Sun and produce Ground Level Enhancements (GLEs). The traditional approach to interpreting and modelling GLE observations assumes particle propagation only parallel to the magnetic field lines of interplanetary space, i.e. it is spatially 1D. Recent measurements by PAMELA have characterised SEP properties at 1 AU for the ~100 MeV-1 GeV range at high spectral resolution. Aims. We model the transport of GLE-energy solar protons using a 3D approach, to assess the effect of the Heliospheric Current Sheet (HCS) and drifts associated to the gradient and curvature of the Parker spiral. We derive 1 AU observables and compare the simulation results with data from PAMELA. Methods. We use a 3D test particle model including a HCS. Monoenergetic populations are studied first to obtain a qualitative picture of propagation patterns and numbers of crossings of the 1 AU sphere. Simulations for power law injection are used to derive intensity profiles and fluence spectra at 1 AU. A simulation for a specific event, GLE 71, is used to compare with PAMELA data. Results. Spatial patterns of 1 AU crossings and the average number of crossings are strongly influenced by 3D effects, with significant differences between periods of A+ and A- polarities. The decay time constant of 1 AU intensity profiles varies depending on the position of the observer and is not a simple function of the mean free path as in 1D models. Energy dependent leakage from the injection flux tube is particularly important for GLE energy particles, resulting in a rollover in the spectrum.

Solar energetic particles↗
NASA NTRS2023

Solar Activity Relations in Energetic Electron Events Measured By the MESSENGER Mission

Aims. We perform a statistical study of the relations between the properties of solar energetic electron (SEE) events measured by the MESSENGER mission from 2010 to 2015 and the parameters of the respective parent solar activity phenomena in order to identify the potential correlations between them. During the time of analysis, the MESSENGER heliocentric distance varied between 0.31 and 0.47 au. Methods. We used a published list of 61 SEE events measured by MESSENGER, which includes information on the near-relativistic electron peak intensities, the peak-intensity energy spectral indices, and the measured X-ray peak intensity of the flares related to the SEE events. Taking advantage of multi-viewpoint remote-sensing observations, we reconstructed, whenever possible, the associated coronal mass ejections (CMEs) and shock waves; and we determined the three-dimensional (3D) properties (location, speed, and width) of the CMEs and the maximum speed of the 3D CME-driven shocks in the corona. We used different methods (Spearman, Pearson, and a Bayesian approach, namely the Kelly method to linear regression) to estimate the correlation coefficients between the flare intensity, maximum speed at the apex of the CME-driven shock, CME speed at the apex, and CME width with the electron peak intensities and with the energy spectral indices. In this statistical study, we considered and addressed the limitations of the particle instrument on board MESSENGER (elevated background intensity level, anti-Sun pointing). Results. There is an asymmetry to the east in the range of connection angles (CAs) for which the SEE events present the highest peak intensities, where the CA is the longitudinal separation between the footpoint of the magnetic field connecting to the spacecraft and the flare location. Based on this asymmetry, we define a subsample of well-connected events as when −65° ≤ CA ≤ +33°. For the well-connected sample, we find moderate to strong correlations between the near-relativistic electron peak intensity and the 3D CME-driven shock maximum speed at the apex (Spearman: cc = 0.53 ± 0.05; Pearson: cc = 0.65 ± 0.04; Kelly: cc = 0.87 ± 0.20), the flare peak intensity (Spearman: cc = 0.63 ± 0.03; Pearson: cc = 0.59 ± 0.03; Kelly: cc = 0.74 ± 0.30), and the 3D CME speed at the apex (Spearman: cc = 0.50 ± 0.04; Pearson: cc = 0.46 ± 0.03; Kelly: cc = 0.60 ± 0.39). When including poorly connected events (full sample), the relations between the peak intensities and the solar-activity phenomena are blurred, showing lower correlation coefficients. Conclusions. Based on the comparison of the correlation coefficients presented in this study using near 0.4 au data, (1) both flare and shock-related processes may contribute to the acceleration of near relativistic electrons in large SEE events, in agreement with previous studies based on near 1 au data; and (2) the maximum speed of the CME-driven shock is a better parameter to investigate particle-acceleration-related mechanisms than the average CME speed, as suggested by the stronger correlation with the SEE peak intensities.

Sun↗
NASA NTRS2023

The 17 April 2021 Widespread Solar Energetic Particle Event

A complex and long-lasting solar eruption on 17 April 2021 produced a widespread solar energetic particle (SEP) event that was observed by five longitudinally well-separated observers in the inner heliosphere that covered distances to the Sun from 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and near-Earth spacecraft. The event was the second widespread SEP event detected in solar cycle 25, and it produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare that showed multiple hard X-ray peaks over a duration of one hour. The event was further accompanied by a medium-fast coronal mass ejection (CME) with a speed of 880 km s -1 that drove a shock, an extreme ultraviolet wave, and long-lasting and complex radio burst activity that showed four distinct type III burst groups over a period of 40 min. We aim to understand the reason for the wide spread of elevated SEP intensities in the inner heliosphere as well as identify the underlying source regions of the observed energetic electrons and protons. We applied a comprehensive multi-spacecraft analysis of remote-sensing observations and in situ measurements of the energetic particles and interplanetary context to attribute the SEP observations at the different locations to the various potential source regions at the Sun. We used an ENLIL simulation to characterize the complex interplanetary state and its role in the energetic particle transport. The magnetic connection between each spacecraft and the Sun was determined using ballistic backmapping in combination with potential field source surface extrapolations in the lower corona. Using also a reconstruction of the coronal shock front, we then determined the times when the shock establishes magnetic connections with the different observers. Radio observations were used to characterize the directivity of the four main injection episodes, which were then employed in a 2D SEP transport simulation to test the importance of these different injection episodes. A comprehensive timing analysis of the inferred solar injection times of the SEPs observed at each spacecraft suggests different source processes being important for the electron and proton events. Comparison among the characteristics and timing of the potential particle sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source for the electron event. In contrast to earlier studies on widespread SEP events, we find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110 o covered by distinct SEP injections, which is also supported by our SEP transport modeling.

sun↗
NASA NTRS2017

Evidence of Significant Energy Input in the Late Phase of A Solar Flare from NuSTAR X-Ray Observations

We present observations of the occulted active region AR 12222 during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaign on 2014 December 11, with concurrent Solar Dynamics Observatory (SDO)/ AIA and FOXSI-2 sounding rocket observations. The active region produced a medium-size solar flare 1 day before the observations, at approximately 18 UT on 2014 December 10, with the post-flare loops still visible at the time of NuSTAR observations. The time evolution of the source emission in the SDO/AIA 335 Å channel reveals the characteristics of an extreme-ultraviolet late-phase event, caused by the continuous formation of new post-flare loops that arch higher and higher in the solar corona. The spectral fitting of NuSTAR observations yields an isothermal source, with temperature 3.8-4.6 MK, emission measure (0.3-1.8) × 1046 cm−3, and density estimated at (2.5-6.0) × 108 cm−3. The observed AIA fluxes are consistent with the derived NuSTAR temperature range, favoring temperature values in the range of 4.0-4.3 MK. By examining the post-flare loops' cooling times and energy content, we estimate that at least 12 sets of post-flare loops were formed and subsequently cooled between the onset of the flare and NuSTAR observations, with their total thermal energy content an order of magnitude larger than the energy content at flare peak time. This indicates that the standard approach of using only the flare peak time to derive the total thermal energy content of a flare can lead to a large underestimation of its value.

gamma rays↗
NASA NTRS2014

On the Solution of the Continuity Equation for Precipitating Electrons in Solar Flares

Electrons accelerated in solar flares are injected into the surrounding plasma, where they are subjected to the influence of collisional (Coulomb) energy losses. Their evolution is modeled by a partial differential equation describing continuity of electron number. In a recent paper, Dobranskis & Zharkova claim to have found an "updated exact analytical solution" to this continuity equation. Their solution contains an additional term that drives an exponential decrease in electron density with depth, leading them to assert that the well-known solution derived by Brown, Syrovatskii & Shmeleva, and many others is invalid. We show that the solution of Dobranskis & Zharkova results from a fundamental error in the application of the method of characteristics and is hence incorrect. Further, their comparison of the "new" analytical solution with numerical solutions of the Fokker-Planck equation fails to lend support to their result.We conclude that Dobranskis & Zharkova's solution of the universally accepted and well-established continuity equation is incorrect, and that their criticism of the correct solution is unfounded. We also demonstrate the formal equivalence of the approaches of Syrovatskii & Shmeleva and Brown, with particular reference to the evolution of the electron flux and number density (both differential in energy) in a collisional thick target. We strongly urge use of these long-established, correct solutions in future works.

Sun: flares↗
NASA NTRS1975

Heavy particle emission of unusual composition from the sun

Sustained emission of low energy (0.6-4 MeV/nuc) solar particles with a composition richer in iron than in oxygen is observed in three time periods of 1974. The relative abundances of Ne, Mg, Si, and S-Ca to O are also enhanced as compared to those of the sun and of solar cosmic rays. During all three periods, a large H-3/H-4 isotope ratio was also observed; for two cases, it is in the order of unity. On the other hand, the d/H-4 and the T/He-4 are less than 0.04 and 0.02 respectively. The solar activity during the time periods of the reported emission was low. At present, it is not clear whether the emission processes which yielded enriched heavy ions also produced the He-3 enhancement. It seems that any explanation of heavy ion enrichments in solar cosmic rays has to explain also the He-3 enrichment.

Hovestadt, D.↗
DOE OSTI2025

Can the Solar Atmosphere Generate Very-High-Energy Cosmic Rays?

The origin and acceleration of high-energy particles, constituting cosmic rays, is likely to remain an important topic in modern astrophysics. Among the two categories galactic and solar cosmic rays, the latter are much less investigated. The primary source of solar cosmic ray particles are impulsive explosions of the magnetized plasma, known as solar flares and coronal mass ejections. These particles, however, are characterized by relatively low energies compared to their galactic counterparts. In this work, we explore the resonance wave–wave (RWW) interaction between the polarized electromagnetic radiation emitted by the solar active regions and the quantum waves associated with high-energy, relativistic electrons generated during solar flares. Mathematically, the RWW interaction problem boils down to analyzing a Klein–Gordon Equation (spinless electrons) embedded in the electromagnetic field. We find that RWW could accelerate the relativistic electrons to enormous energies even comparable to energies in the galactic cosmic rays.

acceleration of particles↗
NASA NTRS2014

Simulation of Energetic Neutral Atoms from Solar Energetic Particles

Energetic neutral atoms (ENAs) provide the only way to observe the acceleration site of coronal-mass-ejection-driven (CME-driven) shock-accelerated solar energetic particles (SEPs). In gradual SEP events, energetic protons can charge exchange with the ambient solar wind or interstellar neutrals to become ENAs. Assuming a CME-driven shock with a constant speed of 1800 km/s and compression ratio of 3.5, propagating from 1.5 to 40 Rs, we calculate the accelerated SEPs at 5-5000 keV and the resulting ENAs via various charge-exchange interactions. Taking into account the ENA losses in the interplanetary medium, we obtain the flux-time profiles of these solar ENAs reaching 1AU.We find that the arriving ENAs at energies above approx.100 keV show a sharply peaked flux-time profile, mainly originating from the shock source below 5 RS, whereas the ENAs below approx. 20 keV have a flat-top time profile, mostly originating from the source beyond 10 Rs. Assuming the accelerated protons are effectively trapped downstream of the shock, we can reproduce the STEREO ENA fluence observations at approx. 2-5 MeV/nucleon. We also estimate the flux of ENAs coming from the charge exchange of energetic storm protons, accelerated by the fast CME-driven shock near 1AU, with interstellar hydrogen and helium. Our results suggest that appropriate instrumentation would be able to detect ENAs from SEPs and to even make ENA images of SEPs at energies above approx.10-20 keV.

sun↗
NASA NTRS1989

Advanced space transportation technologies

A wide range of propulsion technologies for space transportation are discussed in the literature. It is clear from the literature review that a single propulsion technology cannot satisfy the many mission needs in space. Many of the technologies tested, proposed, or in experimental stages relate to: chemical and nuclear fuel; radiative and corpuscular external energy source; tethers; cannons; and electromagnetic acceleration. The scope and limitation of these technologies is well tabulated in the literature. Prior experience has shown that an extensive amount of fuel needs to be carried along for the return mission. This requirement puts additional constraints on the lift off rocket technology and limits the payload capacity. Consider the possibility of refueling in space. If the return fuel supply is guaranteed, it will not only be possible to lift off more payload but also to provide security and safety of the mission. Exploration to deep space where solar sails and thermal effects fade would also be possible. Refueling would also facilitate travel on the planet of exploration. This aspect of space transportation prompts the present investigation. The particle emissions from the Sun's corona will be collected under three different conditions: in space closer to the Sun, in the Van Allen Belts; and on the Moon. It is proposed to convert the particle state into gaseous, liquid, or solid state and store it for refueling space vehicles. These facilities may be called space pump stations and the fuel collected as space fuel. Preliminary estimates of fuel collection at all three sites will be made. Future work will continue towards advancing the art of collection rate and design schemes for pumping stations.

Raj, Rishi S.↗
NASA NTRS1988

COSTEP: A comprehensive suprathermal and energetic particle analyzer for SOHO

The group of instruments involved in the COSTEP (comprehensive suprathermal and energetic particle analyzer) project are described. Three sensors, the LION (low energy ion and electron) instrument, the MEICA (medium energy ion composition analyzer) and the EPHIN (electron proton helium instrument) are described. They are designed to analyze particle emissions from the sun over a wide range of species (electrons through iron) and energies (60 KeV/particle to 500 MeV/nucleon). The data collected is used in studying solar and space plasma physics.

Kunow, Horst↗
NASA NTRS1973

High Energy Phenomena on the Sun

The proceedings of a symposium of high energy phenomena on the sun are presented. The subjects discussed include the following: (1) flare theories and optical observations, (2) microwave and hard X-ray observations, (3) ultraviolet and soft X-ray emissions, (4) nuclear reactions in solar flares, (5) energetic particles from the sun, (6) magnetic fields and particle storage, and (7) radio emissions in the corona and interplanetary space.

Ramaty, R.↗
NASA NTRS1975

Unusual emission of iron nuclei from the sun

Sustained emission of low energy solar particles with a composition richer in iron than oxygen is observed in the time period 1974 May 7 to 17. Between 0.7 and 4 MeV/nucleon the relative abundances of C:O:Fe are 0.24:1:1.35. It was suggested that these observations provide indication for effects of heavy ion enrichment in the lower corona of the sun.

Gloeckler, G.↗
NASA NTRS2017

Cloudy with a Chance of Solar Flares: The Sun as a Natural Hazard

Space weather is a naturally occurring phenomenon that represents a quantifiable risk to space- and ground-based infrastructure as well as society at large. Space weather hazards include permanent and correctable faults in computer systems, Global Positioning System (GPS) and high-frequency communication disturbances, increased airline passenger and astronaut radiation exposure, and electric grid disruption. From the National Space Weather Strategy, published by the Office of Science and Technology Policy in October 2015, space weather refers to the dynamic conditions of the space environment that arise from emissions from the Sun, which include solar flares, solar energetic particles, and coronal mass ejections. These emissions can interact with Earth and its surrounding space, including the Earth's magnetic field, potentially disrupting technologies and infrastructures. Space weather is measured using a range of space- and ground-based platforms that directly monitor the Sun, the Earth's magnetic field, the conditions in interplanetary space and impacts at Earth's surface, like neutron ground-level enhancement. The NASA Goddard Space Flight Center's Space Weather Research Center and their international collaborators in government, industry, and academia are working towards improved techniques for predicting space weather as part of the strategy and action plan to better quantify and mitigate space weather hazards. In addition to accurately measuring and predicting space weather, we also need to continue developing more advanced techniques for evaluating space weather impacts on space- and ground-based infrastructure. Within the Earth's atmosphere, elevated neutron flux driven by atmosphere-particle interactions from space weather is a primary risk source. Ground-based neutron sources form an essential foundation for quantifying space weather impacts in a variety of systems.

neutron flux↗
NASA NTRS1975

Unusual emission of iron nuclei from the sun

Sustained emission of low-energy solar particles with a composition richer in iron than oxygen has been observed during the time period May 7-17, 1974. Between 0.7 and 4 MeV per nucleon the relative abundances of C:O:Fe are 0.65:1:1.35. We suggest that these observations provide an indication of the effects of heavy-ion enrichment in the lower corona of the sun.

Gloeckler, G.↗