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Solar energetic particles

Solar energetic particles (SEPs), also known as solar cosmic rays, are bursts of high-energy charged particles—primarily protons, electrons, helium ions (alphas), and heavier elements up to lead—accelerated by explosive solar events and propagated through interplanetary space along magnetic field lines.^[1] These events typically last from hours to days, with particles reaching energies from tens of keV to several GeV and speeds approaching the speed of light, allowing them to traverse the 93 million miles to Earth in as little as tens of minutes.^[2] SEPs originate during periods of heightened solar activity, such as solar maximum in the 11-year solar cycle, and can penetrate near-Earth space environments, including the lunar surface, though Earth's magnetosphere shields low-altitude, low-latitude regions.^[4]^[2] The acceleration of SEPs occurs through two primary mechanisms, distinguishing impulsive from gradual events. Impulsive SEPs are produced near the Sun in solar jets or active regions via magnetic reconnection and wave-particle interactions, often linked to narrow coronal mass ejections (CMEs) and solar flares, resulting in short-duration bursts enriched in certain isotopes.^[1] In contrast, gradual SEPs arise from diffusive shock acceleration at the leading edges of fast, wide CMEs—traveling at speeds up to 3000 km/s (about 6.7 million mph)—which propagate through the corona and interplanetary medium, reaccelerating ambient solar wind particles or pre-existing suprathermals over larger distances.^[2]^[1] These processes sample plasma from different coronal temperatures: impulsive events from hot active regions (2–4 million K), and gradual events predominantly from cooler coronal plasma (below 1.6 million K).^[1]^[4] Compositionally, SEPs reflect solar abundances but with enhancements that reveal their origins; impulsive events show dramatic enrichments, such as 3He/4He ratios up to 10,000 times the solar wind value (versus a typical 5×10⁻⁴) and iron-to-oxygen ratios about 10 times higher, favoring heavy elements (Z > 50) due to lower ionization states and resonant interactions.^[1] Gradual events, however, exhibit more uniform coronal abundances influenced by the first ionization potential (FIP) effect, where low-FIP elements like iron are enhanced, alongside minor contributions from electrons and heavier ions like carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron.^[1]^[4] This variability allows SEPs to serve as probes of solar plasma dynamics and interplanetary magnetic fields.^[1] As a key component of space weather, SEPs pose substantial risks, delivering intense ionizing radiation that can damage spacecraft electronics, solar panels, and navigation systems while increasing cancer risks and causing acute radiation sickness in astronauts—equivalent to 40 mSv per hour behind minimal shielding during extreme events like the 2000 Bastille Day storm.^[5]^[1] On Earth, they disrupt radio communications, alter upper atmospheric chemistry (including up to 50% ozone depletion in the mesosphere), and modulate galactic cosmic rays, with intensities monitored by neutron observatories.^[1]^[4] Ongoing missions like the Solar Dynamics Observatory and Parker Solar Probe continue to refine models of SEP origins and forecasting to mitigate these hazards for deep-space exploration.^[1]

Fundamentals

Definition and composition

Solar energetic particles (SEPs) are high-energy charged particles that originate from explosive releases of energy in the Sun's atmosphere, encompassing protons, electrons, helium ions, and heavier elements ranging up to lead. These particles achieve energies from tens of keV to several GeV, far exceeding the thermal energies typical of the solar corona.^[6] SEPs represent a key component of solar activity, providing direct samples of the chemical makeup and physical conditions in the solar atmosphere.^[7] The composition of SEPs is dominated by protons, which constitute over 90% of the particle population in most events, followed by helium-4 ions (alpha particles) at approximately 5-10%, with electrons and heavier ions making up the remaining fractions.^[8] Heavy ions, including elements like oxygen and iron, appear in trace amounts but exhibit enhancements relative to solar abundances in certain events; for instance, iron-to-oxygen ratios can be several times higher than coronal values.^[6] A distinctive feature is the isotopic enrichment observed in some SEPs, particularly in impulsive events, where the ratio of helium-3 to helium-4 can reach up to 1000 times the values found in the solar photosphere or solar wind.^[7] SEPs differ markedly from the solar wind, which is a continuous stream of plasma with proton energies typically below 10 keV and a composition reflecting steady coronal outflow without significant acceleration.^[9] In contrast, SEPs are transient and episodically accelerated to much higher velocities, often propagating through interplanetary space as discrete events. Historically, SEPs were referred to as "solar cosmic rays" to differentiate them from galactic cosmic rays originating outside the solar system, though the modern term SEPs is preferred for precision.^[6]

Physical characteristics

Solar energetic particles (SEPs) are characterized by energy spectra that typically follow a power-law distribution, expressed as J(E) \propto E^{-\gamma}, where J(E) is the differential intensity and \gamma ranges from approximately 2 to 5 for protons. These spectra extend over a broad energy range, from about 10 keV to several GeV, reflecting the diverse acceleration processes involved.^[1]^[10] In major SEP events, the integrated fluence for protons above 10 MeV can reach up to $10^9 particles per cm², providing a measure of the total particle exposure at Earth.^[11] The intensity and flux of SEPs vary significantly during events, with peak differential fluxes for protons above 10 MeV reaching up to $10^5 particles cm⁻² s⁻¹ sr⁻¹. Event durations differ by particle type and energy, spanning from tens of minutes for relativistic electrons to several days for lower-energy protons, influenced by injection timing and interplanetary transport.^[12]^[13] SEPs exhibit pronounced anisotropy in their initial distribution, with particles preferentially beamed along interplanetary magnetic field lines due to gyro-motion and weak scattering near the source. Spacecraft measurements reveal angular distributions that evolve from highly directional (anisotropy >0.9) to more isotropic over time as scattering increases.^[14] From a radiation perspective, intense SEP events can produce equivalent dose rates of up to approximately 1.5 Gy/hour or higher for unshielded astronauts, primarily from proton interactions in tissue-equivalent materials. These rates underscore the acute exposure potential, though they decay rapidly with distance from the Sun-Earth line.^[15]

Origins and acceleration

Solar sources

Solar energetic particles (SEPs) primarily originate from two distinct solar phenomena: solar flares and coronal mass ejections (CMEs). Solar flares produce impulsive SEP events through emissions during their impulsive phase, driven by magnetic reconnection in active regions on the Sun's surface. These events release high-energy electrons and ions from localized reconnection sites within magnetically complex active regions. In contrast, CMEs are large-scale eruptions of plasma and magnetic fields from the solar corona, often ejecting billions of tons of material into space at speeds exceeding 1000 km/s, which drive shocks responsible for gradual SEP events characterized by higher proton fluences. Observations indicate that most major SEP events are associated with both flares and CMEs, though the relative contributions vary by event type.^[1]^[16]^[7] The acceleration of SEPs typically occurs in the low solar corona, at distances of 2-10 solar radii (R_S) from the photosphere. For flare-related events, particles are energized near reconnection sites in the active region loops, often below 3 R_S, where magnetic field restructuring releases stored energy. CME-driven shocks form and accelerate particles farther out, around 3-6 R_S, particularly where the shock is strongest in the expanding corona, as evidenced by associations with decametric-hectometric type II radio bursts. These locations align with the transition from closed coronal loops to open interplanetary magnetic fields, facilitating particle escape into the heliosphere.^[17]^[1]^[18] Not all solar eruptions produce significant SEPs; only about 1-2% of observed CMEs are associated with major SEP events exceeding 10 MeV proton intensities, with the probability increasing for fast (>1000 km/s) and wide (>120°) CMEs, which account for over 90% of large gradual events. Flares without accompanying CMEs, often from confined active regions, tend to generate electron-rich SEP events with enhanced 3He and heavy ion abundances but lower overall proton fluxes. These impulsive events are less common and typically weaker than CME-associated ones.^[1]^[19] SEP occurrence exhibits strong dependence on the solar cycle, peaking during solar maximum when sunspot activity is highest. Active regions, the primary sites of flares and CMEs, are concentrated at heliographic latitudes of 0°-30° during maximum phases, following the Sun's differential rotation pattern. This latitudinal preference contributes to higher SEP rates in equatorial zones, with event frequency correlating closely with the 11-year sunspot cycle amplitude.^[1]^[20]

Acceleration mechanisms

Solar energetic particles (SEPs) are accelerated through several physical processes occurring in the solar corona and interplanetary space, primarily driven by explosive solar events. The dominant mechanisms include diffusive shock acceleration at collisionless shocks, shock-drift acceleration along shock surfaces, direct energization via magnetic reconnection in current sheets, and stochastic amplification through turbulence-wave particle interactions. These processes convert magnetic and kinetic energy from solar activity into high-energy particles, with efficiencies depending on plasma conditions, shock geometry, and seed particle populations. Diffusive shock acceleration (DSA), a first-order Fermi process, is the primary mechanism for accelerating ions and electrons at shocks driven by coronal mass ejections (CMEs). In DSA, particles scatter repeatedly across the shock front due to magnetic turbulence, gaining energy from the converging flow on each crossing. The efficiency of DSA is highest at quasi-perpendicular shocks, where the shock normal angle relative to the magnetic field exceeds 45 degrees, and Alfvén Mach numbers greater than 2, as these conditions enhance particle injection and confinement.^[21]^[22] The energy gain in DSA per acceleration cycle can be approximated by the relation

\frac{\Delta E}{E} \approx \frac{4}{3} \frac{u_{\rm shock}}{v_{\rm particle}},

where u_{\rm shock} is the shock speed in the upstream frame and v_{\rm particle} is the particle speed, assuming non-relativistic particles and isotropic scattering. Multiple cycles lead to a power-law energy spectrum, typically with spectral indices around -2 to -3 for observed SEPs. This process efficiently accelerates protons to GeV energies and electrons to hundreds of MeV over the shock's propagation. Shock-drift acceleration is a second-order Fermi-like process where charged particles, particularly electrons and ions, gain energy by drifting along the motional electric field at the shock surface. This mechanism is prominent in impulsive solar flares, where particles experience gyromotion-induced drifts in the de Hoffmann-Teller frame, leading to net energy increase proportional to the shock's curvature and speed. It complements DSA by providing initial injection for higher-energy acceleration, especially for electrons in perpendicular shock geometries.^[23] Magnetic reconnection energizes particles directly in thin current sheets during flares, releasing stored magnetic energy as electric fields that accelerate electrons into beams and, in hybrid scenarios, heavy ions to supra-thermal energies. In reconnection sites, collapsing magnetic fields induce parallel electric fields, slinging electrons to relativistic speeds while hybrid models incorporate downstream shocks for efficient heavy ion acceleration, explaining enhanced abundances of elements like iron in some SEP events. These processes produce non-thermal tails in particle distributions, with electrons reaching keV to MeV energies rapidly.^[24]^[25] Turbulence plays a crucial role in amplifying seed particles from the solar wind or pre-accelerated flare populations through resonant wave-particle interactions, such as cyclotron resonance with Alfvén waves. These interactions scatter particles, enabling stochastic acceleration and enhancing the efficiency of other mechanisms by providing the necessary diffusion. In strong turbulence regimes (\delta B / B > 1), power-law spectra emerge from repeated pitch-angle scattering, bridging low-energy solar wind ions to SEP intensities.^[26]^[27]

Events and propagation

Types of solar energetic particle events

Solar energetic particle (SEP) events are broadly classified into impulsive and gradual types based on their duration, composition, and solar origins, with additional categories such as ground level enhancements (GLEs), energetic storm particles (ESPs), and hybrid events capturing specific subsets or variations.^[28] This classification reflects distinct acceleration processes: impulsive events arise primarily from magnetic reconnection in solar flares or jets, while gradual events are driven by shocks associated with coronal mass ejections (CMEs).^[28] Hybrid events merge characteristics of both, often due to complex interactions in active regions, whereas GLEs and ESPs highlight rare high-energy or locally accelerated phenomena.^[28] Observational signatures, such as electron richness or proton dominance, aid in distinguishing these types during space weather monitoring.^[28] Impulsive events are characterized by short durations of several hours, with a rapid rise time often less than 1 hour, and are rich in electrons and ^3He ions due to acceleration via magnetic reconnection in solar flares or narrow jets without associated CMEs.^[28] These events exhibit ^3He/^4He abundance ratios enhanced by factors up to 1000 or more compared to solar wind values (typically ~4×10^{-4}), along with overabundances of heavy ions like Fe/O ratios exceeding 4 times coronal levels, following a power-law dependence on mass-to-charge ratio (A/Q) with a slope of approximately 3.64.^[28] High-energy electrons in these events can reach up to 100 keV, though ions typically extend to ~10 MeV/amu, and they are often linked to type III radio bursts but lack significant proton enhancements in pure cases.^[28] Source plasma temperatures for impulsive events range from 2.5 to 3.2 MK, reflecting hotter coronal environments in active regions.^[28] Gradual events, in contrast, are proton-dominated and last from hours to several days, originating from diffusive shock acceleration by fast, wide CMEs (often >1000 km/s) that propagate through the corona and interplanetary medium.^[28] These events show lower ^3He/^4He ratios closer to coronal abundances (~10^{-4}), with compositions dominated by protons and He/O ratios of 40–60, and a first ionization potential (FIP) bias enhancing low-FIP elements like Fe in hotter sources (up to 3.2 MK for ~24% of events).^[28] Peak intensities correlate strongly with CME speed (correlation coefficient r = 0.80), and particle energies can extend to GeV scales, with spectral flattening at low energies due to Alfvén wave growth from proton streaming.^[28] Unlike impulsive events, gradual ones often form a "reservoir" of trapped particles over broad longitudes (>180°), with uniform spectra observed across multiple spacecraft.^[28] Ground level enhancements (GLEs) represent rare, extreme subsets of gradual events where relativistic protons (>500 MeV) penetrate Earth's atmosphere to produce detectable increases in secondary neutrons at ground level.^[29] Approximately 77 such events have been recorded since 1942 (as of November 2025), primarily through the global neutron monitor network, which detects nuclear cascades from high-rigidity particles (>430 MeV or 1 GV).^[29]^[30] These events require exceptionally strong shocks from fast CMEs and are associated with major flares, with fluences exceeding 10^6 protons cm^{-2} sr^{-1} in the most intense cases, such as the 1956 event.^[28] GLE spectra are notably hard, enabling ground detection, and they share compositional traits with gradual events, including temperature-dependent abundances from coronal sources at 0.8–1.6 MK (69% of cases).^[28] Energetic storm particles (ESPs) are intensity enhancements observed at the passage of interplanetary shocks driven by CMEs near 1 AU, distinct from direct solar injections as they involve local acceleration rather than remote solar origins.^[31] These events peak sharply during shock crossings, lasting hours to days, and often exhibit softer energy spectra (max ~20 MeV for classic events) compared to preceding SEPs, with seeds drawn from solar wind or remnant suprathermal ions.^[31] ESPs are frequently tied to geomagnetic storms and Forbush decreases, showing anisotropies peaked at 90° pitch angle for spike-like variants (duration 5–20 minutes, max ~5 MeV) or isotropic distributions in longer events accelerated at quasi-parallel shocks.^[31] Heavy ion ratios, such as Fe/He, decrease post-shock, reflecting varying acceleration efficiency based on shock obliquity (θ_{Bn} >60°).^[28] Hybrid events combine features of impulsive and gradual types, typically arising from complex active regions where CME shocks reaccelerate ^3He-rich suprathermal seeds from flares alongside ambient coronal protons, resulting in enhanced ^3He/^4He and Fe/O ratios alongside proton excesses (~10 times normal).^[28] These events often involve fast but narrow CMEs and exhibit variable durations (hours to days) and spatial spreads, with compositions blending impulsive enrichments (e.g., high-Z ions) and gradual proton dominance.^[28] Observations from missions like SOHO have identified hybrids as intermediate cases, challenging pure classifications and highlighting multi-mechanism acceleration in eruptive solar events.

Propagation through interplanetary space

Solar energetic particles (SEPs) propagate from their solar origins through interplanetary space primarily along the interplanetary magnetic field (IMF), which forms an Archimedean spiral structure known as the Parker spiral due to the outward flow of the solar wind at approximately 400 km/s.^[32] Charged particles are tied to these helical field lines, gyrating around them while mirroring at points of increasing field strength, but pitch-angle scattering induced by magnetic turbulence causes deviations from strict field-aligned motion, resulting in diffusive transport perpendicular and parallel to the IMF.^[33] This scattering broadens the spatial distribution of particles over time. Travel times to 1 AU vary with energy: relativistic electrons and GeV protons can arrive in as little as 10 minutes due to near-light-speed velocities, while lower-energy protons (e.g., ~10 MeV) take tens of minutes to several hours, influenced by scattering delays.^[34] The diffusive nature of SEP transport is described by the spatial diffusion coefficient κ, which quantifies the rate of particle spreading and typically ranges from 10^{10} to 10^{13} km²/s, depending on turbulence levels in the IMF and particle rigidity.^[35] In the diffusion approximation, particle intensity J decreases exponentially with heliocentric distance r as J ∝ exp(-r/λ), where λ is the mean free path (often ~0.1–0.5 AU for protons and electrons in the inner heliosphere).^[36] Pitch-angle scattering, parameterized by the diffusion coefficient in velocity space D_{μμ}, drives this process by randomizing particle directions relative to the field, with stronger turbulence near solar activity maxima enhancing scattering and reducing λ.^[37] These models, often solved via focused transport equations, account for additional effects like adiabatic cooling and magnetic focusing, which further modify intensities during transit.^[38] Propagation can be impeded by large-scale structures such as the heliospheric current sheet (HCS), a wavy, sector-boundary surface embedded in the IMF that acts as a barrier, suppressing cross-hemisphere transport of MeV-energy SEPs and limiting intensities in the opposite magnetic hemisphere from the source.^[39] Recent Parker Solar Probe observations have revealed delayed arrivals of higher-energy particles in some events, attributed to magnetic mirroring in regions of enhanced field strength near the Sun, where faster particles are temporarily trapped and reflected before escaping into interplanetary space.^[40] Such nondiffusive effects highlight the role of coronal and inner-heliospheric magnetic topology in modulating transit times beyond simple diffusive expectations.^[41] SEPs are detected in interplanetary space using spacecraft instruments that measure differential energy fluxes, such as the Electron, Proton, and Alpha Monitor (EPAM) on GOES satellites, which covers ~0.8–200 MeV/nucleon for electrons and ions, and the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on SOHO, providing data from ~1 MeV to ~100 MeV for multiple species.^[42] For the highest-energy events producing ground-level enhancements (GLEs), ground-based neutron monitors detect secondary particles from GeV primaries interacting with Earth's atmosphere, offering global coverage of relativistic proton fluxes.^[32] The interplanetary medium's variability, including solar wind speed gradients and coronal mass ejection (CME) sheaths, influences SEP injection and trapping during propagation. Faster solar wind streams can compress the Parker spiral, shortening field lines and accelerating particle arrival, while dense CME sheaths ahead of shocks trap particles through enhanced scattering, prolonging release and altering observed anisotropies at 1 AU.^[43] These effects are particularly pronounced in gradual events, where ongoing shock acceleration interacts with the evolving solar wind structure.^[44]

Impacts and effects

Space weather consequences

Solar energetic particles (SEPs) significantly influence Earth's radiation belts by injecting high-energy electrons and protons into the outer Van Allen belt, leading to enhanced relativistic electron fluxes that can persist for weeks during prolonged solar activity. This enhancement occurs through mechanisms such as interplanetary shocks compressing the magnetosphere and exciting ultra-low-frequency (ULF) waves that drive radial diffusion, as well as very-low-frequency (VLF) chorus waves providing local acceleration to multi-MeV energies on timescales of hours.^[45] SEPs contribute to geomagnetic disturbances indirectly through prompt Forbush decreases in galactic cosmic ray flux, which reduce ionospheric conductivity and modulate the ring current, and via enhanced precipitation of ring current protons triggered by electromagnetic ion cyclotron (EMIC) waves excited during SEP-associated storms. These processes intensify the ring current, causing depressions in the geomagnetic field measured by indices like Dst, with precipitation rates peaking during active geomagnetic conditions (e.g., low AL index values). High-energy protons from SEPs also produce enhanced ionization in the ionosphere's D-region (50-90 km altitude), resulting in polar cap absorption that degrades high-frequency (HF) radio signals by up to 10 dB or more, particularly at frequencies below 30 MHz. Additionally, during major SEP-driven superstorms, variations in the global electric circuit occur, with atmospheric electric field deviations of up to +0.13 kV/m linked to reduced cosmic ray ionization and sustained high-speed solar wind.^[46]^[47]^[48]^[49] Beyond Earth, SEPs impact other planetary environments, such as Mars, where the absence of a global magnetic field allows direct penetration into the atmosphere. The intense SEP event on May 20, 2024, driven by an X12-class solar flare and coronal mass ejection, caused significant atmospheric sputtering—ejecting neutral atoms from the upper atmosphere—and radiation spikes exceeding previous records, with proton fluxes at energies >150 MeV triggering diffuse aurora and radar blackouts observed by MAVEN. A more recent example is the November 11, 2025, X5.1 solar flare, which produced a ground-level enhancement (GLE) with high-energy particles reaching Earth's surface, causing radio blackouts and enhanced radiation monitored globally. At Jupiter, SEPs and associated solar wind dynamic pressure enhancements compress the magnetosphere, shifting auroral ovals poleward by ~3°, thereby modulating particle precipitation and auroral power in the extensive Jovian radiation belts.^[50]^[51]^[52] In space weather forecasting, SEP events are integrated into alert systems by organizations like NOAA's Space Weather Prediction Center, issuing warnings for proton fluxes >10 MeV with lead times typically ranging from 10 to 60 minutes based on real-time solar observations such as flare detections and coronal mass ejection tracking. These short lead times enable protective measures for satellites and aviation, though probabilistic models using historical flare data improve longer-term event likelihood estimates up to 48 hours in advance.^[53]^[54]

Biological and technological hazards

Solar energetic particles (SEPs) present acute and chronic hazards to biological systems in space, primarily through ionizing radiation that can penetrate spacecraft shielding and deliver high doses to astronauts. In major SEP events, unshielded exposure to blood-forming organs can reach up to 2 Sv, sufficient to induce acute radiation syndrome, while skin doses may exceed 25 Gy, leading to severe burns and prodromal symptoms such as nausea and fatigue. These doses equate to roughly 20,000 times the radiation from a standard chest X-ray delivered over mere hours, far surpassing safe annual limits of 50 mSv for radiation workers on Earth.^[55] Long-term exposure elevates cancer risk, with NASA's career limit set at 600 mSv effective dose to minimize stochastic effects like tumorigenesis.^[56] The biological impacts stem from the high linear energy transfer (LET) of heavier ions within SEPs, which deposit energy densely along tracks, causing complex clustered DNA damage including double-strand breaks and base modifications that overwhelm repair mechanisms.^[57] This results in chromosomal aberrations, such as dicentrics and translocations, observable in lymphocytes even at low fluences, increasing mutagenesis and carcinogenesis risks.^[58] Unlike low-LET radiation, high-LET particles from SEPs exhibit no safe exposure threshold for stochastic effects, as even minimal hits can trigger persistent DNA damage responses and cellular senescence.^[59] Technologically, SEPs disrupt satellite operations through single event upsets (SEUs) in microelectronics, where charged particles flip bits in memory or processors, often necessitating autonomous reboots or ground interventions.^[60] Proton bombardment also degrades solar arrays by displacing atoms in photovoltaic cells, contributing to reduced efficiency over multiple events.^[60] These effects have led to temporary outages in imaging systems and command losses across low-Earth orbit and geostationary satellites.^[61] Mitigation strategies for SEP hazards include dedicated storm shelters on the International Space Station providing shielding to limit blood-forming organ doses below 250 mGy-Eq during alerts.^[62] Spacecraft employ radiation-hardened components, like error-correcting memory and triple modular redundancy in processors, to withstand SEUs, while deep-space missions incorporate trajectory adjustments to minimize exposure windows based on solar activity forecasts.^[63] Advanced shielding materials, including polyethylene for hydrogen-rich attenuation of high-LET particles, further reduce biological doses by scattering secondaries.^[63] Historical incidents underscore these risks: the August 1972 SEP event, occurring between Apollo 16 and 17 missions, delivered fluxes that could have exposed EVA astronauts to lethal doses exceeding 1 Sv without the Moon's magnetosphere, nearly endangering a potential lunar walk.^[64] Similarly, the 2003 Halloween storms triggered widespread SEU-induced anomalies and solar array degradation in over 20 satellites, including the SOHO spacecraft's temporary shutdown and power reductions in geostationary assets, highlighting vulnerabilities in unhardened systems.^[65]

Historical and observational context

Early discoveries

The first detections of solar energetic particles (SEPs) occurred through observations of sudden increases in ground-level cosmic ray intensity, recorded by physicist Scott E. Forbush using ionization chambers at high- and mid-latitude stations on February 28 and March 7, 1942.^[66] These impulsive enhancements, lasting several hours, were initially attributed to charged particles emanating from solar activity, marking the earliest indirect evidence of SEPs interacting with Earth's atmosphere.^[67] Post-World War II analyses further confirmed the solar origin of these events. In 1946, Forbush linked a major cosmic ray intensity increase on July 25 to a prominent solar flare near sunspot regions, providing stronger evidence that charged particles from solar eruptions could reach Earth and produce detectable effects.^[68] This event, observed via ground-based ionization chambers, helped solidify the connection between solar phenomena and cosmic ray variations, distinguishing SEPs from steady galactic cosmic rays.^[69] During the 1950s, high-altitude balloon flights advanced direct measurements of SEPs. Notably, during the February 23, 1956, solar proton event, balloon-borne neutron detectors at altitudes above 30 km captured fluxes of protons exceeding 1 GeV that did not penetrate to ground level, revealing the energy spectrum and confirming the proton-dominated composition of these solar emissions.^[70] These flights, equipped with Geiger counters and scintillation detectors, demonstrated that SEPs could achieve relativistic energies, expanding understanding beyond ground observations.^[71] The terminology for these particles evolved alongside observational progress. The term "solar cosmic rays" emerged in the early 1950s to describe flare-associated particle bursts, reflecting their similarity to galactic cosmic rays but tying them explicitly to solar sources.^[72] By the 1970s, as space-based data highlighted their distinct acceleration and composition, the more precise designation "solar energetic particles" (SEPs) gained prominence to differentiate them from broader cosmic ray populations.^[73] Early instrumentation relied on rudimentary yet effective tools for high-altitude and ground-based detection. Ionization chambers measured atmospheric ionization from particle cascades, while Geiger counters on balloons provided directional and energy-resolved data during events.^[74] The International Geophysical Year (IGY) from 1957 to 1958 marked a pivotal advancement, with the deployment of standardized neutron monitors at global stations enabling systematic, real-time monitoring of SEP-induced ground-level enhancements (GLEs).^[75] Key pioneers shaped these foundational efforts. James Van Allen contributed significantly through Geiger-Müller counters aboard Explorer 1 and subsequent satellites in 1958, which detected intense trapped radiation belts around Earth—influenced by SEP injections during solar events—revealing how solar particles populate and energize magnetospheric regions.

Notable historical events

One of the earliest and most significant solar energetic particle (SEP) events was the ground-level enhancement (GLE) on February 23, 1956, recognized as the first well-documented GLE recorded by neutron monitors. This event produced the largest increase in cosmic ray intensity ever observed at ground level, with enhancements detected simultaneously across multiple stations worldwide, attributed to a major solar flare of class 3+ in the visible spectrum. The SEP spectrum was exceptionally hard, extending to relativistic energies above 10 GeV, providing a benchmark for understanding extreme particle acceleration at the Sun.^[76]^[77]^[78] The August 1972 SEP event stands out for its intensity and timing during the Apollo program, occurring between the Apollo 16 and 17 missions and highlighting vulnerabilities in human spaceflight. This multi-day proton storm reached peak fluxes exceeding 10^4 protons cm⁻² s⁻¹ above 30 MeV, with total fluences on the order of 10^9 cm⁻² in that energy range, driven by a series of X-class flares including the "seahorse" flare on August 4. The event's severity prompted NASA to develop emergency radiation sheltering protocols using the lunar module as a storm shelter, influencing crew safety measures for subsequent missions and underscoring the risks of deep-space exposure.^[64]^[79]^[80] In March 1989, a powerful SEP event accompanied by a fast coronal mass ejection (CME) triggered widespread technological disruptions, serving as an early analog to later intense storms like the 2000 Bastille Day event. The proton fluxes peaked at levels causing polar cap absorption that lasted several days, with energies up to 60 MeV, leading to satellite anomalies including communications interruptions and loss of imagery for the GOES-7 weather satellite due to particle-induced effects. The associated geomagnetic storm induced currents that caused the Hydro-Québec power grid blackout, affecting 6 million people for nine hours and damaging transformers across North America.^[81]^[82]^[83] The Halloween solar storms of 2003 featured multiple GLEs on October 28, October 29, and November 2, marking one of the most active periods of SEP activity in the space age. These events, driven by X-class flares from active regions 0486 and 10486, produced proton fluences up to 10^{10} cm⁻² above 10 MeV, with the October 28 GLE reaching ground-level intensities detectable by neutron monitors. The radiation environment extended to Mars, where the Mars Express spacecraft experienced temporary safe mode entry and instrument shutdowns due to high particle fluxes, demonstrating interplanetary reach and impacts on deep-space missions.^[84]^[85]^[86] The July 2012 solar storm represented a near-miss Carrington-level event, with a massive CME erupting from active region 1520 on July 23 that narrowly avoided direct Earth impact by traveling on the Sun's farside. If Earth-directed, it would have rivaled the 1859 Carrington event in geomagnetic intensity, potentially causing widespread power outages and satellite failures, with retrospective modeling estimating an acute radiation dose of approximately 0.66 Sv to unshielded astronauts—comparable to a lethal exposure threshold. This event has been pivotal in refining space weather forecasting models, emphasizing the need for improved heliospheric monitoring.^[87]^[88]^[79] In 2024, near the peak of solar cycle 25, multiple ground-level enhancements underscored the intensity of SEP activity: GLE74 on May 11, associated with an extreme geomagnetic storm from active region 3664; GLE75 on June 8; and GLE76 later that year. These events, driven by powerful X-class flares, produced significant proton fluxes observable by neutron monitors worldwide. On November 11, 2025, an X5.1 flare triggered another GLE, further highlighting the ongoing risks during solar maximum as of November 2025.^[89]

Research and future directions

Key research findings

Studies of solar energetic particles (SEPs) have revealed key insights into their compositional properties, which closely mirror the elemental abundances in the solar corona, particularly for low first ionization potential (low-FIP) elements that are enhanced due to fractionation processes during coronal formation. In ³He-rich SEP events, a hallmark anomaly is the extreme enhancement of ³He relative to ⁴He, often by factors up to 10,000 compared to solar wind values, attributed to resonant interactions with electromagnetic ion-cyclotron waves that preferentially excite and accelerate ³He ions in the corona.^[90] Observations by Solar Orbiter during the October 24–25, 2023, event detected ³He-rich SEPs at 0.47 AU, showcasing irregular heavy-ion enhancements peaking at sulfur and indicating acceleration near magnetic reconnection sites in the low corona.^[91] Advancements in imaging and multi-spacecraft observations have confirmed specific acceleration sites for SEPs. More recently, 2025 research led by Southwest Research Institute (SwRI), drawing on Parker Solar Probe data, identified magnetic reconnection in the nascent solar wind as a novel SEP source, where particles are energized to keV–MeV energies just beyond the corona, expanding the known acceleration environments.^[92] At Mars, the May 20, 2024, SEP event represented the strongest in solar cycle 25, with proton fluxes exceeding prior records and triggering widespread aurorae. MAVEN observations of this event provided the first direct measurements of atmospheric sputtering, quantifying enhanced escape of oxygen and carbon ions driven by SEP impacts on the Martian exosphere.^[93] Recent investigations have linked SEPs to disruptions in Earth's global electric circuit (GEC). A 2025 study of the May 2024 geospace superstorm, using ground-based atmospheric electric field measurements from China's Gar station, showed that enhanced SEP fluxes, alongside secondary cosmic rays, altered ionospheric conductivity and induced variations in the near-surface vertical electric field, potentially affecting thunderstorm electrification and fair-weather currents.^[49]

Current missions and forecasting

Ongoing spacecraft missions play a crucial role in observing solar energetic particles (SEPs) in situ, providing data on their composition, acceleration, and propagation near the Sun and across the heliosphere. The Solar Orbiter, launched in February 2020, has detected multiple 3He-rich SEP events, including extended periods lasting over one day observed in 2023, which highlight the isotope's enhancement in impulsive events close to the Sun.^[94] Similarly, the Parker Solar Probe, launched in August 2018, has conducted multiple close approaches to the Sun, reaching within 3.8 million miles of the solar surface by December 2024, enabling direct measurements that confirm shock acceleration mechanisms for SEPs during coronal mass ejections.^[95] At Mars, the MAVEN spacecraft's Solar Energetic Particle (SEP) instrument analyzed a major SEP event in May 2024, capturing intense particle fluxes associated with solar activity and their impact on the Martian atmosphere.^[50] Multi-spacecraft observations enhance the understanding of SEP events by providing multi-viewpoint data on their interplanetary context. In May 2024, a conjunction between the JUICE spacecraft (en route to Jupiter) and STEREO-A allowed comparative analysis of a widespread SEP event on May 13, revealing differences in particle intensities and timings due to their spatial separation near 1 AU.^[96] Legacy data from SOHO and STEREO missions are integrated into modern analyses, such as modeling shock formation and SEP transport, to contextualize observations from newer probes like Solar Orbiter.^[97] Forecasting SEP events relies on models that incorporate solar flare and coronal mass ejection (CME) data to issue timely warnings. Machine learning approaches, such as those developed in 2025, use multi-source inputs including flares, CMEs, and radio bursts to predict SEP occurrences, achieving lead times of around 30 minutes for proton events.^[98] SEP-specific models address challenges like delayed intensity peaks, which arise from shock propagation delays, by analyzing time distributions between peak fluxes and shock arrivals in historical events.^[99] A notable recent prediction involved the November 11, 2025, X5.1-class solar flare from Active Region 4274, which triggered a ground-level enhancement (GLE) detected by global neutron monitors, demonstrating improved real-time alerting systems for high-energy SEPs reaching Earth's surface.^[51] Looking ahead, the Interstellar Mapping and Acceleration Probe (IMAP), launched on September 24, 2025, will map the heliosphere's boundary and energetic neutral atoms, aiding in the study of SEP interactions with interstellar space.^[100] However, forecasting SEPs above GeV energies remains challenging due to uncertainties in interplanetary propagation, including magnetic field turbulence and diffusion effects that complicate particle trajectories.^[101]

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