Heliospheric Energetic Particles (HEPs)

• Heliospheric energetic particles (HEPs) are non-thermal ions and electrons that populate the heliosphere, sourced from solar, interplanetary, and interstellar processes.
• The topic explores multiple acceleration regimes—from near-Sun flares to termination shocks—and detailed transport through the Parker-spiral magnetic field.
• Empirical data from missions like Parker Solar Probe and Solar Orbiter underpin models that elucidate HEP spectra, anisotropies, radial gradients, and energetic neutral atom diagnostics.

Heliospheric energetic particles are non-thermal ions and electrons whose kinetic energies exceed those of the ambient thermal solar-wind plasma and which populate the heliosphere. In broad heliophysical usage, the category spans solar energetic particles, energetic storm particles, suprathermal ions, anomalous cosmic rays, galactic cosmic rays undergoing heliospheric transport, and particles accelerated at planetary bow shocks (Wimmer-Schweingruber et al., 2024). In parallel, several transport studies use the term for charged particles, mostly protons but also heavier ions and electrons, with energies from a few MeV up to many GeV moving through the heliosphere (Effenberger et al., 2012). A narrower recent usage defines “HEPs” specifically as solar-wind particles accelerated at a closed-in termination shock in a collapsed heliosphere during dense interstellar-cloud encounters (Opher et al., 16 Jan 2026). Across these usages, the subject is unified by a common set of problems: source formation, acceleration, transport through the Parker-spiral magnetic field and heliospheric current sheet, interaction with heliospheric boundaries and the tail, and remote diagnosis through energetic neutral atoms (Gómez-Herrero et al., 2017, Kornbleuth et al., 13 Mar 2026).

1. Terminology and particle populations

In current research literature, “heliospheric energetic particles” is not a single uniformly delimited taxonomic label. One widely used formulation treats HEPs as non-thermal ions and electrons filling the heliosphere and explicitly distinguishes SEPs, ESPs, ACRs, suprathermal ions, and other locally accelerated populations (Wimmer-Schweingruber et al., 2024). A transport-centered formulation emphasizes charged particles, mostly protons but also heavier ions and electrons, with energies from a few MeV up to many GeV, and frames the central problem as propagation through the turbulent heliospheric magnetic field (Effenberger et al., 2012). By contrast, a recent paleospace-environment study introduces HEPs as “solar wind particles accelerated by the closed-in Termination Shock” during a compressed-heliosphere phase, specifically to distinguish them from SEPs and from present-day ACRs (Opher et al., 16 Jan 2026).

The populations commonly discussed under the broader HEP rubric are physically distinct. SEPs are accelerated in the vicinity of the Sun; shocks driven by solar disturbances produce ESPs; ACRs are of mixed interstellar-heliospheric origin; suprathermal ions form the seed population for further acceleration; and planetary bow shocks provide additional local energetic-particle sources (Wimmer-Schweingruber et al., 2024). Quiet-time helium measurements in the inner heliosphere further show that, in the 11.1–49 MeV/nuc range, the observed flux is a mixture of ACR helium and GCR helium, with the ACR contribution dominating much of the interval and the GCR contribution becoming important toward the high-energy end (Xu et al., 25 Feb 2026).

This plurality of usage matters because the same observational diagnostics—spectra, anisotropies, radial gradients, and ENA maps—can refer either to a broad heliospheric transport problem or to a more specialized source class. The literature therefore uses “HEP” both as a regime descriptor and, in some contexts, as a source-specific term.

2. Source populations and acceleration regimes

The principal acceleration regimes of heliospheric energetic particles are distributed across the heliosphere rather than localized to a single site. SEPs are accelerated in the vicinity of the Sun, typically in association with flares and coronal mass ejections, whereas ESPs are accelerated at interplanetary shocks, particularly CME-driven shocks propagating through the solar wind (Wimmer-Schweingruber et al., 2024). ACRs originate when interstellar neutral atoms enter the heliosphere, become ionized and picked up by the solar wind, are transported to the outer heliosphere, and are then accelerated at or near the termination shock and in the heliosheath before re-entering the inner heliosphere (Xu et al., 25 Feb 2026).

Near-Sun current-sheet acceleration constitutes a distinct regime. During Parker Solar Probe Encounter 7, at $r \approx 0.095\ \mathrm{au}$, suprathermal H, He, O, and Fe ions at $>10$–100 keV/nucleon were observed in association with heliospheric current-sheet crossings. The event showed very few heavy ions during the first full crossing, reduced heavy-ion intensities during the second partial crossing, no velocity dispersion, He pitch-angle distributions with up to $\sim 10{:}1$ anti-sunward field-aligned flows near the HCS, He spectra that steepened on either side of the HCS, power laws of the form $\sim E^{-4\text{–}6}$ for He, O, and Fe, and maximum energies scaling as $E_X/E_H \propto [(Q_X/M_X)]^\alpha$ with $\alpha \sim 0.65$–0.76 under standard charge-state assumptions (Desai et al., 2021). These observations were interpreted as inconsistent with solar flares and near-Sun CME-driven shocks and as challenging both direct $E/Q$-organized acceleration and local diffusive or reconnection-only models in their simplest forms (Desai et al., 2021).

A much more extreme source regime appears in compressed-heliosphere scenarios. In the cold-cloud model, the termination shock moves to $r_{\mathrm{TS}} \approx 0.118\ \mathrm{AU}$ and the heliopause to $r_{\mathrm{HP}} \approx 0.22\ \mathrm{AU}$, the shock is quasi-parallel with compression ratio $r \approx 3.6$ at the nose, hybrid simulations generate a downstream suprathermal tail beginning near $>10$0, and a Parker transport calculation extends the spectrum to $>10$1 with negligible flux above $>10$2 (Opher et al., 16 Jan 2026). In that narrow nomenclature, the resulting particles are the HEPs.

3. Transport formalisms, diffusion tensors, and anomalous propagation

The standard large-scale framework for heliospheric energetic-particle transport is the Parker transport equation. In one formulation,

$>10$3

where $>10$4 is the omnidirectional distribution function, $>10$5 is the solar-wind bulk velocity, and $>10$6 is the spatial diffusion tensor (Effenberger et al., 2012). In global heliospheric modeling, the technically important point is that $>10$7 is not uniquely specified by a scalar parallel coefficient and one isotropic perpendicular coefficient. A generalized local tensor with distinct $>10$8, $>10$9, and $\sim 10{:}1$0, transformed to the global frame through a Frenet–Serret trihedron tied to magnetic-field geometry, changes the resulting modulated spectra substantially: computed differential fluxes can deviate by up to $\sim 10{:}1$1 below a few hundred MeV relative to the traditional isotropic-perpendicular formulation (Effenberger et al., 2012).

A second transport strand concerns anomalous rather than Gaussian propagation. For energetic particles upstream of heliospheric shocks, power-law intensity profiles are interpreted as superdiffusion with

$\sim 10{:}1$2

where $\sim 10{:}1$3, and with the Lévy-walk relation $\sim 10{:}1$4 linking the MSD exponent to the step-length exponent $\sim 10{:}1$5 (Perri et al., 2015). The analysis of Ulysses CIR shocks and the Voyager 2 termination-shock event was extended to recover not only $\sim 10{:}1$6, but also the superdiffusion coefficient $\sim 10{:}1$7, the fractional diffusion coefficient $\sim 10{:}1$8, and the transition scale $\sim 10{:}1$9 at which the profiles turn from near-flat to power-law decay (Perri et al., 2015). This matters because superdiffusive shock acceleration predicts spectral behavior different from standard DSA and offers a physically consistent explanation for algebraic upstream intensity profiles where homogeneous background conditions disfavor an exponential solution (Perri et al., 2015).

Large-scale drift remains an additional transport channel rather than a negligible correction. Full-orbit simulations in a Parker spiral plus realistic 2D+slab turbulence found that gradient and curvature drifts are reduced, but only to $\sim E^{-4\text{–}6}$0–$\sim E^{-4\text{–}6}$1 of their values in the corresponding configuration without turbulence, with the weakest suppression at higher proton energies (Laitinen et al., 2024). This is markedly less efficient than several theoretical drift-reduction estimates, particularly at SEP energies, and implies that drift must remain explicit in inner-heliospheric transport treatments (Laitinen et al., 2024).

4. Current sheets, interaction regions, and intermittent transport

The heliospheric current sheet is both a transport boundary and an accelerator. In a flat equatorial HCS embedded in a Parker spiral, full-orbit calculations for 1–800 MeV protons show strong current-sheet drift to distant longitudes, with longitudinal separations exceeding $\sim E^{-4\text{–}6}$2 from the best-connected field line at 1 AU for 100 MeV protons, opposite drift directions in $\sim E^{-4\text{–}6}$3 and $\sim E^{-4\text{–}6}$4 polarity cycles, and only negligible sensitivity to realistic HCS thicknesses between 0 and 40,000 km (Battarbee et al., 2017). The same study found that a flat equatorial HCS limits proton crossing into the opposite hemisphere, especially in the $\sim E^{-4\text{–}6}$5 configuration, and that a single injection can generate multi-component time profiles through the combination of HCS drift and ordinary Parker-spiral transport (Battarbee et al., 2017).

Near the Sun, HCS transport is more sharply structured. The Encounter 7 PSP event showed that the strongest suprathermal He anisotropies track the field-polarity reversal itself, with anti-sunward field-aligned beams closer to the HCS and nearly isotropic distributions farther from it, while intensity maxima occur in separatrix regions rather than inside the reconnection exhaust (Desai et al., 2021). That combination of no velocity dispersion, strong field-aligned PADs, and current-sheet organization implies a source region within $\sim E^{-4\text{–}6}$6 of the spacecraft along the field and places tight constraints on any model invoking local reconnection, direct parallel electric fields, or local diffusive acceleration (Desai et al., 2021).

Corotating interaction regions and their mergers provide a complementary large-scale transport environment. Three-dimensional CRONOS MHD simulations from 0.1 to 10 AU, driven by analytic or WSA-based boundary conditions, form stream interfaces, forward and reverse shocks, and eventually CMIRs self-consistently, and identify CIRs as structures with strongly enhanced magnetic field strengths, turbulence, and shocks that can act as diffusion barriers while also accelerating low-energy cosmic rays (Wiengarten et al., 2014). This establishes a realistic background for stochastic transport solvers and explains why recurrent modulation and phase relationships of GCRs and Jovian electrons require explicit heliospheric structure rather than a spherically symmetric wind (Wiengarten et al., 2014).

At still smaller scales, impulsive SEP “dropouts” reveal how limited cross-field mixing can be. Full-orbit simulations with a spatially compact and instantaneous proton source at 1 AU-like conditions reproduce small-scale intensity gradients and velocity dispersion when the foot-point random motion model is used, but not when a standard two-component turbulence model with stronger resonant scattering is used (Guo et al., 2013). The same study concludes that particle scattering in the solar-wind magnetic field must be infrequent for intensity dropouts to form, and estimates that for 1 MeV protons a condition $\sim E^{-4\text{–}6}$7 is required (Guo et al., 2013). This sharply constrains any SEP transport model that invokes strong perpendicular mixing.

5. Heliospheric boundaries, the tail, and ENA diagnostics

At the heliospheric boundary, energetic-particle transport becomes explicitly kinetic. Voyager 1’s August 2012 transition into the “magnetic highway” was marked by a virtual absence of heliospheric energetic particles and magnetic fluctuations, a jump in magnetic-field magnitude from about $\sim E^{-4\text{–}6}$8 to $\sim E^{-4\text{–}6}$9 without significant rotation, and pitch-angle distributions that developed a strong $E_X/E_H \propto [(Q_X/M_X)]^\alpha$0 peak with depletion near $E_X/E_H \propto [(Q_X/M_X)]^\alpha$1 (Florinski, 2013). A fully kinetic Boltzmann treatment with isotropic angular diffusion, $E_X/E_H \propto [(Q_X/M_X)]^\alpha$2 in the heliosheath and $E_X/E_H \propto [(Q_X/M_X)]^\alpha$3 in the magnetic highway, reproduces this as a double loss-cone or pancake distribution: field-aligned particles escape rapidly along smooth field lines, while $E_X/E_H \propto [(Q_X/M_X)]^\alpha$4 particles linger near the boundary (Florinski, 2013). For 1 MeV protons the model uses $E_X/E_H \propto [(Q_X/M_X)]^\alpha$5, $E_X/E_H \propto [(Q_X/M_X)]^\alpha$6, and $E_X/E_H \propto [(Q_X/M_X)]^\alpha$7 (Florinski, 2013).

Remote ENA imaging places additional constraints on heliosheath and tail HEPs. A comet-like heliosphere coupled to pickup-ion acceleration at the termination shock reproduces hydrogen ENA fluxes from 3 to 88 keV fairly well and shows that similar upwind and downwind ENA fluxes below about 55 keV do not by themselves require a bubble-like heliosphere (Czechowski et al., 2019). In that model, low-energy ENA production is localized near the termination shock because the charge-exchange cross section is large, whereas at higher energies the tail dominates because ions survive long enough to sample the extended heliotail (Czechowski et al., 2019).

A more recent reinterpretation of INCA’s high-energy Belt and Buckle instead argues for a split-tail heliosphere. In a multi-ion MHD model, only the split-tail configuration produces a low-$E_X/E_H \propto [(Q_X/M_X)]^\alpha$8 region at low latitudes in the heliotail, beginning near $E_X/E_H \propto [(Q_X/M_X)]^\alpha$9 and extending about $\alpha \sim 0.65$0 to the heliopause, with $\alpha \sim 0.65$1 and heliosheath-created ion temperatures of $\alpha \sim 0.65$2–$\alpha \sim 0.65$3 (Kornbleuth et al., 13 Mar 2026). Reconnection in that region accelerates a fraction of the heliosheath-created ions into a 5–20 keV tail, producing the 5.2–13.5 keV ENAs of the Buckle and favoring the split-tail geometry over a long comet-like tail (Kornbleuth et al., 13 Mar 2026). The coexistence of these two interpretations indicates that ENA morphology is sensitive both to global shape and to whether in-situ heliosheath acceleration beyond the termination shock is included.

Helium ENAs add an additional composition-specific diagnostic. HSTOF/SOHO measurements of 28–58 keV/n helium ENAs can be explained, within $\alpha \sim 0.65$4, by charge-exchange neutralization of energetic He ions in the inner heliosheath, with the dominant source function arising from $\alpha \sim 0.65$5 charge exchange with neutral H and with the main uncertainty set by the post-termination-shock $\alpha \sim 0.65$6 pickup-ion spectrum (Grzedzielski et al., 2013). This extends ENA diagnosis from the dominant proton component to the helium component of the heliosheath HEP population.

6. Measurement systems and empirical constraints

The modern in-situ experimental basis for HEP studies is strongly shaped by Solar Orbiter and Parker Solar Probe. Solar Orbiter’s Energetic Particle Detector spans electrons from 2 keV to 15 MeV, protons from 3 keV to 100 MeV, and ions from a few tens of keV/nuc to 450 MeV/nuc, using STEP, SIS, EPT, and HET to provide timing, spectra, anisotropy, and composition (Gómez-Herrero et al., 2017). STEP resolves suprathermal electrons and protons at up to 1 s cadence; SIS measures suprathermal ion composition and isotopes; EPT and HET extend coverage into the SEP and low-energy GCR/ACR regimes (Gómez-Herrero et al., 2017). These capabilities were designed explicitly to address how solar eruptions produce energetic particle radiation that fills the heliosphere (Gómez-Herrero et al., 2017).

Multi-spacecraft observations now show how structured that radiation field is. At 1 AU, oxygen differential fluxes over $\alpha \sim 0.65$7–500 MeV/nuc vary by about seven orders of magnitude between quiet and highly active conditions over single Bartels rotations; the 29 November 2020 SEP event extended over more than $\alpha \sim 0.65$8 in heliolongitude; and STEP data near interplanetary shocks reveal suprathermal variability on 10–20 s scales, only a few proton gyroperiods (Wimmer-Schweingruber et al., 2024). The same CME-driven shock observed at 0.07 AU by PSP and at 0.7 AU by Solar Orbiter had comparable $\alpha \sim 0.65$9 and Mach numbers but very different foreshock environments: PSP saw a sharp transition without a developed foreshock, whereas Solar Orbiter saw upstream shocklets and stronger wave activity (Wimmer-Schweingruber et al., 2024). These measurements directly show that acceleration and transport cannot be factorized cleanly into source physics plus a static propagation kernel.

For steady heliospheric populations, Solar Orbiter/HET has now provided direct radial-gradient constraints on ACR helium inside 1 AU. Between 2020 February and 2022 July, after SEP and ICME removal and Carrington-rotation averaging, the radial gradient between about 0.3 and 1 AU was $E/Q$0/au over 11.1–49 MeV/nuc (Xu et al., 25 Feb 2026). After subtracting the GCR contribution with the BON2020 model, the average gradient rose to $E/Q$1/au over 11.1–41.2 MeV/nuc (Xu et al., 25 Feb 2026). These measurements confirm that even the quiet-time HEP background has strong radial structure in the inner heliosphere and that transport properties inside 1 AU differ from those inferred farther out (Xu et al., 25 Feb 2026).

7. Compressed heliospheres, terrestrial exposure, and unresolved problems

The compressed-heliosphere scenario extends the HEP concept beyond present-day space weather into long-timescale heliospheric state changes. In the cold-cloud encounter model, Earth alternates between residence inside a shrunken heliosheath and periods outside the heliosphere altogether; when inside, the relevant HEP flux at $E/Q$2 is at least an order of magnitude more intense than today’s most extreme SEP events and can persist for several months, and when outside, the $E/Q$3 GCR intensity is at least an order of magnitude more intense than today because heliospheric shielding is lost (Opher et al., 16 Jan 2026). The same study states that the modeled HEP spectrum at the termination shock is $E/Q$4 orders of magnitude more intense than the 2003 Oct 29 SEP event at comparable energies and that Earth could have experienced such HEP-rich conditions for a few months out of each year over intervals ranging from $E/Q$5 year to $E/Q$6 years, depending on cloud geometry (Opher et al., 16 Jan 2026). Atmospheric ionization, ozone chemistry, climate effects, and biological consequences are discussed there as plausible consequences, but the magnitude of these impacts remains explicitly uncertain (Opher et al., 16 Jan 2026).

Several open problems remain central across the field. At the Voyager boundary, the true connection length $E/Q$7, radial thickness of the magnetic-highway region, and the detailed turbulence spectrum responsible for the very small $E/Q$8 remain unknown (Florinski, 2013). For HCS transport, the role of a realistic wavy and tilted current sheet, sector structure, and reconnection electric fields is still unresolved relative to idealized flat-sheet calculations (Battarbee et al., 2017). In the tail, the disagreement between comet-like and split-tail ENA interpretations shows that global morphology, charge-exchange physics, and in-situ re-acceleration cannot yet be separated cleanly (Kornbleuth et al., 13 Mar 2026).

These unresolved issues do not obscure the main synthesis. Heliospheric energetic particles constitute a hierarchically organized nonthermal system: seed populations and pickup ions are generated locally or imported from the interstellar medium, multiple acceleration sites operate from the low corona to the termination shock and heliotail, and transport through turbulence, current sheets, interaction regions, and boundaries imprints spectra, anisotropies, radial gradients, and ENA morphologies. The current literature therefore treats HEPs not as a single source class, but as the energetic-particle content of the heliosphere under a wide range of magnetic and plasma regimes.