Production and Loss Processes of Hydrogen Energetic Neutral Atoms in the Heliosphere from 5 eV to 500 keV

Maciej Bzowski; Marzena A. Kubiak; Pawe{\l} Swaczyna

REVIEW 3 major objections 4 minor 78 references

Production and Loss Processes of Hydrogen Energetic Neutral Atoms in the Heliosphere from 5 eV to 500 keV

T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read For hydrogen ENAs in the IMAP energy range, charge exchange with helium atoms becomes a significant production channel at high energies, and above roughly 10 keV the dominant loss switches from charge exchange to electron-stripping…

desk verdict Useful reference synthesis for IMAP-era ENA work; the process categories are sound qualitatively but should not be read with false precision. read the letter →

arxiv 2411.13174 v2 pith:RL6SELHM submitted 2024-11-20 physics.space-ph astro-ph.SRphysics.atom-ph

Pawe{l} Swaczyna , Maciej Bzowski , Marzena A. Kubiak This is my paper · ORCID

classification physics.space-phastro-ph.SRphysics.atom-ph

keywords energeticneutralatomsheliospherechargeexchangeelectronstrippingionizationIMAPmissionatomiccrosssectionsinterstellarhydrogen

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Hydrogen energetic neutral atoms (ENAs) are the remote-sensing messengers of the heliosphere: ions neutralized far from the Sun travel straight to detectors near Earth. This paper computes which collisions create and destroy these atoms across the full energy range (5 eV to 500 keV) that IMAP will observe. It finds that charge exchange of protons with helium atoms, not just hydrogen atoms, produces a significant share of the highest-energy ENAs, and that above roughly 10 keV the dominant loss is no longer charge exchange but electron stripping, mainly by interstellar neutral hydrogen. The paper also supplies new analytic cross-section formulas that correct known biases in the standard proton–hydrogen charge-exchange fit. These results tell ENA modelers which reactions to include in each IMAP energy channel and which can be safely omitted.

What carries the argument

The analysis is carried by two analytic cross-section fits in the Janev and Smith functional form: Equation (A1) for the H+ + H0 to H0 + H+ charge-exchange cross section, fitted to Schultz et al. (2023) theory below 1 keV/u and Barnett (1990) above, and Equation (B1) for the H0 + H0 to H+ + H0 + e- stripping cross section, fitted to the Cariatore and Schultz (2021) recommendation. These cross sections are folded with six representative heliospheric condition sets using the ENA line-of-sight integral, so that production and loss processes are ranked by collision rate and by survival exposure, not by cross section alone.

What would settle it

Observe the heliosheath ENA spectrum with IMAP-Ultra at 30–300 keV: if a model that adds proton–helium charge exchange consistently overproduces the measured flux, while the proton–hydrogen-only model matches, the paper's central production claim fails; equivalently, a laboratory measurement of the H+ + He0 charge-exchange cross section at 10–300 keV/u that differs from the Barnett (1990) values used here by more than the quoted uncertainty would reset the claimed crossover energy.

Watch

Extended reading notes

Core claim

The central discovery is a re-ranking of the reactions that produce and destroy hydrogen ENAs in the heliosphere. In the energy ranges of all three IMAP ENA instruments, proton charge exchange with hydrogen atoms remains the dominant production channel, but for IMAP-Ultra (3–300 keV) charge exchange with helium atoms is always a category-A process, and for IMAP-Hi it is category A at 1 au and category B elsewhere. Above about 10 keV, the loss side changes character: charge exchange with protons and photoionization dominate below that energy, while stripping ionization—especially collisions with ambient interstellar neutral hydrogen—dominates at higher energies because the charge-exchange rate drops steeply. The paper quantifies this through collision rates and exposure factors for six representative heliospheric regions, and it provides new analytic fits for the proton–hydrogen charge-exchange cross section and for the hydrogen-atom stripping cross section, grounding them in recent theory and compiled measurements.

Load-bearing premise

The quantitative importance categories inherit the adopted representative heliospheric densities—especially the interstellar hydrogen density of 0.003 cm-3 at 1 au, the helium density of 0.01 cm-3, and the neglect of secondary helium and of spatial and temporal variation—so the crossing energies and exposure factors would shift if the true densities differ.

Editorial extensions

If this is right

IMAP-Ultra ENA flux models that include only proton–hydrogen charge exchange will miss a category-A production channel; helium charge exchange must be added.
Above roughly 10 keV, the survival of hydrogen ENAs is governed by electron stripping by interstellar neutral hydrogen, so loss models for high-energy channels must include this reaction.
The new analytic fit for H+ + H0 charge exchange reduces the previous up-to-25% overestimate of the Lindsay and Stebbings (2005) cross section near 20 keV/u.
In the heliosheath, ionization losses attenuate ENAs by only a few percent over 50 au, so heliosheath ENA observations are limited by the cooling length of the parent protons, not by ENA losses.
The exposed distance of extraheliospheric ENA sources is limited to a few hundred au in the VLISM because of charge exchange with protons and stripping by hydrogen atoms.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the local ISN hydrogen density at 1 au varies between solar minimum and maximum by a factor of two, the energy at which helium charge exchange becomes a category-A process for IMAP-Hi would shift within its passband, so the category table should be read as representative, not fixed.
The same collision-rate ranking could be applied to helium ENAs, which the paper notes survive much longer; that would give a self-consistent way to plan multi-species ENA imaging.
A direct laboratory cross-section measurement of H+ + He0 charge exchange between 10 and 300 keV/u with better than the ~20–40% claimed uncertainty would sharpen the crossover energy where helium overtakes hydrogen in producing high-energy ENAs.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Short version: this is a reference-grade synthesis, not a discovery paper. It compiles the relevant cross sections for hydrogen ENA production and loss, computes rates in six representative heliosphere regions, and turns them into A/B/C importance categories for the three IMAP instruments. The genuinely new pieces are three analytic fits — H+ + H0 charge exchange (Appendix A), H0 + H0 ionization (Appendix B), and H+ + He+ charge exchange (Eq. 10) — plus the synthesis itself. The fits are transparently labeled as fits to prior data, with parameters, energy ranges, and accuracy stated. The process ranking is the main contribution and it is mostly robust: helium charge exchange does become a significant production channel at high energy, and stripping by neutral hydrogen takes over from charge exchange above roughly 10 keV in the outer regions. The cross-section discussion is careful, with documented uncertainties and explicit notes where data are extrapolated beyond recommended ranges. Citation pattern is fine; the relevant theoretical and experimental literature is well covered. The soft spots are real but not fatal. The A/B/C categories are not uncertainty-propagated. Densities in Table 1 are rounded to one or two significant digits, and the authors say so; factor-of-two changes in n_H or n_He can push borderline entries (H+ + He0 for IMAP-Hi, H0 + H+ stripping for IMAP-Ultra) across category thresholds and shift the stated ~10 keV crossover. Figures 2 and 5 show rates and exposures without error bars. A reader who treats the categories as precise numbers will over-interpret them; the qualitative ordering survives any plausible density change. Also, some cross sections are used below or above their validated ranges, e.g., the Janev & Smith He2+ charge-exchange formula below 0.1 keV/u, but the authors flag each such case in the text. One practical annoyance: the new fit coefficients are not provided in machine-readable form, which matters for a paper whose purpose is to be the standard reference. Who is this for: IMAP data analysts and modelers who need a starting checklist of which processes matter in which energy band. It deserves peer review. A referee should push for error propagation on the categories and exposure factors, and for tabulated fit coefficients in the text or supplementary material.

Referee Report

3 major / 4 minor

Summary. The paper provides a systematic survey of production and loss processes for hydrogen ENAs in the heliosphere over 5 eV to 500 keV, targeting the energy ranges of the IMAP-Lo, IMAP-Hi, and IMAP-Ultra instruments. It defines six representative heliospheric regions (slow/fast solar wind at 1 au and 45 au, heliosheath, VLISM), compiles cross sections from ALADDIN, Barnett (1990), Janev & Smith (1993), and newer theoretical calculations, and computes production and loss collision rates as well as path-integrated exposure factors. Processes are classified into A/B/C significance categories per instrument. The central conclusions are that charge exchange of protons with helium atoms contributes significantly to high-energy ENA production, and that above roughly 10 keV stripping ionization, primarily by neutral hydrogen in the outer heliosphere, replaces charge exchange as the dominant loss mechanism.

Significance. If the conclusions hold, the paper constitutes a useful reference for IMAP-era ENA modeling and for interpreting future observations. Its strengths are the transparency of the cross-section choices with documented accuracy estimates, the explicit analytic fits for the H+ + H0 charge-exchange and H0 + H0 ionization cross sections in Appendices A and B, the numerical evaluation of the electron-impact rate coefficient with a quantitative comparison to the mean-relative-speed approximation, and the clear six-region framework that makes the calculations reproducible. The potential impact is moderate but real: it identifies helium charge exchange as a process that should not be neglected in high-energy ENA modeling. However, the A/B/C categories and the crossover energies are presented without propagated uncertainties, and the abstract generalizes the loss-crossover result to all regions more strongly than the region-by-region analysis supports.

major comments (3)

[§4.5, Table 2; §5.13, Table 3; Figure 5] The quantitative significance categories and crossover energies are not propagated with uncertainties. The densities in Table 1 are explicitly rounded to one or two significant digits, and the text itself notes factor-of-order-two variations (e.g., ISN hydrogen density at 1 au between 0.002 and 0.02 of the termination-shock density). A factor-of-2-3 change in n_H or n_He can move a process across the 10%/1% category thresholds or shift the stated ~10 keV loss crossover. The qualitative ordering of processes is probably robust, but the abstract's wording that helium charge exchange produces a 'significant portion' of high-energy ENAs and that stripping becomes 'the main loss mechanism' is a quantitative claim that requires either a sensitivity analysis or an explicit caveat in the summary. I recommend adding a small parameter-sensitivity study (e.g., varying the ISN hydrogen and helium densities over their plausible ranges and recomputing the categories) or softening the quantitative claims.
[Abstract; §5.13; Table 3] The abstract states that above ~10 keV 'stripping ionization processes, e.g., from collisions with ambient interstellar neutral hydrogen, become the main loss mechanism.' This is not supported for the inner heliosphere. At 1 au, the collision rates in the left panel of Figure 5 show that proton-impact ionization (Section 5.8), not neutral-hydrogen stripping, balances the declining charge-exchange rate above ~20 keV, and the path-integrated exposure categories in Table 3 assign H0+H0 ionization only category B for IMAP-Hi and IMAP-Ultra. The text correctly describes the region dependence ('the situation is slightly different at 45 au'), but the abstract over-generalizes. The abstract should specify that the neutral-hydrogen stripping crossover applies in the outer heliosphere and heliosheath, or present the region-dependent result.
[§4.3; §5.3; Appendix A] Several cross sections are used outside their validated energy ranges without quantifying the impact on the final rates. For example, Section 5.3 extends the Janev & Smith formula below its recommended 100 eV/u lower limit; Section 4.3 recommends Equation (10) above 100 keV/u even though it notes the Barnett (1990) cross section becomes 2.5 times larger at 240 keV/u; and Appendix A claims 20% accuracy for the new H+ + H0 fit down to 1 eV/u, where the underlying theoretical values show quantum oscillations that the analytic form cannot reproduce. Because the collision rates in Figures 2 and 5 are shown without error bars, the reader cannot assess whether the category assignments are robust to these extrapolations. Adding representative uncertainty bands to the rate curves or at least listing the extrapolation-induced error for the affected processes would make the significance categories more defensible.

minor comments (4)

[§2, near Eq. (7)] The sentence 'The maximum error of the approximate formula on the right-hand side of Equation (5) is less than 2.5%' appears to refer to the approximation on the right-hand side of Equation (7), not Equation (5).
[Appendix A] The names 'Lindsy' and 'Linsday' appear in the first paragraph and should be 'Lindsay' (Lindsay & Stebbings 2005).
[Abstract; §1] The abstract and title cover energies up to 500 keV, while the IMAP-Ultra instrument is quoted as covering 3-300 keV. The text should clarify that the analysis intentionally extends beyond the instrument upper bound to 500 keV.
[§4.3, Eq. (10)] Equation (10) uses the notation 'log E' without specifying the base; it appears to be log10 based on the parameter values, but this should be stated explicitly for reproducibility.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the conclusions are computed from external cross-section compilations and explicitly representative heliospheric conditions, not from fitted parameters or load-bearing self-citations.

full rationale

No circular step is present. The paper's central claims—helium charge exchange contributes significantly to high-energy hydrogen ENA production, and stripping ionization dominates ENA losses above roughly 10 keV—are derived by combining externally compiled cross sections (Barnett 1990; Janev & Smith 1993; Schultz et al. 2023; Cariatore & Schultz 2021) with explicitly tabulated representative densities, temperatures, and bulk speeds (Table 1). The rate calculation follows Equations (4)-(6) and contains no fitted parameter that is later relabeled as a prediction. The analytic fits in Appendices A and B are transparent least-squares fits to external recommended cross-section data; they serve as inputs to the analysis, not as outputs of it. The self-citations that do appear are for adopted physical parameters, such as Swaczyna et al. (2020) for the ISN hydrogen density and Swaczyna et al. (2023a, 2023b) for ISN helium flow parameters, and for a prior cross-section fit (Swaczyna et al. 2019b) that is explicitly compared against, rather than adopted as, the recommended formula. None of these citations imports the target result or forecloses alternatives by fiat. The paper itself labels the heliospheric conditions as representative and recommends comprehensive models for actual analyses, explicitly acknowledging that the A/B/C significance categories inherit the uncertainty and variability of these conditions. That is a precision limitation, not circularity.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

The paper's quantitative conclusions rest on adopted cross-section compilations and representative heliospheric conditions, plus new least-squares fits to prior cross-section data. No new physical entities are postulated.

free parameters (3)

A1-A8 parameters of Equation (A1) for H+ + H0 charge exchange cross-section = A1=4.8569, A2=21.906, A3=31.487, A4=0.12018, A5=4.1402e-06, A6=3.7524, A7=8.8476e-12, A8=6.1091
Least-squares fit to Schultz et al. (2023) theoretical cross section (0.1 eV/u to 1 keV/u) and Barnett (1990) recommended values (1 to 630 keV/u); the cross section is the paper's central production/loss input.
B1-B8 parameters of Equation (B1) for H0 + H0 ionization cross-section = B1=14.897, B2=16.097, B3=0.96601, B4=5627.5, B5=0.63515, B6=-6.5455, B7=4.7441e+05, B8=-1.8052
Least-squares fit to Cariatore & Schultz (2021) recommended cross section over 30 eV/u to 10 MeV/u; used for the dominant high-energy loss process.
Coefficients of Equation (10) for H+ + He+ charge exchange cross-section = 1.2375, 0.12616, 176.29
Fit to Faulkner et al. (2019) theoretical results over 50 keV/u to 1 MeV/u, used to extend the Barnett (1990) cross section to higher energies.

assumptions (5)

domain assumption The six representative condition sets in Table 1 adequately represent the heliospheric regions relevant to IMAP observations.
Section 3 adopts densities, bulk speeds, and temperatures for SSW/FSW at 1 au and 45 au, heliosheath, and VLISM from various observations and models; the paper's quantitative importance categories depend on these values.
domain assumption ENA trajectories are straight lines and the ENA energy equals the parent proton energy (negligible momentum exchange in production).
Section 2 and Section 4.5: Equation (1) assumes straight-line motion; Section 4.5 says 'we neglect the momentum exchange in the ENA production processes, and therefore, the energy of the parent proton is the same as the ENA energy.'
domain assumption The collision rate approximation in Equation (6) (sigma times vrel) is adequate for most processes; numerical integration is used for electron impact ionization.
Section 2 and Section 5.7: the mean-relative-speed approximation is used for all processes except electron impact ionization, where it is shown to be inaccurate by up to a factor of ~5 for low-energy ENAs.
domain assumption Cross sections from Barnett (1990), Janev & Smith (1993), Schultz et al. (2023), and Cariatore & Schultz (2021) are accurate within the stated uncertainties, and extrapolations beyond recommended energy ranges are valid.
Sections 4-5 and Appendices; e.g., Section 5.3 says 'we also use the analytic formula for energies below the recommended range.'
domain assumption Species heavier than helium and transfer ionization processes are negligible for hydrogen ENA production and loss.
Section 3 neglects species heavier than helium; Section 4.2 dismisses transfer ionization because it contributes only a few percent of the charge exchange process.

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Cite this review

Pith. "Pith review of Production and Loss Processes of Hydrogen Energetic Neutral Atoms in the Heliosphere from 5 eV to 500 keV." pith.science (2026). https://pith.science/paper/RL6SELHM

@misc{pith2026241113174,
  author       = {Pith},
  title        = {Pith review of: Production and Loss Processes of Hydrogen Energetic Neutral Atoms in the Heliosphere from 5 eV to 500 keV},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/RL6SELHM}},
  note         = {Machine review of arXiv:2411.13174}
}

read the original abstract

Energetic Neutral Atom (ENA) observations provide valuable insights into the plasma conditions in the heliosphere and the surrounding interstellar medium. Unlike plasma detectors, which measure charged particles tied to the magnetic fields at their location, ENA detectors capture former ions that were neutralized in distant regions and traverse the heliosphere in straight trajectories. ENA fluxes near the Sun represent line-of-sight integrals of parent ion fluxes multiplied by neutralization (production) rates and reduced by the probability of ENA reionization (loss) processes. So far, most ENA analyses have focused on charge exchange between hydrogen atoms and protons as the primary source of ENAs. Here, we examine various ENA production and loss processes throughout the heliosphere in the broad energy range (5 eV to 500 keV) encompassing the next-generation ENA instruments aboard the Interstellar Mapping and Acceleration Probe (IMAP) mission. Our study considers binary collisions involving the most abundant species: protons, electrons, {\alpha}-particles, He+ ions, photons, as well as hydrogen and helium atoms. Our findings indicate that, in addition to ENAs produced by charge exchange of energetic protons with hydrogen atoms, a significant portion of high-energy ENAs originate from the charge exchange with helium atoms. Below 10 keV, the dominant ENA loss processes are charge exchange collisions with protons and photoionization. However, stripping ionization processes, e.g., from collisions with ambient interstellar neutral hydrogen, become the main loss mechanism for higher energies because the charge exchange rate rapidly decreases.

Figures

Figures reproduced from arXiv: 2411.13174 by the authors.

**Figure 1.** Comparison of the cross sections for the production of hydrogen ENAs. The cross sections are obtained from the formulae discussed in this paper, see [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗

**Figure 2.** Left column: The collision rates for the ENA production processes in the heliospheric condition sets defined in [PITH_FULL_IMAGE:figures/full_fig_p011_2.png] view at source ↗

**Figure 3.** Reaction rate coefficient for the electron impact ionization of hydrogen ENAs. The left and middle panels show the results of the numerical integration with the Maxwellian distribution, and the approximation given in Equations (6-7), respectively. The right panel presents the difference between the calculated rate coefficients using these two approaches. The red lines in the left and middle panels mark the reaction … view at source ↗

Figures from the paper (1 more)

**Figure 4.** Figure 4: Cross sections for ENA loss processes discussed in Section 5 (except photoionization) as a function of the collision energy. The cross sections for the charge exchange and ionization losses in collisions with the same species are marked with solid and dashed lines of t…

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