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Impact of May 2024 ICME Event on Martian UV Dayglow Emissions using MAVEN/IUVS and EMM/EMUS Observations

T0 review · 2 major / 7 minor · reviewed 2026-07-09 · glm-5.2

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Pith's one-line read Solar storm lit up Mars's entire upper atmosphere at once

desk verdict Genuine two-instrument ICME dayglow result, but the 'planetary-scale' framing leans on a seasonally confounded EMUS baseline. read the letter →

arxiv 2607.07009 v1 pith:FVTR4JRF submitted 2026-07-08 physics.space-ph

Aadarsh Raj Sharma , Vaghamshi Dhruv Dhaneshbhai , Lot Ram , Sumanta Sarkhel This is my paper · ORCID
classification physics.space-ph PACS 96.50.sh96.30.Dz94.05.Dd
keywords MarsUVdayglowICMEprotonauroraelectronprecipitationthermosphereMAVENEMM
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

When a major interplanetary coronal mass ejection struck Mars in May 2024, it simultaneously intensified all three of the planet's main ultraviolet dayglow emissions—hydrogen Lyman-alpha and two atomic oxygen lines at 130.4 and 135.6 nm—across a planetary-scale region spanning the full vertical depth of the thermosphere and broad horizontal ranges of latitude, longitude, local time, and solar zenith angle. By combining near-simultaneous limb profiles from one spacecraft with disk images from another, the authors show that the event did not merely create a localized auroral patch but amplified existing dayglow structures everywhere at once. The concurrent brightening of hydrogen (driven by proton precipitation via charge-exchange producing energetic neutral atoms) and oxygen emissions (driven by electron-impact excitation from solar wind and solar energetic particle electrons) demonstrates that both proton and electron excitation pathways operated simultaneously during the ICME. A key diagnostic is the ratio of the two oxygen lines: because the 130.4 nm line is normally dominated by solar photon scattering while the 135.6 nm line is almost entirely electron-impact excited, a drop in their ratio below unity during the event signals a shift toward particle-driven excitation. The ratio fell to approximately 0.5 at peak ICME intensity, and solar EUV flux did not increase, ruling out enhanced illumination as the cause and pointing to energetic electron precipitation as the amplifier. Crucially, the characteristic peak altitudes and spatial distribution patterns of all three emissions remained essentially unchanged—the ICME intensified the brightness without reorganizing the atmospheric structure.

What carries the argument

The diagnostic machinery rests on three pillars. First, the H Lyman-alpha enhancement traces the proton aurora mechanism: solar wind protons charge-exchange with exospheric hydrogen to produce energetic neutral atoms that precipitate and excite hydrogen emissions. Second, the oxygen emissions trace electron-driven excitation: the O(5S) 135.6 nm line is 95% electron-impact excited while O(3S) 130.4 nm is 80-85% resonant scattering, so their ratio [130.4/135.6] serves as a clean diagnostic of photon-versus-particle dominance. A ratio dropping to ~0.5 during peak ICME, combined with no increase in solar EUV flux, isolates electron precipitation as the driver. Third, the near-simultaneous limb (

What would settle it

If the [O(3S)130.4/O(5S)135.6] ratio were shown to be comparably sensitive to ICME-induced changes in neutral O/CO2 density ratios or atmospheric temperature structure as it is to electron precipitation, then the attribution of the oxygen enhancement to electron-impact excitation would become ambiguous, and the paper's central claim about simultaneous proton and electron mechanisms would need to be re-examined.

Watch

Extended reading notes

Core claim

The central finding is that an intense ICME produces a planetary-scale, vertically and horizontally extensive enhancement of Martian UV dayglow through the simultaneous operation of proton-driven and electron-driven excitation mechanisms, with the oxygen emission line ratio serving as the key diagnostic that distinguishes particle-driven from photon-driven excitation. The ICME amplified existing emission structures rather than creating new ones.

Load-bearing premise

The paper attributes the oxygen dayglow enhancement specifically to electron-impact excitation based on the [O(3S)130.4/O(5S)135.6] ratio dropping below unity, treating this ratio as a clean diagnostic of particle-driven versus photon-driven processes. This ratio, however, also depends on neutral density profiles and atmospheric composition, both of which an ICME could alter through compression or heating independently of electron precipitation.

Editorial extensions

If this is right

  • If the oxygen line ratio is a reliable real-time diagnostic of electron precipitation energy and flux, it could serve as a remote-sensing proxy for thermospheric space-weather monitoring at Mars without requiring in-situ particle detectors.
  • The planetary-scale nature of the response implies that future Mars missions planning atmospheric entry or aerobraking during solar maximum should anticipate thermospheric density and composition perturbations across the entire dayside, not just near the subsolar point.
  • The simultaneous proton and electron excitation during a single ICME suggests that coupled ion-neutral-electron models of the Martian upper atmosphere need to treat both precipitation channels together rather than in isolation.
  • The persistence of enhancement toward the terminator and the observed day-to-night transport signature suggest that ICME-driven space weather modifies the nightside ionosphere through enhanced plasma transport, which could affect radio communication and radar sounding on the nightside.

Reading between the lines

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

  • If the ICME primarily amplifies existing structures rather than reorganizing them, this suggests the Martian thermosphere's basic circulation and composition patterns are robust to even extreme transient forcing—a property that would simplify predictive modeling of space-weather impacts.
  • The broadening of the oxygen emission peak from ~120-140 km to ~100-160 km without a shift in peak altitude could be used to invert for the precipitating electron energy spectrum, since different electron energies deposit at different altitudes; this event's broadening implies a wide energy spectrum from hundreds of eV to tens of keV.
  • The fact that the strongest enhancement occurred during southern summer (Ls ~270°), when the hydrogen corona is most extended, raises the question of whether ICME impacts during other seasons would produce qualitatively different responses—particularly whether proton aurora would be suppressed when the corona is contracted.
  • If the [130.4/135.6] ratio depends on neutral O/CO2 density ratios that themselves change during ICME compression of the atmosphere, then the ratio's interpretive power as a pure electron-precipitation diagnostic may be partially degenerate with neutral composition changes.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 7 minor

Summary. This manuscript presents near-simultaneous MAVEN/IUVS limb and EMM/EMUS disk observations of Martian UV dayglow emissions (H Lyman-alpha, O(3S) 130.4 nm, O(5S) 135.6 nm) during the May 2024 ICME event. The authors report pronounced enhancements in all three emissions relative to quiet-time conditions, with the response extending vertically through the thermosphere and horizontally across latitude, longitude, local time, and SZA. The concurrent enhancement of hydrogen and oxygen emissions is attributed to simultaneous proton and electron precipitation. The oxygen emission ratio [O(3S)130.4/O(5S)135.6] is used as a diagnostic of photon-driven versus particle-driven excitation. The study is well-motivated, the dual-instrument approach is a genuine strength, and the IUVS limb comparison is well-controlled with consecutive orbits. The manuscript is generally clearly written and the figures support the claims.

Significance. The principal novelty is the coordinated near-simultaneous use of IUVS limb profiles (vertical structure) and EMUS disk images (horizontal extent) to characterize the ICME response in both dimensions, which has not been previously demonstrated. The observational approach of matching quiet-time and event-time orbits for SZA and spatial coverage is sound. The check that solar EUV flux did not increase during the event (Figure S7) addresses an important confound. The use of the oxygen emission ratio as a diagnostic of electron-impact excitation, grounded in prior modeling work (Ritter et al., 2019; Soret et al., 2024), provides a physically grounded attribution framework. These are substantive contributions to the characterization of Martian upper atmospheric response to extreme space weather events.

major comments (2)
  1. Section 2 (Data and Methodology) and Section 4 (Discussion): The EMUS disk comparison uses a single quiet-time scan from 20 April 2024 (Ls ~240 deg) versus a single event scan from 18 May 2024 (Ls ~270 deg), a ~4-week seasonal gap. The authors themselves note in Section 4 that Ls ~270 deg is when 'the expanded hydrogen corona favours the most frequent and intense proton aurora events.' This seasonal effect is invoked to explain the strength of the ICME response, but it is not acknowledged as a potential confound for the EMUS baseline comparison. Seasonal expansion of the H corona and possible changes in thermospheric O/CO2 density ratios could independently shift the EMUS baselines, particularly for H Lyman-alpha. Since the EMUS disk observations are the primary evidence for the 'horizontally extensive' and 'planetary-scale' response across latitude, longitude, and local time, a seasonal
  2. Section 4 (Discussion), paragraph on the oxygen emission ratio: The attribution of the oxygen dayglow enhancement specifically to electron-impact excitation rests on the [O(3S)130.4/O(5S)135.6] ratio as a clean diagnostic. The paper states that this ratio 'serves as a diagnostic of photon-driven versus particle-driven processes,' but the ratio also depends on neutral density profiles (O and CO2), solar EUV flux, and electron energy spectra simultaneously. The authors note that EUV did not increase (Figure S7), which is important, but they do not address whether ICME-induced changes in neutral O/CO2 density ratios (e.g., from atmospheric compression or heating) could independently shift the emission ratio. A brief discussion of this potential degeneracy, or a quantitative argument for why neutral density changes are expected to be secondary, would strengthen the attribution claim.
minor comments (7)
  1. Section 3, Figure 2 caption: The caption states that green curves represent orbit 21225, but the text refers to this orbit as showing the strongest ICME impact. Clarifying the UT timing of orbit 21225 in the caption would help readers cross-reference with the EMUS observation time.
  2. Section 3, paragraph on EMUS observations: The text states that the EMUS event observation 'closely coincided with IUVS observations from orbit 21225.' The time difference is ~30 minutes (09:29 UT vs 09:59 UT). A brief note on whether the solar wind conditions were stable over this 30-minute window would strengthen the near-simultaneous claim.
  3. Section 4: The reference to 'Hughes et al. (2024)' in the paragraph on proton aurora does not appear in the reference list. The closest entries are Hughes et al. (2023, 2025, 2026). Please verify and correct.
  4. Section 4: The phrase 'simultaneously enhanced proton-driven and electron-driven excitation pathways' could be tightened. The two pathways operate on different species (H vs O) and are not strictly simultaneous in the sense of coupled processes. Clarifying that they operate concurrently but independently would improve precision.
  5. Figure 4: The SZA bin labels in the legend are not fully legible. Consider enlarging the legend or listing the SZA bins in the figure caption.
  6. Section 2: The quiet-time orbit selection criteria are described briefly. Stating the exact SZA matching tolerance and spatial overlap threshold used for selecting quiet-time orbits would strengthen the methodology.
  7. References: Several references have 2026 publication dates (Parrott et al., 2026; Pranjali et al., 2026; Hughes et al., 2026; Sharma et al., 2026). If these are accepted/in-press, please verify final citation details (volume, page, DOI) before publication.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity; observational study with independent instrument cross-validation and externally cited diagnostics

full rationale

This is an observational study, not a derivation or prediction paper, so the circularity patterns (self-definitional, fitted-input-as-prediction, uniqueness-imported, etc.) are largely inapplicable. The central claim — that UV dayglow emissions brightened during the May 2024 ICME — is grounded in direct measurements from two independent instruments (MAVEN/IUVS limb profiles and EMM/EMUS disk images) and upstream solar wind parameters from MAVEN in-situ instruments (SWIA, MAG, SWEA, SEP). The key diagnostic, the [O(3S)130.4/O(5S)135.6] ratio as an indicator of electron-impact vs. photon-driven excitation, is cited from Ritter et al. (2019), an independent group, and the ratio's physical interpretation is supported by Soret et al. (2024), also external. Self-citations exist (Sharma et al. 2025, Ram et al. 2024) but support secondary points about altitude-broadening patterns and prior single-instrument results, not the central claim. The paper does not fit a parameter to data and then 'predict' the same data, nor does it define a quantity in terms of the result it claims to derive. The EMUS quiet-time baseline seasonal gap (20 April vs. 18 May) is a correctness/confounding concern, not a circularity issue — the paper does not define the baseline in terms of the event outcome. No step in the argument chain reduces to its own inputs by construction.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new physical entities, particles, or forces. It uses established excitation mechanisms (R1-R6) from prior literature. The free parameters are observational selection criteria rather than model-fitting parameters. The key axioms are domain assumptions about the interpretability of the oxygen ratio and the comparability of quiet/event observations.

free parameters (3)
  • Quiet-time orbit selection window = orbits 21212-21218
    Chosen to match SZA and spatial coverage of event-time orbits; not a fitted model parameter but a selection criterion that could influence baseline brightness.
  • EMUS quiet-time reference scan = 20 April 2024, 14:47 UT
    Selected for comparable viewing geometry; a single scan rather than an averaged baseline.
  • Peak emission altitude range for EMUS analysis = 130-150 km
    Stated in Section 3 for Figure 4 spatial analysis; chosen based on prior literature (Ritter et al. 2019).
assumptions (4)
  • domain assumption The [O(3S)130.4/O(5S)135.6] ratio is a clean diagnostic of photon-driven vs particle-driven excitation, independent of neutral density changes.
    Invoked in Section 4 to attribute the ratio drop to enhanced electron precipitation. The ratio also depends on O and CO2 density profiles and electron energy spectra (Ritter et al. 2019; Soret et al. 2024).
  • domain assumption Solar EUV flux did not change between quiet and event times, ruling out photoionization as a driver of the enhancement.
    Stated in Section 4 with reference to Figure S7. This is an observational check, not an axiom per se, but the inference that EUV constancy rules out photon-driven enhancement assumes no time-lag effects in atmospheric response.
  • domain assumption ICME-driven IMF penetration below 300 km creates open field topologies that facilitate broad particle access to the thermosphere.
    Invoked in Section 4, citing Xu et al. (2018), to explain the horizontal extent of the enhancement. This is a cited physical mechanism, not independently verified in this paper.
  • domain assumption The quiet-time and event-time observations are directly comparable because SZA and spatial coverage are matched.
    Stated in Section 2. This assumes that matching SZA and spatial coverage is sufficient to isolate ICME effects, controlling for all other variables (season, dust, orbital geometry).

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Cite this review

Pith. "Pith review of Impact of May 2024 ICME Event on Martian UV Dayglow Emissions using MAVEN/IUVS and EMM/EMUS Observations." pith.science (2026). https://pith.science/paper/FVTR4JRF

@misc{pith2026260707009,
  author       = {Pith},
  title        = {Pith review of: Impact of May 2024 ICME Event on Martian UV Dayglow Emissions using MAVEN/IUVS and EMM/EMUS Observations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FVTR4JRF}},
  note         = {Machine review of arXiv:2607.07009}
}
read the original abstract

We present first near-simultaneous observations of large-scale spatial response of the Martian H Lyman-{\alpha} and atomic oxygen dayglow emissions (O(3S) 130.4 nm and O(5S) 135.6 nm) during May 2024 interplanetary coronal mass ejection (ICME). Using coordinated near-simultaneous observations from MAVEN/Imaging Ultraviolet Spectrograph (IUVS) and the EMM/Emirates Mars Ultraviolet Spectrometer (EMUS), we have identified pronounced enhancements in all three emissions relative to quiet-time conditions. Interestingly, coordinated IUVS limb and EMUS disk observations reveal that ICME-driven response extends vertically through the thermosphere and horizontally across broad ranges of latitude, longitude, local-time, and solar zenith angle. Concurrent enhancement of hydrogen and oxygen emissions indicates the simultaneous contribution of proton and electron-driven excitation mechanisms during ICME. These findings reveal a novel planetary-scale response of the Martian dayglow. By highlighting the critical role of energetic-particle precipitation in amplifying this global response, our work carries crucial implications for understanding Martian thermospheric response during intense solar transients.

Figures

Figures reproduced from arXiv: 2607.07009 by the authors.

Figure 1
Figure 1. (Left) ENLIL-CONE model simulation of 17-18 May 2024 ICME event. (Right) Solar wind variations near Mars (20 April – 20 May 2024): (a) density, (b) velocity, (c) dynamic pressure (SWDP), (d) IMF (|B|), (e) electron density, (f) SEP electron energy flux, and (g) SEP ion density flux. The red and blue dashed line shows EMM/EMUS observation, while red and blue shaded regions show MAVEN/IUVS observations during event an… view at source ↗
Figure 2
Figure 2. IUVS limb profiles of H Lyman-α, O(³S) 130.4 nm, and O(⁵S) 135.6 nm dayglow emissions. Red, blue, and green curves denote event-time, quiet-time, and MAVEN orbit 21225, respectively; shaded regions indicate 1σ variability. The top-row shows H Lyman-α limb profiles for increasing SZA from left to right. At low SZA (43o -62o ; Figure 2a-2c), the event-time profiles exhibit substantial enhancement relative to quite-tim… view at source ↗
Figure 3
Figure 3. EMUS disk images of H Lyman-α, O(³S) 130.4 nm, and O(⁵S) 135.6 nm under quiet (left) and ICME (middle) conditions, with spatial profiles (right) extracted from blue (quiet) and red (event) boxes. Bottom panels show SZA variation [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Variation of H Lyman-α, O(³S) 130.4 nm, and O(⁵S) 135.6 nm brightness with latitude (left), longitude (middle), and local-time (right) for different SZA bins (indicated in the legend). All data are taken from the 130–150 km altitude range. Solid curves represent event-…

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