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arxiv: 2511.19737 · v2 · submitted 2025-11-24 · 🌌 astro-ph.EP

Observations of [O I] emission in Comets C/2014 Q2 (Lovejoy) and C/2007 N3 (Lulin): Possible Influence of Solar Activity on Oxygen Line Ratios

Ella J. Mayfield , Adam J. McKay , Michael S. P. Kelley , Anita L. Cochran This is my paper

Pith reviewed 2026-05-17 04:26 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords cometsoxygen emissionCO2 abundancesolar activityphotochemistry[O I] linesempirical modelscometary coma
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The pith

An empirical model for inferring CO2 from oxygen line ratios matches direct measurements better for a comet observed near solar maximum than near minimum.

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

The paper tests whether solar activity level affects how well oxygen line ratios can serve as a proxy for CO2 abundance in comets. Observations of [O I] emission were taken for Comet Lovejoy near solar maximum and Comet Lulin near solar minimum, then analyzed with both empirical and theoretical release-rate models. The results were compared against contemporaneous space-based CO2 measurements. The empirical model, calibrated on earlier solar-maximum data, reproduced the direct CO2 values more closely for Lovejoy, while neither model matched well for Lulin. A sympathetic reader would care because this suggests the proxy method may need solar-cycle adjustments if it is to be used reliably for remote composition studies of many comets.

Core claim

The central claim is that the accuracy of release rates used to convert oxygen line ratios into CO2 abundances depends on solar activity. Specifically, the empirical model developed from comet observations near solar maximum reproduces the directly measured CO2 abundances better for Lovejoy, observed near solar maximum; neither the empirical nor the theoretical model accurately reproduces the direct measurement for Lulin, observed near solar minimum. The authors conclude that solar-cycle phase influences the photochemistry governing [O I] release in the coma.

What carries the argument

The oxygen line ratio, formed from the intensities of the [O I] 5577 Å green line and the 6300/6364 Å red lines, converted to CO2 production rates using either empirical or theoretical release rates.

If this is right

  • The oxygen line ratio method would require separate release-rate calibrations for different phases of the solar cycle to remain accurate.
  • Direct space-based CO2 measurements stay necessary for validation, especially during solar minimum conditions.
  • Comet composition surveys that rely on this proxy may carry systematic offsets if solar activity is ignored.
  • Observers planning future ground-based [O I] studies should record and account for the prevailing solar cycle phase when choosing a model.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Photochemical models of the cometary coma may need to incorporate time-varying solar UV flux to predict oxygen emission rates correctly across the cycle.
  • Reanalysis of existing comet spectra could apply solar-phase-specific corrections to improve past CO2 abundance estimates.
  • Similar solar-cycle dependencies might appear in other remote-sensing proxies used for planetary and cometary atmospheres.

Load-bearing premise

The difference in model performance between the two comets is caused primarily by their different solar-activity levels rather than by comet-specific chemistry, viewing geometry, or other unaccounted factors.

What would settle it

Direct CO2 measurements paired with oxygen line ratio observations for one or more additional comets observed at solar maximum or minimum that either repeat or contradict the Lovejoy-versus-Lulin pattern.

Figures

Figures reproduced from arXiv: 2511.19737 by Adam J. McKay, Anita L. Cochran, Ella J. Mayfield, Michael S. P. Kelley.

Figure 1
Figure 1. Figure 1: Energy level diagram for atomic oxygen [O I]. In analysis of the oxygen line ratio, the 2972 and 2958Å lines can be ignored as they only comprise 10% of transitions from the 1S level to the 1D level. Note that all atoms that decay through the 5577Å line will then also decay through either the 6300 or 6364Å line. Figure adapted from A. Bhardwaj & S. Raghuram (2012). by: R ≡ N(O( 1S)) N(O( 1D)) = I2958 +I297… view at source ↗
Figure 2
Figure 2. Figure 2: Region of the spectrum obtained of Lovejoy on UT May 11, 2015 that contains the forbidden oxygen line at 6300Å. Note that the cometary line appears redward of 6300Å due to Doppler shifting caused by the comet’s geocentric velocity. The data are represented by a histogram (black) with 1σ errorbars. The data are fitted with two separate Gaussians corresponding to the telluric (red) and cometary (blue). The s… view at source ↗
Figure 3
Figure 3. Figure 3: Spitzer IRAC images of comet C/2014 Q2 (Lovejoy). Top row: 3.6 µm images. Center row: 4.5 µm images. Bottom row: derived CO2 gas maps. All images are displayed with an asinh color scale intended to show both the low-surface brightness instrumental background and the bright cometary core. Arrows indicate the projected Celestial north (N), Sun (⊙) and velocity (v) vectors. Each sub-panel is 8. ′1×8. ′1. imag… view at source ↗
Figure 4
Figure 4. Figure 4: Regions of the spectra obtained of Lovejoy on UT 2015 February 3 that contain the forbidden oxygen lines at 5577Å (top row), 6300Å (middle row), and 6364Å (bottom row). Note that the cometary line appears redward of the telluric in all plots due to Doppler shifting caused by the comet’s geocentric velocity. The data are represented by a histogram (black) with 1σ errorbars. The right column shows the cometa… view at source ↗
Figure 5
Figure 5. Figure 5: Regions of the spectra obtained of Lulin on UT 2009 February 14 that contain the forbidden oxygen lines at 5577Å (top row), 6300Å (middle row), and 6364Å (bottom row). Note that the cometary line appears blueward of the telluric in all plots due to Doppler shifting caused by the comet’s geocentric velocity. The data are represented in the same ways described in [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Water production rates for Lulin from the literature, fitted to the power law prediction by M. R. Combi et al. (2019) for the relationship between water production rate and heliocentric distance post-perihelion. The data generally follow the curve, with the production rate from D. Bodewits et al. (2010) as a clear outlier, and the production rate from T. Ootsubo et al. (2012) a more subtle outlier. The unc… view at source ↗
Figure 7
Figure 7. Figure 7: Top panel: Total solar spectral irradiance for two dates plotted as a function of wavelength. The two dates plotted correspond to two of our observation dates, one each for Lovejoy and Lulin. The bottom panel plots the ratio between the two spectra (green), with the straight black line showing a ratio equal to one for easier visual comparison. The solar quiet spectrum is shown to have a consistently lower … view at source ↗
read the original abstract

Observing [O I] emission to calculate an "oxygen line ratio" has been proposed as a potential proxy for direct CO$_2$ measurement in comets. However, the photochemistry governing [O I] release into the coma is not well understood, and using theoretical release rates often yields different results than using empirical release rates determined in conjunction with direct space-based measurements of CO$_2$. We hypothesize that the accuracy of the release rates could depend on the level of solar activity at the time the comet is observed, which will be influenced by the solar cycle. We present observations and analysis of [O I] emission in one comet observed near solar maximum, C/2014 Q2 (Lovejoy), and one near solar minimum, C/2007 N3 (Lulin). Our [O I] measurements were obtained using two high spectral resolution optical spectrographs: the Tull Coud\'e spectrometer at McDonald Observatory and the ARCES spectrometer at Apache Point Observatory. We use empirical and theoretical models for [O I] emission from the literature to derive multiple sets of inferred CO$_2$ abundances for these comets and compare to contemporaneous space-based measurements of CO$_2$. We find that the empirical model, which was developed based on comet observations obtained near solar maximum, reproduces the directly measured CO$_2$ abundances better for Lovejoy. Neither model accurately reproduces the direct measurement for Lulin. We discuss the implications of our findings for the accuracy and dependencies of the oxygen line ratio method for inferring CO$_2$ abundances in cometary comae.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper reports high-resolution spectroscopic observations of [O I] emission in comets C/2014 Q2 (Lovejoy), observed near solar maximum, and C/2007 N3 (Lulin), observed near solar minimum. Using empirical and theoretical release-rate models from the literature, the authors derive CO2 abundances via the oxygen line ratio and compare the results to contemporaneous direct spacecraft CO2 measurements. They conclude that the empirical model (developed from observations near solar maximum) reproduces the direct CO2 values better for Lovejoy, while neither model accurately matches the direct measurement for Lulin, and discuss possible solar-activity dependence of the method.

Significance. If the result holds, the work would provide initial evidence that solar-cycle phase affects the reliability of the [O I] oxygen-line-ratio proxy for CO2, with implications for interpreting ground-based cometary spectra and for the development of activity-dependent release-rate models. The direct comparison to independent spacecraft CO2 data is a clear strength of the analysis.

major comments (2)
  1. [Abstract and Results] Abstract and results description: the claim of a clear performance difference between models for the two comets is presented without quantitative error bars on the inferred CO2 abundances, without reported sample sizes or integration times for the spectral data, and without explicit details on how model parameters (e.g., release rates) were applied or varied; this leaves the central comparison outcome difficult to assess quantitatively.
  2. [Discussion] Discussion: the attribution of the model-performance difference primarily to solar activity level is not supported by any quantitative assessment or control for confounding variables such as the comets' differing heliocentric distances at observation, intrinsic CO2/H2O ratios, outgassing patterns, or slit-filling factors in the high-resolution spectra. With only two comets, the data cannot isolate solar UV flux as the driver.
minor comments (2)
  1. [Observations] Clarify the precise rest wavelengths adopted for the [O I] lines and any telluric or continuum subtraction procedures applied to the Tull Coudé and ARCES spectra.
  2. [Figures/Tables] Add error bars to any tabulated or plotted comparisons of model-derived versus spacecraft CO2 abundances.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight areas where the manuscript can be clarified and strengthened. We respond to each major comment below.

read point-by-point responses

  1. Referee: [Abstract and Results] Abstract and results description: the claim of a clear performance difference between models for the two comets is presented without quantitative error bars on the inferred CO2 abundances, without reported sample sizes or integration times for the spectral data, and without explicit details on how model parameters (e.g., release rates) were applied or varied; this leaves the central comparison outcome difficult to assess quantitatively.

    Authors: We agree that the results section and abstract would benefit from greater quantitative detail to allow readers to assess the comparisons more rigorously. In the revised manuscript we will add error bars to all reported CO2 abundances derived from the oxygen line ratios. We will also report the number of individual spectra and total integration times used for each comet. In addition, we will expand the methods description to specify the exact release-rate values adopted from the literature for both the empirical and theoretical models and to clarify how these rates were applied to the observed line fluxes. revision: yes

  2. Referee: [Discussion] Discussion: the attribution of the model-performance difference primarily to solar activity level is not supported by any quantitative assessment or control for confounding variables such as the comets' differing heliocentric distances at observation, intrinsic CO2/H2O ratios, outgassing patterns, or slit-filling factors in the high-resolution spectra. With only two comets, the data cannot isolate solar UV flux as the driver.

    Authors: We accept that observations of only two comets cannot fully isolate solar activity from other variables. In the revised discussion we will add explicit quantitative information on the heliocentric distances at the epochs of observation, compare the spacecraft-measured CO2/H2O ratios for the two comets, and discuss possible effects of outgassing patterns and slit-filling factors. We will also qualify the interpretation by stating that the present results are suggestive of a solar-cycle dependence and that a larger sample is required to confirm the role of solar UV flux. These additions will make the limitations of the current dataset transparent while preserving the value of the direct spacecraft comparison. revision: yes

Circularity Check

0 steps flagged

No circularity; analysis rests on direct comparison to independent spacecraft CO2 data

full rationale

The paper presents spectroscopic observations of two comets and derives CO2 abundances using empirical and theoretical [O I] release-rate models taken from the literature. These inferred abundances are then compared to contemporaneous direct spacecraft measurements of CO2. No equations or steps reduce a fitted quantity to a prediction by construction, no self-citation chain is invoked to justify a uniqueness claim, and the central result is an empirical performance difference evaluated against external benchmarks rather than an internal re-derivation of the input models themselves. The derivation chain is therefore self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Central claim depends on the applicability of literature release rates to these specific comets and on the assumption that solar activity is the dominant variable separating the two cases; no new free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Literature empirical and theoretical [O I] release rates remain valid when applied to the observed comets.
    Used to convert measured line ratios into inferred CO2 abundances for comparison with spacecraft data.

pith-pipeline@v0.9.0 · 5627 in / 1145 out tokens · 74515 ms · 2026-05-17T04:26:09.974414+00:00 · methodology

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Reference graph

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