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REVIEW 3 major objections 5 minor 24 references

The evolution of the electron number density in the coma of comet 67P at the location of Rosetta from 2015 November through 2016 March

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A simple radial-flow model of comet 67P's ionosphere goes from overestimating electron densities by a factor of ten to median agreement within 8 percent over five months, and the recovery is not explained by ion-neutral collisional…

desk verdict A useful five-month benchmark: the simple radial-flow model transitions from >10x overestimation to near-agreement, and the paper's honest hedge about the decoupling-radius interpretation is more interesting than the headline number. read the letter →

arxiv 1909.00639 v1 pith:6725LS5I submitted 2019-09-02 astro-ph.EP physics.space-ph

Erik Vigren , Niklas J. T. Edberg , Anders I. Eriksson , Marina Galand , Pierre Henri , Fredrik L. Johansson , Elias Odelstad , Martin Rubin
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Xavier Vallieres
This is my paper · ORCID
classification astro-ph.EPphysics.space-ph
keywords comet67P/Churyumov-GerasimenkoRosettamissionelectronnumberdensitycometaryionosphereion-neutraldecouplingphotoionizationmodelRPC/MIPROSINA/COPS
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

Comet 67P's ionosphere, as seen from the Rosetta spacecraft, is the test bed for a deliberately simple model: assume plasma moves radially outward at the same speed as the neutral gas and ignore dissociative recombination. Earlier case studies found that this model works well at low activity and close to the nucleus, but badly near perihelion. This paper extends the comparison to the five months after perihelion, 2015 November through 2016 March, and establishes that the model's failure is not permanent: the modeled electron densities gradually drop from being more than a factor of 10 too high to a median modeled-to-observed ratio of 1.08 in late February through March 2016. The paper further shows that within the theorized ion-neutral decoupling radius the agreement does not improve as expected, which undercuts the standard story that good fits at low activity signal strong ion-neutral collisional coupling. The result matters because it redraws the validity envelope of the simplest useful cometary ionosphere model and redirects attention to what else, besides collision coupling, could make the bulk ion radial velocity track the neutral outflow.

What carries the argument

The two working parts are (i) the model formula called Eq. (2) in Vigren et al. (2016), which converts a measured neutral density $n_n$ into a modeled electron density through an expression with a fixed pre-factor that encodes a constant H2O photoionization frequency of $7\times10^{-7}$ s$^{-1}$ at 1 AU and assumes radial outflow at the neutral speed with no dissociative recombination, and (ii) the ion-neutral decoupling distance $r_{\rm in} = k_{\rm in} r n_n / u_n$ from Eq. (1), where $k_{\rm in}\sim1.5\times10^{-9}$ cm$^3$ s$^{-1}$ is the ion-neutral collision rate coefficient. The analysis also rests on a reduction scheme in which MIP electron densities are compressed into medians of 100 consecutive SDL measurements, aligned in time with COPS neutral densities and spacecraft ephemerides, so that a modeled-to-observed ratio can be studied as a function of time and of $r/r_{\rm in}$.

What would settle it

One concrete check would be to take the 2,529 median values from 2016 February 20 through March 21 and recompute the modeled-to-observed ratio after applying a systematic factor of 0.8 or 1.2 to the MIP electron densities, a plausible absolute calibration range for mutual impedance data. If the median ratio then moves outside roughly 1.0-1.3, the claimed 'reasonable agreement' rests on an unverified absolute scale rather than on the physical assumptions of radial flow and negligible recombination.

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Extended reading notes

Core claim

The central discovery is a temporal transition in model-observation agreement. For the period 2015 November to 2016 March 21, the authors compare electron densities measured by RPC/MIP in Short Debye Length mode with values computed from ROSINA/COPS neutral densities using Eq. (2), a photoionization-based formula with no free parameters beyond a fixed pre-factor. Early in the interval the model overshoots by more than a factor of 10; by 2016 mid-February, with heliocentric distance $d>2.4$ AU and cometocentric distance $r<40$ km, the modeled and observed densities track each other closely, and for 2,529 median values from 2016 February 20 through March 21 the modeled-to-observed ratio has median 1.08, mean 1.17, and standard deviation of the mean 0.54. A companion result concerns the ratio $r/r_{\rm in}$, where $r_{\rm in}$ is the theorized ion-neutral decoupling distance: plotted against this ratio, the modeled-to-observed ratio shows, if anything, an increase as $r/r_{\rm in}$ decreases. The authors therefore conclude that the data give no support for the idea that the $u_i=u_n$ assumption holds better inside the decoupling distance, and they call for alternative explanations of why the simple model succeeds at low activity.

Load-bearing premise

The load-bearing premise is that the mutual impedance probe's electron densities are accurate in absolute value, with no stated systematic uncertainty, so that the near-unity median ratio in March 2016 is meaningful; if the absolute scale were off by tens of percent, the agreement could be a coincidence.

Editorial extensions

If this is right

  • From 2016 mid-February onward, with $d>2.4$ AU and $r<40$ km, the model reproduces observed median electron densities; the 2,529 SDL median values from February 20 to March 21 give a median modeled-to-observed ratio of 1.08.
  • In the earlier post-perihelion window the same model overestimates by more than a factor of 10, so the formula's validity is restricted in both activity level and distance from the nucleus.
  • The ratio $n_{e,\rm model}/n_{e,\rm MIP}$ shows no improvement, and possibly a worsening trend, as $r/r_{\rm in}$ decreases, so the data do not support collisional coupling inside the decoupling distance as the reason for the model's success at low activity.
  • If the model's success is instead due to the radial component of the ion bulk velocity matching the radial neutral outflow while the full ion speed is higher, then the model's success at low activity does not constrain the total ion-neutral coupling strength.

Reading between the lines

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

  • Not tested in the paper: the March 2016 agreement could partly reflect a systematic offset in the mutual impedance calibration; recomputing the ratio after shifting the MIP densities by a realistic tens-of-percent factor would show whether the median can stay near 1.
  • Not tested in the paper: if the CO2/H2O ratio correlates with the residual model error in late February through March, the H2O-only photoionization assumption would explain part of the transition without any change in ion-neutral coupling.
  • Not tested in the paper: measuring the full 3D ion velocity distribution over this interval would show whether only the radial component tracks the neutral outflow; if so, the $u_i=u_n$ assumption is a statement about one component, not about coupling.
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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

3 major / 5 minor

Summary. The paper compares electron number densities measured by RPC/MIP with values computed from a simple cometary ionosphere model driven by ROSINA/COPS neutral densities at the location of Rosetta during the post-perihelion phase from 2015 November through 2016 March. The model assumes that ions flow radially outward at the neutral gas speed and that dissociative recombination is negligible, with production described by Eq. 2 (photoionization of H2O with a constant frequency and a d^{-1/2} neutral velocity law). The central empirical result is a transition from a gross model overestimate (by more than a factor of 10) in November 2015 to a median modeled-to-observed ratio of 1.08 for 2,529 MIP median values during 2016 February 20 through March 21. The authors also plot the modeled-to-observed ratio against r/r_in and report that, if anything, the ratio increases with decreasing r/r_in, which they interpret as giving no support for the assumption that ui=un holds better inside the ion-neutral decoupling distance.

Significance. If the results are robust, this paper provides the first extended empirical mapping of when and where the simplified radial-flow photoionization model is valid at comet 67P, and it directly challenges the commonly invoked ion-neutral collisional coupling explanation for the model's success at low activity. A notable strength is that no parameters are fitted in this work; the model scale and the photoionization frequency are taken from prior published work, and the comparison uses independent Rosetta data over a long interval with a large sample of median values. The paper is also candid about several acknowledged omissions, including neglect of dissociative recombination (with an estimated 50% effect only in November 2015) and neglect of electron-impact ionization, which are unlikely to erase the order-of-magnitude overestimate that motivates the transition claim.

major comments (3)
  1. [Section 3, median ratio paragraph] The quantitative claim of 'reasonable agreement' rests on the absolute calibration of the RPC/MIP Short Debye Length electron densities and on the absolute model scale set by the prefactor in Eq. 2, but no systematic uncertainty is stated for either quantity. A 30-50% offset in the MIP SDL inversion or in the model prefactor would move the median ratio from 1.08 to roughly 0.6-0.8 or 1.5-1.8, changing the qualitative conclusion from 'reasonable agreement' to a threshold-dependent statement. Please add a sensitivity analysis with plausible offsets in both scales and state the known calibration uncertainty of MIP SDL densities, or at least discuss why such offsets can be excluded.
  2. [Section 3, Figure 2 and surrounding text] The interpretation that Figure 2 gives 'no support for the ui=un assumption to hold better within, or even well within, the ion-neutral decoupling distance' is confounded by the strong correlation between r/r_in and time/activity: the periods with the largest modeled-to-observed ratios (November 2015 and the following months) are the high-activity periods with small r/r_in, while the March 2016 data have large r/r_in and ratios near unity. This temporal/activity dependence could explain the apparent anti-correlation without implying anything about spatial decoupling. The authors should control for time or neutral density (e.g., by analyzing residuals within each of the three periods, or by binning at fixed activity) before drawing the stated conclusion.
  3. [Section 2, Eq. 2 and Section 3, March 2016 window] Eq. 2 applies an H2O photoionization rate to the total COPS neutral number density, but the introduction notes that CO2 outgassing became comparable to H2O at southern latitudes in late February and March 2016 (Gasc et al. 2017). Because COPS reports total neutral density and its pressure sensitivity may differ for CO2, and because the ionization frequency of CO2 differs from that of H2O, the model scale in the very window where the best agreement is found could be biased. Please justify the applicability of the H2O-based prefactor to the total neutral density in this window, or quantify the effect of using species-dependent ionization rates and COPS calibration.
minor comments (5)
  1. [General] The text contains several spelling and grammar issues, including 'nanongrains' (Section 1, should be 'nanograins'), 'cadency' (Section 2, should be 'cadence'), and 'the data is divided' (Figure 2 caption, should be 'the data are divided'). These should be corrected in a final revision.
  2. [Section 3] The phrase 'with a median of 1.08, a mean of 1.17 and a standard deviation of the mean of 0.54' is ambiguous: 0.54 is most likely the standard deviation of the ratio distribution, not the standard error of the mean. Please clarify which quantity is reported.
  3. [Section 3] The sentence 'the closer to perihelion and further from the nucleus the worse is the agreement' combines two effects, but Figure 1 shows that the agreement degrades with decreasing heliocentric distance and with increasing cometocentric distance simultaneously. It would be helpful to state explicitly whether the two dependencies are separable in the data, or to note that they are strongly correlated over the interval.
  4. [Figure 2] The caption says 'The data is divided into three different time periods as indicated in the legend,' and the colors are described as 'red and light blue,' but the third period's color is not named. Please list all three periods and their colors explicitly in the caption.
  5. [Section 1] The definition of the ion-neutral decoupling distance in Eq. 1 is taken from Gombosi (2015), but the text immediately notes a critical weakness in this formulation, citing Vigren & Eriksson (2019). Since the later interpretation in Figure 2 depends on r_in, it would be useful to state more explicitly that the qualitative conclusion is insensitive to alternative definitions of r_in, as the authors partly do in Section 3.

Circularity Check

0 steps flagged · score 2.0 of 10

No construction-level circularity: the model equation and its prefactor are prior inputs, and the March 2016 agreement is an out-of-sample comparison against independent MIP and COPS data; only a minor self-citation for the model normalization is present.

full rationale

The paper's central result is an empirical comparison, not a derivation from the model alone. Modeled densities are computed from ROSINA/COPS neutral densities via Eq. (2), which is explicitly imported from Vigren et al. (2016), and compared to RPC/MIP electron densities. No parameter is fitted to the MIP data in this paper, so the November 2015 overestimate by more than a factor of 10 and the 2016 March median modeled/observed ratio of 1.08 are not statistically forced by any parameter adjusted here. A constant prefactor or a constant absolute calibration offset would shift all ratios in time and cannot by itself produce the observed transition, so the main claim is robust to the calibration concerns noted by the skeptic. The r/rin analysis uses Eq. (1) from Gombosi (2015), and the authors explicitly flag that this definition of rin is questionable, cite Vigren & Eriksson (2019) for an alternative, and add that their point is unaffected by down-scaling rin; this undercuts any self-definitional objection. The only self-citation with quantitative weight is the 0.70 prefactor reduction 'justified in Vigren et al. (2016)', which is an input normalization rather than a quantity fitted to the present data, and the paper's time-dependent agreement claim does not reduce to it. Absolute accuracy of the March agreement depends on unstated systematics in MIP SDL inversion and COPS sensitivity to CO2, but that is a calibration-risk concern, not a circularity concern.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central comparison imports the model equation, its normalization constants, and all rate coefficients from prior literature, mostly the authors' own earlier papers. No new entity is postulated. The free parameters are inherited calibration or scaling choices, not fits performed here, and the axioms are the standard simplifications of the radial-outflow recombination-free model plus the questionable rin scale used for the anti-correlation analysis.

free parameters (2)
  • Global model normalization pre-factor in Eq. 2 (10^-6 with d in AU and r in km) = 10^-6 (equivalently the 0.70 reduction of the Cochran and Schleicher 1993 pre-factor)
    Sets the absolute scale of the modeled electron density. The reduction from 0.85 to 0.70 was justified in the authors' prior paper (Vigren et al. 2016), not derived in this work; the March 2016 agreement claim (median ratio 1.08) depends on this scale, although the temporal transition does not.
  • Neutral expansion velocity law (un = 0.70 (d/1 AU)^-1/2 km/s in the modified form) = about 0.57 km/s at 1.5 AU and 0.45 km/s at 2.5 AU
    The d^-1/2 scaling is adopted from Cochran and Schleicher (1993) and enters Eq. 2 through the photoionization travel time; the paper notes the resulting speeds are roughly 10-30 percent lower than Hansen et al. (2016). The exponent is assumed, not derived.
assumptions (6)
  • domain assumption The H2O photoionization frequency at 1 AU is 7e-7 s^-1, constant over the period and scaling as d^-2
    Used in Section 2 to construct Eq. 2; the paper states this crude treatment agrees within 20 percent of the TIMED/SEE-based rates of Heritier et al. (2018).
  • domain assumption Dissociative recombination does not severely reduce the electron density over the interval
    Invoked in Section 3 citing Heritier et al. (2018) Fig. 6; the authors admit the relative error from its neglect may reach 50 percent in parts of November 2015.
  • domain assumption Electron-impact ionization contributes negligibly to ion production over the period
    Section 2 appeals to Fig. 16 of Heritier et al. (2018); including it would raise modeled densities and worsen the modeled-to-observed ratios, so the omission is conservative for the overestimation claim.
  • domain assumption The ion-neutral decoupling radius of Gombosi (2015) (Eq. 1 here) is a meaningful ordering scale for the data
    Underlies the r/rin analysis in Fig. 2; the paper itself flags this formulation as questionable and notes it implicitly assumes ui = un (Vigren and Eriksson 2019).
  • domain assumption The local COPS neutral density characterizes the upstream ion production column at Rosetta
    Eq. 2 converts a local neutral density into a local electron density; this is part of the simplified radial-outflow model whose validity is the subject under test.
  • domain assumption The coma is effectively H2O-dominated for photoionization purposes
    The introduction notes CO2 outgassing became comparable to H2O at southern latitudes in late February and March 2016 (citing Gasc et al. 2017), and the model still applies an H2O-based rate to the total neutral density, so this assumption is partially violated exactly in the good-agreement window.

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Cite this review

Pith. "Pith review of The evolution of the electron number density in the coma of comet 67P at the location of Rosetta from 2015 November through 2016 March." pith.science (2026). https://pith.science/paper/6725LS5I

@misc{pith2026190900639,
  author       = {Pith},
  title        = {Pith review of: The evolution of the electron number density in the coma of comet 67P at the location of Rosetta from 2015 November through 2016 March},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/6725LS5I}},
  note         = {Machine review of arXiv:1909.00639}
}
read the original abstract

A comet ionospheric model assuming the plasma to move radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov-Gerasimenko. It has been found that at low activity and within several tens of km from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, showed that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare ROSINA/COPS driven model results with RPC/MIP derived electron number densities for an extended time period (2015 November through 2016 March) during the post-perihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.

Figures

Figures reproduced from arXiv: 1909.00639 by the authors.

Figure 1
Figure 1. a gives information on the heliocentric distance, the cometocentric distance, and the latitude of Rosetta during the investigated time period. Note that the selected time period coincides with “southern summer” with generally higher outgassing over southern latitudes. Figure 1b shows the neutral number densities measured by COPS (grey), modelled (black) and observed (red) electron number densities. The displayed MIP… view at source ↗
Figure 3
Figure 3. Density plot of modeled to observed electron number density ratios versus r/rin (based on the scattered data in [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗

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Reference graph

Works this paper leans on

24 extracted references · 22 canonical work pages

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    Vigren1, N

    The evolution of the electron number density in the coma of comet 67P at the location of Rosetta from 2015 November through 2016 March E. Vigren1, N. J. T. Edberg1, A. I. Eriksson1, M. Galand2, P. Henri3, F. L. Johansson1, E. Odelstad4, M. Rubin5 and X. Vallières3

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    (2007) and Trotignon et al

    METHOD We refer to Balsiger et al. (2007) and Trotignon et al. (2007) for descriptions of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor (ROSINA/COPS) and the Rosetta Plasma Consortium/Mutual Impedance Probe (RPC/MIP), respectively. Measurements by COPS and MIP yield, respectively, the neutral number densities, nn, and...

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    The factor 10-6 follows by reducing a pre-factor in Eq

    in Cochran & Schleicher, 1993). The factor 10-6 follows by reducing a pre-factor in Eq. 3 of Cochran & Schleicher (1993) from 0.85 to 0.70 (justified in Vigren et al.,

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    Rosetta approached 67P in August 2014 at a heliocentric distance of d~3.6 AU

    INTRODUCTION The Rosetta mission to comet 67P/Churyumov-Gerasimenko (henceforth 67P) gave the opportunity to study the evolution of a cometary coma during a significant part of its orbit around the Sun (currently characterized by a period of 6.44 years and perihelion/aphelion near ~1.25/5.68 AU). Rosetta approached 67P in August 2014 at a heliocentric dis...

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    Physikalisches Institut, Universität Bern, Switzerland ABSTRACT A comet ionospheric model assuming the plasma to move radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 6...

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    predicted

    or solar wind electrons that have been accelerated by an ambipolar electric field (Madanian et al., 2016, Deca et al., 2017). Another interesting finding concerns the electron density profile observed during the final descent of Rosetta towards the nucleus surface marking the end of the mission (on 2016 September 30). The density profile was found by Heri...

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    Key words: comets: individual (67P) – molecular processes

    the observations to being in reasonable agreement during 2016 March. Key words: comets: individual (67P) – molecular processes

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    In the present study we aim at providing further clues towards understanding the ionization balance of cometary comae by inspection of electron number densities measured during nearly five months of the Rosetta mission covering a period not much dealt with in previous case studies. We look at the post-perihelion phase from 2015 November through 2016 March...

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    Such a crude treatment of the photoionization frequency gives values within 20% of those calculated (from TIMED/SEE L3 V12 data) by Heritier et al

    and assuming a constant H2O photoionization frequency at 1 AU of 7×10-7 s-1 (e.g., Vigren et al., 2015a). Such a crude treatment of the photoionization frequency gives values within 20% of those calculated (from TIMED/SEE L3 V12 data) by Heritier et al. (2018) over the conside...

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    RESULTS AND DISCUSSION Figure 1a gives information on the heliocentric distance, the cometocentric distance, and the latitude of Rosetta during the investigated time period. Note that the selected time period coincides with “southern summer” with generally higher outgassing ov...

    2015

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    The picture that emerges is that the closer to perihelion and further from the nucleus the worse is the agreement between modeled and observed electron number densities. From 2016 mid February, when at a heliocentric distance d > 2.4 AU and a cometocentric distance r < 40 km (...

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    2 that the ne,model/ne,MIP ratio, if anything, shows an increasing trend with decreasing r/rin ratios

    It is seen in Fig. 2 that the ne,model/ne,MIP ratio, if anything, shows an increasing trend with decreasing r/rin ratios. In other words, Fig. 2 gives (surprisingly) no support for the ui=un assumption to hold better within, or even well within the ion-neutral decoupling dista...

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    post-perihelion exceptions

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    to a scenario during lower outgassing where the same type of model reproduces the observations rather well. We find, if anything, an anti-correlation between the modeled-to-observed ratios and the r/rin ratios (where r is the cometocentric distance of Rosetta and rin is the th...

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    Physics of Cometary Magnetospheres, in Magnetotails in the Solar System, edited by A. Kelling, C. M. Jackman and P. A. Delamere, AGU Monograph, 207, pp 169-188 Gombosi, T. I., Burch, J. L., & Horányi, M. 2015, A&A, A23 Hajra, R., Henri, P., Myllis, M., et al. 2018, MNRAS, 480,...

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    and as investigated for 67P in the parameter study by Vigren et al. (2015b). Except for the few observations of direct Rosetta detections of energetic nanograins reported by Burch et al. (2015) and discussed further by Gombosi et al. (2015), the current evidence for their pres...

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    plasma peak about a cometary radius above the surface. The assumptions made at low activity seem to break near perihelion and within the diamagnetic cavity, and this may be caused by: i) Ion acceleration by the ambipolar electric field (e.g., Vigren & Eriksson, 2017). Within t...

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    and ~10-30% lower than the values obtained using Eq. 7 of Hansen et al. (2016). Judging from Fig. 16 of Heritier et al. (2018), with possible exception for 2016 March, we deem it unlikely that electron-impact ionization contributed significantly to the total ionization rate ov...

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    (2016, 2017), Galand et al

    have been applied in case studies by e.g., Vigren et al. (2016, 2017), Galand et al. (2016) and Heritier et al. (2017, 2018). These focused either on relatively low activity levels (pre-perihelion; Vigren et al. 2016; Galand et al., 2016, post-perihelion; Heritier et al., 2017...

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    could possibly prohibit negatively charged nanongrains at low relative speed to reach the spacecraft, so there remains a slight possibility that their numbers could be underestimated. Finally, outside the diamagnetic cavity, assuming a radial flow may be limited; i.e., the sou...

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    from the nucleus. Near perihelion (and when the spacecraft was located beyond a cometocentric distance of 100 km), making similar assumptions, yields an overestimation of the electron number density, compared with the observed ones typically by a factor of 2-5. As for the low ...

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    The cometary ionosphere of 67P has been modeled through MHD- (e.g., Rubin et al., 2014), hybrid- (e.g., Koenders et al., 2015), and Particle-In-Cell (Deca et al.,

    giving rise to a partially ionized coma, the ionization degree of which to first approximation is expected to increase with increasing cometocentric distance and with decreasing heliocentric distance. The cometary ionosphere of 67P has been modeled through MHD- (e.g., Rubin et...

    2014

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