PIC Modeling of Ionospheric Plasma Diagnostics by Hemispherical Probes: Study of the LAP-CSES at Magnetic Conjugates
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis study investigates the behavior of hemispherical Langmuir probes in realistic ionospheric plasma conditions at three geophysical locations: Millstone Hill, the Equator, and Rothera. Using Particle-in-Cell-based (PIC-based) model, PTetra, the authors explores how plasma parameters, probe bias, and magnetic fields influence sheath formation, wake effects, and the probes’ current-voltage (I-V) characteristics. The study demonstrates how plasma parameters modify sheath structure, collected currents, and floating potentials. The influence of magnetic field orientation on sheath asymmetry and electron gyration is also analyzed. Comparisons between simulated currents and in situ measurements reveal moderate to strong correlations, highlighting both the accuracy of the PIC approach and the limitations of simplified analytical models.
Overall, the paper is logically presented. Before accepting the paper for publication, I have several suggestions and comments, mainly regarding the simulation setup and visualization.
- I found it somewhat difficult to navigate between the text, figures, and tables. While many numerical results are already presented in the tables, referencing them more frequently would improve readability. Additionally, presenting the plasma parameters for the three geophysical locations alongside Tables 2 and 3, immediately after introducing the LAP-CSES geometry, would help readers better understand the environments being studied.
- Tables 2–4 could be rearranged to better follow the flow of the article. For instance, the Debye length and gyro radius are key parameters for evaluating the simulation setup, yet they currently appear later in the paper.
- In Figure 1(c), the grid size appears much larger than the sensor diameter. How is the inner boundary handled in the simulation? Specifically, how is the LAP shape represented? Are the extension bars and electronics box treated as ideal conductors? Additionally, please clarify the grid size and the number of particles used.
- In Figure 2, does the background plasma remain charge-neutral? The varying color bars may be confusing. Also, why does the background in panel (a) appear so red compared with panels (b) and (c)? A similar issue occurs in panels (d) and (f).
- In Figure 3, I understand the challenge of presenting the potential around the floating potential while including biased probes. However, the differing background colors may be misleading and could be clarified.
- In Figure 4, the background color in all Equator plots appears lighter. Shouldn’t the background ion density be the same across these plots?
The following points are minor issues:
- Adding arrows to indicate the magnetic field direction on the 2D plots would be helpful, as the magnetic field is an important factor discussed in the paper.
- The text uses “…for floating, negatively and positively biased probes,” while the figure captions use “…around weakly, strongly negative and positive bias LAP…,” which could confuse readers. Consider harmonizing this terminology.
- Since this paper focuses on sheath and wake structures and the current collected by the probes, the authors may consider presenting only a zoomed-in region around the probe in the plots to improve clarity.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsSee attached below.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsTITLE: “PIC Modeling of Ionospheric Plasma Diagnostics by Hemispherical Probes: Study of the LAP-CSES at Magnetic Conjugates”
AUTHORS: Nadia Imtiaz, Saeed Ur Rehman, Liu Chao, Rui Yan and Richard Marchand
DATE: August, 2025
Manuscript ID: plasma-3832622
SUMMARY ASSESSMENT:
In this paper, the authors used the PTetra-based Particle in Cell code, adapted to simulate the plasma-Langmuir probe interaction, with the aim of modeling the probe's I-V curves (and comparing them with in-situ measurements onboard the CSES) using certain plasma parameters (density, temperature, magnetic field) to verify how these parameters affect the sheath structure (e.g., charge distribution) and the fluctuating potential of the Langmuir probe. This is an interesting article that aims to provide a more complete understanding of how the Langmuir probe works in a geophysical context, as defined by the authors. The article is well written, in understandable English, well organized and with a logical development in the presentation of the topics covered. The Abstract, the long Introduction and the Conclusions are clear and precise and reflect the content of the article (aside from a small omission in the Introduction, which is mentioned below). The article has 35 references.
Therefore, I suggest to the Editors that the article be published in “Plasma” journal after being revised according to the small suggestions and recommendations that I set out below.
GENERAL RECOMMENDATIONS:
1- To better understand the paper, some standardization is needed:
i) LP is the used acronym for Langmuir probe, but LAP is used for the authors' Langmuir probe. What then is the meaning of the middle letter, "A"?
ii) In Table 2, the electron temperature appears with units of energy (eV), while the paper's equations are written with , and thus, appears with units of temperature (K).
iii) The electron density is written as in Table 2, as (the plasma density) in the Debye length expression (line 174), as in equation (7), and as in equations (2) and (8).
iv) In Table 3, the thermal velocities of electrons and the two types of ions mentioned are given in m/s, while the electron drift velocity is given in km/s.
v) In Figure 4, the ion density scale appears only associated with the middle figures d-f, and not with all three sets, as is the case with Figures 2 and 3. Although I understand why, I suggest standardizing the scale by placing it after each of the three sets of figures (redundancy is not always a bad thing...).
Also in Figure 4, in the aforementioned associated scale, I don't understand what the authors mean when they write (.). Can you please check?
2- Table 3 (page 9) is not cited in the body of the article. Explicit reference should be made to Table 3, commenting on the parameters (at least some) that appear in it.
There are several points where this reference to Figure 3 could be placed: for example, in lines 212-212 and 213-216; but first, the reason for this Table should be explained.
3- I suggest that the comments on the results presented in Figures 2 and 3 (between lines 160-216) be revised to clarify which figure(s) (2 and/or 3) are being analysed. It seems to me that, as the article is currently written, Figure 2 is not sufficiently commented on. Below, I suggest some possible changes to the text.
4- I find it preferable to indicate the name of the first author of an article and only then the reference number. For example, in the current version of the article, sometimes a sentence begins with a reference, which is unpleasant (at least for me). In this sense, I suggest that the authors change the following sentences:
- line 51: "... has been presented by [9]." => ... has been presented by Jao et al. [9];
- lines 82-83: "For instance, [19] presented..." => For instance, Dorian et al. [19] presented...;
- lines 98-99: "... was introduced into the literature [20]." => "... was introduced into the literature by Lira and Marchand [20].;
- line 106: "[22] investigated the effects of..." => Brask et al. [22] investigated the effects of...;
- line 112: "[17] used..." => Olowookerer and Marchand [17] used...
- line 184: "[30] also presented..." => Marchand [30] also presented...
- line 238: "[29] provides examples..." => Lafleur [29] provides examples...";
- line 270: "[31] reported that the..." => Brask et al. [31] reported that the...
5- In Figures 2-4, the axis system shown in the upper left corner is very small and difficult to visualize, particularly in Figure 3.
SOME OTHERS SPECIFIC CONSIDERATIONS AND SUGGESTIONS:
- Although the Introduction is already long, it still lacks a concise description (with the necessary bibliographic reference(s)) of the PTetra code used, as mentioned in the Abstract and in line 153. This could also be done in a new (and shorter) section.
- In Table 2 (page 4): What are the coordinates in which the components of the magnetic induction field vector are presented? Cartesian, spherical? It seems important to indicate this next to the vector. For example, . I suggest a detailed explanation where the relationship with the orientation of the simulated probe is presented.
Also in this Table 2, I suggest that the units be placed close to the symbol of the respective physical quantity without overloading the numerical values ​​presented, as is currently the case. Example: .
- In Table 3 (page 9), the drift velocity is that of electrons, correct? Or is it the electromagnetic drift velocity ExB, the same for electrons and ions? Some clarification is needed, and should be provided in the comments that should be introduced in the text of the article regarding this figure.
- Why is it that in Table 3 the thermal velocity of the ions is about 1.5% that of the electrons, while in Table 2 the thermal energy of the ions is about 40% that of the electrons at Millstone Hill and the same at Equator? Could the authors please clarify?
- Line 166: where "Figure 2(a-c)" is, shouldn't it be Figure 3(a-c)?
- Line 167: “When a probe is charged or a biased…” => When a probe is charged or biased…
Lines 182-184: "The magnetic field parallel to the payload axis reduces the overall electron influence and significantly decreases the rocket's negative buoyancy potential [19]" => For example, Darian et al. [19] show that the magnetic field parallel to the payload axis reduces the overall electron influx and decreases the rocket’s negative floating potential significantly.
- Line 186: “Our analysis reveals that the floating potential… “ => As can be seen in Figure 3, our analysis reveals that the floating potential…
- Lines 189-191: “The interaction between differentially biased probes and the surrounding plasma also leads to the formation of negative electron and positive ion sheaths around the probes.” => Figure 2 shows that the interaction between differentially biased probes and the surrounding plasma also leads to the formation of negative electron and positive ion sheaths around the probes.
- Line 204: “Several studies have established to study the effect of the…” => Several studies have been conducted to study the effect of the...
Also, references to these studies are missing.
- Lines 201-211: "As expected, Millstone Hill exhibits comparatively...". Why? Are the B-field intensities so different at Millstone Hill and Rothera?
- line 222: where is "...with is..." => ...where is...
- lines 261-262: “… which correspond to the thin, intermediate, and thick sheath regimes for the Sensor 1, respectively.” => … which correspond to the thin, intermediate, and thick sheath regimes, respectively, for Sensor 1.
- Lines 272-273: "The electron retardation region corresponding to the exponential part of the I-V curve for the electrons following a Maxwellian distribution [23]:" => The current in the electron retardation region corresponding to the exponential part of the I-V curve for the electrons following a Maxwellian distribution is obtained from the equation [23]:
- Line 298: “The two sensors collect a large electron current…” => The two sensors collect a larger electron current…
- Reference [15]: does not seem correct/complete to me: the title of the Thesis (or report, but not article!) is: "Theory of Spherical and Cylindrical Langmuir Probes in a Collisionless, Maxwellian Plasma at Rest", Copyright©1966 University of Toronto [J. G. Laframboise “Theory of Spherical and Cylindrical Langmuir Probes in a Collisionless, Maxwellian Plasma at Rest,” University of Toronto, Toronto, 1966] [https://apps.dtic.mil/sti/pdfs/AD0634596.pdf]
2025 August 26
Comments for author File: Comments.pdf
Author Response
Please see the attached file.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper has been greatly improved now. Thus I recommend this paper now published as is.
