Earth&rsquo;s magnetosheath: A comparison of plasma flow direction between models and observations

Vandas, Marek; Romashets, Evgeny

doi:10.5194/egusphere-2025-4672

Preprints

https://doi.org/10.5194/egusphere-2025-4672

Preprints

21 Oct 2025

| 21 Oct 2025

Earth’s magnetosheath: A comparison of plasma flow direction between models and observations

Marek Vandas and Evgeny Romashets

Abstract. Observations of the plasma ﬂow direction in the Earth’s magnetosheath are compared with the help of three analytical magnetic-ﬁeld models, namely Kobel and Flückiger (1994), Romashets and Vandas (2019), and Vandas and Romashets (2019), which all assume current-free ﬁelds in the magnetosheath. 47 magnetosheath passages by spacecraft are analyzed in detail and performance of the models are evaluated. It is concluded that the performances are comparable and that they are satisfactory on average. Therefore, a usage of the model by Kobel and Flückiger (1994) is recommended, because it is the simplest one and yields results much faster.

Received: 23 Sep 2025 – Discussion started: 21 Oct 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 695 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (695 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

25 Feb 2026

Earth's magnetosheath: a comparison of plasma flow direction between models and observations

Marek Vandas and Evgeny Romashets

Ann. Geophys., 44, 137–148, https://doi.org/10.5194/angeo-44-137-2026,https://doi.org/10.5194/angeo-44-137-2026, 2026

Short summary

Marek Vandas and Evgeny Romashets

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-4672', Anonymous Referee #1, 02 Nov 2025

Referee Report on EGUsphere Preprint 2025-4672 entitled “Earth’s magnetosheath: A comparison of plasma flow direction between models and observations”
This is a simple, yet interesting, paper that compares observations of magnetosheath magnetic fields and plasma flow directions with the predictions of simple analytical models for these parameters. The paper will eventually be suitable for publication, but there are some important tasks to be completed before it can be published.
The first task is to properly reference past research. Both Howe and Binsack (JGR, 77, 3334, 1972) and Crooker et al. (JGR, 89, 9711, 1984) have compared flow directions within the magnetosheath with theoretical and model predictions. They are not referenced in the present paper.
The present paper finds discrepancies between observed flows and model predictions, particularly in the vicinity of the magnetopause, but makes no attempt to explain why this might be the case. The authors should consider the various predictions described by Crooker et al. and see if these help explain their results.
The authors scale magnetopause and bow shock positions by the solar wind dynamic pressure, but they must surely be aware that magnetic reconnection and erosion cause the magnetopause (and therefore the bow shock) to move inward during periods of southward IMF orientation. They should note that they are ignoring this effect near line 73.
The authors tell where the bow shock and magnetopause shapes come from for Magnetic field model 2 and 3, but there is no statement about this for Magnetic field model 1 in the vicinity of line 80.
In Section 3, the authors report that they require bow shock and magnetopause crossings to be clearly identifiable, but they do not tell what they do when there are multiple magnetopause and bow shock crossings. Which one do they choose?
The authors shift solar wind observations to the bow shock nose, but how do they shift them to match spacecraft observations further downstream? Some of the intervals in Table 1 occur near the terminators.
Could the authors please comment on what the typical cause of high delta? Is it a poor prediction for the flow direction in the Y-Z plane or a poor predictions of VX?
Successful prediction of magnetosheath parameters may depend on the location of the solar wind monitor. Monitors far off the sun-Earth line may not see the features that reach Earth. Can the authors please present the dependence of delta on the distance of the solar wind monitor from the Sun-Earth line?
Reconnection at the magnetopause may affect plasma flow patterns in the magnetosheath. Could the authors please present the dependence of delta on the north/south component of the IMF orientation?
Magnetic fiield directions and strengths may be more important in the magnetosheath when the solar wind Mach number is low. Could the authors please present the dependence of delta on the solar wind Mach number?
The dashed line ‘fits’ to the point in each panel of Figure 6 are not very compelling. The authors should provide the correlation coefficients for each fit.
It would be good if the authors make an effort to explain why the discrepancies between observed and predicted flow direction are greatest in the subsolar magnetosheath for large cone angle.

Citation: https://doi.org/10.5194/egusphere-2025-4672-RC1
- AC1: 'Reply on RC1', Evgeny Romashets, 02 Nov 2025
  
  We thank the referee for the valuable comments and suggestions.
  
  We shall incorporate them into a revised version.
  
  Citation: https://doi.org/10.5194/egusphere-2025-4672-AC1
RC2:
'Comment on egusphere-2025-4672', Anonymous Referee #2, 30 Nov 2025
Referee report on 'Earth’s magnetosheath: A comparison of plasma flow direction between models and observations', by Vandas and Romashets

This manuscript describes comparisons of model magnetosheath magnetic field (from three different analytic models) as a proxy for the bulk plasma velocity (for radial IMF) against the observed magnetosheath velocity, for a few dozen magnetosheath traversals. The comparisons are performed to ascertain which analytic model performs best. This is an interesting study to assess the various analytic models in the scientific literature. However, there are several questions to be answered and some needed clarifications before this manuscript can be considered to be publishable in Annales Geophysicae. These are described in detail below.
In general, there are several qualitative and subjective judgmental comments such as ‘performances are comparable’, ‘comparisons are satisfactory’, and ‘reported a fairly good agreement’. It would be helpful to the reader if such comments had more quantitative support. For example, does ‘fairly good agreement’ mean within 10%? Or within a factor of 2? Or something else??

The description of the selection criteria in Lines 100-102 appears to be incomplete. For example, what quantitative criterion was used for the selected cases to be close to radial IMF (or aligned with the solar wind flow)? What criterion was used for the steadiness of the IMF during the magnetosheath pass? Are there any other applied criteria?

Related to the previous point – if these cases occurred during times of nearly radial IMF, it was expected that the magnetosheath magnetic field and velocity components would be similar to each other. Instead, the sample intervals in Figs. 1,2 show several changes in the sign (polarity) of By, Bz that are not seen in the velocity components. Please explain in the manuscript why the magnetic field and velocity components are so different.

For the comparisons between models and observations, how do the authors match model boundary crossings with the actual crossings? In general, model and observed boundary crossings are co-located in a statistical sense. Not so much during individual traversals.

Related to the previous point, how did the authors treat multiple crossings of boundary during a given pass? Is this part of the selection criteria for the data set? Additional details regarding the selection criteria would be helpful for the reader.

Spreiter and Rizzi (Acta Astro., 1974) describe how a field aligned flow in the solar wind is field-aligned everywhere; and their references show that this was known as early as 1960 (and perhaps earlier). Some other potentially relevant references not mentioned in the manuscript are provided below.
Other suggested references:
Farrugia, C. J., et al. (2010), Magnetosheath for almost-aligned solar wind magnetic field and flow vectors: Wind observations across the dawnside magnetosheath at X = −12 Re, J. Geophys. Res., 115, A08227, doi:10.1029/2009JA015128.
Merka, J., A. Szabo, J. Safranková, and Z. Nemecek (2003), Earth's bow shock and magnetopause in the case of a field-aligned upstream flow: Observation and model comparison, J. Geophys. Res., 108(A7), 1269, doi:10.1029/2002JA009697.
Pi, G., J.-H. Shue, K. Grygorov, H.-M. Li, Z. Němeček, J. Šafránková, Y.-H. Yang, and K. Wang (2017), Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions, J. Geophys. Res., 122, 4051–4063, doi:10.1002/2015JA021809.
Samsonov, A., Z. Němeček, J. Šafránková, K. Jelínek (2012), Why does the subsolar magnetopause move sunward for radial interplanetary magnetic field?, J. Geophys. Res., 117, doi:10.1029/2011JA017429.
Shue, J.-H., J.-K. Chao, P. Song, J.P. McFadden, A. Suvorova, V. Angelopoulos, K.H. Glassmeier, and F. Plaschke (2009), Anomalous magnetosheath flows and distorted subsolar magnetopause for radial interplanetary magnetic fields, Geophys. Res. Lett., 36, L18112, doi:10.1029/2009GL039842.
Spreiter, J.R., and A.W. Rizzi (1974), Aligned magnetohydrodynamic solution for solar wind flow past the earth’s magnetosphere, Acta Astron., 1, 15–35, doi:10.1016/0094-5765(74)90006-X.

Minor suggested edits (technical corrections):
Line 35: hypotheses -> hypothesis
Line 38: ‘in hands here’ -> ‘in hand’
Line 41: procede -> proceed
Line 56: way -> method
Lines 56, 95, 107: abberated -> aberrated
Line 165: ‘models better describes’ -> ‘models better describe’
Line 168: ‘much more faster’ -> ‘much faster’
Figures: Please consider using different colors. Individuals with red-green color-blindness may not be able to discriminate between the different traces.
Citation: https://doi.org/10.5194/egusphere-2025-4672-RC2
- AC2: 'Reply on RC2', Evgeny Romashets, 30 Nov 2025
  
  Dear Referee,
  Thank you for valuable comments and corrections.
  We agree and will modify the manuscript accordingly.
  Authors
  
  Citation: https://doi.org/10.5194/egusphere-2025-4672-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-4672', Anonymous Referee #1, 02 Nov 2025

Referee Report on EGUsphere Preprint 2025-4672 entitled “Earth’s magnetosheath: A comparison of plasma flow direction between models and observations”
This is a simple, yet interesting, paper that compares observations of magnetosheath magnetic fields and plasma flow directions with the predictions of simple analytical models for these parameters. The paper will eventually be suitable for publication, but there are some important tasks to be completed before it can be published.
The first task is to properly reference past research. Both Howe and Binsack (JGR, 77, 3334, 1972) and Crooker et al. (JGR, 89, 9711, 1984) have compared flow directions within the magnetosheath with theoretical and model predictions. They are not referenced in the present paper.
The present paper finds discrepancies between observed flows and model predictions, particularly in the vicinity of the magnetopause, but makes no attempt to explain why this might be the case. The authors should consider the various predictions described by Crooker et al. and see if these help explain their results.
The authors scale magnetopause and bow shock positions by the solar wind dynamic pressure, but they must surely be aware that magnetic reconnection and erosion cause the magnetopause (and therefore the bow shock) to move inward during periods of southward IMF orientation. They should note that they are ignoring this effect near line 73.
The authors tell where the bow shock and magnetopause shapes come from for Magnetic field model 2 and 3, but there is no statement about this for Magnetic field model 1 in the vicinity of line 80.
In Section 3, the authors report that they require bow shock and magnetopause crossings to be clearly identifiable, but they do not tell what they do when there are multiple magnetopause and bow shock crossings. Which one do they choose?
The authors shift solar wind observations to the bow shock nose, but how do they shift them to match spacecraft observations further downstream? Some of the intervals in Table 1 occur near the terminators.
Could the authors please comment on what the typical cause of high delta? Is it a poor prediction for the flow direction in the Y-Z plane or a poor predictions of VX?
Successful prediction of magnetosheath parameters may depend on the location of the solar wind monitor. Monitors far off the sun-Earth line may not see the features that reach Earth. Can the authors please present the dependence of delta on the distance of the solar wind monitor from the Sun-Earth line?
Reconnection at the magnetopause may affect plasma flow patterns in the magnetosheath. Could the authors please present the dependence of delta on the north/south component of the IMF orientation?
Magnetic fiield directions and strengths may be more important in the magnetosheath when the solar wind Mach number is low. Could the authors please present the dependence of delta on the solar wind Mach number?
The dashed line ‘fits’ to the point in each panel of Figure 6 are not very compelling. The authors should provide the correlation coefficients for each fit.
It would be good if the authors make an effort to explain why the discrepancies between observed and predicted flow direction are greatest in the subsolar magnetosheath for large cone angle.

Citation: https://doi.org/10.5194/egusphere-2025-4672-RC1
- AC1: 'Reply on RC1', Evgeny Romashets, 02 Nov 2025
  
  We thank the referee for the valuable comments and suggestions.
  
  We shall incorporate them into a revised version.
  
  Citation: https://doi.org/10.5194/egusphere-2025-4672-AC1
RC2:
'Comment on egusphere-2025-4672', Anonymous Referee #2, 30 Nov 2025
Referee report on 'Earth’s magnetosheath: A comparison of plasma flow direction between models and observations', by Vandas and Romashets

This manuscript describes comparisons of model magnetosheath magnetic field (from three different analytic models) as a proxy for the bulk plasma velocity (for radial IMF) against the observed magnetosheath velocity, for a few dozen magnetosheath traversals. The comparisons are performed to ascertain which analytic model performs best. This is an interesting study to assess the various analytic models in the scientific literature. However, there are several questions to be answered and some needed clarifications before this manuscript can be considered to be publishable in Annales Geophysicae. These are described in detail below.
In general, there are several qualitative and subjective judgmental comments such as ‘performances are comparable’, ‘comparisons are satisfactory’, and ‘reported a fairly good agreement’. It would be helpful to the reader if such comments had more quantitative support. For example, does ‘fairly good agreement’ mean within 10%? Or within a factor of 2? Or something else??

The description of the selection criteria in Lines 100-102 appears to be incomplete. For example, what quantitative criterion was used for the selected cases to be close to radial IMF (or aligned with the solar wind flow)? What criterion was used for the steadiness of the IMF during the magnetosheath pass? Are there any other applied criteria?

Related to the previous point – if these cases occurred during times of nearly radial IMF, it was expected that the magnetosheath magnetic field and velocity components would be similar to each other. Instead, the sample intervals in Figs. 1,2 show several changes in the sign (polarity) of By, Bz that are not seen in the velocity components. Please explain in the manuscript why the magnetic field and velocity components are so different.

For the comparisons between models and observations, how do the authors match model boundary crossings with the actual crossings? In general, model and observed boundary crossings are co-located in a statistical sense. Not so much during individual traversals.

Related to the previous point, how did the authors treat multiple crossings of boundary during a given pass? Is this part of the selection criteria for the data set? Additional details regarding the selection criteria would be helpful for the reader.

Spreiter and Rizzi (Acta Astro., 1974) describe how a field aligned flow in the solar wind is field-aligned everywhere; and their references show that this was known as early as 1960 (and perhaps earlier). Some other potentially relevant references not mentioned in the manuscript are provided below.
Other suggested references:
Farrugia, C. J., et al. (2010), Magnetosheath for almost-aligned solar wind magnetic field and flow vectors: Wind observations across the dawnside magnetosheath at X = −12 Re, J. Geophys. Res., 115, A08227, doi:10.1029/2009JA015128.
Merka, J., A. Szabo, J. Safranková, and Z. Nemecek (2003), Earth's bow shock and magnetopause in the case of a field-aligned upstream flow: Observation and model comparison, J. Geophys. Res., 108(A7), 1269, doi:10.1029/2002JA009697.
Pi, G., J.-H. Shue, K. Grygorov, H.-M. Li, Z. Němeček, J. Šafránková, Y.-H. Yang, and K. Wang (2017), Evolution of the magnetic field structure outside the magnetopause under radial IMF conditions, J. Geophys. Res., 122, 4051–4063, doi:10.1002/2015JA021809.
Samsonov, A., Z. Němeček, J. Šafránková, K. Jelínek (2012), Why does the subsolar magnetopause move sunward for radial interplanetary magnetic field?, J. Geophys. Res., 117, doi:10.1029/2011JA017429.
Shue, J.-H., J.-K. Chao, P. Song, J.P. McFadden, A. Suvorova, V. Angelopoulos, K.H. Glassmeier, and F. Plaschke (2009), Anomalous magnetosheath flows and distorted subsolar magnetopause for radial interplanetary magnetic fields, Geophys. Res. Lett., 36, L18112, doi:10.1029/2009GL039842.
Spreiter, J.R., and A.W. Rizzi (1974), Aligned magnetohydrodynamic solution for solar wind flow past the earth’s magnetosphere, Acta Astron., 1, 15–35, doi:10.1016/0094-5765(74)90006-X.

Minor suggested edits (technical corrections):
Line 35: hypotheses -> hypothesis
Line 38: ‘in hands here’ -> ‘in hand’
Line 41: procede -> proceed
Line 56: way -> method
Lines 56, 95, 107: abberated -> aberrated
Line 165: ‘models better describes’ -> ‘models better describe’
Line 168: ‘much more faster’ -> ‘much faster’
Figures: Please consider using different colors. Individuals with red-green color-blindness may not be able to discriminate between the different traces.
Citation: https://doi.org/10.5194/egusphere-2025-4672-RC2
- AC2: 'Reply on RC2', Evgeny Romashets, 30 Nov 2025
  
  Dear Referee,
  Thank you for valuable comments and corrections.
  We agree and will modify the manuscript accordingly.
  Authors
  
  Citation: https://doi.org/10.5194/egusphere-2025-4672-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Publish subject to revisions (further review by editor and referees) (05 Dec 2025) by Christos Katsavrias

AR by Evgeny Romashets on behalf of the Authors (30 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Feb 2026) by Christos Katsavrias

RR by David Sibeck (07 Feb 2026)

RR by Anonymous Referee #2 (08 Feb 2026)

Suggestions for revision or reasons for rejection

Referee report on 'Earth’s magnetosheath: A comparison of plasma flow direction between models and observations', by Vandas and Romashets

This manuscript describes magnetosheath comparisons between models and observations. The observed magnetosheath velocity is compared against a model velocity. The model velocity has been determined from three different analytic magnetosheath magnetic field models, using a radial IMF as input to determine the magnetosheath magnetic field, which then serves as a proxy for the bulk plasma velocity. This is done for a few dozen magnetosheath traversals. Observed and modeled magnetosheath magnetic fields using observed IMF conditions are also shown in case studies. The observed and modeled velocity comparisons in the magnetosheath are performed to ascertain which analytic magnetic field model performs best. This is an interesting study to assess the various analytic models in the scientific literature. The revisions by the referees have improved this manuscript. While the authors have adequately answered this referee’s questions and provided clarifications, there remains a couple minor suggested revisions to improve the overall message of the study. Please consider these suggestions, which are described in detail below.

It might help the reader to provide (e.g., around Line 139) a clear statement such as “The use of a radial upstream magnetic field (IMF) is used to model the magnetosheath magnetic field (B(rmod)), which is then used as a proxy for a modeled magnetosheath velocity vector.”. [readers can quickly understand the concise term ‘proxy measure’]

Figures 1-5 are still confusing to the general reader, because it appears that the same magnetosheath model run is used to create both the blue traces of the left side (velocity components) and right side (magnetic field) panels. It is suggested that the Figure(1) caption clearly mentions that the left side panel model velocity components are proxy values, created using a radial upstream magnetic field; while the blue (model) magnetosheath magnetic field components of the right side panels use the actual upstream magnetic field observations. Perhaps different shades of blue (with explanation in the caption) for the left and right panels would also be useful to distinguish these two different runs.

Lines 173-174: ‘…that the flow direction is modeled worser for low cone angles…’ -> ‘…the average deviation between observed and modeled velocities is larger for small cone angle…’

Lines 191-192: ‘There is not trend in delta.’ -> ‘No clear trend in the values of delta with distance d is observed.’

Hide

ED: Publish subject to revisions (further review by editor and referees) (08 Feb 2026) by Christos Katsavrias

AR by Evgeny Romashets on behalf of the Authors (13 Feb 2026) Author's response Manuscript

EF by Katja Gänger (17 Feb 2026) Author's tracked changes

ED: Referee Nomination & Report Request started (17 Feb 2026) by Christos Katsavrias

RR by Anonymous Referee #2 (18 Feb 2026)

RR by Anonymous Referee #1 (18 Feb 2026)

ED: Publish as is (19 Feb 2026) by Christos Katsavrias

AR by Evgeny Romashets on behalf of the Authors (19 Feb 2026) Author's response Manuscript

Journal article(s) based on this preprint

25 Feb 2026

Earth's magnetosheath: a comparison of plasma flow direction between models and observations

Marek Vandas and Evgeny Romashets

Ann. Geophys., 44, 137–148, https://doi.org/10.5194/angeo-44-137-2026,https://doi.org/10.5194/angeo-44-137-2026, 2026

Short summary

Marek Vandas and Evgeny Romashets

Viewed

Total article views: 351 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
244	75	32	351	79	69

HTML: 244
PDF: 75
XML: 32
Total: 351
BibTeX: 79
EndNote: 69

Views and downloads (calculated since 21 Oct 2025)

Month	HTML	PDF	XML	Total
Oct 2025	89	12	6	107
Nov 2025	67	17	11	95
Dec 2025	38	26	11	75
Jan 2026	30	13	2	45
Feb 2026	20	7	2	29

Cumulative views and downloads (calculated since 21 Oct 2025)

Month	HTML	PDF	XML	Total
Oct 2025	89	12	6	107
Nov 2025	67	17	11	95
Dec 2025	38	26	11	75
Jan 2026	30	13	2	45
Feb 2026	20	7	2	29

Viewed (geographical distribution)

Total article views: 346 (including HTML, PDF, and XML) Thereof 346 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 25 Feb 2026

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (695 KB)
Metadata XML

Short summary

Observations of the plasma flow direction in the Earth’s magnetosheath are compared with the help of three analytical magnetic-fieldmodels, namely Kobel and Flückiger (1994), Romashets and Vandas (2019), and Vandas and Romashets (2019), which all assume current-free fields in the magnetosheath. 47 magnetosheath passages by spacecraft are analyzed in detail and performance of the models are evaluated. It is concluded that the performances are comparable and that they are satisfactory on average.


Total:	0
HTML:	0
PDF:	0
XML:	0