Statistical distribution of mirror-mode-like structures in the magnetosheaths of unmagnetized planets – Part 2: Venus as observed by the Venus Express spacecraft

Volwerk, Martin; Simon Wedlund, Cyril; Mautner, David; Rojas Mata, Sebastián; Stenberg Wieser, Gabriella; Futaana, Yoshifumi; Mazelle, Christian; Rojas-Castillo, Diana; Bertucci, César; Delva, Magda

doi:https://doi.org/10.5194/angeo-41-389-2023

Articles | Volume 41, issue 2

https://doi.org/10.5194/angeo-41-389-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/angeo-41-389-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 41, issue 2

Regular paper

|

10 Oct 2023

Regular paper |

| 10 Oct 2023

Statistical distribution of mirror-mode-like structures in the magnetosheaths of unmagnetized planets – Part 2: Venus as observed by the Venus Express spacecraft

Martin Volwerk, Cyril Simon Wedlund, David Mautner, Sebastián Rojas Mata, Gabriella Stenberg Wieser, Yoshifumi Futaana, Christian Mazelle, Diana Rojas-Castillo, César Bertucci, and Magda Delva

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Final revised paper (published on 10 Oct 2023)
Preprint (discussion started on 26 Jul 2022)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-645', Anonymous Referee #1, 01 Aug 2022

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-645/egusphere-2022-645-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2022-645-RC1
- AC1: 'Reply on RC1', Martin Volwerk, 05 Oct 2022
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-645/egusphere-2022-645-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-645-AC1
RC2:
'Comment on egusphere-2022-645', Anonymous Referee #2, 18 Aug 2022

Review of manuscript:

Statistical distribution of mirror mode-like structures in the magnetosheaths of unmagnetised planets:

2. Venus as observed by the Venus Express spacecraft

submitted to Ann.Geophys. August 2022 by M.Volwerk et al.

General verdict:

This manuscript describes an extended analysis of low frequency wave structures (ULF) observed in the

Venus magnetosheath by the Venus Express magnetometer. The analysis tries to identify structures

of a type which they call 'mirror-mode-like'.

The paper is an extension of a previous study by the same authors (Volwerk et al. 2016) discussing mainly the

occurrence rate of MM-like structure in dependence on solar activity. The main difference

compared to the earlier study is that the data set has been extended and a comparison

with a study on ion temperature observations by Rojas-Mata et al. (2022) is given.

Thus the present paper contains some new material though conclusions are similar to the

previous publication. I have two main points of critique on the method of the paper:

1. Mirror mode waves are generated by ion temperature anisotropies and usually identified by combining

magnetic and ion observations. As shown by previous studies (Song 1994, Ruhunusiri 2015, Fraenz 2017, see below)

most ULF waves in the Earth, Mars and Venus magnetosheaths are of Alfvenic type.

ULF waves without field rotation are largely of fast Alfvenic type

and only a small percentage are of mirror mode type. When using only a magnetic criterion one

would expect that what is called 'mirror-mode-like' in the present study identifies mainly

fast Alfvenic waves. Thus the term 'mirror-mode-like' is rather misleading.

2. Why mirror mode occurrence should depend on solar activity is not clear. The proposed

dependence on pick-up of exospheric ions is rather speculative. If this would rearly

be the case a comparison between the occcurrence in Venus and Earth magnetosheath

should be done. Since at magnetosheath location at Earth pick-up is very small

a clear difference to Venus and Mars should be observed. More important than the pick-up

could be the bow shock normal angle dependenece for the plasma downstream of the shock.

In conclusion I recommend a major revision of the paper where

1. the results are discussed in relation to the more robust studies by (Song 1994, Ruhunusiri 2015, Fraenz 2017)

and the term 'MM-like' should probably be replaced by fast Alfvenic.

2. the physical influence of the pick-up process should be better proven or justified

why it should have major influence only just behind the bow shock.

Minor comments by line number:

32: it should be stressed that the theory of MMs usually only considers the ion temperature

since the instability evolves on ion scales. The role of the electron temperature in this is less clear.

50: it should be mentioned here which processes are relevant in the Venus magnetosheath.

63: maximum

66: completely ignored in this introduction is a series of papers which analysis ULF plasma wave types

using both magnetometer and ion spectrometer data and thus has a superior wave type identification:

Song, Russell, Gary, JGR, 99,6011 (1994): Identification of low-frequency fluctuations in the terrestrial magnetosheath.

Ruhunusiri et al. GRL, DOI: 10.1002/2015GL064968 (2015): Low-frequency waves in the Martian magnetosphere

and their response to upstream solar wind driving conditions

Fraenz et al., PSS, DOI: 10.1016/j.pss.2017.08.011 (2017): Ultra low frequency waves at Venus: Observations

by the Venus Express spacecraft

Specifically the last paper applies the Song-Russell method to VEX magnetometer and ASPERA-4 ion and electron

data to obtain a statistical wave type identification. Here the high temporal resolution (4s) of the electron

spectrometer is used to obtain properties of the ion distribution under the assumption quasi-neutrality.

By this method the authors show that ULF waves of mirror mode type at the dayside of Venus occur only close to the MPB

with a share of about 15% of all ULF wave types. We regard these results as much more robust than the results

presented in the current manusccript.

75: yes, it is doubtful whether ASPERA-4 IMA data alone with 192s resolution and restricted field of view can

be used in this context. Faenz et al. 2017 try to overcome this problem.

85: Simon Wedlund(2022a) discusses that the identification of MM-like structures based on magnetometer data alone

does not make much sense.

100: a bow shock model based on VEX observations was derived by Martinecz et al., JGR, DOI: 10.1029/2008JE003174 (2009)

and later improved by Chai et al., JGR, DOI: 10.1002/2014JA19878 (2014).

113: section reference missing.

Figure.2: variation direction

It would be more interestin to see examples which also show MPB crossings because closer to the MPB usually

more MM structures can be identified.

The conclusion of this section seems to be that the CSW method is more accurate.

142: the definition of VSO is incorrect: Z_VSO points to Venus orbital North, not 'solar North'.

It is not clear why the division into solar min and maximum is made. Physically it would make more sense

to divide into observation behind a quasi-parallel and quasi-perpendicular bow shock.

155: it is confusing to discuss Fig. 7&8 at this point while they appear much later in the paper.

170: since no temperature data are used in this analysis the conclusion is pure speculation. If you compare with

results by Fraenz 2017 it is found that the 'MM-like' structure behind the bow shock are just fast alfvenic waves.

190: also this discussion indicates that a separation of the data set according to upstream bow shock type

would make more sense. The expansion of the exosphere during solar maximum causes mainly more ICWs.

202: is there any conclusion from Figs. 4&5? Otherwise they can also be omitted.

Figure 6: what is the reference for the MPB or ionopause location?

226: it is not clear why pick-up production should be higher just behind the bow shock. The hydrogen exosphere

has an exponential fall in density independent of the bow shock.

274: show

Citation: https://doi.org/10.5194/egusphere-2022-645-RC2
- AC2: 'Reply on RC2', Martin Volwerk, 05 Oct 2022
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-645/egusphere-2022-645-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-645-AC2

Peer review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (25 Oct 2022) by Dalia Buresova

AR by Martin Volwerk on behalf of the Authors (17 Apr 2023) Author's response Author's tracked changes

EF by Lorena Grabowski (19 Apr 2023) Manuscript

ED: Referee Nomination & Report Request started (04 May 2023) by Dalia Buresova

RR by Anonymous Referee #2 (09 May 2023)

RR by Anonymous Referee #3 (07 Aug 2023)

Suggestions for revision or reasons for rejection

Referee report on ‘Statistical Distribution of Mirror Mode-like Structures in the Magnetosheaths of Unmagnetised Planets: 2. Venus as Observed by the Venus Express Spacecraft’ by Volwerk et al.

This study is one of a series of papers that investigates the presence of mirror mode-like structures within the magnetosheath regions of unmagnetized planets. This particular paper examines mirror mode-like structures in the Venusian magnetosheath, using spaceraft observations over several years. This work is also an extension of the authors’ previous published study in the Venus magnetosheath. This work is of interest and may be publishable after significant revisions.

• In general, the paper is too lengthy; primarily because it is quite disorganized. It meanders from case studies to histograms, to long time series, and then to statistical maps. Then more histograms, statistical maps, and case studies. Some analysis is provided, and then more statistical maps. While description of figures are usually provided as the figures are introduced, some important aspects are neglected until much later in the manuscript. Specific comments are provided below.

Specific comments and questions:

• Line 138: Why are these specific eigenvalue ratios used? Is there a rationale and/or reference?

• Line 141: {Bx,By,Bz} What coordinate system is this? VSO? MVA? Something else?

• Line 207: ‘total observational rate is ~50% higher’. It is not clear where this ratio comes from. It’s not obvious that any parameter or ratio of parameters in Table 1 supports this ~50% conclusion.

• Line 211: This is one example of the disorganization of this manuscript. Figs 4-6 are completely skipped, in order to describe Figs 7 and 8.

• Lines 223-225: ‘In a marginal way,…’. This statement is not at all obvious from the Figures; even with a lot of imagination.

• Fig.8 caption: It is said in the caption that the format is the same as that of Fig.6. But it is not the same, because the bow shock boundaries (solid black curve and dashed magenta curve) have been switched.

• Line 267 and Fig.9: This figure is very confusing. It is identical to Fig.6, and the caption does not match the figure.

• Figs.10 and 15: One way to better organize the paper would be to combine Figs. 10 and 15; plotting the histograms on a logarithmic scale and including the exponential fits of Fig.15. The mix/max ratio can then be plotted in a separate panel (and a 10* factor then wouldn’t be needed).

• Line 391: ‘these plots shows’. Which plots and from which paper? Please add a bit more description.

• Lines 413-414: ‘decrease of these numbers from the bow shock away,’. This clause is confusing. Do you mean: ‘decrease of this parameter with increasing distance from the bow shock towards the planet,’?

• Line 430: Regarding the low perentage of events: This obvious and important point should have been brought up when Fig.12 was first described; rather than waiting until this part of the manuscript.

• Line 441: ASPERA-4 data: Is this ion or electron data?

• Lines 442-445: These two sentences are extremely confusing and unclear. Please re-write.

• Line 456-458: This sentence is presented as a Conclusion, but it is actually an initial assumption based on earlier studies (Zhang et al. (2008); Russell et al. (1988))

Additional minor items:

• Line 58: ‘bow shock normal,, with this angle’ -> ‘bow shock normal, with’

• Line 75: maximume -> maximum

• Line 91: ‘exponential for’ -> ‘exponent coefficient between’

• p.4, footnote 1: whichs -> which

• Lines 119, 141: caculated -> calculated

• Line 128: ‘different, but more accurate, from’ -> ‘different from, but more accurate than’

• Line 145: ‘crossings based on’ -> ‘crossings used is based on’

• Fig.2: Spacecraft coordinates are labeled as MSO. Should this be VSO?

• Fig.2 caption, 2nd line: variantion -> variation

• Figs.2,3: Upper titles: The ratio threshold of maximum to intermediate eigenvalues is written as 2.5; but the threshold appears to be 3 in the 4th panels, and is written as 3 on Line 138.

• Line 204: ‘asymmetric division between solar miMiNum and maximum’ -> ‘asymmetric division of the sampling epoch between solar miNiMum and maximum’

• Line 209: ‘periapsis’. Is this the same as ‘pericentre’ in the caption of Fig.3? If yes, then please use consistent nomenclature.

• Line 242: excentricity -> eccentricity

• Line 258: ‘Putting indeed together’ -> ‘Putting together’

• Lines 264: ‘1-second events per day.’ -> ‘events per day.’

• Fig.5 caption: ‘<N> is the median’. This notation is typically used for the mean (average), not median. Also, Line 260 says that this is the average.

• Line 302: ‘flux onto the creation’ -> ‘flux for the creation’

• Line 315-316 (2 places): ‘with 30 deg around’ -> ‘within 30 deg of’

• Line 318: recorden -> recorded

• Fig.12 caption: quasi-perpendiculare -> quasi-perpendicular

• Line 341: ‘(Fig. 14.’ -> ‘(Fig. 14).’

• Line 343: ‘in which’ -> ‘during which’

• Fig.14: Grey and green bars are obscuring the traces. Please rearrange the order so that the traces are plotted on top of the gray and green bars.

• Line 372: ‘that only the generation’ -> ‘that the generation’

• Line 374: ‘depth, not through increased’ -> ‘depth; independent of’

• Line 375: ‘could be a temporal development’ -> ‘could be some temporal development’

• Line 417: ‘mean (red) and standard deviation (green) are overplotted.’. For people who are red-green color-blind, these traces cannot be discriminated. Please choose a better color scheme.

• Line 427: ‘various angles ranges’ -> ‘various angle ranges’

• Lines 434-435: ‘similar as in our present study.’ -> ‘similar to the current study.’

• Line 436: ‘except a few random’ -> ‘except for a few random’

• Line 441: noticed -> noted

• Lines 452-453: ‘This points at an extra necessity for MM-like structures to start to develop, apart from’ -> ‘This also suggests that MM-like structures are more likely to start to develop during solar maximum, separate from the’

• Line 459: ‘lies higher’ -> ‘is higher’

• Line 459: ‘solar minimum even when’ -> ‘solar minimum, after’

• Line 481: ‘What are the extra conditions’ -> ‘Are there additional conditions’

• Lines 482-484: ‘Also, the location of the solar minimum bow shock seems to take a special place, as during solar maximum it is the boundary where the probability, P, strongly increases.’ -> ‘Also, the bow shock appears to be a boundary where the probability, P, strongly increases, independent of the phase of the solar cycle.’

• Line 488: Missing closing parenthesis.

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ED: Publish as is (15 Aug 2023) by Dalia Buresova

AR by Martin Volwerk on behalf of the Authors (18 Aug 2023) Manuscript

Short summary

Freshly created ions in solar wind start gyrating around the interplanetary magnetic field. When they cross the bow shock, they get an extra kick, and this increases the plasma pressure against the magnetic pressure. This leads to the creation of so-called mirror modes, regions where the magnetic field decreases in strength and the plasma density increases. These structures help in exploring how energy is transferred from the ions to the magnetic field and where around Venus this is happening.