the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Statistical distribution of mirror mode-like structures in the magnetosheaths of unmagnetised planets: 2. Venus as observed by the Venus Express spacecraft
Abstract. In this series of papers, we present statistical maps of mirror mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on Extreme Ultraviolet (EUV) solar flux levels (high and low).
The detection of these structures is done through magnetic field-only criteria and ambiguous determinations are checked further. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution facilitating comparisons between planetary and cometary environments.
Applied to the magnetometer data of the Venus Express (VEX) spacecraft from May 2006 to November 2014, we detect structures closely resembling MMs lasting in total more than 93,000s, corresponding to about 0.6 % of VEX's total time spent in the Venus's plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 10 % at most for any given controlling parameter.
In general, MM-like structures appear in two main regions, one behind the shock, the other close to the induced magnetospheric boundary, as expected from theory. For solar maximum, the active region behind the bow shock is further inside the magneosheath, near the solar minimum bow shock location. The ratios of the observations during solar minimum and maximum are slightly dependent on the depth Δ B / B of the structures, deeper structures are more prevalent at solar maximum. A dependence on solar EUV (F10.7) flux is also present, where at higher F10.7 flux the events occur at higher values than the daily average value of the flux. Combining the plasma data from the Ion Mass Analyser with the magnetometer data shows that the instability criterion for MMs is reduced in the two main regions where the structures are measured, whereas it is still enhanced in the region in-between these two regions, implicating that the generation of MMs is transferring energy from the particles to the field.
This study is the second of two on the magnetosheaths of Mars and Venus, and a third paper comparing the results obtained at the two planets will follow.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
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
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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
-
AC1: 'Reply on RC1', Martin Volwerk, 05 Oct 2022
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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: showCitation: 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
-
AC2: 'Reply on RC2', Martin Volwerk, 05 Oct 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
-
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
-
AC1: 'Reply on RC1', Martin Volwerk, 05 Oct 2022
-
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: showCitation: 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
-
AC2: 'Reply on RC2', Martin Volwerk, 05 Oct 2022
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Cited
Cyril Simon Wedlund
David Mautner
Sebastian Rojas Mata
Gabriella Stenberg Wieser
Yoshifumi Futaana
Christian Mazelle
Diana Rojas-Castillo
Cesar Bertucci
Magda Delva
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(9455 KB) - Metadata XML