the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2023
Proton Plasma Asymmetries between the Convective-Electric-Field Hemispheres of Venus' Dayside Magnetosheath
Abstract. Proton plasma asymmetries with respect to the convective electric field (E) are characterized in Venus’ dayside magnetosheath using measurements taken by an ion mass-energy spectrometer and a magnetometer. Investigating the spatial structure of the magnetosheath plasma in this manner provides insight into the coupling between solar-wind protons and planetary ions. A previously developed methodology for statistically quantifying asymmetries is further developed and applied to an existing database of proton bulk-parameter measurements in the dayside magnetosheath. The density and speed exhibit weak asymmetries favoring the hemisphere in which E points towards the planet, while the magnetic-field strength has a weak asymmetry favoring the opposite hemisphere. The temperatures perpendicular and parallel to the background magnetic field as well as their ratio present no significant asymmetries. Deflection of the solar wind due to momentum exchange with planetary ions is revealed by (1) the O+ Larmor-radius trends of the asymmetries of the bulk-velocity components perpendicular to the upstream solar-wind flow and (2) the E×BIMF -drift trends of the bulk-velocity component along the cross-flow component of the interplanetary magnetic field (BIMF). These interpretations are enabled by comparisons to studies of solar-wind deflection at Mars and comet 67P/Churyumov-Gerasimenko, highlighting the benefits of comparative planetology studies.
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RC1: 'Comment on egusphere-2023-2570', Anonymous Referee #1, 09 Jan 2024
The paper is highly technical and not the easiest to read, probably requiring that the reader read several of the preceding work. This isn’t a red card, this is how science works, but I want to flag that it makes this particular paper quite hard going. This review is written from the point of view of someone familiar with Venus Express and IMA, but not with the previous research in this specific field. Apologies for any misunderstandings, this paper is quite dense.
The introduction of paper would benefit greatly from a cartoon or simple explanatory figure describing the overall geometry of the Venusian magnetosphere, the different hemispheres, the actual regions being studied. While I am sure this is second nature to the authors, the paper requires a developed 3D mental picture of the induced magnetosphere of Venus and I am concerned will be quite difficult to follow for any reader who does not already have this in their mind.
This is a highly technical paper. The brief description of the novel methodology used to calculate the data products used is quite terse and the paper would stand alone a lot better if it could hold the readers hand through the process. All I got from it was that some form of gaussian distribution is being assumed and some sort of curve fitting is going on? It is not very clear. If so, how have the authors ensured that the field of view obscurations of IMA have not skewed results?
Regarding Fig 1, I appreciate that the authors have clarified that the ‘error’ bars are actually first and third quartiles with the central square presumably as the median of the distribution. From this, as a first time reader, what I take away is that all the data are mostly the same for all parameters across all VSE latitudes. No asymmetries appear present in the data. The authors claim an asymmetry in velocity, and I do indeed see how it seems to trend to higher velocities at high negative latitudes.
“The plasma speed appears lower closer to the central parallel”. What is the central parallel? If this is VSE 0o, can you please throw the reader a lifeline and say this? Am I supposed to be looking at both the top and bottom panels, or am I just supposed to be looking at the bottom panels (normalized) with the top as reference? If so, why?
If this is what is being claimed, I have a few concerns with the claim of a trend.
All data points appear to overlap within ‘errors’. How can a trend be statistically claimed? I have exactly the same concern for Fig 6 of Rojas Mata et al. (2023) which this paper relies upon.
In the raw data (panel b) the apparent trend comes mostly from the bottom few data points, beginning at but, as the authors themselves say, “Bins 75° or farther contain much fewer scans (< 25) so these data may have lower statistical reliability.” If I cover these up, then the ‘trend’ is far less clear to my eye.
In the normalized data (panel g), again, covering up the bottom 3 data points where the statistics are dubious, I am also unconvinced that a trend exists.
Please perform a rigorous and appropriate statistical test on the binned data to prove that the trends are real and not just a random pattern from poorer sampling that has been biased by some unknown reason. If the authors have doubt about the statistical veracity of data at 75 degrees and greater, then they could consider cutting off their analysis at some threshold latitude that was better supported by the data and where the statistical sampling is significant enough.
The claim “This indicates that the convective electric field has little influence on average magnetosheath properties, especially compared to the bow shock geometry.” Is similarly very strong. An alternative conclusion could be that there’s simply too much noise in the data to extract any meaningful trends. I recommend further statistical analysis to demonstrate statistically that the distributions are the same, and give the certainty to this.
The conclusion that “Both density and speed are slightly higher in the −E hemisphere, whereas the magnetic-field strength is slightly higher in the +E hemisphere” does not appear to be supported by the data. Density appears constant in both hemispheres (Fig 1a,g). Likewise with Magnetic field strength (Fig c,i, especially if one ignores the data points at high latitudes which the authors have flagged as having poor statistics). I thus cannot agree with this conclusion at present.
The conclusion that “The y and z components of the bulk velocity and their asymmetries exhibit trends with the upstream O+ Larmor radius.” Is in apparent contradiction to the earlier acknowledgment that “IMA’s limited FOV combined with the spacecraft’s orientation sometimes leads to measurements with poor constraints on vz. Reviewing the measurements to correct vz is beyond the scope of this work”. If the data is suspect, can the authors please explain why conclusions are being drawn upon it?
Because of these concerns, I have not dived further into the comparison with other planets, but I would be happy to in a future draft once these foundational questions have been answered.
Minor comments
The caption of Fig 1 should take the opportunity to clarify once again that these are magnetosheath measurements.
How have the authors ensured that data were selected only from the magnetosheath?
Citation: https://doi.org/10.5194/egusphere-2023-2570-RC1 -
AC1: 'Reply on RC1', Sebastián Rojas Mata, 12 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2570/egusphere-2023-2570-AC1-supplement.pdf
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AC1: 'Reply on RC1', Sebastián Rojas Mata, 12 Apr 2024
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RC2: 'Comment on egusphere-2023-2570', Anonymous Referee #2, 13 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2570/egusphere-2023-2570-RC2-supplement.pdf
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AC2: 'Reply on RC2', Sebastián Rojas Mata, 12 Apr 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2570/egusphere-2023-2570-AC2-supplement.pdf
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AC2: 'Reply on RC2', Sebastián Rojas Mata, 12 Apr 2024
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