the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Foreshock cavitons and spontaneous hot flow anomalies: A statistical study with a global hybrid-Vlasov simulation
- 1Department of Physics, University of Helsinki, Helsinki, Finland
- 2Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico
- 3Finnish Meteorological Institute, Helsinki, Finland
- 1Department of Physics, University of Helsinki, Helsinki, Finland
- 2Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico
- 3Finnish Meteorological Institute, Helsinki, Finland
Abstract. The foreshock located upstream of Earth's bow shock hosts a wide variety of phenomena related to the reflection of solar wind particles from the bow shock and the subsequent formation of ultra-low frequency (ULF) waves. In this work, we investigate foreshock cavitons, which are transient structures resulting from the non-linear evolution of ULF waves, and spontaneous hot flow anomalies (SHFAs), which evolve from cavitons as they accumulate suprathermal ions while being carried to the bow shock by the solar wind. Using the global hybrid-Vlasov simulation model Vlasiator, we have conducted a statistical study in which we track the motion of individual cavitons and SHFAs in order to examine their properties and evolution. In our simulation run where the interplanetary magnetic field (IMF) is directed at a sunward-southward angle of 45 degrees, continuous formation of cavitons is found up to ~ 11 Earth radii (RE) from the bow shock (along the IMF direction), and caviton-to-SHFA evolution takes place within ~ 2 RE from the shock. A third of the cavitons in our run evolve into SHFAs, and we find a comparable amount of SHFAs forming independently near the bow shock. We compare the properties of cavitons and SHFAs to prior spacecraft observations and simulations, finding good agreement. We also investigate the variation of the properties as a function of position in the foreshock, showing that the transients close to the bow shock are associated with larger depletions in the plasma density and magnetic field magnitude, along with larger increases in the plasma temperature and the level of bulk flow deflection. Our measurements of the propagation velocities of cavitons and SHFAs agree with earlier studies, showing that the transients propagate sunward in the solar wind rest frame. We show that SHFAs have a greater solar wind rest frame propagation speed than cavitons, which is related to an increase in the magnetosonic speed near the bow shock.
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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.
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Preprint
(3896 KB)
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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.
Journal article(s) based on this preprint
Vertti Tarvus et al.
Interactive discussion
Status: closed
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RC1: 'Comment on angeo-2020-87', Anonymous Referee #1, 26 Mar 2021
- AC1: 'Reply on RC1', Vertti Tarvus, 03 Jun 2021
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RC2: 'Comment on angeo-2020-87', Anonymous Referee #2, 23 Apr 2021
This manuscript presents statistical results of transient features present in the foreshock of a global hybrid Vlssov simulation, namely the so-called cavitons and Spontaneous Hot Flow Anomalies (SHFAs). Results on formation, propagation, evolution, and properties are presented. The work is very thorough, however, some of the results require further work to be more compelling. If addressed I believe that this work would be suitable for publication.
Major comments
Throughout there is very little explicit comparison of the properties of the transients compared to the foreshock in general, let alone the ambient foreshock at the transient's location. Instead mostly only values in the pristine solar wind are used for comparison. However, understanding how the structures differ from their surroundings is of vial importance and needs to be incorporated into the work throughout. This affects numerous aspects of the work, including:
- Are the choices of properties and thresholds for detection of the transients suitable? How does a 20% decrease in density compare to the variability in density associated with the foreshock ULF wave field? Is plasma beta a sensible parameter to use to distinguish between cavitons and SHFAs (I would have thought a temperature criterion would have been more appropriate) and how does a value of 10 compare to the typical foreshock and its variability?
- In Figure 2, how do the density of suprathermals and temperatures of cavitons and SHFAs compare to typical foreshock conditions? Are the velocities in these structures significantly different from the ambient?
- In Figure 3, are the correlations presented simply extensions of the overall foreshock or do they constitute distinct populations?
These are important considerations in fully understanding the context of the results presented.
I also have concerns over the results surrounding the suprathermal ions. The method employed of distinguishing between core and suprathermals uses the velocity and temperature of the pristine solar wind. This seems unsuitable for transients associated with flow anomalies, as the authors concede on line 200, and thus many of the results are likely micharacterising the solar wind and suprathermal ions in these structures. I would suggest the authors reprocess the data separating out regions in phase space using a distance condition in velocity space (based on the temperature in the pristine solar wind) either from the bulk or peak phase space density.
Related to the above, many conjectures around how the solar wind beam vs. the suprathermals are affecting the moments of the distribution are made, however, no velocity distributions are presented within the manuscript. It is known that the distrbutions within foreshock transients can evolve from multicomponent to single component plasmas, whereas the authors posit only the former.
Finally, the results with relation to the "nose angle" (which may be better described in the manuscript as meridional angle or solar zenith angle throughout) need to be understood in terms of the theta_Bn angle that the transient is magnetically connected to, since this largely controls the physics of the foreshock. This may aid in the interpretation of the results.
Minor and specific points
Lines 20-21: "before it is deflected by the magnetopause" This could do with rewording, since the bow shock also deflects the solar wind and the pressure gradients present throughout the magnetosheath (between bow shock and magnetopause) act to deflect the plasma around the boundary.
Line 23: "far back into the upstream." This is not true for the entire region of the shock connected to the IMF, as the sentence suggests, only in the quasi-parallel case. Please reword this sentence, for example, removing the word "far".
Line 59: "SHFAs evolve" I would say they are "thought to evolve" since this is point requires further evidence in general and the results of the manuscript show it be the case only for some SHFAs.
Line 188: "SHFAs tend to be more depleted than cavitons" This could simply be an effect of the plasma beta condition so needs further comment.
Figure 4: PDFs would be more helpful to readers than CDFs to see the regions where the transients actually form, rather than cumulatively from the bow shock up to some region where a certain proportion form. Some of the cumulative numbers can remain in the text, however.
Table 1: Minimina and maxima of probability distributions are not robust statistics, the 25th and 75th percentile would be more appropriate columns to use. This would also remove potential confusion between the minimum and maximum value for each a particular transient used in the left column, which is appropriate.
Figure 4: The label states these are counts, but they are proportions
- AC2: 'Reply on RC2', Vertti Tarvus, 03 Jun 2021
Peer review completion
Interactive discussion
Status: closed
-
RC1: 'Comment on angeo-2020-87', Anonymous Referee #1, 26 Mar 2021
- AC1: 'Reply on RC1', Vertti Tarvus, 03 Jun 2021
-
RC2: 'Comment on angeo-2020-87', Anonymous Referee #2, 23 Apr 2021
This manuscript presents statistical results of transient features present in the foreshock of a global hybrid Vlssov simulation, namely the so-called cavitons and Spontaneous Hot Flow Anomalies (SHFAs). Results on formation, propagation, evolution, and properties are presented. The work is very thorough, however, some of the results require further work to be more compelling. If addressed I believe that this work would be suitable for publication.
Major comments
Throughout there is very little explicit comparison of the properties of the transients compared to the foreshock in general, let alone the ambient foreshock at the transient's location. Instead mostly only values in the pristine solar wind are used for comparison. However, understanding how the structures differ from their surroundings is of vial importance and needs to be incorporated into the work throughout. This affects numerous aspects of the work, including:
- Are the choices of properties and thresholds for detection of the transients suitable? How does a 20% decrease in density compare to the variability in density associated with the foreshock ULF wave field? Is plasma beta a sensible parameter to use to distinguish between cavitons and SHFAs (I would have thought a temperature criterion would have been more appropriate) and how does a value of 10 compare to the typical foreshock and its variability?
- In Figure 2, how do the density of suprathermals and temperatures of cavitons and SHFAs compare to typical foreshock conditions? Are the velocities in these structures significantly different from the ambient?
- In Figure 3, are the correlations presented simply extensions of the overall foreshock or do they constitute distinct populations?
These are important considerations in fully understanding the context of the results presented.
I also have concerns over the results surrounding the suprathermal ions. The method employed of distinguishing between core and suprathermals uses the velocity and temperature of the pristine solar wind. This seems unsuitable for transients associated with flow anomalies, as the authors concede on line 200, and thus many of the results are likely micharacterising the solar wind and suprathermal ions in these structures. I would suggest the authors reprocess the data separating out regions in phase space using a distance condition in velocity space (based on the temperature in the pristine solar wind) either from the bulk or peak phase space density.
Related to the above, many conjectures around how the solar wind beam vs. the suprathermals are affecting the moments of the distribution are made, however, no velocity distributions are presented within the manuscript. It is known that the distrbutions within foreshock transients can evolve from multicomponent to single component plasmas, whereas the authors posit only the former.
Finally, the results with relation to the "nose angle" (which may be better described in the manuscript as meridional angle or solar zenith angle throughout) need to be understood in terms of the theta_Bn angle that the transient is magnetically connected to, since this largely controls the physics of the foreshock. This may aid in the interpretation of the results.
Minor and specific points
Lines 20-21: "before it is deflected by the magnetopause" This could do with rewording, since the bow shock also deflects the solar wind and the pressure gradients present throughout the magnetosheath (between bow shock and magnetopause) act to deflect the plasma around the boundary.
Line 23: "far back into the upstream." This is not true for the entire region of the shock connected to the IMF, as the sentence suggests, only in the quasi-parallel case. Please reword this sentence, for example, removing the word "far".
Line 59: "SHFAs evolve" I would say they are "thought to evolve" since this is point requires further evidence in general and the results of the manuscript show it be the case only for some SHFAs.
Line 188: "SHFAs tend to be more depleted than cavitons" This could simply be an effect of the plasma beta condition so needs further comment.
Figure 4: PDFs would be more helpful to readers than CDFs to see the regions where the transients actually form, rather than cumulatively from the bow shock up to some region where a certain proportion form. Some of the cumulative numbers can remain in the text, however.
Table 1: Minimina and maxima of probability distributions are not robust statistics, the 25th and 75th percentile would be more appropriate columns to use. This would also remove potential confusion between the minimum and maximum value for each a particular transient used in the left column, which is appropriate.
Figure 4: The label states these are counts, but they are proportions
- AC2: 'Reply on RC2', Vertti Tarvus, 03 Jun 2021
Peer review completion
Journal article(s) based on this preprint
Vertti Tarvus et al.
Model code and software
Vlasiator: hybrid-Vlasov simulation code Minna Palmroth and the Vlasiator team https://doi.org/10.5281/zenodo.3640594
Analysator: python analysis toolkit Markus Battarbee and the Vlasiator team https://doi.org/10.5281/zenodo.4462515
Vertti Tarvus et al.
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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.
- Preprint
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