|The authors should clarify their claim that jets can be spontaneously born in the magnetosheath and reach levels of dynamic pressure larger than the background magnetosheath plasma state . Which physical process would allow for such extreme nonlinear plasma behavior ? Also, the authors conjecture that some experimental observations of jets might be „contaminated” by spontaneously born structures like the ones observed in their simulations, looks too strong. |
The argument put forward to explain why a truncated approach disregarding electron kinetics is valid, is not convincing : „As for electrons, the electron kinetic physics can be neglected for magnetosheath jets mainly because the kinetic pressure of the electrons downstream of the Earth’s bow shock is smaller by about a factor of 10 with respect to the ion pressure. Additionally, the fact that the jet dimensions are between fluid and ion kinetic scales is also an indication that an ion kinetic model should be sufficient. Naturally the electron physics inside jets is important to maintain quasi-neutrality and one will certainly find electron kinetic fluctuations that are relevant to the local thermodynamical properties of the plasma, but that is:a separate problem. Hence, we conclude that the 2D3V approach neglecting kinetic electrons is sufficient to investigate the jet formation and transfer through the magnetosheath.” I understand the discussion on the electron to ion kinetic pressure ratio, however, the environment simulated numerically is a collisionless magnetized plasma where the motion of particles is dominated by the magnetic and electric fields. It was shown that electron dynamics at kinetic scales, i.e. the one where the spatial scales of the order of electron Larmor radius is resolved, play a crucial role for the electric and magnetic configuration (see classical plasma textbooks like, e.g., G. Schmidt, 1966). Obviously, I do not ask the authors to perform global fully kinetic Vlasov simulations, I believe they could use some insight from theory and local 3D fully kinetic particle in cell simulations to put their simulations in the right context.
A note on the normalization used in figure 5: all the physical parameters are normalized to solar wind conditions. As I asked in my previous report, to which exactly values of the solar wind variables ? Insofar simulation results are concerned, this is rather clear. I assume the normalization is performed with respect to the solar wind parameters given in Table 1. But what about normalization of MMS data? I understand the solar wind data are taken from OMNI but which values, at which moment of time ? To be more explicit: there are 6142 jets in MMS data base. I assume one value per jet is included in the histograms shown in the 3rd column of figures 5 and 6. The question is which value of the OMNI solar wind density, velocity, dynamic pressure, magnetic field, temperature is used to normalize each of the MMS data shown in Figure 5 and 6. Is this an OMNI sample extracted at exactly same UT as the MMS sample ? Or the normalizing value is derived from an average over a time interval ? Do the authors consider some time delay between OMNI and MMS data ? This question seems important to me given the variability of solar wind parameters, thus choosing one or another solar wind sample might significantly change the shape of MMS distributions shown in figures 5 and 6.
I have a remark on authors claim: “Figure 5 shows an excellent overall agreement especially between the Vlasiator jets at random times and the MMS jets.” When one looks at the statistical results, the overall agreement is not so excellent. Indeed, Figure 5 shows significant differences between the numerical simulations and MMS histograms/distributions: (1) extend of jets: the standard deviation of MMS observations is one order of magnitude larger than simulations meaning that observed jets span a much broader range of scales; (2) density of jets: the MMS distribution is skewed towards larger (normalized) values while simulations distribution is more Gaussian meaning that MMS observations indicate a preference towards larger (normalized) densities; (3) maximum value of velocity: the skewness of numerical simulations and MMS are significantly different, indicating different probabilities in numerical versus MMS data for different ranges of velocities; (4) maximum of the dynamic pressure: numerical simulations show a flat top distribution while the MMS distribution is skewed with one peak.
I note the authors conjecture: „As we can see from Figures 8 and 9, the jets constitute a region of plasma that has very different properties than the surrounding magnetosheath. It is perhaps like the injection of a bubble of cold air into hotter air, or a low-pressure weather system”. I tend to believe they are right. Nevertheless, this brings back my criticism about neglecting the kinetic physics of electrons. Indeed, earlier theoretical papers and recent local three-dimensional, fully electromagnetic kinetic (particle in cell) simulations of jets (also called plasmoids in some earlier publications) indicate that the edges of the „bubble” are precisely the key sites where electron kinetic physics is important and effective on the dynamics of the jet, regardless the electron to ion pressure ratio.