1. What is Vlasiator simulation?
Vlasiator is a global magnetospheric, high-performance hybrid-Vlasov simulation modeling protons as velocity distribution functions and electrons as a massless charge-neutralizing fluid. It evolves proton distribution functions according to the Vlasov equation and electromagnetic fields according to Maxwell's equations and Ohm's law, including the Hall term. Vlasiator is intrinsically 6-dimensional, with 3 position space dimensions and 3 velocity space dimensions. It simulates the Geocentric Solar Ecliptic xy-plane, capturing the solar wind, foreshock, dayside magnetosheath, and magnetosphere, and partially the nightside. The simulation uses periodic boundaries in the out-of-plane directions, homogeneous Neumann conditions for the +-y and -x boundaries, and a perfect conductor at a radius of 5 R E from the origin as the inner boundary. The high solar wind velocity was chosen to facilitate quick development of the bow shock and magnetosheath in the simulation. The Alfven Mach number, an important parameter for realistic evolution of the plasma environment near Earth's bow shock, is within the normal range of observations at Earth in all the runs.
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2. How are magnetosheath jets identified and tracked in the study?
In the study, magnetosheath jets are identified and tracked using methods developed by Palmroth et al. (2021) and Suni et al. (2021). The search box is chosen to focus on the subsolar magnetosheath in each run, and the tracking duration is limited by the simulation duration of each run, starting at 290 seconds. Jets are defined as regions in the magnetosheath where the instantaneous dynamic pressure is at least twice the 3-minute moving time average of the dynamic pressure. The regions fulfilling this criterion are delineated with green contours in Fig. 1a. The parameters of the different simulation runs used in the study are provided in Table 1. Non-FCS-jets and FCS-jets are marked with black and red dots, respectively. The jets that form at the bow shock are categorized into FCS-jets and non-FCS-jets based on their contact with FCSs. The regions fulfilling the FCS criteria are delineated with grey contours in Fig. 1b. The tracking of jets is limited by the simulation duration, and jets that are identified at only one time step are discarded. The magnetosheath is defined using the temperature criterion, and the x-directional Mach number is used as a proxy for the bow shock location. The density boundary is not used for this purpose due to its fluctuations at the quasi-parallel shock. The study focuses on the subregion of the dayside magnetosheath for jet identification and tracking.
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3. How are non-FCS-jets classified based on their directions of propagation in the spacecraft frame v SC?
Non-FCS-jets are classified based on their directions of propagation in the spacecraft frame v SC. Jets whose propagation velocity vector is within 45 degrees from the antisunward (-x) direction are classified as 'antisunward jets'. The remaining jets are classified as 'flankward jets'. In cases where the maximum cross-correlation of the dynamic pressure time series between any VSC pair is less than 0.8, the classification is based on v tr instead. After categorization, the formation sites and times of the jets are visually inspected, and jets not connected with the bow shock or part of the same structure as any previously identified jet are discarded. This classification helps in understanding the propagation characteristics of non-FCS-jets in the magnetosheath.
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4. What are the differences in plasma and magnetic field properties for flankward and antisunward jets?
The differences in plasma and magnetic field properties for flankward and antisunward jets can be observed in their formation sites, surrounding conditions, and jet propagation velocities. Flankward jets form at the bow shock several R E duskward of the subsolar point, while antisunward jets form downstream of the deep ULF foreshock. The magnetic field upstream and downstream of the jets exhibit different characteristics, with flankward jets showing a magnetosonic Mach boundary sunward of the other two bow shock boundaries, and antisunward jets having a magnetosonic Mach boundary earthward of the other two. The jet propagation velocities for both types of jets are super-Alfvenic but submagnetosonic. Additionally, the formation of flankward jets is associated with a large increase in plasma density and deflection of plasma flow from v x-dominated to v yz-dominated, while antisunward jets show an increase in density and magnetic field strength, with the B yz component contributing more than B x. The timing analysis also reveals differences in the ambient bulk velocity and temperature anisotropy between the two types of jets.
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