The Local Bow Shock Environment during Magnetosheath Jet Formation: Results from a Hybrid-Vlasov Simulation

Abstract. Magnetosheath jets are plasma structures that are characterised by enhanced dynamics pressure and/or plasma velocity. A jet generation mechanism that has been widely discussed in observational and numerical studies is steepened Ultra Low Frequency (ULF) waves interacting with the bow shock. However, other formation mechanisms have also been proposed. In Suni et al. (2021), we studied jets in four 2D simulation runs of the global magnetospheric hybrid-Vlasov model Vlasiator and found that 75 % of the jets were associated with regions of enhanced magnetic field and dynamic pressure in the foreshock that we called foreshock compressive structures (FCS). In this study we focus on investigating the jets in the same simulation runs that are not associated with FCSs (non-FCS-jets), and then compare them to FCS-associated jets (FCS-jets). The data set consists of 791 jets in total, of which 562 (71 %) are FCS-jets and 229 (29 %) are non-FCS-jets. We find that the non-FCS-jets can be divided into two categories based on their direction of propagation, either predominantly antisunward or predominantly towards the flanks of the magnetosphere. Using this new categorisation methodology, we compare the plasma and magnetic field properties of flankward-propagating jets, antisunward-propagating jets, and FCS-jets at and around their formation times and locations, using both case studies and statistical analysis. We find that 120 (52 %) of the non-FCS-jets propagate antisunward, are associated with dynamic pressure and magnetic field enhancements in the foreshock, and are very reminiscent of FCS-jets in their properties. Thus 86 % of the jets in the data set are associated with dynamic pressure and magnetic field enhancements in the foreshock. The remaining 109 (48 %) propagate toward the flanks and exhibit higher perpendicular than parallel temperature with respect to the magnetic field, suggesting that they could consist of quasi-perpendicular magnetosheath plasma. We propose that these jets could be associated with local turning of the shock geometry from quasi-parallel to quasi-perpendicular due to bow shock reformation at the oblique shock caused by foreshock ULF wave activity. As antisunward-propagating jets can potentially impact the magnetopause and have effects on the magnetosphere, understanding which foreshock and bow shock phenomena are associated with them and which are not is important.