Abstract
Abstract. We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density. For two approximately hour-long intervals in November, 2015 we found six jets, which are of two distinct types. (a) Two of the jets are associated with the magnetic field discontinuities at the boundary between the quasi-parallel and quasi-perpendicular magnetosheath. Straddling the boundary, the leading part of these jets contains an ion population similar to the quasi-parallel magnetosheath, while the trailing part contains ion populations similar to the quasi-perpendicular magnetosheath. Both populations are, however, cooler than the surrounding ion populations. These two jets also have clear increases in plasma density and magnetic field strength, correlated with a velocity increase. (b) Three of the jets are found embedded within the quasi-parallel magnetosheath. They contain ion populations similar to the surrounding quasi-parallel magnetosheath, but with a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the magnetosphere, and contains ions similar to the jets inside the quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets are associated with emissions of whistler, lower hybrid, and broadband electrostatic waves, as well as approximately 10 s period electromagnetic waves with a compressional component. The latter have a Poynting flux of up to 40 µW m−2 and may be energetically important for the evolution of the jets, depending on the wave excitation mechanism. Only one of the jets is likely to have modified the surrounding magnetic field into a stretched configuration, as has recently been reported in other studies. None of the jets are associated with clear signatures of either magnetic or thermal pressure gradient forces acting on them. The different properties of the two types also point to different generation mechanisms, which are discussed here. Their different properties and origins suggest that the two types of jets need to be separated in future statistical and simulation studies. Keywords. Magnetospheric physics (magnetosheath; plasma waves and instabilities; solar wind–magnetosphere interactions)
Highlights
Small-scale, transient increases in magnetosheath dynamic pressure have recently attracted increased attention (e.g. Nemecek et al, 1998; Savin et al, 2008; Hietala et al, 2009; Archer et al, 2012; Plaschke et al, 2013)
We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density
We have used MMS data to carry out a detailed analysis of the large- and meso-scale properties of magnetosheath jets with a modern suite of plasma instruments, based on the current understanding of such jets
Summary
Small-scale, transient increases in magnetosheath dynamic pressure have recently attracted increased attention (e.g. Nemecek et al, 1998; Savin et al, 2008; Hietala et al, 2009; Archer et al, 2012; Plaschke et al, 2013). Magnetosheath jets are most commonly observed downstream of the quasi-parallel bow shock, and the upstream IMF is steady for a large majority of the jets (Archer and Horbury, 2013; Plaschke et al, 2013). Based on the observation that some jets are associated with IMF discontinuities, Archer et al (2012) suggested that the interaction of solar wind discontinuities with the bow shock was responsible for the jet formation. The second main theory was suggested by Hietala et al (2009) and Hietala and Plaschke (2013) and was based on the observation that the majority of jets occur behind the quasiparallel bow shock They suggested that jets can form as the solar wind interacts with the rippled bow shock associated with the quasi-parallel configuration.
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