Abstract
We perform 2D local hybrid simulations of collisionless shocks in order to study the properties of simulated magnetosheath jets as a function of shock properties, namely their Alfvénic Mach number (MA) and geometry (angle between the upstream magnetic field and the shock normal, θBN). In total we perform 15 simulations with inflow speeds of Vin = 3.3 CA (Alfvén velocity), 4.5 CA and 5.5 CA and θBN = 15°, 30°, 45°, 50°, and 65°. Under these conditions, the shock MA varied between 4.28 and 7.42. In order to identify magnetosheath jets in the simulation outputs, we use four different criteria, equivalent to those utilized to identify subsets of magnetosheath jets, called high-speed jets (Plaschke and Hietala and Angelopoulos, Ann. Geophys., 2013, 31, 1877–1889), transient flux enhancements (Němeček et al., Geophys. Res. Lett., 1998, 25, 1273–1276), density plasmoids (Karlsson et al., J. Geophys. Res., 2012, 117, a–n; Karlsson et al., J. Geophys. Res., 2015, 120, 7390–7403) and high-speed plasmoids (Gunell et al., Ann. Geophys., 2014, 32, 991–1009). In our simulations, the density plasmoids were produced only by shocks with MA ≥5.7, while the high-speed plasmoids only formed downstream of shocks with MA ≥6.97. We show that higher MA shocks tend to produce faster jets that tend to have larger surface area, mass, linear momentum and kinetic energy, while these quantities tend to be anticorrelated with θBN. In general, the increase of θBN to up to 45° results in increased jet formation rates. In the case of high-speed jets in runs with Vin = 3.3 CA and high-speed plasmoids, the jet formation anticorrelates with θBN. The jet production all but ceases for θBN = 65° regardless of the shock’s MA. The maximum distances of the magnetosheath jets from the shocks were ≲140 di (upstream ion intertial lengths), which, estimating 1 di ∼100–150 km at Earth, corresponds to 2.4–3.3 Earth radii. Thus, none of the simulated jets reached distances equivalent to the average extension of the Earth’s subsolar magnetosheath, which would make them the equivalents of geoeffective jets. Higher MA shocks are probably needed in order to produce such jets.
Highlights
IntroductionMagnetosheath jets (see Plaschke et al, 2018, and the references therein) is an umbrella term for different kinds of phenomena discovered in the Earth’s magnetosheath (Lucek et al, 2005) that produce localized increases of the solar wind (SW) dynamic pressure (Pdyn)
Magnetosheath jets is an umbrella term for different kinds of phenomena discovered in the Earth’s magnetosheath (Lucek et al, 2005) that produce localized increases of the solar wind (SW) dynamic pressure (Pdyn)
3.1.2 Transient Flux Enhancements We show the properties of transient flux enhancements (TFEs) in the Figure 4
Summary
Magnetosheath jets (see Plaschke et al, 2018, and the references therein) is an umbrella term for different kinds of phenomena discovered in the Earth’s magnetosheath (Lucek et al, 2005) that produce localized increases of the solar wind (SW) dynamic pressure (Pdyn) They were first reported by Němeček et al (1998) as increases of ion flux (product of ion density and velocity) located downstream of the quasi-parallel bow-shock of Earth. Plasmoids were classified as diamagnetic (paramagnetic), if the magnetic field magnitude inside them diminished (increased) and embedded (fast) if the velocity inside them remained the same (increased) compared to the ambient values It was proposed by Karlsson et al (2015) that diamagnetic plasmoids may arrive from the SW and cross into the magnetosheath, while the paramagnetic plasmoids are produced at the bow-shock. In their local hybrid simulations of collisionless shocks, Preisser et al (2020a) found that plasmoids may form due to magnetic reconnection just behind the quasiparallel shocks
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