Asymmetries in the Earth's dayside magnetosheath: results from global hybrid-Vlasov simulations

Abstract

Abstract. Bounded by the bow shock and the magnetopause, the magnetosheath forms the interface between solar wind and magnetospheric plasmas and regulates solar wind–magnetosphere coupling. Previous works have revealed pronounced dawn–dusk asymmetries in the magnetosheath properties. The dependence of these asymmetries on the upstream parameters remains however largely unknown. One of the main sources of these asymmetries is the bow shock configuration, which is typically quasi-parallel on the dawn side and quasi-perpendicular on the dusk side of the terrestrial magnetosheath because of the Parker spiral orientation of the interplanetary magnetic field (IMF) at Earth. Most of these previous studies rely on collections of spacecraft measurements associated with a wide range of upstream conditions which are processed in order to obtain average values of the magnetosheath parameters. In this work, we use a different approach and quantify the magnetosheath asymmetries in global hybrid-Vlasov simulations performed with the Vlasiator model. We concentrate on three parameters: the magnetic field strength, the plasma density, and the flow velocity. We find that the Vlasiator model reproduces the polarity of the asymmetries accurately but that their level tends to be higher than in spacecraft measurements, probably because the magnetosheath parameters are obtained from a single set of upstream conditions in the simulation, making the asymmetries more prominent. A set of three runs with different upstream conditions allows us to investigate for the first time how the asymmetries change when the angle between the IMF and the Sun–Earth line is reduced and when the Alfvén Mach number decreases. We find that a more radial IMF results in a stronger magnetic field asymmetry and a larger variability of the magnetosheath density. In contrast, a lower Alfvén Mach number leads to a reduced magnetic field asymmetry and a decrease in the variability of the magnetosheath density, the latter likely due to weaker foreshock processes. Our results highlight the strong impact of the quasi-parallel shock and its associated foreshock on global magnetosheath properties, in particular on the magnetosheath density, which is extremely sensitive to transient quasi-parallel shock processes, even with the perfectly steady upstream conditions in our simulations. This could explain the large variability of the density asymmetry levels obtained from spacecraft measurements in previous studies.

Ion Acceleration Efficiency at the Earth’s Bow Shock: Observations and Simulation Results

“…The simulation plane, which corresponds to the X−Z plane in GSE coordinates, is different from the MMS orbit, which is mainly in the X−Y plane. However, in the context of the bow shock this simulation setup corresponds to a Parker spiral like setup, see (Turc et al 2020). The solar wind in the simulation consists of a purely proton plasma and has a flow speed of 750 km/s in the −X direction with a plasma density of 1 cm −3 .…”

Proton Plasma Asymmetries Between Venus' Quasi‐Perpendicular and Quasi‐Parallel Magnetosheaths

“…At Earth, the upstream IMF's orientation and its role in processes captured by MHD models provide partial insight into the physics (Dimmock & Nykyri, 2013; Dimmock, Nykyri, et al., 2015; Haaland et al., 2017; Longmore et al., 2005; Němeček et al., 2002; B. M. Walsh et al., 2012). However, persisting discrepancies with MHD and hybrid simulations suggest IMF orientation may not even be a controlling factor at all (Dimmock, Nykyri, et al., 2015; Turc et al., 2020; B. M. Walsh et al., 2012). Thus, boundary phenomena at the magnetopause (e.g., subsolar magnetic reconnection or flux transfer events) and/or kinetic phenomena in the magnetosheath (e.g., instabilities and turbulence) must also be involved (Dimmock et al., 2017; Dimmock, Nykyri, et al., 2015, and references therein).…”

Asymmetric Interaction of a Solar Wind Reconnecting Current Sheet and Its Magnetic Hole With Earth's Bow Shock and Magnetopause

“…Backstreaming foreshock ions travel far distances upstream of the shock along the magnetic field line and perturb the solar wind. As such, the upstream structure arrives at and crosses the quasi‐perpendicular side of the bow shock before the quasi‐parallel side (Turc et al., 2020). This asymmetric interaction across the bow shock will inevitably transfer downstream and create asymmetric interaction zones at the magnetopause boundary (Keika et al., 2009; Webster et al., 2021).…”