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 accurately the polarity of the asymmetries, 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. We investigate how the asymmetries change when the angle between the IMF and the Sun-Earth line is reduced and when the Alfven 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 Alfven Mach number leads to a reduced magnetic field asymmetry and a decrease in the variability of the magnetosheath density and velocity, the latter likely due to weaker foreshock processes. Our results highlight the strong impact of the foreshock on global magnetosheath properties, in particular on the magnetosheath density, which is extremely sensitive to transient foreshock processes.

The impact of Radial and Non-Radial IMF on the Earth's Magnetopause Size, Shape, and Dawn-Dusk Asymmetry from Global 3D Kinetic Simulations.

“…11, and 12, IMF, density, temperature and ion velocity are tested at the the dusk-dawn flanks asymmetry in the magnetosheath. In the magnetosheath (left to the vertical blue dashed line) all plasma parameters are asymmetric which is consistent with recent results obtained by Turc et al (2020). Furthermore, for both IMF orientations, the magnetopause is more extended on the duskside (red dashed line) than on the dawnside (green dashed line).…”

The impact of Radial and Non-Radial IMF on the Earth's Magnetopause Size, Shape, and Dawn-Dusk Asymmetry from Global 3D Kinetic Simulations.

“…hybrid, Hall-less and Hall-MHD simulations have been tested in one study by for the analysis of MR regimes with the conclusion that dawn-dusk asymmetry is obtained and should be related to ions flow. Recently, Turc et al (2020) used the hybrid -Vlasiator 2D-3V code to study asymmetries in the Earth magnetosheath for different IMF conditions. For reference, the Vlasiator code provides a kinetic description of ions, solving directly the Vlasov equation for the particle distribution function in 2D-3D space, but assumes a fluid description for electrons (e.g., Palmroth et al, 2018).…”

“…Uncovering its origin is important for better understanding, modeling and prediction of space weather phenomena (e. g., S. Lu et al, 2016). Asymmetry is observed by Cluster spacecraft in north-south magnetotail planes (Haaland et al, 2017;Samsonov, 2006) and in dawn-dusk planes (A. P. Walsh et al, 2014;Samsonov, 2011;Dimmock et al, 2017;Turc et al, 2020).…”

“…Paularena et al (2001) reported the same kind of asymmetry in different regions in the dayside magnetosphere in the Sun-Earth and the Dusk-Dawn plane. In their recent study,Turc et al (2020) discussed the magnetosheath asymmetry in terms of IMF, solar wind -29-…”

The Impact of Radial and Non‐Radial IMF on the Earth's Magnetopause Size, Shape, and Dawn‐Dusk Asymmetry From Global 3D Kinetic Simulations

“…Three other models i.e., hybrid, Hall-less, and Hall-MHD simulations have been tested in one study by Karimabadi et al (2004) for the analysis of MR regimes with the conclusion that dawn-dusk asymmetry is obtained and should be related to ions flow. Recently, Turc et al (2020) used the hybrid -Vlasiator 2D-3V code to study asymmetries in the Earth magnetosheath for different IMF conditions. For reference, the Vlasiator code provides a kinetic description of ions, solving directly the Vlasov equation for the particle distribution function in 2D-3D space, but assumes a fluid description for electrons (e.g., Palmroth et al, 2018).…”