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

Lucile Turc,Yann Pfau-Kempf,Urs Ganse,Maxime Grandin,Minna Palmroth,Vertti Tarvus,Andrew Dimmock,Andreas Johlander,Maxime Dubart,Markus Battarbee

doi:10.5194/egusphere-egu2020-9211

Abstract

&lt;p&gt;The magnetosheath is the region bounded by the bow shock and the magnetopause which is home to shocked solar wind plasma. At the interface between the solar wind and the magnetosphere, the magnetosheath plays a key role in the coupling between these two media. Previous works have revealed pronounced dawn-dusk asymmetries in the magnetosheath properties, with for example the magnetic field strength and flow velocity being larger on the dusk side, while the plasma is denser, hotter and more turbulent on the dawn side. 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 that have been processed to obtain the 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 due to the different processing methods. We investigate how the asymmetries change when the IMF becomes more radial and when the Alfv&amp;#233;n Mach number decreases. When the IMF makes a 30&amp;#176; angle with the Sun-Earth line instead of 45&amp;#176;, we find a stronger magnetic field asymmetry and a larger variability of the magnetosheath density. In contrast, a lower Alfv&amp;#233;n Mach number leads to a decrease of the magnetic field asymmetry level and of the variability of the magnetosheath density and velocity, likely due to weaker foreshock processes.&lt;/p&gt;

Highlights

The interaction of the supermagnetosonic solar wind with the Earth’s magnetosphere forms a standing bow shock which decelerates the incoming flow to submagnetosonic speeds in front of the obstacle
The colour scheme is chosen to highlight the areas of the magnetosheath where the normalised magnetic field strength is above or below 4, which is the upper limit for the magnetic field compression at the bow shock crossing according to the Rankine– Hugoniot jump conditions (Treumann, 2009)
The simulations enable us to investigate the asymmetry levels at low Alfvén Mach numbers (MA ∼ 3.5, Run 2B), while the statistical data set compiled from THEMIS measurements does not contain enough data points at such low MA to derive the asymmetry of the magnetosheath parameters

Summary

Introduction

The interaction of the supermagnetosonic solar wind with the Earth’s magnetosphere forms a standing bow shock which decelerates the incoming flow to submagnetosonic speeds in front of the obstacle. Since the early gasdynamic model of Spreiter et al (1966), the magnetosheath has been subject to intensive scrutiny (e.g. Petrinec et al, 1997; Paularena et al, 2001; Longmore et al, 2005; Lucek et al, 2005; Dimmock and Nykyri, 2013; Lavraud et al, 2013; Dimmock et al, 2017) These studies revealed that the magnetosheath properties display significant spatial variations, as a function of the distance from the boundaries, with, for example, the formation of the plasma depletion layer near the magnetopause during northward interplanetary magnetic field (IMF) conditions (e.g. Zwan and Wolf, 1976; Wang et al, 2004), and as a function of the distance from the Sun–Earth line, with pronounced dawn–dusk asymmetries (see the reviews by Walsh et al, 2014; Dimmock et al, 2017, and references therein). We concentrate on the impact of the bow shock properties on the large-scale distribution of the magnetosheath properties

Methods

Results

Discussion

Conclusion