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

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.
