Decay of space plasma turbulence downstream of bow shocks: MMS magnetosheath unbiased campaign observations

Abstract

Turbulence properties in the solar wind change with increasing heliospheric distance. Observations from different missions have shown how spectral indices converge towards a $f^{-5/3}$ spectrum in the inertial range as the solar wind reaches one astronomical unit, and how the cascade rate decays as the plasma moves away from the Sun. We evaluate whether these properties display similar trends in a smaller astrophysical system, that of a planetary magnetosheath. Using data from the 2023 Magnetospheric Multiscale (MMS) unbiased magnetosheath campaign, we determine the evolution of the spectral indices and the cascade rate along 15 inbound magnetosheath crossings. WIND observations permitted to monitor the state of the solar wind during the campaign and allowed to study the impact of the upstream plasma conditions on the evolution of the cascade rate downstream the bow shock. Our results show an increase of the cascade rate between two to three orders of magnitude after the plasma crosses the bow shock into the magnetosheath. Once in the magnetosheath, turbulence develops with increasing distance to the bow shock in a way similar to solar wind observations, despite the smaller size of the system. Incompressible Hall-MHD scaling-law estimates of the cascade rate show a clear power-law decay of the cascade rate in the magnetosheath, particularly for the cases with upstream slow wind. This suggests that the way energy is injected into the system plays a role in the decay of turbulence as the plasma moves away from the bow shock.