Stability of the Earth's Dayside Magnetosheath: Effects of Upstream Solar Wind Structures and Downstream Jets

Abstract The constant stream of plasma from the Sun, the solar wind, forms a bow shock as it interacts with Earth's magnetic field. The plasma properties change drastically as it crosses the shock and enter the downstream magnetosheath. This extended region hosts various kinetic ion plasma instabilities, such as mirror modes, ion cyclotron waves, and firehose instabilities, which develop due to local temperature anisotropies induced in the plasma. We use THEMIS satellite measurements from 2008–2023 combined with OMNI solar wind data to present the largest, to date, statistical analysis of temperature anisotropy‐driven plasma instabilities in the dayside magnetosheath. We separate the magnetosheath into quasi‐parallel and quasi‐perpendicular regions and into the region close to the bow shock and close to the magnetopause, revealing fundamentally different plasma stability conditions in each region. The plasma conditions appear mostly stable, in particular in the quasi‐parallel magnetosheath. There are nonetheless substantial populations beyond classical instability thresholds at increased temperature anisotropy, primarily occurring near the magnetopause and downstream of the quasi‐perpendicular shock. Additionally, we show for the first time how impacting upstream solar wind structures alter parameters relevant for plasma stability in Earth's magnetosheath, revealing that coronal mass ejections cause the most extreme departure from nominal distributions. We apply our analysis to dynamic pressure enhancements known as magnetosheath jets. We find the plasma inside jets to be typically more stable to ion plasma instabilities. The plasma stability inside jets however is dependent on the choice of jet criteria, hinting at different jet sub‐types being at play.