High-speed jets downstream of the Earth’s bow shock

Abstract

The solar wind is decelerated at the Earth’s bow shock, after which it flows around the magnetosphere in the magnetosheath. Amidst the shocked plasma, faster flows can be often observed. These dynamic pressure enhancements, called magnetosheath jets, form as part of shock dynamics, and some of them can ultimately collide into the boundary of the magnetosphere, launching disturbances inside the magnetosphere. In this thesis, we study various aspects of magnetosheath jets to improve our under- standing of their contribution to solar wind–magnetosphere interaction.

We use subsolar magnetosheath observations of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data to study where and under which solar wind conditions jets occur. The orientation of the interplanetary magnetic field (IMF) strongly controls jet occurrence and we find that jets are nine times more common downstream of the quasi-parallel shock than the quasi-perpendicular shock. Further analysis reveals that low IMF magnitude, high Alfv´enic Mach number, and high plasma beta increase their occurrence in the quasi-perpendicular regime. We create a statistical model of jet occurrence to reconstruct their yearly occurrence throughout solar cycles 23 and 24. Our results suggest that there is no substantial variation across the solar cycle.

We also investigate acceleration of electrons at bow waves/shocks ahead of the fastest jets. We use a one-dimensional Monte Carlo model, where electrons are energized by shock drift acceleration at the jet-driven bow wave/shock amplified by a collapsing trap that forms between the bow wave and the magnetopause. We find that the simulated energy flux increases of suprathermal electrons are comparable to previous observational results, indicating that such a process could explain the observed energization. Transient structures both upstream and downstream of the Earth’s bow shock contribute to the total acceleration in the system.

Finally, we study whether jets can be expected to alter conditions for magnetopause reconnection. Our analysis of THEMIS data shows that magnetic field is more variable inside jets than typically in the magnetosheath. Most jets contain periods of polarity opposite to the prevailing IMF, suggesting that jets may potentially modulate local conditions for reconnection. Magnetopause reconnection is a significant process in solar wind–magnetosphere interaction and our results motivate future studies where direct and indirect signatures of jets influencing reconnection should be investigated. Their total contribution on the system must arise from their numbers.