Statistical Study of Magnetosheath Jet‐Driven Bow Waves

Abstract

When a magnetosheath jet (localized dynamic pressure enhancements) compresses ambient magnetosheath at a (relative) speed faster than the local magnetosonic speed, a bow wave or shock can form ahead of the jet. Such bow waves or shocks were recently observed to accelerate particles, thus contributing to magnetosheath heating and particle acceleration in the extended environment of Earth's bow shock. To further understand the characteristics of jet‐driven bow waves, we perform a statistical study to examine which solar wind conditions favor their formation and whether it is common for them to accelerate particles. We identified 364 out of 2,859 (~13%) magnetosheath jets to have a bow wave or shock ahead of them with Mach number typically larger than 1.1. We show that large solar wind plasma beta, weak interplanetary magnetic field (IMF) strength, large solar wind Alfvén Mach number, and strong solar wind dynamic pressure present favorable conditions for their formation. We also show that magnetosheath jets with bow waves or shocks are more frequently associated with higher maximum ion and electron energies than those without them, confirming that it is common for these structures to accelerate particles. In particular, magnetosheath jets with bow waves have electron energy flux enhanced on average by a factor of 2 compared to both those without bow waves and the ambient magnetosheath. Our study implies that magnetosheath jets can contribute to shock acceleration of particles especially for high Mach number shocks. Therefore, shock models should be generalized to include magnetosheath jets and concomitant particle acceleration.

Electron Acceleration by Magnetosheath Jet‐Driven Bow Waves

“…In the accompanying paper, Liu et al (2020) employ a statistical study showing that magnetosheath jets that have a bow wave have a higher probability to exhibit higher electron energy than those without a bow wave.…”

“…Liu, Hietala, et al (2019) searched the event list reported by Plaschke et al (2013) for magnetosheath jets and found 364 events (out of 2,859) that have a bow wave or shock-like structure at their leading edge. The detailed selection criteria can be found in the accompanying paper (Liu et al, 2020). We selected three representative events that have electron energy flux enhancements associated with the bow wave for case studies.…”

“…Therefore, 2V/cosθ Bn is typically around several thousand km/s (e.g., 8,300 km/s in Event 3). Based on statistical study by Liu et al (2020), we estimate that the encounter rate of bow waves by electrons ranges from at least~0.05 to 0.5 per hour depending on the solar wind conditions. If we assume that each bow wave can exist and accelerate electrons for 1-2 min, the average velocity increase gained by electrons is~50% × 1.5 min × (0.05 to 0.5) hr −1 × thousands of km/s to several to tens of km/s (e.g., 5-50 km/s in Event 3).…”

“…Each time electrons enter the magnetosheath, jet-driven bow waves could provide additional several to tens of km/s on average. Under favorable solar wind conditions, such as high solar wind Alfvén Mach number (Liu et al, 2020), jet-driven bow waves could result in first-order modification (10%) to the diffusive shock acceleration model. Upstream of shocks in the foreshock, foreshock transients can also drive secondary shocks and accelerate particles (e.g., Liu et al, 2016Liu et al, , 2017.…”

“…In this study, we apply case studies using multipoint Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations to investigate how electrons interact with jet-driven bow waves. In the accompanying paper, Liu et al (2020) present a statistical study to further confirm particle acceleration by jet-driven bow waves.…”