Kinetic Generation of Whistler Waves in the Turbulent Magnetosheath
Presentation Type
Talk
Presenter Format
In Person Meeting Talk
Topic
Dayside Science
Start Date
10-5-2022 4:00 PM
Abstract
The Earth’s magnetosheath is governed by numerous physical processes which shape the particle velocity distributions and contribute to the heating of the plasma. Among them are whistler waves which can be excited through cyclotron resonance with electrons if the electron temperature anisotropy is high. In this work, we investigate whistler waves detected by NASA’s Magnetospheric Multiscale (MMS) mission in the turbulent magnetosheath downstream of a quasi-parallel shock. We find that even though statistically whistler waves tend to occur in regions predicted unstable to the whistler temperature anisotropy instability, they are also observed in regions predicted to be stable to wave generation. For such waves the electron pitch-angle distribution often has the so-called butterfly shape (with minima in both the parallel and perpendicular directions) and is located in ion scale magnetic field minima. Using a linear dispersion solver, we show that the butterfly distribution is unstable to the whistler waves observed by MMS. While previous studies have reported this phenomenon in mirror modes and magnetic holes, our findings show that it also occurs on smaller scales (a few ion inertial lengths) in more turbulent environments, such as the quasi-parallel magnetosheath. The result emphasizes the importance of considering complete 3D particle distribution functions when studying plasma wave instabilities.
Kinetic Generation of Whistler Waves in the Turbulent Magnetosheath
The Earth’s magnetosheath is governed by numerous physical processes which shape the particle velocity distributions and contribute to the heating of the plasma. Among them are whistler waves which can be excited through cyclotron resonance with electrons if the electron temperature anisotropy is high. In this work, we investigate whistler waves detected by NASA’s Magnetospheric Multiscale (MMS) mission in the turbulent magnetosheath downstream of a quasi-parallel shock. We find that even though statistically whistler waves tend to occur in regions predicted unstable to the whistler temperature anisotropy instability, they are also observed in regions predicted to be stable to wave generation. For such waves the electron pitch-angle distribution often has the so-called butterfly shape (with minima in both the parallel and perpendicular directions) and is located in ion scale magnetic field minima. Using a linear dispersion solver, we show that the butterfly distribution is unstable to the whistler waves observed by MMS. While previous studies have reported this phenomenon in mirror modes and magnetic holes, our findings show that it also occurs on smaller scales (a few ion inertial lengths) in more turbulent environments, such as the quasi-parallel magnetosheath. The result emphasizes the importance of considering complete 3D particle distribution functions when studying plasma wave instabilities.
