Speaker
Description
Ambipolar electric fields are often observed in compressed plasma layers throughout the near-Earth space environment and have been shown to be important for plasma flows, wave generation, broadband turbulence, and dissipation mechanisms. When an ambipolar electric field self-consistently forms perpendicular to a background magnetic field, E×B velocity shear is generated. Shear-driven instabilities in the lower hybrid frequency range arise in a shear layer of width less than an ion gyro-diameter in which electrons are magnetized but the ions are effectively unmagnetized due to gyro averaging effects. These are called the electron-ion hybrid (EIH) instability. The EIH instability recently has been shown to play an important role in plasma dynamics occurring in compressed magnetotail current sheets.
The electrostatic EIH instability has been extensively studied in the laboratory in order to document the wave characteristics and understand the effects of electric fields, density gradients, and neutral collisions. New spatially resolved spectral measurements in the Space Physics Simulation Chamber at the Naval Research Laboratory show that electromagnetic EIH waves are broadly distributed in frequency and wavenumber. Azimuthal mode numbers up to m=5 have been resolved, with observations of two bands in the dispersion relation. The lower band has a frequency much less than the electron cyclotron frequency (Ωce) with a positive group velocity and azimuthal and radial components. The higher band has a frequency of approximately 0.7Ωce with a negative group velocity and a purely azimuthal component. The empirical parallel and transverse dispersion relation of the waves is extracted and compared to theoretical calculations. These laboratory experiments are critical to understanding how the EIH instability driven turbulence interacts with the background plasma flow and gradient profiles, and how anomalous dissipation mechanisms (i.e. resistivity and viscosity) arise.
Knowledge of the turbulence characteristics gained from laboratory experiments is being applied to observations of compressed current sheets in the magnetotail to help understand the role of the EIH instability in the current sheet development toward magnetic reconnection. Using NASA’s Magnetospheric Multiscale (MMS) mission data, electrostatic lower hybrid waves are observed to be localized to a region with a strong transverse ambipolar electric field located at the center of a compressed gyro-scale current sheet with a strong guide field in the magnetotail. The waves were found to be driven by electron E×B velocity shear. The terms in the generalized Ohm’s Law are estimated directly from MMS data, and analysis shows that the wave effects (resistivity, viscosity, and diffusion) and pressure anisotropy effects were comparable. It was also found that the quasi-static electric field gradient generates a non-gyrotropic electron distribution function, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. These results stress the importance of the ambipolar electric field in gyro-scale current sheets and in reconnection physics.
This work is supported by the Naval Research Laboratory Base Program.
