Nonlinear Phenomena Associated with Large Amplitude Whistler Pulses

Abstract

In a magnetized laboratory plasma (n 10 11 cm -3 , kT e ≥ 1 eV, B 0 ≥ 10 G, 1 m diam x 2.5 m) large amplitude current pulses (150 A, 0.2 μs) are excited in the parameter regime described by Electron MHD (EMHD ; ω ci << ω << ω ce ). The currents are transported by low-frequency whistlers forming wave packets with topologies resembling 3D spherical vortices. The generalized vorticity, Ω = ⊇ x (v + eA/m e ), is shown to be frozen into the electron fluid, ∂Ω/∂t ⊇ x (v x Ω). The nonlinearity in v x Ω is negligible since v and Ω(r,t) are found to be nearly parallel. Thus, large amplitude pulses [Ω(r, t) ≤ Ω 0 = eB 0 /m e ] show the same behavior as small amplitude pulses. However, the associated large currents with v drift ≥ (kT e /me) 1/2 lead to strong electron heating which can modify the damping of whistlers in collisional plasmas (ω ∼ v ei T e -3/2 ). Observations show that a heated flux tube provides a filament of high Spitzer conductivity which permits a nearly collisionless propagation of whistler pulses. This filamentation effect is not associated with density modifications as in modulational instabilities but arises from conductivity modifications. The heated flux tube also generates a quasi-dc magnetic field driven by thermoelectric currents. These assume self-consistently a Taylor state with ⊇ x B kB.