Nonlinear Phenomena Associated with Large Amplitude Whistler Pulses

In a magnetized laboratory plasma (n w 10'' ~ m ~ , kTe 2 1 eV, Bo > 10 G, 1 m diam x 2.5 m) large amplitude current pulses (150 A, 0.2 ps) are excited in the parameter regime described by Electron MHD (EMHD; wci << w << wee). The currents are transported by low-frequency whistlers forming wave packets with topologies resembling 3D spherical vortices. The generalized vorticity, R = V x (v + eA/me), is shown to be frozen into the electron fluid, dR/dt w V x (v x R). The nonlinearity in v x R is negligible since v and n ( r , t ) are found to be nearly parallel. Thus, large amplitude pulses [R(r,t) < Ro = eBo/me] show the same behavior as small amplitude pulses. However, the associated large currents with vdrift 1 (kTe/me)'12 lead to strong electron heating which can modify the damping of whistlers in collisional plasmas (w u,i cx TF~I'). 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 V x B w kB.