Quasi-longitudinal propagation of nonlinear whistlers with steep electrostatic fluctuations

The quasi-longitudinal whistlers are recently reported in magnetized laboratory plasmas, i.e., at densities considerably higher than the space or magnetospheric plasmas. Given their oblique nature, these whistlers are known to be accompanied by density perturbations which undergo strong nonlinear steepening exclusively for their propagation close to resonant cone angle [Yoon et al., J. Geophys. Res. 119, 1851 (2014)]. This aspect is examined in the parameter regime of laboratory experiments where quasi-longitudinal whistler fluctuations are reported. A systematic study by a set of dedicated single mode numerical solution of the fully nonlinear model of quasi-longitudinally propagating whistlers is presented predominantly covering the high-density (low magnetic field) regime relevant to the laboratory whistler experiments. Following the recovery of existing computational results available for low-density cases, the computations in the newer regime are performed in the present study. The evolution recovered in both these regimes finds the sharp density structures or oscillations to be of resonant origin. While structures accompanying the whistlers' low-density resonant cone readily agree with the upper hybrid resonance frequency, the freshly covered high-density regime shows that the strong nonlinear nature of the whistler is capable of producing a modification in the resonant frequency, causing it to downshift from its linearly expected upper hybrid frequency.