Observations of elongated whistler waves in the inertial regime

<jats:p>Experimental observations of a new regime of whistler propagation in the laboratory are reported in this paper. The experiments are carried out in a large laboratory unbound uniform plasma with a density of ne ∼ 109–1011 cm−3 and a magnetic field of 1–20 G. Studies are performed in the electron magnetohydrodynamic regime, which is governed by electron dynamics with Le ≪ L ≪ Li and τci ≫ τ  ≫  τce, where L and τ are the spatial and temporal scale lengths of the perturbations, Le and Li the electron and ion Larmor radii, respectively, and τci and τce the temporal scales corresponding to the ion gyrofrequency and electron gyrofrequency, respectively. The complete topology of the perturbed wave magnetic field is unravelled by mapping it on a two dimensional grid over repeated plasma shots. It is observed that the excited waves are elongated whistlers in the propagation direction. However, in the perpendicular direction, the extent is limited to scale lengths of the order of natural scale length of plasma, i.e., the skin depth (∼c/ωpe), rather than being oblique as predicted by theory and observed in other experiments. The wave shows a feeble resonance cone at an angle of ∼10° but no obliqueness. Also, the secondary induction, which helps the wave to propagate further into the plasma, does not manifest within the spatial scale lengths characteristic to this regime. However, the wave is still able to maintain the whistler speed calculated for the given plasma and pulsed current parameters. It is speculated that the natural scale length (skin depth) and in turn the electron inertia may play a significant role, leading to the observed results. A physical model is also presented that could possibly explain the observed phenomena.</jats:p>