Whistler wave propagation and interplay between electron inertia and Larmor radius effects

Abstract

The influence of Larmor radius effects on the propagation of whistler waves is investigated experimentally in laboratory plasma. The waves are excited using a loop antenna of diameter less than the electron skin depth, the natural scale length in this regime. In an earlier experiment [G. Joshi et al., Phys. Plasmas 24, 122110 (2017)], it was shown that such waves assume an elongated shape with perpendicular dimensions of the order of skin depth. In the present work, we show that wave propagation is significantly modified when the external guiding magnetic field is decreased. The wave spreads in the perpendicular direction in spite of starting of as an elongated whistler due to electron inertia effects. In the near region, the antenna field becomes dominant even forming null points, with the physical processes taking shape and wave still being guided by the net background magnetic field. However, the feeble external magnetic field in the region away from the antenna is unable to guide the wave any further and the wave spreads. In spite of a large current pulse, the wave remains linear (ΔB/B0 ≤ 1). The observed results are attributed to the interplay between electron inertia and finite Larmor radius effects and are explained in terms of a modified physical model.