Observation of reflected electrons driven quasi- longitudinal (QL) whistlers in large laboratory plasma

Abstract

This paper reports experimental and theoretical investigations on plasma turbulence in the source plasma of a Large Volume Plasma Device. It is shown that a highly asymmetrical localized thin rectangular slab of strong plasma turbulence is excited by loss cone instability. The position of the slab coincides with the injection line of the primary ionizing energetic electrons. Outside the slab, in the core, the turbulence is weaker by a factor of 30. The plasma turbulence consists of oblique [θ=tan−1(k⊥/k||)≈87°] Quasi-Longitudinal (QL) electromagnetic whistlers in a broad band of 40kHz<f≤80 kHz with k⊥∼1.2 cm−1 and k||∼0.06cm−1. Experimental observations suggest that the primary agent for the turbulence is not driven by primary ionizing energetic electrons but by the loss cone feature in the velocity distribution of reflected energetic electrons. A magnetic mirror is formed in the Electron Energy Filter when it is energized. It is shown that it is this mirror which is responsible for both reflection of the energetic electrons and imposing loss cone feature on it. Theoretical framework is based upon Oblique whistler approximation by Sharma and Vlahos [Astrophys. J. 280, 405 (1984)] and Verkhoglyadova et al. [J. Geophys. Res. 115, A00F19 (2010)] and Quasi Longitudinal (QL) whistlers by Booker and Dyce [Radio Sci. J. Res 69D (1965)] for excitation of the plasma turbulence in the magnetosphere.