High‐voltage antenna‐plasma interaction in whistler wave transmission: Plasma sheath effects

Abstract

We study the plasma sheath surrounding an antenna that transmits whistler mode waves in the inner magnetosphere in order to investigate the feasibility of conducting controlled experiments on the role of wave‐particle interactions in the pitch angle diffusion of relativistic radiation belt electrons. We propose a model for an electrically short antenna‐sheath‐plasma system with transmission frequencies below the electron characteristic frequencies and much higher than the ion characteristic frequencies. The ion current is neglected. We analytically solve a time‐dependent one‐dimensional situation by neglecting the effects of the wave's magnetic field. In our model, the antenna is charged to a large negative potential during a steady transmission. Positive charge occurs in the sheath and the sheath is free of electrons and conduction current. The net charge on the antenna and in the sheath is zero. The volume, or the radius in a cylindrical case, of the sheath varies in response to the charge/voltage variation on the antenna. The oscillating radius of the sheath translates to a current in the plasma, which radiates waves into the plasma. A whistler wave transmission experiment conducted by the RPI‐IMAGE has shown that the model may describe the most important physical processes occurring in the system. The system response is predominately reactive, showing no evidence for significant sheath current or sheath resistance. The negligibly small sheath conduction electron current can be understood if the antenna is charged to a substantial negative potential, as described by the model. Quantitatively, the model may underestimate the sheath capacitance by about 20%.