Direct comparison of whistler mode excitation efficiency between an electric and loop dipole antenna.*

High energy electrons from either solar wind or from human activity may become trapped inside the Van Allen radiation belts or create an artificial radiation belt that can persist for long periods of time. Spacecraft flying through these belts may be susceptible to damage from these trapped electrons. Whistler waves are known to precipitate electrons into the atmosphere, so a proposed solution is using spacecraft to carry compact electron beams or antennas to remediate these trapped electrons. Results of a laboratory plasma experiment comparing the efficiency of exciting whistler waves by an electric dipole and loop antennas are presented here. For the first time, the complex impedance on a loop antenna has been directly measured by measuring the voltage and current directly on the antenna loop. A significant decrease in the real part of the impedance is measured as the plasma density is decreased. Characterization of the radiation impedance will help further understand the coupling efficiency for whistler waves in a magnetized plasma since old and newly proposed models disagree in how the antenna and plasma parameters effect the impedance. This is clear in the order of magnitude difference for their predicted impedance values. The results from this experiment are pertinent to active space-based experiments on artificial whistler wave excitation.