Electrodynamic tether currents in the day/night ionosphere: Correlations during the Plasma Motor Generator mission

Ground-based radar observations during NASA's Plasma Motor Generator (PMG) electrodynamic tether mission are reported, which indicate the existence of ion-acoustic or soliton wave packets produced as ionospheric excitations by the orbiting PMG tether system. These appear to be the first such radar measurements of traveling ionospheric disturbances propagating along geomagnetic field lines during an active orbiting ionospheric tether-plasma experiment. The presence of such excitations gives added importance to earlier studies of the plasma radiation modes involved in tether electrodynamics and the part they play in the ionospheric energy balance mechanisms. In addition, tether-current measurements studied during postflight data analysis show a pronounced current decrease when the spacecraft passed from daylight to darkness, which may be related to current limiting. The significance of this observed day/night correlation in conjunction with the soliton-like wave packet radiation modes calls for a detailed investigation of the PMG tether-current measurements in terms of local and global interactions of the PMG with Earth's ionosphere, the initial work for which we show here. This includes an analysis of the radiation impedance due to Alfven and lower hybrid bands, ion-acoustic (soliton) modes, and whistler waves. A decomposition of the measured tether currents into Hall, Pedersen, Cowling, and Birkeland conductivities is presented.