Lower hybrid waves generated in the wake of the Galileo spacecraft

During the first Earth flyby of the Galileo spacecraft, which occurred on December 8, 1990, spin modulated bursts of broadband electrostatic noise were observed with an intensification near the local lower hybrid frequency. These bursts occurred while the spacecraft was passing through the plasmasphere, where both the plasma density and the magnetic field strength are relatively high. By analyzing the spin modulation, which consisted of one burst per spacecraft rotation, it is shown that the waves are generated in the spacecraft wake. As a possible explanation for the observations, it is suggested that plasma density gradients in the wake could produce the waves via the electrostatic lower-hybrid-drift instability (LHDI). Numerical solutions of the dispersion relation show that strong wave growth occurs over a broad frequency range, with a peak growth rate near the lower hybrid resonance frequency. The numerical analysis also demonstrates that the LHDI occurs over a wide range of plasma parameters, including those that are believed to exist in the wake. These results also suggest that lower hybrid waves previously believed by other researchers to be of natural origin may in fact have been generated in spacecraft wakes via the LHDI or other wake-related mechanism.