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 hybr
id frequency. These bursts occurred while the spacecraft was passing t
hrough the plasmasphere, where both the plasma density and the magneti
c field strength are relatively high. By analyzing the spin modulation
, which consisted of one burst per spacecraft rotation, it is shown th
at the waves are generated in the spacecraft wake. As a possible expla
nation for the observations, it is suggested that plasma density gradi
ents in the wake could produce the waves via the electrostatic lower-h
ybrid-drift instability (LHDI). Numerical solutions of the dispersion
relation show that strong wave growth occurs over a broad frequency ra
nge, 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. (C) 1997 Elsevier Science Ltd.