AN ASSESSMENT OF LIGHTNING OR INSITU INSTABILITIES AS A SOURCE FOR WHISTLER MODE WAVES IN THE NIGHT IONOSPHERE OF VENUS

Burst rate and polarization studies have shown that at least half of the impulsive 100-Hz signals detected at low altitudes in the night ionosphere of Venus by the Pioneer Venus Orbiter electric field detector are whistler mode waves. At the Earth, whistler mode waves are generated both by lightning and by in situ instabilities, and we investigate the likelihood of either being a source for the VLF bursts at Venus. Because the magnetic field is weak, the nightside ionosphere at Venus can be a high beta(e). plasma, where beta(e) is the ratio of thermal to magnetic energy density for electrons. For beta(e) approximately 1, electron thermal speeds are comparable to whistler mode phase speeds and Landau resonant instabilities will not apply to the Venus ionosphere. However, gyroresonant instabilities may operate at Venus. Linear stability analysis shows that the damping by ambient electrons is weak for beta(e) much less than 1 and whistler mode waves could be generated by either lightning or instabilities driven by superthermal electrons if beta(e) is low. On the other hand, the waves would be associated with high beta(e) if they are generated by temperature anisotropy in the ambient electron population. The dependence of 100-Hz wave intensity on the plasma parameters shows that the waves are most intense for low beta(e), and hence the waves cannot be generated through temperature anisotropy of the ambient electrons. The 100-Hz waves are also most intense when the whistler mode damping is weakest, but not when the growth rate due to superthermals is greatest. Moreover, the 100-Hz wave intensity does not depend on electron density, as would be expected if superthermal electrons were responsible for the waves, since the growth rates are largest when the beam to ambient density ratio is greatest. Consequently. whistler mode instabilities do not appear to explain the 100-Hz waves at Venus, and atmospheric lightning is the most likely source for these waves.