SIGNATURES OF SOLAR-WIND INTERACTION WITH THE NIGHTSIDE IONOSPHERE OFVENUS

Plasma and field relationships observed across the nightside of Venus evidence a chaotic variety of interactions between the ionosphere and the combined effects of the solar wind and interplanetary magnetic fie ld draped about the planet. Close examination of these data reveal wit hin the chaos a number of repeatable signatures key to understanding f undamental field-plasma interactions. Observed from the Pioneer Venus Orbiter, (PVO), nightside conditions range from extensive, ''full-up'' ionospheres with little evidence of dynamic or energetic perturbation s, to an almost full depletion, sometimes described as ''disappearing ionospheres''. Between these extremes, the ionospheric structure is of ten irregular, sometimes exhibiting well-defined density troughs, at o ther times complex intervals of either abundant or minimal plasma conc entration. Consistently, large B-fields (typically exceeding 5-10 nano teslas) coincide with plasma decreases, whereas stable, abundant plasm a distributions are associated with very low-level field. We examine h undreds of nightside orbits, identifying close correlations between re gions of elevated magnetic fields featuring polarity reversals, and (a ) exclusive low-frequency or distinctive broadband noise, or both, in the electric field data, (b) turbulent, superthermal behavior of the t he ions and electrons. We review extensive studies of nightside fields to show that the correlations observed are consistent with theoretica l arguments that the presence of strong magnetic fields within ''norma l'' ionospheric heights indicates the intrusion of magnetosheath field s and plasma within such regions. We find abundant evidence that the ' 'ionosphere'' is frequently disrupted by such events, exhibiting a cha otic, ''auroral-like'' complexity appearing over a wide range of altit ude and local time. We show that field-plasma disturbances, widely sug gested to be similar to conditions in the Earth's auroral regions, are tightly linked to the electric field noise otherwise attributed to li ghtning. Owing to the coincidence inherent in this relationship, we su ggest that natural, predictable plasma instabilities associated with t he plasma gradients and current sheets evident within these events pro duce the E-field noise. The data relationships argue for a more detail ed investigation of solar wind induced E-field noise mechanisms as the appropriate scientific procedure for invoking sources for the noise p reviously attributed to lightning. Consistent with these views, we not e that independent analyses have offered alternative explanations of t he noise as arising from ionospheric disturbances, that repeated searc hes for optical evidence of lightning have found no such evidence, and that no accepted theoretical work has yet surfaced to support the inf erence of lightning at Venus.