Ionization troughs are frequently observed in the Venus nightside ionosphere by the orbiter ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter (PVO). These events typically exhibit density depletions which may range from a factor of 2 to an order of magnitude or more, often feature sharp density gradients, and frequently are associated with superthermal ion fluxes. Although their full spatial extent cannot be determined, the ion trough dimensions as perceived along the PVO orbit track extend from 1¿ to more than 10¿ in latitude and longitude. Troughs have been encountered at altitudes ranging from more than 1000 to less than 250 kilometers. The nightside trough boundaries are similar to both the ionization gradients which mark the Venus dayside ionopause and the ionization gradients which define the high-latitude ionization trough in the earth ionosphere. In each of these Venus and earth phenomena there is a close association between sharp gradients in thermal ion distributions and energetic and dynamic processes stimulated by the interaction between the solar wind/interplanetary magnetic field (IMF) and the thermal ionosphere. A close correlation is identified between ion trough events and prominent published examples of 100-Hz plasma waves detected by the orbiter electric field detector (OEFD), which have been interpreted by Scarf and Russell (1983) and by Ksanfomaliti et al. (1983) as whistler mode propagation stimulated by lightning in the lower atmosphere and frequently occurring over mountains. While the occasional coincidence between the detection of whistler waves and the overflight of Venus mountain ranges is also observed for the ion troughs, neither phenomenon is seen to exhibit convincing evidence of an association with the mountains. Overall, these findings lead us to believe that many of the 100-Hz noise bursts previously attributed to lightning are, like the ion troughs, actually the result of the interaction between the ionosphere and the combined effects of the solar wind and the draped IMF. If this interpretation is correct, there may be little evidence for lightning from the PVO measurements. Similarly, a supporting argument for active volcanism as a possible source for the deduced lightning events would also be discounted.