Pioneer Venus Orbiter search for Venusian lightning
Authors: Borucki William J. , Dyer John W. , Phillips James R. , Pham Phan 
Published: February 2008

During 1988 and 1990, the star sensor aboard the Pioneer Venus orbiter (PVO) was used to search for optical pulses from lightning on the nightside of Venus. In our previous effort to use the star sensor for this purpose [Borucki et al., 1981], we reported an upper limit to the lightning activity, but we were unable to show that lightning was being detected because the signal rate was indistinguishable from the false alarm rate. Because the periapsis altitude has increased by nearly a factor of ten since 1979, the star scanner views a much larger area of Venus than it did previously. This increased viewing area should provide an increased signal rate because the amplitude of optical pulses should still be above the detection threshold of the sensor if the flashes are as bright as terrestrial flashes. Because the false alarm rate did not increase, the increased viewing area allows a more sensitive search for lightning activity. Useful data were obtained for 53 orbits in 1988 and 55 orbits in 1990. During this period, approximately 83 s of search time plus 7749 s of control data were obtained. Our results again find no optical evidence for lightning activity. Within the region that was observed during 1988, the results imply that the upper bound to short‐duration flashes is 4×10−7 flashes/km²/s for flashes that are at least 50% as bright as typical terrestrial lightning. During 1990, when the 2‐Hz filter was used, the results imply an upper bound of 1×10−7 flashes/km²/s for long‐duration flashes at least 1.6% as bright as typical terrestrial lightning flashes or 33% as bright as the pulses observed by the Venera 9. The upper bounds to the flash rates for the 1988 and 1990 searches are twice and one half the global terrestrial rate, respectively. These two searches covered the region from 60°N latitude to 30°S latitude, 250° to 350° longitude, and the region from 45°N latitude to 55°S latitude, 155° to 300° longitude. Both searches sampled much of the nightside region from the dawn terminator to within 4 hours of the dusk terminator. These searches covered a much larger latitude range than any previous search. Our results show that the Beta and Phoebe Regio areas previously identified by Russell et al.[1988] as areas with high rates of lightning activity were not active during the two seasons of our observations. When we assume that our upper bounds to the nightside flash rate are representative of the entire planet, the results imply that the global flash rate and energy dissipation rate derived by Krasnopol'sky [1983] from his observation of a single storm are too high. Experimental measurements carried out to determine the amount of scattering occurring in the star scanner optics indicate that our previously determined upper bound to lightning activity is too low. The present results supplant the past results. The apparent conflict between the radio data that indicate the presence of a large amount of lightning activity and the optical data that indicate no lightning on the nightside, can be resolved by noting that theoretical considerations indicate lightning activity is to be expected only on the dayside.