Statistical study of lightning-generated whistler-mode waves observed by Venus Express

From 2006 through 2014, Venus Express regularly observed lightning-generated whistler-mode waves in the ionosphere of Venus. Since the ionopause of Venus is relatively low on the dayside, we restrict our statistical results to below 400 km altitude. Although the detections were of comparable rates for solar minimum and solar maximum (5.4% and 5.0%, respectively), the distribution of the waves was significantly different. During solar minimum the waves were observed most often between 0600 LT and 1200 LT, while in solar maximum the waves were mostly observed near midnight. However, for both periods the peak observation rates occurred at an altitude of 250 km. At this altitude, whistler-mode waves were observed ~8% of the time on average throughout the mission. The median Poynting flux of the signals was 1.4 × 10⁻⁹W/m² and the peak values occurred at low altitudes, implying a lightning source from below the ionosphere. Since the solar cycle should not affect the lightning rate, only the efficiency of energy transfer, we calculate the solar cycle independent rate of flashes to be 320 s⁻¹.