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Statistical Study of Lightning-Generated Whistler-Mode Waves Observed by Venus Express


The existence of lightning on Venus has been a topic of debate for over four decades and arguably for centuries. The auroral-like phenomenon known as the Ashen Light was first reported in 1634 and lightning was one of the potential causes. Astronomers continued to report on sightings of the mysterious nightglow for hundreds of years with no conclusive explanation as to the source. In the late 20th century, the USSR and the US began sending probes to our sister planet with the search for lightning as one of the objectives. Since then there have been numerous observations of lightning reported. The Venera landers detected electrical signals determined to be from lightning during their descents. The Venera 9 orbiter detected flashes of light with its visible spectrometer. Pioneer Venus Orbiter (PVO) observed very low frequency (VLF) radio waves on numerous occasions with the source determined to be lightning. An Earth-based campaign observed seven flashes of light on the nightside of Venus. During a gravity assist flyby of Venus, the Galileo spacecraft detected radio signals inferred to be from lightning. The Venus Express mission regularly detected whistler-mode waves as expected to be from lightning. These are the same type of waves detected by PVO. Most recently, the Akatsuki spacecraft has detected a flash in the clouds while in orbit around Venus.We use the Venus Express (VEX) 128 Hz fluxgate magnetometer data to search for lightning-generated whistler-mode waves in the ionosphere of Venus. Whistler-mode waves are right-hand circularly polarized, transverse plasma waves that are guided by the ambient magnetic field. Using these known properties of whistlers, we developed code to scan all mission data under 1000 km altitude and catalog any observed whistler-mode waves. Since the ionopause of Venus is at a relatively low altitude on the dayside, we restrict our statistical analysis to below 400 km altitude, a reasonable limit for the north polar observations of VEX. Although the detection rates were comparable 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 near periapsis ∼8% of the time, on average, throughout the mission. The median Poynting flux of the signals was 1.4�10−9 W m−2 and the peak values occurred at low altitudes, implying a lightning source from below the ionosphere. These results agree with previous studies from the PVO era. Because of the global greenhouse effect, the Venus atmospheric conditions do not vary greatly around the planet. Therefore, under the assumption that lightning near the north pole is not significantly different than any other location, we estimate the global rate of flashes to be 320 s−1.

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