Lightning Occurrence Statistics from Venus Explorer Observations of ELF Emissions

Venus Express has now recorded ELF emissions (up to 64 Hz) in the low-altitude Venus ionosphere since mid-2006. These signals are most prevalent when the ionosphere magnetic field dips into the atmosphere, enabling the electromagnetic signal to enter the ionosphere. The signals can extend over the full bandwidth of the instrument, up to 64 Hz. The waves are nearly circularly polarized and are right-hand polarized, as expected for whistler-mode propagation generated by lightning. When isolated bursts of signal occur, frequently dispersion is seen in which the high-frequency waves arrive first. This is the expected signature generated by impulsive electric discharges. These observations suggest that the rate of lightning occurrence on Venus is not unlike the terrestrial rate where atmosphere chemistry is affected measurably by these discharges. When Venus Express was inserted into its 24-hour elliptical polar orbit, periapsis was near 80 degrees and later precessed up to 88 degrees. Now in the orbit, Venus Express has precessed over the pole and has reached lower latitudes than on arrival. The occurrence rate of lightning-associated ELF signals has increased. Here we report on the latest statistics of these lightningproduced signals and compare the occurrence statistics on the most recent orbits with those at orbit insertion.