ROLE OF THE MODE THEORY ON LIGHTNING GENERATED WAVES PROPAGATING IN VENUS-IONOSPHERE WAVE-GUIDE

The wave mode either electromagnetic or electrostatic of the OEFD meas ured signals is unknown and needs to be ascertained by indirect means. Considering these waves to be generated by global Venus lightning, th e wave attenuation along the Venus-ionosphere wave-guide has been comp uted. The effective wave attenuation versus frequency shows a ''knee'' characterizing minimum attenuation 6 kHz. This ''knee'' is consistent with the amplitude spectral peak of OEFD signals around this frequenc y. This feature of the OEFD signals implies that the Venus lightning m ay have global occurrence. The lightning generated signals may travel considerable distance within the Venus-ionosphere waveguide. The encou nter of these signals with plasma ''holes'' or ion ''troughs'' in the nightside of Venus wake region results in the leakage of the signals o ut of the shielding ionosphere. These signals are occasionally recorde d by the OEFD. The occurrence rate statistics shows that the leakage o f 5.4 kHz signals through the plasma ''holes'' in the vicinity of PVO periapsis are most effective. The maximum occurrence rate of higher fr equency signals change in altitude slightly from one PVO season to ano ther.