Lightning-generated waves escaping out through plasma holes in the nightside Venus ionosphere (original) (raw)
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Role of the Mode Theory on Lightning Generated Waves Propagating in Venus-Ionosphere Waveguide
Journal of geomagnetism and geoelectricity, 1993
The wave mode either electromagnetic or electrostatic of the OEFD measured signals is unknown and needs to be ascertained by indirect means. Considering these waves to be generated by global Venus lightning, the wave attenuation along the Venus-ionosphere wave-guide has been computed. 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 frequency. This feature of the OEFD signals implies that the Venus lightning may have global occurrence. The lightning generated signals may travel considerable distance within the Venus-ionosphere waveguide. The encounter of these signals with plasma "holes" or ion "troughs" in the nightside of Venus wake region results in the leakage of the signals out of the shielding ionosphere. These signals are occasionally recorded by the OEFD. The occurrence rate statistics shows that the leakage of 5.4 kHz signals through the plasma "holes" in the vicinity of PVO periapsis are most effective. The maximum occurrence rate of higher frequency signals change in altitude slightly from one PVO season to another.
Journal of Geophysical Research, 1993
A theoretical and numerical analysis of the propagation of electromagnetic waves in the nightside Venus ionosphere is presented. The special case of propagation parallel to the magnetic field is considered. The model assumes a source of electromagnetic radiation in the Venus atmosphere, such as produced by lightning, and specifically addresses wave propagation for realistic ionospheric parameters in the altitude range z = 130-160 km at the four frequencies detectable by the Pioneer Venus Orbiter Electric Field Detector (OEFD): 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. The results are summarized as follows. The f = 100 Hz and 730 Hz waves can propagate as whistler waves, provided f < fce, where fce is the electron cyclotron frequency, and provided that the ionospheric electron density is sufficiently small that the waves are not strongly attenuated by collisional effects (i.e., Pedersen conductivity). The attenuation length scale X0 associated with the Pedersen conductivity is •o-2(c/Cøpe)[•e3/Cø(Ven + Vei)2]l/2; for parameters typical of the nightside Venus ionosphere above 140 km, electron-ion collisions are more frequent than electron-neutral collisions and are therefore responsible for the attenuation of the waves. A parameterization of the wave intensities and Poynting flux as a function of magnetic field and peak electron density is presented. The waves are found to propagate most easily for high magnetic field and low electron density conditions (i.e., ionospheric holes); this result is consistent with observational data. The incident Poynting flux at the bottom of the ionosphere (z-130 km) is estimated to be S • 0.1 W/m 2 for the 100 Hz waves. The f = 5.4 kHz and 30 kHz cannot propagate through the nightside Venus ionosphere, as expected, because fpe > f > fce.