Lightning-Generated Whistler-Mode Waves in the Ionosphere of Venus

Abstract:

Lightning produces extremely low frequency (ELF) radio waves, known as whistler-mode waves, that propagate along magnetic field lines to higher altitudes in the ionosphere. Venus lacks an intrinsic magnetic dipole, but the interplanetary magnetic field (IMF) drapes around the planet, magnetizing the ionosphere and forming a comet-like tail. The field lines tend to be nearly horizontal, parallel to the surface around much of the planet, except in the tail where it is more radial. Regions with a radial component to the field allow propagation to higher altitudes. Therefore, a wave on the dayside is less likely to enter the ionosphere at the zenith of its source and more likely to enter at angles towards the horizon, where the field lines and wave path are more aligned.

The dual fluxgate magnetometer onboard Venus Express (VEX) was able to detect ELF signals up to 64 Hz throughout the ionosphere of Venus. The mission was in orbit from 2006-2014 and in that time there were over 5 cumulative hours of whistler observations. In some cases, there was continuous activity for over a minute, implying a connection to an electrical storm below. Most signals were observed when the spacecraft was within 200-300 km altitude at a rate of ~4% of the time. Whistlers may propagate significant distances in the ionosphere before detection, so the observation region may be 1000’s of kilometers from the source region. Therefore, it must be emphasized that these observations alone do not tell us where the lightning itself is occurring. For that we need to employ ray tracing techniques, which is left to future work.

Pioneer Venus (PVO) was able to detect the electric component of lightning-generated waves at 100 and 700 Hz, but on the nightside and at lower latitudes in contrast to the North polar observations of VEX. The improved capability of VEX over PVO has greatly increased our knowledge of Venus lightning, albeit with limited geographic coverage. The study would be greatly improved with global observations at ~250 km altitude, where the majority of lightning whistlers were detected. A magnetometer with twice the sampling rate of the VEX magnetometer would cover the full bandwidth expected of field-guided whistlers. A more thorough ray tracing investigation could then provide the locations of the regions with the most active electrical storms.