Journal Article

Non‐Lightning‐Generated Whistler Waves in Near‐Venus Space

Conner, D., Bale, S. D., Malaspina, D. M., Goodrich, K., George, H., Curry, S., Ramstad, R., Ma, Y.

2023

Overview

The occurrence of Venusian lighting has been debated for decades. Terrestrial lightning generates whistler waves, and many whistlers have been observed in Venus's ionosphere and induced magnetosphere. Venusian lightning occurrence rates derived from these whistler observations are relatively high. However, optical flashes on Venus are exceedingly rare and Venus encounters by multiple spacecrafts have not detected lightning. These non‐detections and rare optical observations are consistent with low Venusian lightning occurrence rates, which is incompatible with the high whistler‐derived rates. We present observations of whistlers during a Parker Solar Probe Venus gravity assist and eliminate lightning as a possible source. These waves are observed at an altitude of 0.39 Venus radii on Venus' nightside with planetward propagation and are simultaneous with Langmuir waves. This provides a mechanism for whistler generation near Venus that does not require lightning, and suggests that whistler‐based lightning occurrence rates may be overestimated. Plain Language Summary Whistler waves are a type of plasma wave. These waves can be generated in several ways, including by lightning. Every lightning strike on Earth generates a whistler wave, but only some of the whistler waves in near‐Earth space are generated by lightning. Many whistler waves have been detected near Venus and have been used to argue that lightning likely occurs on Venus at a relatively high rate. However, other signatures of lightning (including flashes of light in the sky) on Venus are very rare, which indicates that Venusian lightning must occur at a very low rate. The discrepancy between these different signatures of lightning on Venus means that we do not know how often Venus actually experiences lightning. We use data from Parker Solar Probe during a Venus gravity assist to study whistler waves that occurred on the nightside of Venus, very close to the planet. We observe that these waves are traveling toward Venus, which means they could not have been generated by lightning. This shows that whistler waves can occur near Venus without being generated by lightning and indicates that the occurrence rates of Venusian lightning based on whistler wave observations might be overestimated. Key Points Whistler waves were observed at an altitude of 0.39 Venus radii on Venus's nightside with planetward Poynting vector Langmuir waves occurred simultaneously with the whistlers, suggesting electron beam driving with magnetotail origins Lightning is eliminated as a possible generation mechanism for these whistler waves

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Gravity / Ionosphere / Langmuir waves / Lightning / Lightning strikes / Optical observations / Planetary magnetospheres / Plasma waves / Signatures / Solar probes / Venus / whistler wave / Whistler waves / Whistlers