Atmospheric Electricity at the Ice Giants

Abstract

Lightning was detected by Voyager 2 at Uranus and Neptune, and weaker electrical processes also occur throughout planetary atmospheres from galactic cosmic ray (GCR) ionisation. Lightning is an indicator of convection, whereas electrical processes away from storms modulate cloud formation and chemistry, particularly if there is little insolation to drive other mechanisms. The ice giants appear to be unique in the Solar System in that they are distant enough from the Sun for GCR-related mechanisms to be significant for clouds and climate, yet also convective enough for lightning to occur. This paper reviews observations (both from Voyager 2 and Earth), data analysis and modelling, and considers options for future missions. Radio, energetic particle and magnetic instruments are recommended for future orbiters, and Huygens-like atmospheric electricity sensors for in situ observations. Uranian lightning is also expected to be detectable from terrestrial radio telescopes.

Convective storms and atmospheric vertical structure in Uranus and Neptune

“…Intense convective storms large enough to be observed from Earth are frequent in Jupiter [37] and rare in Saturn [22,38]. In both planets, radio emissions emitted from lightning have been observed by different spacecraft [39], and lightning flashes have been observed in some of the most intense storms [40,41], including Jupiter locations with no evident storms in the observed cloud field [42,43], but lightning in Uranus and Neptune has only been inferred from radio signals [44][45][46].…”

Ice giant system exploration in the 2020s: an introduction

“…Our knowledge of ice giant atmospheres was reviewed shortly after the Voyager encounters by Lunine [29], and constraints on planetary origins from atmospheric composition and structure were reviewed by Mousis et al [30]. These reviews have been recently updated: Hueso & Sanchez-Lavega [31] review atmospheric dynamics and winds; Fletcher et al [32] review ice giant banding and global circulation; and Aplin et al [33] review measurements of atmospheric electricity on Uranus and Neptune. Ice giant atmospheres featured prominently on the first day of the discussion meeting, and new insights are revealed in this special issue: Hueso et al explore convective motions in hydrogen-rich atmospheres; Moses et al review the chemistry at work on the ice giants; and Melin et al and Moore et al describe the properties of the upper atmospheres and ionospheres.…”

Non‐Detection of Lightning During the Second Parker Solar Probe Venus Gravity Assist

“…While extraterrestrial lightning has been observed at Jupiter, Saturn, Uranus, and Neptune (Aplin et al., 2020), its existence at Venus remains an open question. Lorenz (2018) reviewed claims of detections and nondetections of lightning at Venus, concluding that while some type of electromagnetic activity is present (Russell et al., 2008), its properties are different from terrestrial lightning.…”

“…At radio frequencies, lightning produces strong, transient broadband emissions commonly known as "sferics" (Smith et al, 2002). Terrestrial sferics typically exhibit a f −2 power spectrum at frequencies below a few MHz, steepening to f −4 at higher frequencies (Levine & Meneghini, 1978).While extraterrestrial lightning has been observed at Jupiter, Saturn, Uranus, and Neptune (Aplin et al, 2020), its existence at Venus remains an open question. Lorenz (2018) reviewed claims of detections and nondetections of lightning at Venus, concluding that while some type of electromagnetic activity is present (Russell et al, 2008), its properties are different from terrestrial lightning.…”