Show Summary Details

Page of

PRINTED FROM the OXFORD RESEARCH ENCYCLOPEDIA, PLANETARY SCIENCE (oxfordre.com/planetaryscience). (c) Oxford University Press USA, 2019. All Rights Reserved. Personal use only; commercial use is strictly prohibited (for details see Privacy Policy and Legal Notice).

date: 17 June 2019

Atmospheric Electricity in the Solar System

Summary and Keywords

Electricity occurs in atmospheres across the Solar System planets and beyond, spanning spectacular lightning displays in clouds of water or dust, to more subtle effects of charge and electric fields. On Earth, lightning is likely to have existed for a long time, on the basis of evidence from fossilized lightning strikes in ancient rocks, but observations of planetary lightning are necessarily much more recent. The generation and observations of lightning and other atmospheric electrical processes, both from within-atmosphere measurements, and spacecraft remote sensing, can be readily studied using a comparative planetology approach, with the Earth as a model.

All atmospheres contain charged molecules, electrons, and/or molecular clusters created by ionization from cosmic rays and other processes, which may affect an atmosphere’s energy balance both through aerosol and cloud formation and direct absorption of radiation. Several planets are anticipated to host a “global electric circuit” by analogy with the circuit occurring on the Earth, where thunderstorms drive the current of ions or electrons through weakly conductive parts of the atmosphere. This current flow may further modulate an atmosphere’s radiative properties through cloud and aerosol effects.

Lightning could potentially have implications for life through its effects on atmospheric chemistry and particle transport. It has been observed on many of the Solar System planets (Earth, Jupiter, Saturn, Uranus, and Neptune), and it may also be present on Venus and Mars. On Earth, Jupiter, and Saturn, lightning is thought to be generated in deep water and ice clouds, but discharges can be generated in dust, as for terrestrial volcanic lightning, and on Mars. Other, less well-understood mechanisms causing discharges in non-water clouds also seem likely. The discovery of thousands of exoplanets has recently led to a range of further exotic possibilities for atmospheric electricity, though lightning detection beyond our Solar System remains a technical challenge to be solved.

Keywords: lightning, ionization, cosmic rays, electric field, conduction current, global electric circuit

Introduction: What is Atmospheric Electricity and Why Is It Important?

Electrical charge exists in atmospheres in the form of an ionized gas, or as an electrical discharge, the most well-known manifestation of which is lightning. Cosmic rays, highly energetic particles created in supernovae and other astrophysical cataclysms, travel through space at high speed. When they encounter a gas, for example in an atmosphere, they ionize it: this process makes atmospheric electricity universal. Some atmospheres also have electrical discharges, which occur when clouds become charged beyond a sustainable level. The combination of background charging and electrical discharges means that some planets can support an electrical circuit, a concept known as a “global electric circuit.”

Lightning and other forms of electrical discharge, discussed in more detail below, present a hazard to humans, vehicles, and electronic systems. The high currents involved in an electrical discharge can kill (Elsom et al., 2018) and damage telemetry and circuitry (Lorenz, 2008). In addition to this practical motivation, there are several more reasons why planetary atmospheric electricity is important. The famous Miller and Urey (1959) experiment found that electrical discharges generated in conditions mimicking the early Earth’s atmosphere produced amino acids, the starting point for life. Early-21st-century work suggests that electrons accelerated in the electric fields in Martian dust storms can affect chemical processes that may also be relevant for life (Harrison et al., 2016). The possibility of life elsewhere in the universe, and the conditions needed for it, remain crucial scientific questions facing mankind and provide a major motivation for the study of planetary atmospheric electricity.

More generally, electrical processes affect (and are affected by) the atmosphere they take place in, so they can be used to deduce information about a planet. For example, as all known lightning generation mechanisms involve atmospheric convection, electrical discharges can indicate the existence of convection. The fundamental physical processes associated with lightning, such as resonances within the surface-atmosphere waveguide, or effects of the propagating medium, can be exploited to deduce the properties of an atmosphere. A global electric circuit leads to the presence of an electric field in the atmosphere away from storms; this may have subtle effects on particle transport and other atmospheric processes (e.g., Harrison et al., 2017). Ions and electrons from ionization can attach to other atmospheric particles, charging them and affecting their properties; charge can also assist in particle growth, leading to electrical effects on the planetary radiative balance (e.g., Curtius et al., 2006; Aplin, 2006).

Electrical processes couple the upper and lower atmosphere, for example, cosmic rays can deliver energy of several GeV from outer space directly to the lower atmosphere. The upper conductive part of an atmosphere, the ionosphere, acts as a boundary between the more weakly conducting lower atmosphere, mainly influenced by meteorological effects, and the solar-terrestrial coupling that modulates the ionosphere and other parts of the upper atmosphere. Electrical discharges can indirectly provide information about the surface of a planet through the propagation of electromagnetic waves (Simões et al., 2008). Atmospheric electricity therefore also provides interdisciplinary coupling between planetary science and space physics. This article will firstly describe electrical discharges, including those in the upper atmosphere (known as transient luminous events) and then consider other electrical properties of atmospheres, with Earth used as a guiding example throughout.

Electrical Discharges

A lightning flash is a transient, high-current discharge with a path length of a few kilometers. It usually lasts for a fraction of a second and consists of several sub-discharges, which are called strokes. The typical current amplitude in such strokes is 30 kA. The high current causes a sudden heating of the lightning channel up to about 30,000°C; the air in and around the channel then expands rapidly, causing an audible signal known as thunder (Uman, 1987). Lightning flashes are dangerous, and they have always produced fear and respect in mankind. This is evident from the role that lightning played in ancient religions and mythologies. Early statues of Buddha show him carrying a thunderbolt. The Nordic god Thor was believed to produce lightning by striking his anvil with a hammer. In many languages the name of the fifth day of the week goes back to Thor, for example “Thursday” in English, “Donnerstag” (Donner = thunder) in German, or “Torsday” in Danish. The Greek god Zeus (later corresponding to the Roman god Jupiter) was thought to use lightning as a weapon to show his power. In American Indian art one can find drawings of a mystical thunderbird whose flapping wings produced the sound of thunder. Lightning produces forest fires, and the use of fire has played a large role for the development of civilization.

Other types of discharge have much smaller currents. A corona discharge occurs when a small current flows from an electrode into the surrounding air creating a region of ionized air (plasma) around the electrode. Such coronas often occur as bluish glow at highly charged conductors with sharp points, and they can cause a significant power loss for high-voltage electric utilities. A similar glow is also produced in gas discharge lamps. When the electric field increases above a threshold, a corona discharge turns into a streamer discharge with filamentary discharge channels. This happens when electrons generated by ionization go on to strike more molecules in a chain reaction causing an electron avalanche. The threshold voltage is also called breakdown voltage, and for dry air at a pressure of 1,000 hPa this value is 3 × 106 Vm−1 (or 30 kV for a gap of 1 cm). It was thought that local enhancements of electric fields in thunderclouds can lead to breakdown and the initiation of lightning (see the Section “Generation of Electrical Discharges”). However, the strongest electric fields measured in thunderclouds are around 105 Vm−1, an order of magnitude lower than the conventional breakdown field (Dywer & Uman, 2014). These values are of macroscopic fields that are measured by launching balloons into developing thunderstorms; they do not necessarily represent the intense fields that exist on much smaller scales. The breakdown field also decreases in the lower ambient pressure in the cloud and in the presence of hydrometeors (liquid or solid water particles suspended in air), but not by enough to explain the disagreement between the measured electric fields and the existence of lightning (Dubinova et al., 2015). There is also another theory called “runaway breakdown.” Relativistic electrons moving at almost the speed of light may create a relativistic runaway electron avalanche at smaller electric fields of about 105 Vm−1 (Gurevich, Milikh, & Roussel-Dupre, 1992). High-energy electrons from cosmic rays are needed to start this runaway process.

Luminous discharges called sprites, blue jets, and elves (referred to collectively as transient luminous events [TLEs]) exist above thunderstorms. Sprites appear at 40–90 km as large red to orange streamers with tendrils. They may be produced by runaway breakdown due to strong electrostatic fields from the thunderstorm below. Blue jets propagate upward from the top of thunderclouds in the form of blue cone-shaped structures. They typically go up to an altitude of 40 km, but there are also gigantic jets that establish a direct path of electrical contact between the thundercloud tops and the lower ionosphere. Elves are radially expanding, horizontal circles of light from a point high above the causative lightning flash, occurring near a height of 90 km (Pasko, Yair, & Kuo, 2012). Due to their low luminosity, all these TLEs were detected and investigated relatively recently, starting in the 1990s. The Nobel laureate C. T. R. Wilson suggested on theoretical grounds that electrical breakdown could occur at higher altitudes, and he may also have personally witnessed a sprite (Wilson, 1956). The first recording of a sprite was made by chance in summer 1989 when scientists from the University of Minnesota tested a low-light video camera (Franz, Nemzek, & Winkler, 1990). So far, TLEs have only been observed on Earth, but they may also exist on other planets (Yair et al., 2009; Pérez-Invernón et al., 2017).

Ball lightning, bead lightning, or volcanic lightning constitute unusual types of natural discharge (Rakov & Uman, 2003). There are numerous credible witness reports for ball lightning, but hardly any real scientific documentation exists due to its rarity. On the other hand, bead lightning is a well-documented optical phenomenon in which the visible lightning channel breaks up into luminous fragments, and these “beads” appear to last longer (~1–2 s) than a typical lightning flash, which lasts about 30 μ‎s. Impressive volcanic lightning has often been observed in the ejected material above active volcanoes. This material consists of rock fragments, ash, and ice particles that collide and produce static charges. Fractoemission may also contribute to volcanic lightning by producing charge when rocks break (see, e.g., Aplin, Bennett, Harrison, & Houghton, 2016; Rakov & Uman, 2003).

Generation of Electrical Discharges

Thunderclouds have been known to contain charged particles since the mid-18th century, when Benjamin Franklin conducted his famous kite experiment: electric sparks jumped from a key tied to the bottom of a conducting kite string to his knuckles. Franklin was not fully aware of the danger of his experiment, since a direct lightning strike to the kite in the thundercloud could have killed him. His work can also be seen as the first atmospheric electrical experiment (Rakov & Uman, 2003). Nowadays, lightning flashes can be artificially triggered by launching a small rocket with a thin wire that bridges the gap between the ground and a charged cloud overhead.

The main source of lightning is a cloud type termed “cumulonimbus.” Convective clouds form when large masses of warm and moist air rise to higher altitudes where they cool. The moisture condenses on aerosols to form the water droplets constituting the visible cloud. At temperatures from 0 to −40°C liquid water droplets coexist with ice particles, and electrification occurs in this mixed-phase region. In the 21st century the most widely accepted electrification process in thunderclouds is charge transfer by collisions between graupel (soft hail) and ice crystals in the presence of water droplets at subzero temperatures, mostly below −10°C (Rakov & Uman, 2003). In this process the heavier graupel particles usually acquire a negative charge whereas the lighter ice crystals get charged positively. The smaller positively charged particles are carried to higher altitudes leading to a cloud that is usually positively charged in its upper region and negatively charged below. A typical thundercloud also consists of a negative screening layer on the top and pockets of positive charge at the cloud bottom (Stolzenburg, Rust, & Marshall, 1998). So the major ingredients for forming thunderstorms are the vertical convection of warm and moist gas, the presence of particles that can be charged by collisions, and a large-scale charge separation process.

The two most common types of lightning discharges are intracloud (IC) lightning and cloud-to-ground (CG) discharges. About 90% of all CG lightning is initiated by a downward-moving negatively charged lightning leader. (There is also positive lightning created by positively charged leaders going to ground; e.g., most sprites are related to large positive lightning flashes.) The optically faint leader produces a negatively charged path below the cloud, which goes down in steps with a typical length of several tens of meters. The leader current is in the range of 1 kA, and a few Coulombs of charge are distributed along the channel. When the stepped leader approaches the ground, raised objects such as towers or trees can emit connecting streamers, and the merging of such a streamer with the leader is called “attachment.” The tip of the leader is suddenly raised to the potential of the Earth, and a potential wave is driven up to the cloud base in several tens of microseconds with speeds up to 108 ms−1. This is the so-called first return stroke in which the largest current flows. The first return stroke can be followed by several subsequent return strokes, which themselves are initiated by dart leaders. A high-speed video of a lightning flash will reveal a faint downward-going stepped leader followed by a bright upward-going return stroke, since the light intensity is strongly correlated with the size of the current (Idone & Orville, 1985).

Observation Techniques

A variety of photoelectric sensors have been used to observe lightning from space, from aircraft, and from ground-based stations. In the days of analogue photography, images of lightning were taken with a streak camera that employed a continuous relative motion between the lens and the film to resolve short-time phenomena. In the 21st century, of course, digital photography has taken over, and high-speed video cameras can record films with frequencies of millions of frames per second.

Lightning leads to large changes in the electrostatic field at close distances. The electrostatic field change due to a flash at 5 km is about 104 Vm−1, and still about 1 Vm−1 at a distance of 100 km (Uman, 1987). Such electric fields can be measured by field mills or other electric field measuring systems employing an elevated flat plate antenna. More importantly, lightning can be detected from large distances, and it is known to emit significant electromagnetic energy over a large range of frequencies. At extremely low frequencies, lightning flashes excite global electromagnetic resonances called Schumann resonances in the cavity formed by the Earth’s surface and the ionosphere. They can be registered by highly sensitive electric and magnetic field measurements at research stations around the world, and the fundamental first resonance is at 7.8 Hz. Impulsive signals such as lightning are very broadband, and the radio spectrum of lightning extends from a few Hz up to about 300 MHz, with a peak in the energy density spectrum around 10 kHz. This is due to the fact that CG-lightning can be seen as a quarter-wave antenna with an average length of 7–8 km, and an electromagnetic wave with a wavelength of 4 × 7.5 = 30 km has a frequency of 10 kHz. Radio atmospheric signals called “sferics,” in the VLF range (Very Low Frequency, from 3 to 30 kHz) can easily be received thousands of kilometers from their source, since they are mostly reflected by the terrestrial ionosphere. However, some of the energy can also escape from the Earth-ionosphere waveguide and enter the magnetosphere. The signal then gets dispersed by the plasma near the Earth and forms a so-called whistler signal, which means that lower frequencies travel slower—this makes a whistling sound (when transformed to an audible signal). Whistlers propagate along magnetic field lines where they can be detected by satellites or by a ground station at the magnetically conjugate point on the other hemisphere. Sferics at higher frequencies, above the ionospheric cutoff frequency of about 10 MHz, can directly pass the ionosphere and freely propagate to an antenna system on a satellite. The electromagnetic pulses of single strokes typically last only a few tens of microseconds, but a complete flash, consisting of several strokes, can last for a fraction of a second. The dispersed whistler signals can even last for a few seconds.

Lightning can also be detected at microwave frequencies (300 MHz to 300 GHz), and obviously in the visible (around 5 × 1014 Hz). At even higher frequencies, bursts of X-rays and gamma rays generated by thunderstorms were first discovered in 1994 by an experiment on-board the Compton Gamma Ray Observatory, a NASA satellite (Fishman et al., 1994), and named TGFs (Terrestrial Gamma-Ray Flashes).

Lightning on Earth

The detection of terrestrial lightning from space has become increasingly important. There have been at least a dozen satellites using optical detectors, radio instruments, or at best a combination of both. For example, the FORTE (Fast On-Orbit Recording of Transient Events) satellite and the future Taranis (Tool for the Analysis of RAdiation from lightNIng and Sprites) mission have many instruments to specifically investigate thunderstorms and lightning on Earth.1 Many ground-based lightning locating systems have been established to determine the location, intensity, and movement of thunderstorms in real time. This information is used by aviation and weather services or by insurance companies to locate lightning-caused damage. Such detection networks can use many different location techniques based on electric field amplitudes, magnetic field direction finding with loop antennas, time of arrival of the radiation field, radar returns from lightning and rain, or visible light direction. The World Wide Lightning Location Network currently uses 70 VLF stations around the world to produce maps of lightning activity.2 Other important networks are the U.S. National Lightning Detection Network (NLDN) or EUCLID (European Cooperation for Lightning Detection), which is a collaboration of national European lightning detecting networks.3

The global terrestrial flash rate is about 100 flashes per second, and at any moment, about 2,000 thunderstorms are active. These estimates were first made by Brooks (1925) on the basis of “thunder days” (a day when a thunder is heard). Modern measurements with satellites have modified these values to (44±5) s−1 for the flash rate (Christian et al., 2003) and to 1,100 thunderstorms per hour (Mezuman, Price, & Galante, 2014)

Figure 1 shows the global distribution of lightning. Most lightning activity takes place over land because there is less convective intensity in oceanic thunderstorms compared to those over land (Cooray, 2015). The main three lightning hotspots are the tropical regions of South America, central Africa, and Southeast Asia. High activity can also be found in northern Australia and on the land masses around the Gulf of Mexico. There is almost no lightning in Arctic and Antarctic regions.

Atmospheric Electricity in the Solar SystemClick to view larger

Figure 1. Global distribution of lightning with the color code referring to the annual number of lightning flashes per km² with data from April 1995 to Feb. 2003 from NASA’s Optical Transient Detector and Lightning Imaging Sensor.

Credit: NASA MSFC Lightning.

Planetary Lightning

In the following sections we discuss electric discharges on the terrestrial planets Venus and Mars, the gas giants Jupiter and Saturn, and the icy giants Uranus and Neptune. Due to the tenuous atmospheres of Mercury and the dwarf planet Pluto no lightning should exist there. Lightning may also exist on some of the numerous exoplanets that were detected in the last two decades. However, due to the large distances, no exoplanetary lightning has been identified so far (e.g., Hodosán, Rimmer, & Helling, 2016).

Venus

Venus is almost as big as Earth, and it has a substantial atmosphere consisting mainly of carbon dioxide. Due to its close distance to the Sun (~0.7 AU) and a substantial greenhouse effect its surface temperature is around 470°C, at a pressure of 90 bars. It is thought that lightning could take place in Venus’s thick sulphuric acid clouds, located at an altitude around 60 km (Russell, 1993). However, the existence of lightning remains controversial (Yair, 2012), since the clouds are mostly stratiform, showing few signs of convection and instability except in the equatorial region. There is some evidence in optical, VLF, and high-frequency radio emission data for lightning, but all observations are one-offs with possible alternative explanations, and no combined observations (e.g., simultaneous optical and radio) exist (Fischer, Gurnett, & Yair, 2011a).

There are two tentative optical lightning detections for Venus. The first was made by the airglow spectrometer onboard the Soviet spacecraft Venera 9, which detected several pulses with a characteristic duration of 0.25 s (Kransnopolsky, 1983). The second was a telescopic observation from Earth with a 153-cm telescope and a CCD detector (Hansell, Wells, & Hunten, 1995). Despite the tremendous progress in digital photography since 1990s, many attempts to repeat this experiment have failed, as well as searches with optical instruments on spacecraft. A correct identification of short optical bursts is not trivial, since pixel errors, cosmic ray impacts, or transient stray light can lead to false-positive detections. The most comprehensive search for TLEs on the nightside of Venus was carried out recently with the visible channels of the VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument onboard the Venus Express spacecraft (Cardesín Moinelo et al., 2016). Thousands of signals were detected, but all of them can be explained by cosmic rays impinging on the detector.

There is more evidence for Venus lightning in the radio frequency range. The Russian Venera landers recorded impulsive VLF radio signals during their descent onto the surface of Venus (Ksanfomaliti, 1980). The Pioneer Venus Orbiter Electric Field Detector (OEFD) had four frequency channels and measured impulsive signals in the nightside ionosphere of Venus that were interpreted as lightning whistlers. As it is difficult to identify whistlers with such a limited frequency resolution, these bursts provoked much discussion (Taylor & Grebovsky, 1985). Similarly, bursts detected by the magnetometer onboard Venus Express were also attributed to lightning whistlers (Russell et al., 2007). The Galileo plasma wave instrument detected a few weak impulsive bursts at high frequencies during the 1990 Venus flyby (Gurnett et al., 1991). However, such bursts were not detected by the Cassini RPWS (Radio and Plasma Wave Science) instrument during two flybys of Cassini in 1998 and 1999 (Gurnett et al., 2001). The correct identification of bursty radio signals is also non-trivial, since they can easily be caused by noise in electric circuits, particle impacts, or other radio frequency interference.

Mars

Mars probably had a much thicker atmosphere in its history, but it was blown away with time by the solar wind due to the lower gravity and the absence of a significant magnetic field compared to Earth (Haberle et al., 2017). Today the tenuous atmosphere of Mars mainly consists of carbon dioxide, with a surface pressure of only 6 hPa. Atmospheric electricity on Mars does not involve its thin cirrus-like clouds, but Martian dust storms are thought to host electrical activity (Melnik & Parrot, 1998). Triboelectric charging of sand and dust can produce strong electric fields in terrestrial dust storms and dust devils (e.g., Harrison et al., 2016), and similar phenomena are expected for Mars. Farrell et al. (1999) suggested that the electric fields in a Martian dust devil are limited by the low breakdown voltage of 10 kVm−1 in the low-pressure Martian atmosphere. A breakdown could occur by corona or glow discharge or by a spark discharge similar to terrestrial volcanic lightning. Gurnett et al. (2010) searched for impulsive radio signals during dust storms with the radar receiver onboard Mars Express, but none were found, so any discharges may be weak. No instrument package dedicated to atmospheric electricity has arrived at Mars, but the future ExoMars2020 surface platform contains an instrument that may detect lightning discharges (Kolmasova, Santolik, & Skalsky, 2017).

Jupiter

The gas giants Jupiter and Saturn mainly consist of hydrogen and helium. Both have a three-layered cloud structure with uppermost ammonia ice clouds (at around 1,000 hPa), an intermediate ammonium hydrosulphide cloud layer, and a deep-water cloud layer. The temperature at 1,000 hPa is around −140°C for both Jupiter and Saturn, but the atmosphere of Saturn is generally more extended due to its lower gravity. The freezing level of 0°C is in the water cloud layer, at 5,000–6,000 hPa for Jupiter (~100 km below cloud tops), but 8,000–10,000 hPa for Saturn (~200 km below cloud tops). Below that level the icy water cloud particles gradually turn into liquid droplets, with particle electrification and subsequent discharges assumed to take place in this mixed-phase region, similar to Earth.

The existence of lightning on Jupiter is uncontroversial since many spacecraft detected optical flashes on its nightside. The first evidence came from the camera on Voyager 1 (Cook, Duxbury, & Hunt, 1979), and subsequently cameras on Voyager 2, Galileo, and Cassini all imaged flashes. New Horizons detected optical flashes on both hemispheres up to high latitudes of 80°, with particularly active regions around 50°N. Additional confirmation came from the detection of lwhistlers (Gurnett, Shaw, Anderson, & Kurth, 1979), and the Lightning and Radio Emission Detector (LRD) of the Galileo probe observed VLF lightning sferics inside Jupiter’s atmosphere during the probe’s descent (Lanzerotti et al., 1996). Interestingly, no sferics in the high-frequency range were detected so far, and this could be related to a strong damping of the signals in Jupiter’s ionosphere (Zarka, 1985). The most recent Jovian lightning detection comes from the Juno spacecraft, which is in a close polar orbit around Jupiter since mid-2016. Several lightning whistlers were detected by the Juno Waves instrument (Kurth et al., 2017), and surprisingly the Juno MicroWave Radiometer (MWR) detected signals attributed to lightning around 1 GHz (Janssen, personal communication).

Saturn

Besides Earth, Saturn is probably the planet for which the most about lightning is known. This is due to the long Cassini mission that stayed in Saturn’s orbit for 13 years from 2004 until 2017. The first indication of lightning in Saturn’s atmosphere was obtained in November 1980 by the radio instrument onboard Voyager 1 as it flew past. Strong impulsive signals in the frequency range of a few MHz were detected and termed SEDs for Saturn Electrostatic Discharges (Warwick et al., 1981). However, no corresponding storm clouds were identified in the Voyager images. The Cassini mission clearly established the atmospheric origin of SEDs by combining imaging and radio. Its cameras found bright cloud features in Saturn’s so-called storm alley at a planetocentric latitude of 35°South, and the clouds were brighter when the SED rates were high (Dyudina et al., 2007; Fischer et al., 2007). Strong vertical convection may push the cloud particles above the uppermost ammonia cloud layer forming a 2,000 km–sized bright spot, which was observed by amateur astronomers on Earth as well as by Cassini (Fischer et al., 2011b). Since Saturn lightning is deeper in the atmosphere than Jupiter lightning, the optical flashes were more difficult to image, but it became possible around Saturn equinox (August 2009) when ring-shine at the nightside was minimized. The Cassini camera spotted flash-illuminated cloud tops with a diameter of about 200 km (Dyudina et al., 2010) as can be seen on the left side of Figure 2. The optical energy was 109 J, similar to the optical energy of Jovian lightning flashes. This suggests that Jovian and Saturnian lightning are superbolt-like with total energies around 1012 J, whereas the average terrestrial flash has a total energy of only 109 J. SEDs are also about 10,000 times stronger than radio emissions from terrestrial lightning in the frequency range of a few MHz (Fischer et al., 2008). This has led to their first detection from Earth, with the large Ukrainian radio telescope UTR-2 (Konovalenko et al., 2013).

Atmospheric Electricity in the Solar SystemClick to view larger

Figure 2. (Left) Cassini images of a Saturn storm cloud (2000 km across) at 35°South on the planet’s nightside from November 2009. The bright spots with diameters around 200 km are flash-illuminated cloud tops. (Right) Cassini image of the Great White Spot (GWS) on Saturn at 35°North from February 2011.

Images from NASA/JPL/SSI.

The SEDs were detected by the Cassini RPWS instrument in more than a dozen storms, with many of them located in “storm alley” at 35°South. These storms lasted for a few days up to several months, and there was one storm that almost lasted throughout the year 2009. There were also long periods of quiet with no SEDs, especially during the last four years of the mission. There are two different classes of Saturn lightning storms: the regular 2,000 km–sized thunderstorms with flash rates of a few SEDs per minute (Fischer et al., 2008), and the rare and giant Great White Spots (GWS) with SED rates of 10 s−1 (Fischer et al., 2011c). A GWS usually occurs only once per Saturn year (29.5 Earth years), and Cassini was lucky to see one in late 2010, located at 35°North, and reaching a latitudinal extension of 10,000 km after about three weeks. The storm developed an elongated eastward tail that wrapped around the planet (a distance of 300,000 km) by February 2011 (see right side of Figure 2). The westernmost head of the storm with most of the SED activity collided with a large anticyclonic vortex in mid-June 2011, which caused a large drop in SED activity. After the collision SEDs became intermittent and finally disappeared at the end of August 2011.

Cassini RPWS also looked for lightning on Saturn’s enigmatic moon Titan, which has a substantial atmosphere (1,500 hPa at the surface) consisting mainly of nitrogen and methane. The Cassini cameras spotted convective methane clouds on Titan, but no radio bursts that would clearly indicate the existence of Titan lightning were found by RPWS during numerous close flybys (Fischer & Gurnett, 2011).

Uranus and Neptune

The icy giants Uranus and Neptune have an envelope of hydrogen and helium, but at deep levels they consist of heavier elements, most likely including oxygen, carbon, nitrogen, and sulphur. They have a similar cloud structure to Jupiter and Saturn, with an uppermost methane cloud layer. So water clouds also exist on Uranus and Neptune and could be the place where lightning is created. Both Uranus and Neptune were only visited by one spacecraft so far, Voyager 2.

At Uranus 140 SED-like events were detected during the Voyager 2 flyby in January 1986 and termed UEDs for Uranus Electrostatic Discharges (Zarka & Pedersen, 1986). Their frequency range was 0.9 to 40 MHz (upper limit of the Voyager radio instrument) with a mean duration of 120 ms, and they were about an order of magnitude weaker than SEDs. No optical signals of lightning or whistlers were detected at Uranus. During the Neptune flyby of Voyager 2 in August 1989 sixteen whistler-like signals were detected by the Plasma Wave System (Gurnett et al., 1990) from 6–12 kHz, and four sferics at high frequencies were tentatively identified by Kaiser, Zarka, Desch, and Farrell (1991).

“Fair Weather” Atmospheric Electricity: Atmospheric electricity in non-stormy regions arises from air ionization and, in some environments, the effects of distant storms. A more detailed understanding of atmospheric electrical processes was naturally developed for Earth before any other planets. Canton was the first to detect that electricity was present in the air in cloudless skies, in the mid-18th century, and it was subsequently found that the air supported an electric field and was weakly conductive in conditions well away from thunderstorms or disturbed weather (Aplin, Harrison, & Rycroft, 2008). This ultimately led to the description of “fair weather” atmospheric electricity that is used, in the absence of other terminology, to describe non-storm electrical processes in all planetary atmospheres.

On Earth, there is a fair weather electric field of magnitude of about 100 Vm−1 at the surface (sea level). This electric field originates from a concept that has become known as the “global electric circuit” or GEC (Figure 3), first proposed by CTR Wilson (Wilson, 1921, 1929). Thunderstorms are continually active somewhere on Earth and deliver current to the ionosphere, a relatively conductive region where solar UV radiation can ionize air. As the atmosphere is only weakly conductive in comparison to the ionosphere and the surface (Rc in Figure 3), a potential difference of about 300 kV, VI in Figure 3, arises between the ionosphere and the surface. Ions formed by cosmic rays and natural radioactivity move in this electric field, providing a GEC-driven conduction current density (Jc in Figure 3) of typically 2 pAm−2 . The atmospheric electric field has a characteristic diurnal variation in universal time, arising from thunderstorms being generated in the late afternoon and early evening in local time across the globe, mainly above continents in the three well-defined regions, (SE Asia, Africa and S. America) sometimes referred to as “chimneys”. This diurnal variation, with a maximum at 19 UT, has become known as the “Carnegie curve” after the geomagnetic survey vessel that first detected it (e.g., Harrison, 2013). The development of the GEC concept required some scientific precursors, defined as “central tenets” by Aplin et al. (2008), such as the discovery of the ionosphere. Planetary GECs may exist, and are discussed in Global electric circuits in planetary atmospheres.

Atmospheric Electricity in the Solar SystemClick to view larger

Figure 3. Conceptual diagram of the terrestrial global electric circuit (GEC) (Aplin et al., 2008). Red arrows refer to current flow in the GEC. Black arrows refer to ionizing material.

Atmospheric Ions and Ionization

As explained in the first section, energetic particles called “cosmic rays” enter every atmosphere. Muons, or secondary cosmic rays, are the only particles energetic enough to ionize through deep atmospheres such as Venus and the giant planets. UV radiation can also cause daytime ionization in the tropospheres of planets without UV absorbers, such as Mars (e.g., Aplin, 2006). Atmospheric ionization profiles are dominated by cosmic rays, almost always with a minimum near the surface and a maximum near the level where most cosmic rays decay, typically in the upper troposphere or lower stratosphere (known as the Pfotzer-Regener maximum, e.g., Carlson & Watson, 2014). Local modulation of this profile can arise, for example, the ionization rate over the terrestrial surface decreases with altitude up to about 1 km due to the contribution from radioactive gases emitted from the surface. There are also latitudinal and solar cycle effects at planets with magnetic fields (e.g., Aplin, 2006).

Composition of Atmospheric Ions

Energetic particles ionize atmospheres to create positive ions and electrons; what happens next depends on atmospheric chemistry. Positive ions become clustered with polar species through a complex series of reactions to become stable terminal ions, frequently clustered with the species with locally highest proton affinity (Harrison & Carslaw, 2003; Shuman, Hunton, & Viggiano, 2015). If electrophilic, (“electron-loving”) species are present, free electrons attach rapidly to them to create a negative ion such as O2 or CO2. Other polar species may then join the core negative ion to create a cluster-ion. Atmospheres therefore usually contain terminal positive ions and some or all of electrons, negative ions, or negative cluster-ions. Cluster ions are usually described as X+(Y)n for a positive ion with core species X and clustered by ligands Y. On Earth, n is 2–10 but for other planets, models are less well constrained (Aplin, 2008). Ligands can be species such as H2O, NH3, HNO3, and organics; there is some evidence for a humidity dependence in terrestrial ion clustering (e.g., Harrison & Aplin, 2007).

As data is limited, ion species in planetary atmospheres are predicted by modeling, summarized in Table 1. The atmospheres of Earth and Venus contain electrophilic species and therefore negative cluster-ions, whereas at Neptune and Uranus, only free electrons are anticipated, and Mars and Titan are expected to contain a mixture of electrons and negative ions. The 2005 Huygens probe used chemical and electrical instruments to measure atmospheric and surface properties and better constrained Titan’s positive ion composition to nitrogenated species such as NH4+(NH3)n and HCNH+(HCN)n (e.g., Aplin, 2008, 2013).

Physical Properties and Relevance

The presence of ions and/or electrons makes an atmosphere weakly conductive, with a positive (or negative) conductivity σ‎ related to the volumetric number concentration of the positive (or negative) ions and/or electrons n and their mean mobility μ‎ expressed most simply by σ‎= neμ‎.

Mobility is defined as the velocity of an ion in a unit electric field, and is related to mass (e.g., Tammet, 2012). The typical mobility of a terrestrial surface atmospheric cluster-ion is 10−4 m2V−1s−1, and its size is 0.5 nm. Atmospheres containing free electrons are therefore many orders of magnitude more conductive than those containing more massive cluster-ions. For this reason, contributions from positive and negative conductivity can be unequal, see Table 1.

Ions are important in atmospheric physics because of their interaction with aerosol (small suspended particles). Ions attach to aerosol particles and charge them, reducing air conductivity. Charge on aerosols can change cloud growth and droplet lifetime through effects such as enhancement of collision efficiency. This can indirectly modulate a planet’s radiative balance. For example, models suggest that charged aerosol is important for particle growth on Titan, (e.g., Lindgren et al., 2017) and highly charged aerosols in Venus’s clouds could make the atmosphere conductive enough to suppress lightning, by limiting the build-up of charge that is required for lightning (Michael, Tripathi, Borucki, & Whitten, 2009).

Ions may also assist in the growth of aerosol (nucleation), which could affect cloud formation, weather, and climate. In a supersaturated environment such as a cloud chamber, condensation onto charged particles is enhanced (sometimes called “Wilson” nucleation after the eponymous cloud chamber). On Earth, water vapor concentrations are insufficient by two orders of magnitude for Wilson nucleation to occur, and the role of ion-induced nucleation in climate change is still hotly debated. Charge can assist nucleation (e.g., Hirsikko et al., 2011), but the effects on clouds and climate have yet to be fully quantified (e.g., Snow-Kropla et al., 2011; Pierce & Adams, 2009). In planetary environments, Wilson nucleation of methane seems likely to occur on Neptune (Moses, Allen, & Yung, 1992; Aplin & Harrison, 2016), Uranus (Aplin & Harrison, 2017) and has been suggested for sulphuric acid for Venus (Aplin, 2006). Ion-induced Wilson nucleation is also the only known explanation for the existence of nitrogen ice clouds on Neptune’s moon Triton (Delitsky, Turco, & Jacobson, 1990).

Measurements

The only in situ measurements of electric properties of a planetary atmosphere were provided by the Huygens probe on its descent to Titan’s surface in January 2005. The lander contained two types of conductivity instruments, a relaxation probe, and a mutual impedance probe. The rate of charge exchange (“relaxation”) of an isolated probe initially set at a fixed potential is related to the air conductivity. The mutual impedance probe consists of electrodes driven by an alternating current, where the mutual impedance (related to the conductivity) is given by the ratio of the voltage measured at a receiving dipole to the current injected into the medium by a nearby transmitting dipole (Hamelin et al., 2007). The relaxation probe can measure lower conductivities (and is used for terrestrial measurements (e.g., Aplin & Harrison, 2000), whereas the mutual impedance probe is sensitive to higher conductivities. Despite data loss from the relaxation probe, a peak conductivity at the Pfotzer-Regener maximum was recorded by both instruments, showing consistency with each other and with theory. The ill-fated Schiaparelli lander on Exomars also included a relaxation probe for atmospheric electric field and conductivity measurements (Harrison et al., 2016). Opportunistic measurements of atmospheric ions taken by an electron spectrometer on the Cassini spacecraft found that massive charged clusters in Titan’s upper atmosphere were precursors to its thick orange haze (Waite et al., 2007). Measurement of cluster-ions may be possible through their infra-red absorption properties, though work is needed to characterize the signatures of each species (Aplin, 2008; Aplin & Lockwood, 2015). Atmospheric electrical measurements have also been proposed on missions to Venus (Chassefière et al., 2009), and the outer planets (Mousis et al., 2017).

Table 1. Ion Properties for the Tropospheres and Stratospheres of Solar System Planets. The Ionization Environments at Uranus and Neptune are Similar and are Considered Together. The Conductivity at the Surface and at the Pfotzer-Regener Ionization Maximum are Given Unless Stated Otherwise

Planet

Nature of Lower Atmosphere Charge Carriers

Conductivity (Surface and/or at Maximum)

Role of Ionization

Notes and References

Positive

Negative

Positive

Negative

Venus

H3O+(H2O)n (n = 3 or 4), H3O+(SO2) and H3O(H2O+)(CO2)

Sulphate cluster-ions, with free electrons above 60 km

10 fSm−1 (surface)

10 fSm−1 (surface)

Possible “Wilson” nucleation and suppression of lightning

Modeling, discussed by Michael et al. (2009) and Aplin (2013)

10 pSm−1 (at 60 km, when dominated by free electrons)

Earth

Dominated by organics near the surface. In stratosphere H3O+(H2O)n (n = 2-10) more common

Cluster-ions mainly based on inorganic acids such as nitrate, sulphate, and iodic acids

1-10 fSm−1 (surface)

1-10 fSm−1 (surface)

Nucleation; infra-red absorption

Measurements, see Hirsikko et al. (2011) and Aplin and Lockwood (2015)

1 pSm−1 (20 km)

1 pSm−1 (20km)

Mars

H3O+(H2O)4

Free electrons;

1-100 fSm−1 (surface to 70 km)

1-10 pSm−1 (surface); 1 nSm−1 (70 km)

Free electrons may contribute to oxidant chemistry

Modeling. Peak ionization at the surface; conductivity increases to 70 km (Michael et al., 2008). Large diurnal variability

CO3(H2O)2

Jupiter

NH4+(NH3)n; CnHm+

Free electrons

1 aSm−1 (clear and cloudy air at 1000 hPa)

10 pSm−1 (clear air at 1000 hPa); 1 aSm−1 (in clouds)

Charge-assisted chemistry may be significant

Modeling (Whitten et al., 2008); and Lab measurements (Loeffler & Hudson, 2018)

Saturn

C2H9+, C3H11+

Free electrons

10 pSm−1 (peak at 1,000 hPa)

10 nSm−1 (peak at 1,000 hPa)

Modeling (Capone et al., 1977)

Titan

NH4+(NH3)n, HCNH+(HCN)n

Free electrons, some cluster-ions

1 fSm−1 (surface), 10 pSm−1 (70 km)

10 pSm−1 (surface), 3 nSm−1 (70 km)

Charge assists particle growth

Measurements and modeling; for example Aplin (2013), and Lindgren et al. (2017)

Uranus and Neptune

C2H9+, C3H11+

Free electrons

1 pSm−1 (peak at 35 hPa)

1 nSm−1 (peak at 35 hPa)

Evidence for “Wilson” nucleation

Modeling and Voyager 2/telescope data; Capone et al. (1977); Lellouch et al. (1994); Moses et al. (1992); and Aplin and Harrison (2016, 2017)

Global Electric Circuits in Planetary Atmospheres

The first suggestion of a GEC in another planetary atmosphere was in a 1998 undergraduate dissertation by Fillingim (no longer available, but referenced in Aplin, 2006), and a journal article (Farrell & Desch, 2001), both identifying Mars as a suitable candidate for a GEC. Venus and Titan were also suggested as possible Solar System hosts for an Earth-like GEC (Aplin, 2006, 2013). A planet must meet all the following conditions (Aplin et al., 2008) to have a GEC:

  • an upper conductive layer

  • a lower conductive layer

  • discharges or precipitation, to “charge” the circuit

  • current flow in fair weather, to “discharge” the circuit

Aplin et al. (2008) reviewed optimal measurements for GEC identification, and concluded that the Schumann resonance (defined in the section “Observation Techniques”) would be the most useful single parameter. The Schumann resonance requires both upper and lower conducting layers and electrical discharges, although detection of Schumann resonances would still not prove the existence of a GEC. An example of this is the detection of Schumann resonances at Titan, which are not caused by electrical discharges (consistent with the non-detection of Titan lightning described in section “Saturn”). Instead, they are thought to originate from excitation of Titan’s ionosphere by Saturn’s magnetosphere (Beghin et al., 2007). Despite this, Titan could still have a GEC, but instead of electrical discharges, methane raindrops could act to charge the circuit (see Aplin, 2013 and references therein). This is similar to Earth, where charged rain may contribute about 10% of the GEC charging current (Peterson et al., 2017). Venus could also potentially have a GEC like Earth’s, but this would require lightning, which is not yet unambiguously identified (see section “Venus”).

As was described in the first section, a GEC can couple the space environment with a planet’s troposphere, bringing the influence of space weather to planetary weather through the mechanisms described above. A GEC continuously distributes ions throughout an atmosphere, broadening their region of influence in particle formation and chemical processes (see Table 1). This may be particularly important at planets distant from the Sun where electrical influences on the weather and climate are relatively more significant.

Conclusions

Lightning exists on Earth, Jupiter, Saturn, Uranus, and Neptune, and it may also be present at Venus and Mars. Other electrical processes exist in all planetary atmospheres, with a range of potential effects. The global electric circuit concept is helpful in providing conceptual links between the space and planetary environment, and, within the planetary environment, both “fair” and “disturbed” weather. More generally, it offers a framework in which to understand and categorize other apparently unconnected atmospheric processes.

Measurements so far are limited to remote sensing of lightning’s electromagnetic signals, and in situ data from the atmospheres of Earth, Titan, and Jupiter. The Lightning and Airglow Camera on the Japanese Akatsuki mission is currently searching for lightning from Venus orbit. Mars is likely to be the next planet with in situ atmospheric electrical measurements, with several instruments on failed or cancelled landers; in particular, there is a plasma experiment capable of detecting electrical discharges on the Exomars 2020 lander. Atmospheric electrical measurements, both for fair and disturbed weather, have also been proposed for missions to Venus (Chassefière et al., 2009) and the outer planets (Mousis et al., 2017), but these are years away from becoming a reality.

For Earth, one of the biggest scientific questions is how lightning will be affected by climate change, a complex feedback process. Lightning has a significant influence on atmospheric chemistry, most notably through the generation of nitrogen oxides (Rakov & Uman, 2003) or as the trigger for wildfires, which shape the evolution of ecosystems. It was estimated by Romps et al. (2014) that climate change will increase the number of lightning flashes by about 12%/°C, due to the increase in the amount of energy available for vertical convection. However, Finney et al. (2018), using a more realistic model representation of thundercloud charging processes, predicts a 15% decrease in lightning rate for a strong warming scenario. Planetary environments clearly do not suffer from anthropogenic climate change, and long-term measurements of their atmospheres, including lightning, are becoming more possible due to long-lived space missions and the improvement in Earth-based planetary lightning detection. There are many sources of variability in planetary atmospheres affecting dynamics, convection, and lightning. These need further investigation through modeling and terrestrial analogue experiments as well as data from space missions.

The second big scientific question is how and where ionization affects the atmosphere on Earth; several mechanisms may contribute to cloud formation and lifetime, temperature and rainfall rate, but the overall effect on weather and climate is not well known. This second question is far less controversial for planetary environments, several of which almost certainly support ionization-related cloud formation. Detection of a GEC, for example, on Mars, would assist in understanding how atmospheric electricity enhances chemistry that may be involved in the development of life.

Selected Bibliography

Aplin, K. L., Harrison, R. G., & Rycroft, M. J. (2008). Investigating Earth’s atmospheric electricity: A role model for planetary studies. Space Science Reviews, 137, 1–4, 11–27.Find this resource:

Aplin, K. L. (2013). Electrifying atmospheres: Charging, ionisation and lightning in the Solar System and beyond. Dordrecht, The Netherlands: Springer.Find this resource:

Chalmers, J. A. (1967). Atmospheric electricity (2nd ed.). Oxford, U.K.: Pergamon Press.Find this resource:

Cooray, V. (2015). An introduction to lightning. Dordrecht, The Netherlands: Springer.Find this resource:

Dwyer, J. R., & Uman, M. A. (2014). The physics of lightning. Physics Reports, 534, 147–241.Find this resource:

Fischer, G., Gurnett, D. A., & Yair, Y. (2011). Extraterrestrial lightning and its past and future investigation. In M. D. Wood (Ed.), Lightning: Properties, formation and types (pp. 19–38). New York, NY: Nova Science.Find this resource:

Leblanc, F., Aplin, K. L., Yair, Y., Harrison, R. G., Lebreton, J.-P., & Blanc, M. (Eds.). Planetary Atmospheric Electricity. New York, NY: Springer, 2008.Find this resource:

Pasko, V. P., Yair, Y., & Kuo, C.-L. (2012). Lightning related transient luminous events at high altitude in the Earth’s atmosphere: Phenomenology, mechanisms and effects. Space Science Reviews, 168, 475–516.Find this resource:

Rakov, V. A., & Uman, M. A. (2003). Lightning—physics and effects. Cambridge, U.K.: Cambridge University Press, 2003.Find this resource:

Uman, M. A. (1987). The lightning discharge. Orlando, FL: Academic Press.Find this resource:

Yair, Y. (2012). New results on planetary lightning. Advances in Space Research, 50, 293–310.Find this resource:

References

Aplin, K. L. (2006). Atmospheric electrification in the Solar System. Surveys in Geophysics, 27(1), 63–108.Find this resource:

Aplin, K. L. (2008). Composition and measurement of charged atmospheric clusters. Space Science Reviews, 137, 1–4, 213–224.Find this resource:

Aplin, K. L. (2013). Electrifying atmospheres: Charging, ionisation and lightning in the Solar System and beyond. Dordrecht, The Netherlands: Springer.Find this resource:

Aplin, K. L., Bennett, A. J., Harrison, R. G., & Houghton, I. M. P. (2016). Electrostatics and in situ sampling of volcanic plumes In S. Mackie, C. Cashman, H. Ricketts, A. Rust, & M. Watson (Eds.), Volcanic ash: Hazard observation and monitoring. Amsterdam, The Netherlands: Elsevier.Find this resource:

Aplin, K. L., & Harrison, R. G. (2000). A computer-controlled Gerdien atmospheric ion counter. Reviews of Scientific Instruments, 71(8), 3037–3041.Find this resource:

Aplin, K. L., & Harrison, R. G. (2016). Determining solar effects in Neptune’s atmosphere. Nature Communications, 7, 11976.Find this resource:

Aplin, K. L., & Harrison, R. G. (2017). Solar-driven variability in the atmosphere of Uranus. Geophysical Research Letters, 44.Find this resource:

Aplin, K. L., Harrison, R. G., & Rycroft, M. J. (2008). Investigating Earth’s atmospheric electricity: A role model for planetary studies. Space Science Reviews, 137, 1–4, 11–27.Find this resource:

Aplin, K. L., & Fischer, G. (2017). Lightning detection in planetary atmospheres. Weather, 72, 46–50.Find this resource:

Aplin, K. L., & Lockwood, M. (2015). Further considerations of cosmic ray modulation of infra-red radiation in the atmosphere. Astroparticle Physics, 68, 52–60.Find this resource:

Béghin, C., Simões, F., Kasnoselskikh, V., Schwingenschuh, K., Berthelier, J. J., Besser, B. P., . . . Tokano, T. (2007). A Schumann-like resonance on Titan driven by Saturn’s magnetosphere possibly revealed by the Huygens Probe. Icarus, 191, 251–266.Find this resource:

Brooks, C. E. P. (1925). The distribution of thunderstorms of the globe. Geophysical Memoirs, 13, 147–164. London, U.K.: Air Ministry, Meteorological Office.Find this resource:

Capone, L. A., Whitten, R. C., Prasad, S. S., & Dubach, J. (1977). The ionospheres of Saturn, Uranus, and Neptune. The Astrophysical Journal, 215, 977–983.Find this resource:

Cardesín Moinelo, A., Abildgaard, S., García Muñoz, A., Piccioni, G., & Grassi, D. (2016). No statistical evidence of lightning in Venus night-side atmosphere from VIRTIS-Venus Express visible observations. Icarus, 277, 395–400.Find this resource:

Carlson, P., & Watson, A. A. (2014). Erich Regener and the ionisation maximum in the atmosphere. History of Geo- and Space Sciences, 5, 175–182.Find this resource:

Chassefière, E., Korablev, O., Imamura, T., Baines, K. H., Wilson, C. F., Titov, D. V., . . . EVE team. (2009). European Venus explorer: An in-situ mission to Venus using a balloon platform. Advances in Space Research, 44, 106–115.Find this resource:

Christian, H. J., Blakeslee, R. J., Boccippio, D. J., Boeck, W. L., Buechler, D. E., Driscoll, K. T., . . . Stewart, M. F. (2003). Global frequency and distribution of lightning as observed from space by the Optical Transient Detector. Journal of Geophysical Research, 108(D1), 4005.Find this resource:

Cook, A. F., Duxbury, T. C., & Hunt, G. E. (1979). First results on Jovian lightning. Nature, 280, 794.Find this resource:

Cooray, V. (2015). An introduction to lightning. Dordrecht, The Netherlands: Springer.Find this resource:

Curtius, J., Lovejoy, E. R., & Froyd, K. D. (2006). Atmospheric ion-induced aerosol nucleation. Space Science Reviews, 125(1–4), 159–167.Find this resource:

Delitsky, M. L., Turco, R. P., & Jacobson, M. Z. (1990). Nitrogen ion clusters in Triton’s atmosphere. Geophysical Research Letters, 17(10), 1725–1728.Find this resource:

Dubinova, A., Rutjes C., Ebert, U., Buitink, S., Scholten, O., & Trinh, G. T. N. (2015). Prediction of lightning inception by large ice particles and extensive air showers. Physical Review Letters, 115(1), 015002.Find this resource:

Dwyer, J. R., & Uman, M. A. (2014). The physics of lightning. Physics Reports, 534, 147–241.Find this resource:

Dyudina, U. A., Ingersoll, A. P., Ewald, S. P., Porco, C. C., Fischer, G., Kurth, W. S., . . Ferrier, J. (2007). Lightning storms on Saturn observed by Cassini ISS and RPWS during 2004–2006. Icarus, 190, 545–555.Find this resource:

Dyudina, U. A., Ingersoll, A. P., Ewald, S. P., Porco, C. C., Fischer, G., Kurth, W. S., . . . West, R. A. (2010). Detection of visible lightning on Saturn. Geophysical Research Letters, 37, L09205.Find this resource:

Elsom, D. M., Enno, S.-E., Horseman, A., & Webb, J. D. C. (2018). Compiling lightning counts for the UK land area and an assessment of the lightning risk facing UK inhabitants. Weather, 73(6), 171–179.Find this resource:

Farrell, W. M., & Desch, M. D. (2001). Is there a Martian atmospheric electric circuit? Journal of Geophysical Research: Planets, 106(E4), 7591–7595.Find this resource:

Farrell, W. M., Kaiser, M. L., Desch, M. D., Houser, J. G., Cummer, S. A., Wilt, D. M., . . . Landis, G. A. (1999). Detecting electrical activity from Martian dust storms. Journal of Geophyical Research, 104(E2), 3795–3802.Find this resource:

Finney, D. L., Doherty, R. M., Wild, O., Stevenson, D. S., MacKenzie, I. A., & Blyth, A. M. (2018). A projected decrease in lightning under climate change. Nature Climate Change, 1, 210–213.Find this resource:

Fischer, G., & Gurnett, D. A. (2011). The search for Titan lightning radio emissions. Geophysical Research Letters., 38, L08206.Find this resource:

Fischer, G., Gurnett, D. A., Kurth, W. S., Akalin, F., Zarka, P., Dyudina, U. A., . . . Kaiser, M. L. (2008). Atmospheric electricity at Saturn. Space Science Reviews, 137, 271–285.Find this resource:

Fischer, G., Gurnett, D. A., & Yair, Y. (2011a). Extraterrestrial lightning and its past and future investigation. In M. D. Wood (Ed.), Lightning: Properties, formation and types (pp. 19–38). New York, NY: Nova Science.Find this resource:

Fischer, G., Dyudina, U. A., Kurth, W. S., Gurnett, D. A., Zarka, P., Barry, T., . . . Wesley, A. (2011b). Overview of Saturn lightning observations. In H. O. Rucker (Ed.), Planetary radio emissions VII (pp. 135–144). Vienna: Austrian Academy of Sciences Press.Find this resource:

Fischer, G., Kurth, D. A., Gurnett, P., Zarka, U. A., Dyudina, A. P., Ingersoll, S. P., . . . Delcroix, M. (2011c). A giant thunderstorm on Saturn. Nature, 475, 75–77.Find this resource:

Fischer, G., Kurth, W. S., Dyudina, U. A., Kaiser, M. L., Zarka, P., Lecacheux, A., . . . Gurnett, D. A. (2007). Analysis of a giant lightning storm on Saturn. Icarus, 190, 528–544.Find this resource:

Fishman, G. J., Bhat, P. N., Mallozzi, R., Horack, J. M., Koshut, T., Kouveliotou, C., . . . Christian, H. J. (1994). Discovery of intense gamma-ray flashes of atmospheric origin. Science, 264, 1313–1316.Find this resource:

Franz, R. C., Nemzek, R. J., & Winckler, J. R. (1990). Television image of a large upward electrical discharge above a thunderstorm system. Science, 249, 48–51.Find this resource:

Fulchignoni, M., Ferri, F., Angrilli, F., Ball, A. J., Bar-Nun, A., Barucci, M. A., . . . Coradini, M. (2005). In situ measurements of the physical characteristics of Titan’s environment. Nature, 438(7069), 785.Find this resource:

Gurevich, A. V., Milikh, G. M., & Roussel-Dupre, R. (1992). Runaway electron mechanism of air breakdown and preconditioning during a thunderstorm. Physics Letters A, 165, 463–468.Find this resource:

Gurnett, D. A., Morgan, D. D., Granroth, L. J., Cantor, B. A., Farrell, W. M., & Espley, J. R. (2010). Non-detection of impulsive radio signals from lightning in Martian dust storms using the radar receiver on the Mars Express spacecraft. Geophysical Research Letters, 37, L17802.Find this resource:

Gurnett, D. A., Zarka, P., Manning, R., Kurth, W. S., Hospodarsky, G. B., Averkamp, T. F., . . . Farrell, W. M. (2001). Non-detection at Venus of high-frequency radio signals characteristic of terrestrial lightning. Nature, 409, 313–315.Find this resource:

Gurnett, D. A., Shaw, R. R., Anderson, R. R., & Kurth, W. S. (1979). Whistlers observed by Voyager 1—Detection of lightning on Jupiter. Geophysical Research Letters, 6, 511–514.Find this resource:

Gurnett, D. A., Kurth, W. S., Roux, A., Gendrin, R., Kennel, C. F., & Bolton, S. J. (1991). Lightning and plasma wave observations from the Galileo flyby of Venus. Science, 253, 1522–1525.Find this resource:

Gurnett, D. A., Kurth, W. S., Cairns, I. H., & Granroth, L. J. (1990). Whistlers in Neptune’s magnetosphere: Evidence of atmospheric lightning. Journal of Geophyical. Research, 95, A12, 20967–20976.Find this resource:

Haberle, R. M., Clancy, R. T., Forget, F., Smith, M. D., & Zurek, R. W. (2017). The atmosphere and climate of Mars. Cambridge, U.K.: Cambridge University Press.Find this resource:

Hamelin, M., Béghin, C., Grard, R., López-Moreno, J. J., Schwingenschuh, K., Simões, F., . . . Falkner, P. (2007). Electron conductivity and density profiles derived from the mutual impedance probe measurements performed during the descent of Huygens through the atmosphere of Titan. Planetary and Space Science, 55(13), 1964–1977.Find this resource:

Hansell, S. A., Wells, W. K., & Hunten, D. M. (1995). Optical detection of lightning on Venus. Icarus, 117, 345–351.Find this resource:

Harrison, R. G., & Aplin, K. L. (2007). Water vapour changes and atmospheric cluster ions. Atmospheric Research, 85(2), 199–208.Find this resource:

Harrison, R. G., Barth, E., Esposito, F., Merrison, J., Montmessin, F., Aplin, K. L., . . . Zimmerman, M. (2016). Applications of electrified dust and dust devil electrodynamics to Martian atmospheric electricity. Space Science Reviews, 1–4, 299–345.Find this resource:

Harrison, R. G., Nicoll, K. A., & Aplin, K. L. (2017). Evaluating stratiform cloud base charge remotely. Geophysical Research Letters, 44(12), 6407–6412.Find this resource:

Harrison, R. G. (2013). The Carnegie curve. Surveys in Geophysics, 34(2), 209–232.Find this resource:

Harrison, R. G., & Carslaw, K. S. (2003). Ion‐aerosol‐cloud processes in the lower atmosphere. Reviews of Geophysics, 41(3).Find this resource:

Hirsikko, A., Nieminen, T., Gagné, S., Lehtipalo, K., Manninen, H. E., Ehn, M., . . . Mirme, A. (2011). Atmospheric ions and nucleation: A review of observations. Atmospheric Chemistry and Physics, 11(2), 767–798.Find this resource:

Hodosán, G., Rimmer, P. B., & Helling, C. (2016). Is lightning a possible source of the radio emission on HAT-P-11b? Monthly Notices of the Royal Astronomical Society, 461(2), 1222–1226.Find this resource:

Idone, V. P., & Orville, R. E. (1985). Correlated peak relative light intensity and peak current in triggered lightning subsequent return strokes. Journal of Geophyical. Research, 90(D4), 6159–6164.Find this resource:

Kaiser, M. L., Zarka, P., Desch, M. D., & Farrell, W. M. (1991). Restrictions on the characteristics of Neptunian lightning. Journal of Geophyical. Research, 96, 19043–19047.Find this resource:

Kolmasova, I., Santolik, O., & Skalsky, A. (2017). Anticipated plasma wave measurements onboard Exomars 2020 surface platform. In G. Fischer, G. Mann, & P. Zarka (Eds.), Planetary radio emissions VIII (pp. 487–493). Vienna: Austrian Academy of Sciences Press.Find this resource:

Konovalenko, A. A., Kalinichenko, N. N., Rucker, H. O., Lecacheux, A., Fischer, G., Zarka, P., . . . Gurnett, D. A. (2013). Earliest recorded ground-based decameter wavelength observations of Saturn’s lightning during the giant E-storm detected by Cassini spacecraft in early 2006. Icarus, 224, 14–23.Find this resource:

Krasnopolsky, V. A. (1983). Venus spectroscopy in the 3000–8000 Å region by Veneras 9 and 10. In D. M. Hunten, L. Colin, T. M. Donahue, & V. I. Moroz (Eds.), Venus (pp. 459–483). Tucson: University of Arizona Press.Find this resource:

Ksanfomaliti, L. V. (1980). Discovery of frequent lightning discharges in the clouds on Venus. Nature, 284, 244–246.Find this resource:

Kurth, W. S., Imai, M., Hospodarsky, G. B., Gurnett, D. A., Tetrick, S. S., Ye, S.-Y., . . . Levin, S. M. (2017). First observations near Jupiter by the Juno Waves investigation. In G. Fischer, G. Mann, & P. Zarka (Eds.), Planetary radio emissions VIII (pp. 1–12). Vienna: Austrian Academy of Sciences Press.Find this resource:

Lanzerotti, L. J., Rinnert, K., Dehmel, G., Gliem, F. O., Krider, E. P., Uman, M. A., & Bach, J. (1996). Radio frequency signals in Jupiter’s atmosphere. Science, 272, 858–860.Find this resource:

Lellouch, E., Romani, P. N., & Rosenqvist, J. (1994). The vertical distribution and origin of HCN in Neptune’s Atmosphere. Icarus, 108, 112–136.Find this resource:

Lindgren, E. B., Stamm, B., Chan, H. K., Maday, Y., Stace, A. J., & Besley, E. (2017). The effect of like-charge attraction on aerosol growth in the atmosphere of Titan. Icarus, 291, 245–253.Find this resource:

Loeffler, M. J., & Hudson, R. L. (2018). Coloring Jupiter’s clouds: Radiolysis of ammonium hydrosulfide (NH4SH). Icarus, 302, 418–425.Find this resource:

Lorenz, R. D. (2008). Atmospheric electricity hazards. Space Science Reviews, 137(1–4), 287–294.Find this resource:

Melnik, O., & Parrot, M. (1998). Electrostatic discharge in Martian dust storms. Journal of Geophyical Research, 103(A12), 29107–29118.Find this resource:

Mezuman, K., Price, C., & Galanti, E. (2014). On the spatial and temporal distribution of global thunderstorm cells. Environmental Research Letters, 9, 124023.Find this resource:

Michael, M., Tripathi, S. N., & Mishra, S. K. (2008). Dust charging and electrical conductivity in the day and nighttime atmosphere of Mars. Journal of Geophysical Research: Planets, 113(E7).Find this resource:

Michael, M., Tripathi, S. N., Borucki, W. J., & Whitten, R. C. (2009). Highly charged cloud particles in the atmosphere of Venus. Journal of Geophysical Research: Planets, 114(E4).Find this resource:

Miller, S. L., & Urey, H. C. (1959). Organic compound synthesis on the primitive Earth. Science, 130, 245–251.Find this resource:

Moses, J. I., Allen, M., & Yung, Y. L. (1992). Hydrocarbon nucleation and aerosol formation in Neptune’s atmosphere. Icarus, 99(2), 318–346.Find this resource:

Mousis, O., Atkinson, D. H., Cavalie, T., Fletcher, L., Amato, M. J., Aslam, S., . . . .Villanueva, G. L. (2017). Scientific rationale for Uranus and Neptune in situ explorations. Planetary and Space Science.Find this resource:

Pasko, V. P., Yair, Y., & Kuo, C.-L. (2012). Lightning related Transient Luminous Events at high altitude in the Earth’s atmosphere: Phenomenology, mechanisms and effects. Space Science Reviews, 168, 475–516.Find this resource:

Perez-Invernon, F. J., Luque, A., & Gordillo-Vazquez, F. J. (2017). Three-dimensional modelling of lightning-induced electromagnetic pulses on Venus, Jupiter, and Saturn. Journal of Geophyical. Research, 122(7), 7636–7653.Find this resource:

Peterson, M., Deierling, W., Liu, C., Mach, D., & Kalb, C. (2017). A TRMM/GPM retrieval of the total mean generator current for the global electric circuit. Journal of Geophyical Research, 122, 10025–10049.Find this resource:

Pierce, J. R., & Adams, P. J. (2009). Uncertainty in global CCN concentrations from uncertain aerosol nucleation and primary emission rates. Atmospheric Chemistry and Physics, 9(4), 1339–1356.Find this resource:

Rakov, V. A., & Uman, M. A. (2003). Lightning—physics and effects. Cambridge, U.K.: Cambridge University Press.Find this resource:

Romps, D. M., Seeley, J. T., Vollaro, D., & Molinari, J. (2014). Projected increase in lightning strikes in the United States due to global warming. Science, 346, 851–854.Find this resource:

Russell, C. T. (1993). Planetary lightning. Annual Reviews of Earth and Planetary Science, 21, 43–87.Find this resource:

Russell, C. T., Zhang, T. L., Delva, M., Magnes, W., Strangeway, R. J., & Wei, H. Y. (2007). Lightning on Venus inferred from whistler-mode waves in the ionosphere. Nature, 450, 661–662.Find this resource:

Shuman, N. S., Hunton, D. E., & Viggiano, A. A. (2015). Ambient and modified atmospheric ion chemistry: From top to bottom. Chemical Reviews, 115(10), 4542–4570.Find this resource:

Simões, F., Grard, R., Hamelin, M., López-Moreno, J. J., Schwingenschuh, K., Béghin, C., . . . Tokano, T. (2008). The Schumann resonance: A tool for exploring the atmospheric environment and the subsurface of the planets and their satellites. Icarus, 194(1), 30–41.Find this resource:

Snow-Kropla, E. J., Pierce, J. R., Westervelt, D. M., & Trivitayanurak, W. (2011). Cosmic rays, aerosol formation and cloud-condensation nuclei: Sensitivities to model uncertainties. Atmospheric Chemistry and Physics, 11(8), 4001–4013.Find this resource:

Stolzenburg, M., Rust W. D. and Marshall T. C. (1998). Electrical structure in thunderstorm convective regions: 3. Synthesis. Journal of Geophyical Research, 103(D12), 14097–14108.Find this resource:

Tammet, H. (2012). The function-updated Millikan model: A tool for nanometer particle size-mobility conversions. Aerosol Science and Technology, 46(10), i–iv.Find this resource:

Taylor, H. A., & Grebowsky, J. M. (1985). Venus nightside ionospheric troughs—Implications for evidence of lightning and volcanism. Journal of Geophyical Research, 90, 7415–7426.Find this resource:

Uman, M. A. (1987). The lightning discharge. Orlando, FL: Academic Press.Find this resource:

Waite, J. H., Young, D. T., Cravens, T. E., Coates, A. J., Crary, F. J., Magee, B., & Westlake, J(2007). The process of Tholin formation in Titan’s upper atmosphere. Science, 316(5826), 870–875.Find this resource:

Warwick, J. W., Pearce, J. B., Evans, D. R., Carr, T. D., Schauble, J. J., Alexander, J. K., . . . Barrow, C. H. (1981). Planetary radio astronomy observations from Voyager 1 near Saturn. Science, 212, 239–243.Find this resource:

Whitten, R. C., Borucki, W. J., O’Brien, K., & Tripathi, S. N. (2008). Predictions of the electrical conductivity and charging of the cloud particles in Jupiter’s atmosphere. Journal of Geophysical Research: Planets, 113(E4).Find this resource:

Wilson, C. T. R. (1926). Notebook entitled ‘atmospheric electricity’ ref CW/B/7. Library and Archives of the Royal SocietyFind this resource:

Wilson, C. T. R. (1929). Some thundercloud problems. Journal of the Franklin Institute, 208(1), 1–12.Find this resource:

Wilson, C. T. R. (1921). Investigations on lightning discharges and on the electric field of thunderstorms. Philosphical Transactions of the Royal Society of London, A 221(73).Find this resource:

Wilson, C. T. R. (1956). A theory of thundercloud electricity. Proceedings of the Royal Society of London A, 236, 297–317.Find this resource:

Yair, Y. (2012). New results on planetary lightning. Advancesin Space Research, 50, 293–310.Find this resource:

Yair, Y., Takahashi, Y., Yaniv, R., Ebert, U., & Goto, Y. (2009). A study of the possibility of sprites in the atmospheres of other planets. Predictions of the electrical conductivity and charging of the cloud particles in Jupiter’s atmosphere, 114, E09002.Find this resource:

Zarka, P., & Pedersen, B. M. (1986). Radio detection of Uranian lightning by Voyager 2. Nature, 323, 605–608.Find this resource:

Zarka, P. (1985). On the detection of radio bursts associated with Jovian and Saturnian lightning. Astronomy and Astrophysics, 146, L15–L18.Find this resource: