A review of the impact of transient luminous events on the atmospheric chemistry: Past, present, and future

In this article we explore theoretically the role of lightning and atmospherical discharges in destabilizing Ultra Low Frequency (ULF) waves in the Ionosphere. The idea, which could seem quite exotic, takes its origin from the experimental observation that the occurrence of Ultra Low Frequency ULF ~ 6–50 Hz) waves is strongly related to the occurrence of positive polarity cloud-to-ground flashes (+CG) The interest in the subject, on the other hand, comes from the fact that the occurrence of ULF waves is not only related to the occurrence of positive polarity flashes but also to the occurrence of newly discovered transient luminous phenomena as sprites, blue jets and gigantic jets. Sprites, as well as blue jets and gigantic jets, are luminous events which have been detected in discharges between thunderclouds (~ 10 km from Earth's surface) and the lower Ionosphere (mesosphere ~90 km) These transient luminous phenomena strongly affect the atmospheric electricity, due to transfer of large amounts of charge between different regions of the atmosphere (a gigantic jet can remove as much as 0.02% of the total atmospheric charge), and suggest that some important components of the global electric circuit have still to be identified and incorporated in the theoretical framework. Another important aspect is that, during the charge transfer process (electrical discharge), sprites, blue jets and gigantic jets modify the chemistry of a large portion of the stratosphere and mesosphere, with poorly understood influences on global climate changes. In order to survey the total rate of occurrence and the implications of such phenomena on continental and global scale one needs a signature that can be easily detected by remote measurements and independent from in situ video observations. To understand whether the ULF waves can be used as sprites signatures, we explore the relation between positive polarity lightning and ULF waves by presenting a theory which explains the origin and threshold of the observed ULF waves. As a result, we obtain that very large positive polarity flashes give rise to ULF Dust Acoustic waves in the Ionosphere. We also find that the ULF waves may be destabilized, in absence of positive polarity flashes, by mean of very large Ionosphere to thunderclouds discharges, such as gigantic blue jets, which create large dipole electric fields, comparable with the ones produced by +CG lightning.

The serendipitous observation in 1989 of electrical discharge in the high atmosphere induced by thundercloud lightning launched a new field of geophysical investigation. From this single unexpected observation sprang a vigorous and fertile new research field that simultaneously encompasses geophysical disciplines that are normally pursued independently, such as meteorology and lightning, plasma and gas discharge physics, atmospheric chemistry, ionospheric physics, and energetic particle physics.Transient electrical discharge in the upper atmosphere spans the full range of altitudes between the tropopause and the ionosphere and takes a variety of forms that carry the whimsical names red sprites, blue jets, gigantic jets, elves (emissions of light and very low frequency perturbations from electromagnetic pulse sources), and sprite halos, collectively known as transient luminous events (TLEs). To date, TLEs have been observed from ground and airborne or space borne platforms above thunderstorm systems worldwide, and radio observations made concomitantly with optical observations have shown that they are produced by the transient far fields of thundercloud lightning. TLEs appear to be large‐scale (tens of kilometers in dimension), upper atmospheric versions of conventional gas discharge akin to weakly ionized, collision‐dominated systems found in laboratory discharge devices (millimeter‐centimeter dimensions), with characteristic energies of a few electron volts. The dominant physical processes have been identified as described by the familiar kinetic theory of the photochemistry of the upper atmosphere, but with electric field—driven electron impact ionization playing the role of photolysis or energetic precipitating particle—induced ionization.

Transient luminous events (TLEs) such as sprites, blue jets (BJs) and elves have been studied intensively during the last three decades, and much is now known of their properties. This progress is caused by several factors including satellite optical observations, ground-based measurements of sprite-produced electromagnetic fields, the use of high-speed video observations and telescopic cameras with high resolution that enables one to trace the dynamics of sprite and BJ development. In this paper, we review various types of TLEs, including recently discovered dancing sprites, gnomes, ultraviolet (UV) atmospheric flashes and other effects. The sprite initiation, visible evolution, streamer structure, and their relationship with intra-cloud (IC) process are discussed. Considerable study has been given to ULF/ELF measurements which can provide us with important information on the delayed sprite generation and the role played by IC processes in the perturbation of the lower ionosphere above the sprite. A set of electrodynamic and transport kinetic equations describing the TLEs are complicated because the number densities, mobilities of electrons and ions, reaction constants and other parameters are strongly dependent on altitude. Because of this, the majority of theoretical study of TLEs and other large-scale optical phenomena at high altitude are based on numerical modeling of the basic kinetic, transport and electrodynamic equations describing TLEs evolution, whereas the analytical theory remains a formidable task to be accomplished. In this paper, we review a few analytical results, which have been recently derived from simple physical models of the TLEs phenomena. In the remainder of this paper, we focus our attention on the properties of UV flashes in the mesosphere, which have been observed onboard Russian microsatellites “Universitetsky-Tanyana” and “Vernov.” Such a kind of optical flash is referred to as a transient atmospheric event, which differs from the TLEs in optical energy, duration and other parameters.

For many years it has been suggested that transient luminous events (TLE) occurring over thunderstorms may produce significant modifications to neutral atmospheric chemistry. Some have speculated that large ionisation increases from red sprites, one type of TLE, could result in enhancements of odd nitrogen. In this study we make use of nighttime NO2 observations by the GOMOS instrument to test whether TLE are producing significant NOx enhancements in the middle atmosphere on a regional scale. Comparing regional variations of NO2 with 2–3 order of magnitude variations in lightning activity, we show that there is no significant impact of red sprites, giant jets or blue jets upon NOx levels in the stratosphere and mesosphere (20–70 km), within the detection levels of the instrument. While individual TLE may cause a local variation in NOx, these do not appear to be significant on regional scales (or beyond).

In 1999, the first sprites were observed above European thunderstorms using sensitive cameras. Since then, Eurosprite campaigns have been conducted to observe sprites and other transient luminous events (TLEs), expanding into a network covering large parts of Europe and coastal areas. In 2009 through 2013, the number of optical observations of TLEs reached a peak of 2000 per year. Because of this unprecedented number of European observations, it was possible to construct a climatology of 8394 TLEs observed above 1018 thunderstorm systems and study for the first time their distribution and seasonal cycle above Europe and parts of the Mediterranean Sea. The number of TLEs per thunderstorm was found to follow a power law, with less than 10 TLEs for 801 thunderstorms and up to 195 TLEs above the most prolific one. The majority of TLEs were classified as sprites, 641 elves, 280 halos, 70 upward lightning, 2 blue jets and 1 gigantic jet. The climatology shows intense TLE activity during summer over continental areas and in late autumn over coastal areas and sea. The two seasons peak, respectively, in August and November, separated by March and April with almost no TLEs, and a relative minimum around September. The observed TLE activity, i.e. mostly sprites, is shown to be largely consistent with lightning activity, with a 1/1000 of observed TLE-to-lightning ratio in regions with most observations. The overall behaviour is consistent among individual years, making the observed seasonal cycle a robust general feature of TLE activity above Europe.

An overview of general phenomenology and proposed physical mechanisms of large scale electrical discharges termed ‘blue jets’ and ‘gigantic jets’ observed at high altitude in the Earth's atmosphere above thunderstorms is presented. The primary emphasis is placed on summarizing available experimental data on the observed morphological features of upward jet discharges and on the discussion of recently advanced theories describing electrodynamic conditions, which facilitate escape of conventional lightning leaders from thundercloud tops and their upward propagation toward the ionosphere. It is argued that the filamentary plasma structures observed in blue jet and gigantic jet discharges are directly linked to the processes in streamer zones of lightning leaders, scaled by a significant reduction of air pressure at high altitudes.

Blue jets and blue starters are considered as positive streamer coronas expanding from the streamer zones of conventional lightning leaders under conditions when large‐scale electric fields near the thundercloud tops exceed the minimum field required for the propagation of positive streamers in air. Results from a three‐dimensional fractal model based on a phenomenological probabilistic approach to the modeling of streamer coronas indicate, in particular, that blue jets and blue starters can be formed by a fast (∼1 s) accumulation of ∼110–150 C of positive thundercloud charge distributed in a volume with effective radius ∼3 km near the cloud top at ∼15 km. The model simulates the propagation of branching streamer channels constituting blue jets and blue starters as a three‐dimensional growth of fractal trees in the electric field created by thundercloud charges and self‐consistently accounts for the electric field effects due to the propagating streamers. Model results closely resemble blue jet and blue starter characteristics in terms of their altitude extents, transverse dimensions, and conical structure and indicate that blue starters are related to the initial phases of blue jets. The proposed model is supported by the recent spectroscopic observations of blue jets and blue starters, in particular, by the documentation of the 427.8 nm (first negative N2+) emission in blue starters as well as by the low‐pressure laboratory experiments on emission spectroscopy of corona streamers in air. The model results also appear to be in excellent agreement with the recent discovery of the streamer structure of blue jets.

High-altitude electrical discharges associated with thunderstorms and lightning

A small space-telescope equipped with a micro-electro-mechanical system (MEMS) micro-mirror is applied to space missions for observing random, rare and temporal events like transient luminous events (TLEs). The measurement of TLEs with fine time resolution will show the different temporal profiles predicted by the various models for sprites, blue jets, elves and halos. The proposed space-telescope consists of three components: two sub-telescopes with different focal lengths and a spectrometer. The trigger telescope with a short focal length surveys a wide field of view. The zoom-in telescope with a long focal length looks into a small field of view area that is part of the trigger telescope’s wide field of view. Upon identifying a candidate TLE, the trigger telescope determines the location of the event and provides the location to the MEMS micro-mirror. Then, the micro-mirror, which is placed as a pinhole in front of the zoom-in telescope, rotates its mirror plane by such an angle that the zoom-in telescope will watch the small field of view around the center of the event. In this manner, the zoom-in telescope achieves the zoom-in designed by its long focal length. The first such small-space telescope, the MEMS Telescope for Extreme Lightning (MTEL), was launched into space in 2009 and identified a few candidates sprites. However a power failure (over-charge of the solar battery) of the main satellite occurred, and the MTEL was not able to continue space operation to acquire sizable statistics for TLE events. We developed and constructed the second small-space telescope, called MTEL-II, to continue to observe TLE events in space. In this paper, we present the performance of MTEL-II based on ground tests.

The ISUAL gigantic jets (GJs) are categorized into three types from their generating sequence and spectral properties. Generating sequence of the type I GJs resembles that reported previously; after the fully developed jet (FDJ) established the discharge channel, the ISUAL photometers registered a peak that was from a return‐stroke‐like process. The associated ULF (ultra‐low‐frequency) sferics of these type I GJs indicates that they are negative cloud‐to‐ionosphere discharges (−CIs). Type II GJs begin as blue jets and then developed into GJs in ∼100 ms. Blue jets also frequently occurred at the same region before and after the type II GJs. No identifiable ULF sferics of the type II GJs were found, though an extra event that has +CI ULF signature is probably a type II GJ. The FDJ streamer brightness of the type I GJs is ∼3.4 times of that of the type II GJs. These evidences suggest that the type II GJs are composed of positive streamers. Type III GJs were preceded by lightning, and a GJ subsequently occurred near this preceding lightning. The spectral data of the type III GJs are dominated by lightning signals and the ULF data have high background noise; thus both cannot be properly analyzed. However, the average brightness of the type III GJs falls between those of the other two types of GJs. We propose that the discharge polarity of the type III GJs can be either negative or positive, depending on the type of the charge imbalance left by the trigger lightning.

[1] On 22 July 2007, 37 blue jets/starters and 1 gigantic jet occurring over a thunderstorm in the Fujian province of China were observed from the Lulin observatory on the central mountain ridge of Taiwan. The majority of the jets were observed to occur in a 5 min window during the mature phase of the jet-producing thunderstorm. These jets have significant red band emissions. However, the blue emissions from these jets were not discernible due to severe atmospheric scattering. A model estimation of the emissions from a streamer reveals that the red emissions in blue starters and blue jets are mainly from the nitrogen first positive band (1PN2). The type II gigantic jet is the first of this type that was observed from the ground. The generation sequence of the gigantic jet begins with a blue starter, then a blue jet occurs at the same cloud top after ∼100 ms and finally develops into a gigantic jet ∼50 ms later. Using “optical strokes” as surrogates of the lightning strokes, the correlations between jets and the cloud lightning are explored. The results indicate that the occurrence of jets can be affected by the preceding local cloud-to-ground (CG) lightning or nearby lightning (intracloud (IC) or CG), while in turn the jets might also affect the ensuing lightning activity.

NCKU ISUAL team has routinely carried out ground TLE campaigns in Taiwan and performed a global survey of TLEs using the ISUAL payload onboard the FORMOSAT-2 satellite since 2004. The occurrence of TLEs, including sprite, elve, halo, blue jet and gigantic jet, is known to be closely linked to the electrical discharges in thunderclouds. However, the optical and spectral analyses provide little insight into the characteristics of the electric discharge processes that induce the TLEs. The lightning discharges are known to radiate the bulk of electromagnetic energy at the bands of the ultra low frequency (ULF) and the very low frequency (VLF) bands. An ULF magnetic field and an ELF/VLF magnetic/electric fields recording systems currently are operating at low electromagnetic noise sites in Taiwan. With both systems, we have the capability to monitor the sferics emitted by the electric discharges that produced the observed TLEs and to infer their electromagnetic signatures. The important scientific results obtained from the radio observation in Taiwan, including the TLE activities in a typhoon and sferics associated with blue jets as well as electromagnetic signatures of the TLE-associated discharges, are highlighted in this talk. With a recently installed low frequency (LF) magnetic field recording system, a new algorithm based on Hilbert-Huang transform is developed to analyze the electromagnetic features in this band. The preliminary results on the LF measurements will also be presented.

Sources of middle atmosphere nitrogen oxides, including transport from the troposphere and production in situ by energetic electrons, are currently not well characterized. Production of nitrogen oxides (NOx) in the middle atmosphere by transient luminous events (TLEs), such as red sprites and blue jets has previously been estimated from satellite observations and modeling studies. This is the first laboratory attempt to estimate NOx production by TLEs, following studies that have confirmed electrical similarities between laboratory discharges and TLEs. A pressure‐controlled chamber and high‐voltage power supplies simulated middle atmosphere discharges. Chemiluminescence NOx analyzers sampled NOx following the completion of the chamber discharges, which was used to calculate total NOx production for each discharge as well as NOx per ampere of current and NOx per Joule of discharge energy. Three different production efficiencies in NOx/J as a function of pressure pointed to three different production regimes: one for tropospheric pressures (100–500 mb), one for stratospheric pressures (1–100 mb), and one for upper stratospheric to mesospheric pressures (no greater than 1 mb). Discharges at jet‐like pressures are measured to produce 1.7 × 1016 to 6.40 × 1017 molecules of NOx per discharge, while discharges at sprite‐like pressure produce 6.97 × 1013 to 8.57 × 1013 molecules of NOx per discharge. Blue jets were calculated to produce 1.7 × 1022 to 7.4 × 1026 molecules of NOx, while red sprites were calculated to produce 6.8 × 1023 to 6.3 × 1027 molecules of NOx. On the basis of global sprite frequency estimates global annual NOx production by sprites is estimated to be between 7 × 1023 and 2 × 1028 molecules per second.

Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave observations in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfvén waves and particle precipitation related to solar and magnetospheric processes. We review ionospheric processes as well as surface and space weather phenomena that drive the coupling between the troposphere and the ionosphere. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface perturbations and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and to solve inverse problems and outline in a final section a few challenging subjects that are important to advance our understanding of tropospheric-ionospheric coupling.

CTR Wilson Meeting 2016