High-altitude Discharge Research Articles

Low‐frequency radio emissions from high‐altitude sprite discharge

When lightning strikes from a towering cumulonimbus cloud down to the ground, the electrical discharge can perturb the atmosphere's electric field, potentially triggering a second event—sprite discharge. This more elusive type of electrical discharge, which produces lightning that is red in color, initiates from high altitudes, with streamers propagating down toward the top of the cumulonimbus cloud. Coincident with the dramatic displays, researchers have previously identified low‐frequency radio emissions, which they suggest may be produced in association with the sprite discharge. Investigating this hypothesis, Qin et al. used a two‐dimensional plasma model to calculate the radio emissions that should be produced by a single sprite streamer.

The effect of charge density in tropospheric thunderclouds on production of sprites and terrestrial gamma ray flashes

[1] We demonstrate that positive cloud-to-ground lightning flash (most often responsible for sprites) from stratiform tropospheric thundercloud leading to charge moment change iΔMq high above the sprite “threshold” 500 C km really triggers the high-altitude discharge exciting the air luminescence at sprite altitudes with brightness close to the observed brightness. Specifically, we present results for three configurations with the same charges Qcl = 200 C located at altitudes 6 and 8 km (iΔMq = 1200 C km for +Qcl at 6 km and iΔMq = 1600 C km for +Qcl at 8 km) differing by a magnitude of charge density, namely, σ = 1.86, 2.88, and 4.41 C km−2. All three configurations produce luminescence at sprite altitudes with brightness close to the observed one. In addition, the configuration with σ = 2.88 C km−2 produces observable γ emission, which can be detected in near space as terrestrial gamma-ray flashes (TGFs). The configuration with σ = 4.41 C km−2, provided it really could develop, would produce a blue fluorescence directly near the cloud top and extremely intensive γ emission not observed till now. The results obtained for smaller σ prove that too small-charge density in stratiform thunderclouds and consequently too weak electric field above them could account for the lack of Reuven Ramaty High Energy Solar Spectroscopic Imager TGFs correlated with large-charge moment change.

Recent Results from Studies of Electric Discharges in the Mesosphere

The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.

Electromagnetic Pulse in the Magnetosphere Generated by Impulsive Current near the Lower Boundary of the Ionosphere

A solution to the problem of current spreading is constructed in the case of relaxation of electric charges, which have arisen in the mesosphere for one reason or other. These currents penetrate into the conductive region with anisotropic conductivity of the D- and E-layers of the ionosphere, being transformed to a MHD-wave that propagates into the magnetosphere. Based on this solution, the form and spectrum of the generated MHD signal are calculated for Alfvenic and magnetosonic modes coming out to the ionosphere and magnetosphere. Electric charges and currents can arise, for example, in the space between a thunderstorm cloud and the ionosphere, or between the shock wave from a ground explosion and the ionosphere. Some signal parameters accepted in the model are close to those expected for high-altitude electric discharges of the Red Sprite type. The conditions are determined under which the Alfven impulse with an amplitude of up to 100 nT propagates in the magnetosphere above high-altitude discharge of this type. Such an impulse was recorded by the AUREOL-3 satellite after the ground explosion MASSA-1. Recently, this impulse was hypothesized to originate as a result of a high-altitude electric discharge. The hypothesis on a similar MHD pulse allows one to explain in a semiquantitative way the short burst of electron field-aligned acceleration observed by the DE-2 satellite over the Debbie hurricane. The high-altitude atmospheric discharge of this type can be a powerful, though short-time and local, source of electrons with kiloelectronvolt energies at low and middle latitudes. One could expect that such an effect causes a modified character of the so-called Trimpi-effect (a short-term disturbance of propagation of VLF waves in the ionosphere), and thus, it can be observable.

Optical characteristics of blue jets produced by runaway air breakdown, simulation results

The results of numerical calculations of the intensity and spectra of optical emissions from blue jets produced by runaway air breakdown in the atmosphere are presented. It is found that a positive runaway streamer develops in the altitude range 20–34 km following an intracloud discharge that possesses a continuing current of ∼1.7 kA. The ionization front of the runaway streamer propagates upward with a velocity ∼90 km/s and produces optical emissions with a maximum intensity ∼400 kR and a duration ∼53 ms. The comparison between theory and observation yields good agreement for such important blue jet characteristics as maximum intensity of optical emissions, color, front velocity, duration, maximum radius and vertical dimensions and supports the viability of runaway air breakdown as a driving mechanism for this particular type of high altitude discharge.

Evidence of a high-altitude discharge process responsible for radar echoes at 24.4 MHz

We present preliminary evidence of a high-altitude, electrical discharge process that can produce radar echoes at 24.38 MHz. This conclusion is drawn from pulsed-radar observations of near time-coincident occurrences of impulsive electromagnetic radiation with radar echoes that originated at altitudes well above those of mesoscale convective systems.

On blue jet dynamics

A model is proposed for blue jets [Sentman and Wescott, 1995; Wescott et al., 1995a], which explains their dynamics, in particular the jet vertical velocity of about 100 km/s, the terminal altitudes of about 40–50 km, and the competitive occurrence of blue jets and the recently discovered blue starters [Wescott et al., 1995b]. A blue jet is considered to be formed by an attachment‐controlled ionizing wave, which moves upward via an electron avalanche in the wavefront due to the mainly vertical, downward directed quasi‐electrostatic field, caused by the extraordinarily large (>100 C) charge transfer in a high‐altitude intracloud discharge or in a positive cloud‐to‐ground discharge with a long continuing current.