The comment by R. L. Dowden does not properly represent the results presented in our recent paper [ Inan et al., 1995] and attempts to prematurely classify the VLF events reported in our papers as being produced by elves. There is no experimental evidence that the VLF events reported in our papers are necessarily associated with the optical phenomena of elves. In our recent paper [ Inan et al., 1996a] we provide a physically based mechanism which accounts for the forward scatter property of the VLF events observed in our data.
Abstract Terrestrial Gamma‐ray Flashes (TGFs), discovered in 1994 by the Compton Gamma‐Ray Observatory, are high‐energy photon bursts originating in the Earth's atmosphere in association with thunderstorms. In this paper, we demonstrate theoretically that, while TGFs pass through the atmosphere, the large quantities of energetic electrons knocked out by collisions between photons and air molecules generate excited species of neutral and ionized molecules, leading to a significant amount of optical emissions. These emissions represent a novel type of transient luminous events in the vicinity of the cloud tops. We show that this predicted phenomenon illuminates a region with a size notably larger than the TGF source and has detectable levels of brightness. Since the spectroscopic, morphological, and temporal features of this luminous event are closely related with TGFs, corresponding measurements would provide a novel perspective for investigation of TGFs, as well as lightning discharges that produce them.
Farges and Blanc (2010) reported inverted‐chirp infrasonic signals with high frequencies arriving before low frequencies, possibly emitted by sprite discharges and observed on the ground at close range (<100 km) from the source. In the present work a parallel version of a 2‐D FDTD model of infrasound propagation in a realistic atmosphere is applied to demonstrate that the observed morphology of infrasound signals is consistent with general scaling of diameters of sprite streamers inversely proportionally to the air density. The smaller structures at lower altitudes radiate higher infrasonic frequencies that arrive first at the observational point on the ground, while the low frequency components are delayed because they originate at lower air densities at higher altitudes. The results demonstrate that strong absorption of high frequency infrasonic components at high altitudes (i.e., ∼0.2 dB/km for 8 Hz at 70 km) may also contribute to formation of inverted‐chirp signals observed on the ground at close range.
A three‐dimensional Finite Difference Time Domain (FDTD) model of the Earth‐ionosphere cavity with a realistic conductivity profile is employed to study the global lightning activity using the observed intensity variations of Schumann resonances (SR). Comparison of the results derived from our FDTD model and the previous studies by other authors on related subjects shows that Schumann resonance is a good probe to indicate the seasonal variations of lightning activity in three main thunderstorm regions (Africa, southeast Asia, and South America). An inverse method based on genetic algorithms is developed to extract information on lightning intensity in these three regions from observed SR intensity data. Seasonal variations of the lightning activity in three thunderstorm centers are clearly observed in our results. Different SR frequency variations associated with seasonal variations of global lighting activity are also discussed.
A finite‐difference time‐domain (FDTD) model of infrasound propagation in a realistic atmosphere is used to provide quantitative interpretation of infrasonic waves produced by auroral arcs moving with supersonic speed. The Lorentz force and Joule heating are discussed in the existing literature as primary sources producing infrasound waves in the frequency range 0.1–0.01 Hz associated with the auroral electrojet. The results are consistent with original ideas of Swift (1973) and demonstrate that the synchronization of the speed of auroral arc and phase speed of the acoustic wave in the electrojet volume is an important condition for generation of magnitudes and frequency contents of infrasonic waves observable on the ground. The reported modeling also allows accurate quantitative reproduction of previously observed complex infrasonic waveforms including direct shock and reflected shockwaves, which are refracted back to the earth by the thermosphere.
Lightning leaders advance in space by creating a heating conversion zone in their tips (i.e., streamer‐to‐leader transition) in which Joule heating produced by currents of many non‐thermal corona streamers transforms into a hot and conducting leader channel. It is believed that the initial stages of transient luminous events termed gigantic jets (GJs) propagating toward the lower ionosphere are directly related to leaders initiated by conventional intra‐cloud lightning discharges and escaping upward from thundercloud tops. In the present work we provide quantitative description of speeds of these leaders as a function of leader current and ambient air density (altitude). The direct comparisons with available experimental data indicate that the initial speeds of GJs of ∼50 km/s are consistent with leaders possessing currents 2–8 A. The observed acceleration of GJs can be explained by growth of the leader current, and at high altitudes (low air densities) may be significantly affected by predominance of non‐thermal (i.e., streamer) discharge forms.
Modeling studies indicate that double‐headed streamers originating from single electron avalanches in lightning‐driven quasi‐static electric fields at mesospheric altitudes accelerate and expand, reaching transverse scales from tens to a few hundreds of meters and propagation speeds up to one tenth of the speed of light, in good agreement with recent telescopic, high‐speed video and multichannel photometric observations of sprites. The preionization of the medium ahead of a streamer by the ionizing UV photons originating from a region of high electric field in the streamer head (i.e., photoionization) significantly modifies the streamer scaling properties as a function of air pressure in comparison with those predicted by similarity laws. The photoionization leads to lower peak electric fields in the streamer head, lower streamer electron densities, wider initial streamer structures, and lower acceleration and expansion rates of streamers at sprite altitudes 40–90 km, when compared to the ground level. The primary reason for the observed differences is that the effective quenching altitude of the excited states of the molecular nitrogen b 1 Π u , b ′ 1 , and c ′ 4 1 that give photoionizing radiation is about 24 km. The quenching of these states is therefore negligible at sprite altitudes, leading to a substantial enhancement of the electron‐ion pair production ahead of the streamer tip because of the photoionization, when compared to the ground level. The maximum radius of the expanding streamers is predominantly controlled by the combination of the absorption cross section χ min = 3.5 × 10 −2 cm −1 Torr −1 of the molecular oxygen (O 2 ) at 1025 Å and the partial pressure of O 2 in air, Streamers exhibit branching when their radius becomes greater than Model results indicate a lower branching threshold radius for positive streamers in comparison with negative streamers, under otherwise identical ambient conditions. These results are in good agreement with recent results of high‐speed photography of laboratory streamers in near‐atmospheric pressure N 2 /O 2 mixtures and similar morphology documented during recent telescopic and high‐speed video observations of sprites.
Abstract This paper contributes to the understanding of the influence of conductivity perturbations on the ionospheric potential in the Earth's global electric circuit (GEC). The conductivity perturbations appearing in the middle atmosphere produced by γ ray bursts from magnetars are studied first. The transient response of the ionospheric potential is modeled in this case, and timescales of interest are identified (0.01–10s). In this case modification of ionospheric potential is small. Additionally, the principal effects of topography and reduction of conductivity inside the thundercloud are studied. Both of these factors effectively increase the ionospheric potential for a classic source in the GEC represented by a current dipole leading to formation of two main charge centers of the thunderstorm. On the other hand, for GEC including topography and conductivity reduction in thunderclouds the contribution of sequence of negative cloud‐to‐ground lightning discharges to the ionospheric potential is decreased. Simulation results show a very good agreement with equivalent circuit models for conductivity perturbations with horizontal dimensions exceeding 20 km.
Key Points Statistical models from forecast schemes can explain up to 48% of annual North Atlantic TC frequency Hindcast period analogous to warmer climate keeps only surface winds and SST gradient in regression The CLLJ is a major modulator of future North Atlantic TC frequency
