ELF Pulses Research Articles

Impact of the Elf Receiver on Characteristics of the Observed Q-bursts

We consider the impact of a typical Schumann-resonance receiver on the waveforms of the ELF transients, or Q-bursts. For this purpose, the Q-bursts were calculated both in the frequency and the time domains. The data obtained within the framework of the frequently used models of the propagation constant are compared. The effect of a typical ELF receiver containing a 4–40-Hz bandpass filter combined with a 50-Hz notch filter is evaluated. The main attention is given to distortions in the pulsed signal waveform. It is found to what extent a Schumann-resonance receiver retards the pulse, how the ratio between the amplitudes of the positive and negative half-waves at the pulse front is modified, and how the antipodal and round-the-world subpulses are distorted. Since such distortions are able to impede correct identification of the ELF pulses and determination of the source polarity and the source-to-observer distance, we discuss the possible ways of compensating for the deformations produced by the receiver.

Analysis of ELF electromagnetic field pulses recorded by the Hylaty station coinciding with terrestrial gamma‐ray flashes

Terrestrial gamma‐ray flashes (TGFs) were registered the first time by the NASA's Compton Gamma Ray Observatory. The physical mechanism of TGF generation is not fully known, but there is a consensus among researchers that the radiation is produced by bremsstrahlung of relativistic electrons in the thunderstorm regions of the atmosphere. Therefore, TGFs have been linked to positive‐polarity intracloud lightning discharges, strong positive cloud‐to‐ground discharges or upward discharges from a thundercloud top. The currently operating Fermi Gamma‐ray Space Telescope is equipped with a Gamma‐ray Burst Monitor that can detect terrestrial gamma‐ray flashes. It opens up a new possibility to search for lightning discharges responsible for TGFs. Ground‐based lightning monitoring systems in the ELF, LF and VLF ranges can be used for that purpose. The ELF systems are especially useful, since they provide a large monitoring range of several thousand kilometers for strong atmospheric discharges (charge moments above several tens of C km). In this paper we have described the data analysis method for ELF electromagnetic field pulses and applied it to study our first examples of TGFs registered by Fermi GBM coinciding with ELF pulses recorded by the Hylaty ELF station located in the Carpathian Mountains in Poland. Using our ELF electromagnetic wave propagation model we have evaluated charge moments for the two registered events to be 320 and 110 C km and provided upper limits for the remaining events.

Some results of observations of positive lightning discharges and relative phenomena in the east of Siberia

We have considered spatial distributions of positive lightning discharges in the east of Siberia for the summer seasons of 2003–2007 and properties of their electromagnetic signals with the ELF “slow tail”, which, as is known, can be accompanied by sprites. There are two main regions of positive discharges located in the south and west of Yakutsk. Two other “centers” (the northeastern and the eastern) are located in high-mountainous regions. In these regions the positive discharges intensity can exceed the negative discharge intensity. The electromagnetic signals in the ELF range (usually in the form of two half-cycles) were observed after the VLF atmospherics were recorded in the high-latitude regions. The delay of ELF pulses relative to the corresponding atmospherics was 0–7 ms. The long (up to 350 ms) events of quasi-periodic ELF oscillations with the period of about 7 ms (which corresponds to the quasi-period of ELF pulses) were revealed.

Lightning sferics and stroke-delayed pulses measured in the stratosphere: Implications for mesospheric currents

[1] During the Brazil Sprite Balloon Campaign 2002–03, the vector ELF to VLF (25 Hz–8 kHz) electric and magnetic fields driven by cloud-to-ground (CG) lightning strokes at horizontal distances of 75–600 km were measured at altitudes of 30–35 km. Electric field changes were measured for each of the 2467 CG strokes detected by the Brazilian Integrated Lightning Network. ELF pulses that occur 4–12 ms after the retarded time of the lightning sferic, which have been previously attributed to sprites, were found for 1.4% of 934 strokes examined. Thus, lightning-driven electric and magnetic field changes were common and stroke-delayed ELF pulses were rare, which disagrees with results from the Sprites99 Balloon Campaign.

Time domain presentation for ELF pulses with accelerated convergence

A compact time domain solution is constructed for natural electromagnetic pulses propagating in the Earth‐ionosphere cavity. An algorithm is discussed for accelerating the convergence of the time series presenting the pulsed waveform. Compact formulae are derived and computational results are presented for a set of the source distances.

Current moment in sprite-producing lightning

Studies of sprite-producing lightning have revealed much of what we currently know about the mechanisms responsible for this phenomenon. With a combination of new and previous results, we summarize the currently known quantitative characteristics of this special class of lightning. This information has come primarily from the quantitative analysis of the electromagnetic fields produced by distant lightning. The long range of this technique has made it especially powerful, but there are important limitations on what can be measured that are related to the bandwidth of the measurement system. The lightning charge moment change required to initiate sprites varies across a relatively wide range, from approximately 100– 2000 C km . Note that this is not the total charge moment change in sprite producing lightning, which is by definition greater than the initiation threshold. This range is in very good agreement with the predictions of streamer-based sprite modeling. We also summarize the strong evidence, from a variety of sources, in favor of sprite currents as the origin of ELF pulses seen in a significant fraction of sprite events. The largest events show sprite current moment amplitudes of ∼1000 kA km and sprite charge moment changes of at least 1200 C km , and perhaps significantly more. Lastly, we show that delayed sprites are generated from very strong continuing currents (20– 60 kA km ) following a +CG return stroke. In the few cases analyzed, this current generates a charge moment change from 2000 to 6000 C km at the time of sprite initiation, which appears to be consistent with theoretical predictions of larger charge moment changes required for delayed sprite initiation. This current can be detected and analyzed with distant, sensitive ULF magnetic field measurements. Although much is known about sprite-producing lightning, there remain some fundamental yet unanswered questions about the lightning–sprite relationship. We expect that at least some of these will be answered in the relatively near future.

Acceleration of the Convergence of Time Domain Presentations for the ELF Pulses from the Lighting Strokes

Acceleration of the Convergence of Time Domain Presentations for the ELF Pulses from the Lighting Strokes

Time-Domain Representation of ELF Pulses Generated by Lightning Discharges

Time-Domain Representation of ELF Pulses Generated by Lightning Discharges

ELF sferic energy as a proxy indicator for sprite occurrence

Broadband ELF/VLF measurements of sferics near Ft. Collins, Colorado, demonstrate that ELF sferic energy is a proxy for sprite occurrence which can be used to estimate the number of sprites produced by a thunderstorm. Ultra‐long range (∼12,000 km) measurements at Palmer Station, Antarctica, confirm the application of this proxy to storms where no video observations of sprites are available. Comparison with high‐resolution photometer measurements demonstrate the simultaneity of sprite luminosity and an ELF “second pulse” believed to be radiated by electrical currents within the sprite body [Cummer et al., 1998]. Measurements of the second ELF pulse are used to identify a quantitative relationship between the current in sprites and total sprite luminosity.

Natural electromagnetic pulses in the ELF range

We analyze the propagation of electromagnetic waves in the ELF. We apply the modal solution for the spherical Earth‐ionosphere cavity and obtain the time domain fields using the FFT algorithm. The Schumann resonance amplitude over the frequency‐distance plane consists of summits and depressions exhibiting separate extended zones. When the source‐observer distance is small, the field maximum is found at some hundred hertz. This component rapidly decays with the distance. The time domain ELF pulse travels with a constant velocity and “bounces” from the source antipode. Its width increases along the path due to absorption of the high frequency component. The distance dependence of the pulse amplitude depends on both the geometrical focusing and the high frequency absorption.

On ELF pulses from remote lightnings triggering sprites

ELF waveforms at large distances from strong CG (cloud‐to‐ground) discharges are evaluated in the framework of Greifinger's night‐time propagation model. It is shown that if the CG discharges triggering red sprites involve, as now generally accepted, ≳10² C of charge, then they should generate remote ELF atmospherics with considerably larger magnitudes than measured in the upper ELF (> 300 Hz) range.

Coupling of ELF/ULF energy from lightning and MeV particles to the middle atmosphere, ionosphere, and global circuit

In an attempt to explain numerous atmospheric electrical phenomena, the elements of the global electrical circuit are reexamined. In addition to being a “quasistatic “DC” generator” and source of radiated energy at VLF and higher, the thunderstorm is found to be a pulse generator, with most of the external energy contained in ELF and ULF pulse currents to the ionosphere (and Earth). The pulse energy is found to deposit largely in the middle atmosphere above the thunderstorm. The VLF and above components are well understood, as are the ULF components due to the conductivity gradient. However, a previously poorly understood ELF component on the millisecond timescale, or “slow tail,” contains a large fraction of the electrical energy. This component couples strongly to the ionosphere and also launches a unipolar transverse electromagnetic (TEM) wavelet in the radial Earth‐ionosphere transmission line. The increase in charge with distance associated with such wavelets, and their ensemble sum at a point, may explain some large mesospheric “DC” fields but there are still difficulties explaining other than rare occurences, except for antipodal reconvergence. These millisecond duration unipolar wavelets also couple to the ionosphere and may trigger other lightning at a distance. A schema is elucidated by which the charge of MeV particles deposited in the middle atmosphere persists for much longer than the local relaxation time. This also gives rise to unipolar waves of global extent which may explain lower‐latitude field perturbations associated with solar/geomagnetic events.

On the excitation of the Earth-ionosphere waveguide by pulsed ELF sources

A theoretical treatment is given of the problem of the excitation of ELF pulses by lightning discharges in the Earth-ionosphere waveguide. This is done for the Greifinger and Greifinger [(1978) Radio Sci. 13, 831 and (1979) Radio Sci. 14, 998] approximation of ELF wave propagation. We have obtained characteristic waveforms of the pulses excited by various model sources under different conditions. The dependence of ELF pulse delay on propagation distance is investigated; this is found to be close to linear at distances $ ̌ 10,000 km. Peculiarities of the form of the ELF pulse excited by a horizontal, or nearly horizontal, lightning discharge have also been analyzed.

The ELF spectrum of artificially modulated D/ E-region conductivity

Artificial currents flow when the conductivity of the ionospheric D- and E-regions is time modulated in the presence of an auroral d.c. electric field. These currents have been evaluated with a collisional theory of ohmic heating and cooling. Using parameters appropriate to the Max-Planck-Institut für Aeronomie (MPAE) Heating facility near Tromso, Norway (e.g. heater frequency = 2.8 MHz, 100% squarewave amplitude modulation at ELF pulsing rate of 525 Hz, effective radiated power = 200 MW), the electron temperature has been found as a function of time for a series of ELF pulses. At a sufficiently long time after the start of heating the temperature fluctuation becomes a steady-state waveform. The corresponding waveform of the conductivity perpendicular to the magnetic field has been computed and Fourier analyzed as a function of altitude. This establishes the source spectrum of ionospheric currents which radiate ELF waves. The results show that there are about three different height regions with characteristic spectra. Source heights of waves observed in space have been deduced by comparing spacecraft spectra with the theory. Absolute current strengths and spectral ratios predicted with a day-time model for a fundamental frequency of 525 Hz agree with observations. Important discrepancies between the computations and ground-based observations indicate a need for a more elaborate theory for interpreting such data.

Propagation of ELF Pulses in the Earth‐Ionosphere Cavity and Application to ‘Slow Tail’ Atmospherics

The propagation of ELF electromagnetic pulses in the earth‐ionosphere cavity is considered with special reference to ‘slow tail’ atmospherics. The problem is formulated in the frequency domain; inversion to the time domain is achieved by a numerical evaluation of the Fourier transform. The propagational model employed involves a spherical earth with a concentric, spherically stratified, inhomogeneous, isotropic ionosphere. The pulse source assumed represents the median return stroke of a lightning discharge. Both short‐range propagation and long‐range propagation are considered. Particular attention is paid to the nature of the electric‐ and magnetic‐field components of the pulse in the vicinity of the antipode of the source.

Distortion of an ELF pulse after propagation through an antipode

We consider propagation of the slow tail portion of atmospheric waveforms. Using an idealized model of the earth-ionosphere cavity, the transient fields are calculated for a pulse-excited electric dipole source. It is shown that the frequency-independent phase shift at the antipode will lead to a pronounced change in the observed shape of the atmospheric waveform.