Earth-ionosphere Cavity Research Articles

The thunderstorm-driven diurnal variation of the ELF electromagnetic activity level

We present a numerical model which describes the global distribution and the thunderstorm-driven electromagnetic excitation in the extremely low frequency-(ELF) range. The model, in its present stage, builds on the parameterised world-wide distribution and temporal occurrence of thunderstorms and on electromagnetic wave propagation in the Earth-ionosphere cavity. The ionospheric D layer and the surface of the Earth are treated as very good conductors, and the ELF propagation in the Earth-ionosphere waveguide is considered to be isotropic but damped. The return strokes are regarded as the only transmitters and for the purpose of a qualitative simulation these strokes are approximated by vertical Hertz dipoles with normalized dipole moments. Our model has been checked against data from two full days of ELF observations at a remote field station (located at 50.4° N, 9.3° E). The comparison, while qualitatively satisfactory, has revealed some quantitative limitations of the model which lead to suggestions for its improvement. These include a more accurate description of the world-wide lightning occurrence, with refined spatial and temporal resolution and possibly an account of systematic variations in the lighting dipole moment with respect to geographic latitude and frequency.

Further evidence for a global correlation of the Earth‐ionosphere cavity resonances

The Stanford ELF/VLF Radio Noise Survey provides digital time series in the ELF range and VLF analog recordings of one horizontal magnetic field component at different locations around the globe. Three sites, Arrival Heights in Antarctica, Søndrestrømfjord in Greenland and Stanford in California, are chosen to investigate the natural magnetic noise in the frequency range of the Earth‐ionosphere cavity resonances (6–60 Hz). The resonances are interpreted as pure resonance phenomena and the associated parameters are derived in a robust manner by use of the complex exponential algorithm. The resulting amplitudes of the first and second resonances exhibit a pronounced diurnal variation and a distinct day to day variability of the same order during the time interval from January to April 1990. The day to day variability is well correlated between all of the three different sites. They also exhibit a 20–30 day variation that is related to the solar rotation period, expressed by means of sunspot numbers.

Sprites, ELF Transients, and Positive Ground Strokes

In two summertime mesoscale convective systems (MCSs), mesospheric optical sprite phenomena were often coincident with both large-amplitude positive cloud-to-ground lightning and transient Schumann resonance excitations of the entire Earth-ionosphere cavity. These observations, together with earlier studies of MCS electrification, suggest that sprites are triggered when the rapid removal of large quantities of positive charge from an areally extensive charge layer stresses the mesosphere to dielectric breakdown.

Schumann resonances in magnetic field components

The Institut für Geophysik Göttingen has facilities to record the time varying magnetic components H and D at extremely low frequencies (ELF) (0.1–20 Hz) continuously over several days, with a sampling rate of 100 Hz. The lower ELF-range is characterized by anthropogenic noise and the first two Schumann resonance modes of the Earth-ionosphere cavity. The local observations near Göttingen are disturbed by thunderstorms during the summer that contribute a small part of the energy to the global thunderstorm activity. Transient natural signals with amplitudes of about 10 pT are superimposed on a continuous noise level of about 1 pT; both exhibit the Schumann resonance periodicities. The signals show a tendency to repeat after about 2 s which may suggest excitation by whistler-trains. The Schumann resonances are investigated by an analysis of the autocovariance matrix yielding a robust estimation of the amplitude, damping and frequency. All these parameters show a characteristic directional dependence and variability during the day. The amplitudes represent the excitation with different source locations of thunderstorm activity around the world, generating a dipole field within the resonator, while the damping and centre-frequency are related to solar activity coupling to the lower ionosphere.

Midlatitude detection of ELF whistlers

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2 floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospheric ELF noise source.

The ionosphere as a load on the global atmospheric electrical circuit

Low-frequency electric-mode excitation in the earth-ionosphere cavity is considered with the aid of Maxwellequation vector mode transformation with allowance for field penetration into the ionosphere and magnetosphere. It is shown that for atmospheric current generators, the ionosphere can exhibit the properties of a conductor or an insulator, depending, on frequency. The penetration of atmospheric electric fields from the earth into the ionosphere can cause corresponding unitary variatary variations of geophysical parameters.

Investigations of the lower ionosphere over Antarctica via ELF-VLF radiowaves

The paper discusses ELF-VLF investigations of the low Antarctic ionosphere. Two new methods of lower ionospheric diagnostics are based on an investigation of the VLF electromagnetic field structure in the Earth-ionosphere cavity. One method deals with the analysis of local transverse cavity resonances in the near field of an emitter (a horizontal antenna) with a frequency (~1.5-8 kHz) in the range of the first few resonances. The other method, based on tweek investigations, is applicable under night conditions and enables the characteristics of the low ionosphere to be determined over a signal propagation path. The use of the ELF-VLF transmitter at Siple Station provides a unique opportunity to implement these methods, as does the ELF multipoint recording in the Schumann resonance band of atmospherics excited by powerful lightning discharges.

Gravitational-to-electromagnetic wave conversion in electrostatic field of earth-ionosphere resonator

The conversion of a plane gravitational wave into electromagnetic radiation in the presence of the fair-weather static electric field of the earth-ionosphere resonator is examined. The spatial and frequency characteristics of the induced electromagnetic field of the earth-ionosphere cavity are studied in the range of Schumann resonance frequencies (f=10–100 Hz). Numerical estimates are obtained for the amplitudes of the electromagnetic fields, and the possibilities of a natural gravitational-wave detector are considered.

Electrical conductivity of Jupiter's shallow interior and the formation of a resonant planetary-ionospheric cavity

The hydrogenic ion composition and electrical conductivity are evaluated for the shallow interior of Jupiter extending from the 1-bar surface to a depth of 4000 km below this level. The atmosphere is assumed to be a mixture of hydrogen and helium with a standard mass density ratio of 78/22. The pressure and temperature are modeled assuming the gas is adiabatic, obeys the perfect gas law, and is in hydrostatic equilibrium. In this preliminary study the effects of impurities are ignored. The densities of free electrons, molecular and atomic hydrogen, and their positive and negative ions are computed assuming the atmosphere is in local thermodynamic equilibrium. From these quanteties are evaluated the electron- and ion-neutral collision frequencies and mobilities, and the electrical conductivity. Results of the calculation show that for a hydrogen/helium atmosphere electrical currents within the shallow interior of Jupiter are carried primarily by molecular hydrogen ions, but with nonnegligible contributions from electrons near the surface, and atomic hydrogen ions at greater depths. The electrical conductivity is sufficiently large to form a conducting “boundary” to electromagnetic waves propagating within the shallow interior. The location of this boundary is frequency dependent, ranging from a depth of 1000 km at 1 mHz to 3000 km at 1 MHz. The existence of a conducting boundary suggests that Jupiter possesses a closed planetary-ionosphere cavity similar to the earth-ionosphere cavity. Preliminary calculations yield eigenfrequencies of approximately 0.76, 1.35, and 1.93 Hz for the lowest three transverse-magnetic normal modes, respectively, with corresponding Q-factors of roughly 7. It is speculated that the normal modes of this system may be excited by Jovian lightning in a manner similar to the terrestrial Schumann resonances. Measurements of these modes could be used to diagnose the conductivity of the shallow interior.

Electromagnetic resonances in the earth-ionosphere cavity

Electromagnetic resonances in the earth-ionosphere cavity

Allowance for near fields in the problem of local height inhomogeneity of the earth-ionosphere cavity

Allowance for near fields in the problem of local height inhomogeneity of the earth-ionosphere cavity

Relativistic magnetospheric electrons: Lower ionospheric conductivity and long-term atmospheric variability

Long-term (1979–88) observations of relativistic electrons in the earth's outer magnetosphere show a strong solar cycle dependence with a prominent intensity maximum during the approach to solar minimum (1983–85). This population therefore corresponds to the presence of high-speed solar wind streams emanating from solar coronal holes. Using a numerical code, we have calculated the precipitating electron energy deposition in the earth's upper and middle atmosphere. Observed events (typically persisting several days) would have maximum effect in the 40–60 km altitude range. We suggest that this electron population could play an important long-term role in modulating lower D-region ionization and middle atmospheric ozone chemistry. We describe methods of observing middle atmospheric and lower ionospheric effects of the electrons. A particularly promising approach may involve the monitoring of global Schumann resonance modes which are sensitive to global changes in the properties of the earth-ionosphere cavity. Ongoing work indicates that Schumann resonance properties are moderately correlated with the flux of precipitating relativistic electrons thus offering the possibility of continuously monitoring this aspect of magnetosphere-atmosphere coupling.

Detection of elliptical polarization and mode splitting in discrete Schumann resonance excitations

Elliptical polarization and mode splitting have been detected in the magnetic component of discrete, well defined Schumann resonance excitations. These ELF excitations, which are large electromagnetic transients of approximately l s duration, are called Q-bursts and typically occur every few minutes. They are believed to be the signature of the impulsive excitation of the Earth-ionosphere cavity by ultra-large lightning currents. In this paper the magnetic polarization and spectral characteristics of four large Q-bursts are examined in detail using a new analysis technique. Two events display right-hand polarization and two display left-hand polarization. The theoretical polarization properties of the central and side multiplets of the Schumann resonances are used to define a local orthogonal coordinate system in the measurement frame in which these components may be separated. Maximum entropy spectrums computed separately for what are identified to be the central and side multiplets in this coordinate system show distinctly different eigenfrequencies for the lowest mode near 7.5 Hz. For the limited number of cases examined the magnitude of the line splitting detected using this technique is roughly 1.4–1.8 Hz, larger by nearly a factor of two than theoretical or observed values of the splitting previously reported. The frequencies of the side multiplets may lie either above or below the frequency of the central multiplet.

High resolution spectral characteristics of the Earth-ionosphere cavity resonances

The natural resonances of the Earth-ionosphere cavity at frequencies between 5 and 100 Hz have been studied since the fundamental paper by Schumann. While the gross features of the phenomena are now well understood, considerable work remains to be done on their detailed behaviour. In the present study a high resolution, data adaptive spectral technique is applied to digital electromagnetic data obtained at a moderate latitude. A particular feature of the method employed is that spectral properties become available on the same time scale as many ELF events, thus both time local and time averaged resonance features can be readily established. The technique can thus be applied to both dynamic and steady-state descriptions of the cavity's properties. For the data set considered, the technique adequately resolves the first six resonance modes on a time scale of 0.75 s. The presence of higher order modes is also indicated. The time averaged frequencies obtained are in accord with those of previous experimental determinations. When the time local properties of individual transient waveforms are examined, however, we observe a number of detailed effects which are predicted by theory. The precise spectral structure of the resonance modes appears influenced by the differing locations of the sources of transient excitation. In the case of the first order resonance mode, the properties of the cavity consistently support both singlet and doublet resonance behaviour.

Scattering of elf radio waves on global inhomogeneities of the earth-ionosphere cavity

Using the Born approximation in the Stratton-Chu equation, a solution was obtained for the vertical electric component of an elf wave in the earth-ionosphere cavity with day-night and polar inhomogeneities. Calculations were made at a 62-GHz frequency using three models of a cavity inhomogeneity for different orientations of the beam path relative to the inhomgeneity. Relative perturbations of the field and diurnal variations in the observed elf field are obtained for a cavity with an inhomogeneity.

Geophysical Variables and Behavior: XXX. Intense Paranormal Experiences Occur during Days of Quiet, Global, Geomagnetic Activity

25 well-documented (and published by Stevenson in 1970) cases of intense paranormal (“telepathic”) experiences concerning death or illness to friends or family were analyzed according to the global geomagnetic activity (the aa index) at the times of their occurrence. The characteristics of these cases were representative of the general literature and occurred between the years 1878 and 1967. All 25 experiences were reported to have occurred on days when the geomagnetic activity was less than the means for those months. Repeated-measures analyses of variance for the daily aa indices for the 7 days before to the 7 days after the experiences confirmed the observation that they occurred on days that displayed much less geomagnetic activity than the days before or afterwards. These results are commensurate with the hypothesis that extremely low frequency fields, generated within the earth-ionospheric cavity but disrupted by geomagnetic disturbances, may influence some human behavior.

Lightning-induced electron precipitation

The broadband very low frequency (VLF, 0.3–30 kHz) radiation from lightning propagates in the Earth–ionosphere cavity as impulsive signals (spherics) and in the dispersive plasma regions of the ionosphere and magnetosphere it propagates as tones of descending or rising frequency (whistlers)1. VLF radio waves propagating in the magnetospheric plasma scatter energetic electrons by whistler-mode wave–particle interactions (cyclotron resonance) into the atmosphere2–6. These electrons, through collisions with the atmospheric constituents, cause localized ionization, conductivity enhancement, visual and ultraviolet light emissions, and brems-strahlung X rays. We have reported previously on the precipitation of energetic electrons from the radiation belts by the controlled injection from the ground of VLF radio waves7,8. Here we report the first satellite measurements of electron precipitation by lightning. The measured energy deposition of these conspicuous lightning-induced electron precipitation (LEP) bursts (∼ 10−3 erg cm−2) is sufficient to deplete the Earth's radiation belts and to alter subionospheric radiowave propagation (≲1 MHz). A one-to-one correlation is found between ground-based measurements of VLF spherics and whistlers at Palmer, Antarctica, and low-altitude satellite (S81-1) measurements of precipitating energetic electrons.

Cross correlation of power spectra of natural elf noise

The correlation function of the power spectra of natural elf noise at different frequencies is obtained. It is assumed that the earth-ionosphere cavity is excited by a sequence of pulses of radiation from random, mutually independent, thunderstorm discharges. It is shown that the coefficient of mutual correlation of the spectral power densities of noise at different frequencies as a function of the difference between these frequencies has a wide-band part, equal approximately to 2/3, and a narrow-band part, about 1/3, which is related to the interference of separate pulses. The reason for the low stability of estimates of elf spectra, when realizations of a signal with a duration not exceeding 1 min are used in experiments, is indicated.

Energy spectra of the transverse resonances in the earth-ionosphere cavity

Energy spectra of the transverse resonances in the earth-ionosphere cavity

Application of a simplified theory of ELF propagation to a simplified worldwide model of the ionosphere

The approximate theory of ELF propagation in the Earth-ionosphere transmission line described by Booker (1980) is applied to a simplified worldwide model of the D and E regions, and of the Earth's magnetic field. At 1000 Hz by day, reflection is primarily from the gradient on the underside of the D region. At 300 Hz by day, reflection is primarily from the D region at low latitudes, but it is from the E region at high latitudes. Below 100 Hz by day, reflection is primarily from the gradient on the underside of the E region at all latitudes. By night, reflection from the gradient on the topside of the E region is important. There is then a resonant frequency (∼ 300 Hz) at which the optical thickness of the E region for the whistler mode is half a wavelength. At the Schumann resonant frequency in the Earth-ionosphere cavity (∼ 8 Hz) the nocturnal E region is almost completely transparent for the whistler mode and is semi-transparent for the Alfven mode. Reflection then takes place from the F region. ELF propagation in the Earth-ionosphere transmission line by night is quite dependent on the magnitude of the drop in ionization density between the E and F regions. Nocturnal propagation at ELF therefore depends significantly on an ionospheric feature whose magnitude and variability are not well understood. A comparison is made with results based on the computer program of the United States Naval Ocean Systems Center.