Modelling the penetration of thundercloud electric fields into the ionosphere

In a previous paper, we considered the penetration of DC thundercloud electric fields E into the ionosphere and also into the region between the ionosphere and the ground (Velinov and Tonev, 1994). In the present paper, we extend the analysis by making a more precise approximation of the electric conductivity profiles by 5–10 piecewise exponential functions of altitude instead of the two functions used up to now. This allows a much more realistic representation of the atmospheric conductivity profile. Besides, Maxwell's equations are solved for more general boundary conditions, taking into account that the electrosphere is not a perfect conductor. This leads to the appearance not only of the transverse E_r (as had been assumed until now), but also of the geomagnetic field-aligned E_z component of the penetrating thundercloud electric fields. The computations show that both E_r and E_z cause significant variations of the electron density profiles N(z) in the ionosphere.     

The angular distribution of radio waves partially reflected from the lower ionosphere

Measurements of radio waves partially reflected from the D-region made using two antennae of very different beamwidth are reported. The arrays are composed of 40 and 4 dipoles respectively. It is shown that the gain of the larger array over the smaller is often variable—both in height and time. These results can be used to estimate the off-vertical angles from which significant energy is returned. For altitudes less than 80 km angles less than 10° seem to be usual but at higher altitudes the angles increase to values of the order of 15°–20°. Other important properties of the echoes, such as the probability distribution of the amplitude were also measured. The results are discussed with particular reference to the differential absorption method of measuring electron densities and also to the nature of the irregularities responsible for the partial reflections.     

Diagnostics of the lower ionosphere by the method of resonance scattering of radio waves

Results are presented on measurements of lower ionosphere parameters (electron concentration, coefficient of ambipolar diffusion, etc.) by the method of resonance scattering of radio waves by periodic artificial irregularities. The method of resonance scattering is based on the generation of periodic irregularities in the ionospheric plasma by powerful radio emission and investigation of the characteristics of the back scattering of diagnostic radio waves by these irregularities.     

Simulation of the global structure of stationary planetary waves in the mesosphere and lower thermosphere

When simulating the global structure of stationary planetary waves (SPW) the problem of obtaining the numerical solution in the equatorial region appears. It results from the presence of apparent singularities in the operator of the SPW latitudinal structure when the Coriolis parameter is small. The new method based on SPW latitudinal operator inversion is proposed. This method permits the difficulties arising from the simulation of stationary large scale disturbances at low latitudes to be avoided. The global structure of SPW with zonal wave number m = 1 at the mesosphere and lower thermosphere heights has been calculated for the background zonal wind distribution representing a climatic picture of the solstice conditions. In the region of the mean zonal westerlies the SPW penetration across the equator is obtained. The SPW at low latitudes are shown to appear most significantly in the zonal component of the wind velocity. The influence of planetary wave motions on the distribution of longlived species in the ionospheric D-region and at the heights of lower thermosphere are discussed.     

Lunar and planetary influences upon the peak electron density of the ionosphere

Observable effects of semidiurnal tidal character caused by the Sun (radiation and gravity) and Moon (gravity only) seem to be influenced by the planet Jupiter and, during approach, by Venus. These effects can be identified during summer for suitably selected planetary configurations. The importance of the lunar effect is found to depend on the solar hour, season and planetary constellation.     

Numerical simulation of the penetration and reflection of a whistler beam incident on the lower ionosphere at very low latitude

By the full-wave algorithm with Fourier synthesis, 3-D propagation of a whistler beam incident on the pre-dawn lower ionosphere at very low latitude is numerically investigated. Processes of transmission, reflection, and coupling with the Earth-ionosphere waveguide are discussed via the wave energy and polarisation distributions and their dependence on the wave parameters and the ionospheric profile (such as the E_s-layer). It is shown that the dominant wave above 90 km altitude has the propagation characteristics of the magneto-ionic whistler mode, and absorption, spreading, reflection and mode conversion mainly occur at, and are greatly affected by, the bottom of the ionosphere. It is found that the transmitted energy density along the Earth's surface is reduced by 20 dB or more. Beam transmission loss varies asymmetrically with the incident angle, but changes little with the frequency. In the region 150 km (for 5 kHz) away from the ‘exit area’ where whistlers emerge, the bearing measurements using ground-based VLF direction-finders may be in error because direction-finding algorithms assume plane wave propagation. Only a small portion (about −25 dB at 5 kHz) of the incident energy is reflected up to an altitude of 150 km, and major reflection takes place in a small range of altitude at the bottom of the ionosphere with little spreading and lateral shift with respect to the incident beam. Reflection is enhanced considerably at lower frequency. Our results also suggest that an E_s-layer or an ionospheric gradient refracting waves to higher latitudes would be favorable factors for multi-hop echoes to be received on the ground.     

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density.In concert with the increase in magnetic activity to K_p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms⁻¹ occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm⁻¹ westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f₀F2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.     

General formulae for absorption of radio waves in the ionosphere

The relationship between group path, phase path and absorption of radio waves is discussed and new approximations relating these quantities are presented. The new relationships include dependence on the angle between the wave normal and magnetic field directions and so, in contrast to other approximations they are not restricted to quasi-longitudinal or quasi-transverse situations. For deviative absorption it is found that the ordinary mode quasi-transverse approximation introduces errors of less than 5% except in the case of purely longitudinal propagation. For non-deviative absorption, use of the quasi-longitudinal approximation can introduce significant errors which particularly affect the determination of latitudinal variations in absorption.     

Nonlinear disturbances in the ionosphere due to acoustic gravity waves

The influence of the higher harmonics of an internal gravity wave on the formation of nonlinear quasi-periodic disturbances in the F-region of the Earth's ionosphere is considered. It is shown that the Boussinesq approximation cannot be used in describing a plane nonlinear gravity wave as nonlinearities associated with the compressibility of the atmosphere have to be taken into account.     

Dynamical coupling processes between the middle atmosphere and lower ionosphere

A variety of dynamical processes are important in coupling motions within the middle atmosphere and lower ionosphere. These processes can generally be classified as either advective, wave-like or diffusive. Within the middle atmosphere, wave-like processes, and especially gravity waves, are of crucial importance. Turbulent diffusion and advection probably play lesser, but not insignificant, roles. However, whether these same concepts apply to coupling for regions outside the middle atmosphere—and especially between the upper middle atmosphere and the lower ionosphere—is not clear. In this paper the current knowledge about coupling processes between these important regions is reviewed. We discuss coupling processes in the middle atmosphere with the objective of examining how well these concepts may be extended to the topic of middle atmosphere to ionosphere coupling. Different approaches to understanding, and the different experimental procedures used by workers studying the middle atmosphere and ionosphere, also make comparisons difficult. The importance of the measurement of common parameters is highlighted, and the need for coordinated campaigns to examine the interactions between the regions is emphasized.     

Penetration of hydromagnetic waves of cylindrical symmetry through the high latitude ionosphere

The problem is formulated, and boundary conditions are developed, in order to solve numerically the equations for the penetration of hydromagnetic waves of horizontal cylindrical symmetry through a stratified high latitude ionosphere and atmosphere due to sources above the ionosphere. There are two orthogonal polarization modes. The numerical results are shown for various cases. It is found that near the ground the horizontal components of both the electric and magnetic fields along the circumference direction are much smaller than the radial components.     

Large-amplitude standing planetary waves induced in the troposphere by the Sun

Investigations have been made of the global morphologies of two different Sun-weather relationships: the effect of the 27.5-day solar rotations and the effect of the 11-year sunspot cycles on tropospheric pressure. In both cases solar-induced perturbations occur in the form of large-amplitude standing planetary waves (or some combination of waves) of the (0, n), (1, n) and (2, n) types. Certain spatial features of Sun-weather relationships which have hitherto appeared puzzling are merely manifestations of these planetary waves.The range of the solar-induced 27.5-day oscillations of the height of the 500-mbar level reaches 14 Dm (decametres) in certain parts of the World. Such oscillations are of considerable practical importance; for example, the average pressure gradient responsible for driving the westerlies at upper-middle latitudes in winter varies by as much as a factor of two during the solar rotations. It appears that other short-term changes of solar radiation may also evoke a response in the lower atmosphere; the implications for theoretical studies of tropospheric dynamics are considerable.     

Variations of the lower ionosphere parameters measured by the resonance scattering method

This paper presents the results of diagnostics of the lower ionospheric parameters by using the resonance scattering method (RSM) of radio waves on periodic artificial irregularities (PAI). The following ionospheric parameters have been measured by the heating facilities ‘Zimenki’ and ‘Sura’: electron density including the E-F interlayer valley and the lowest height of the ionosphere; the velocity of the vertical wind; the relaxation times of the PAI characterizing coefficients of ambipolar diffusion, and coefficients of attachment and detachment of electrons from negative ions. Variations of these parameters have been considered as a function of height, local time, season, solar activity as well as being induced by passing acoustic-gravity waves and by ionospheric disturbances.     

Disturbances in the lower ionosphere caused by a low altitude nuclear explosion

Observations of the lower ionospheric disturbance caused by a low altitude nuclear explosion are presented. A forward scatter radar, frequency 41 MHz, power 2.5 kW, was used to study these disturbances. The first radar scattering signal consisting of three peaks appeared 40 s after the explosion. It was due to early ionization by delayed y-rays. The second kind of disturbance generated after 190 s was clearly different from the first. The scattering signal had a constant component which indicated a strong specular reflection. The field strength increased by more than 20 db. This disturbance was produced by the direct shock wave. The third kind of disturbance began after 8 min, lasted 5.0 min, and was probably dominated by the fireball/smoke cloud oscillation when it reached its stabilization altitude and approached hydrodynamical equilibrium with the ambient atmosphere. Using numerical computation techniques, we have explained the above results well.     

ELF fields for an inhomogeneous ionosphere with diffusive lower boundary

An ELF field equation is derived for an inhomogeneous ionosphere with diffusive lower boundary. Numerical results are presented to show that consistent ELF fields independent of the stratification of the ionosphere can be obtained by using this field equation.     

Interference of tidal and gravity waves in the ionosphere and an associated sporadic E-layer

Observations made on 10 July 1987 with the EISCAT UHF radar are presented. The F-region measurements of both electron density and field-aligned ion velocity show that an upward propagating gravity wave with a period of about 1 h is present. The origin of the gravity wave is probably auroral. The E-region ion velocities show a tidal wave and both upward and downward propagating gravity waves. The gravity waves have three dominant periods with a possible harmonic relationship and similar vertical wavelengths. These waves are either reflected at a single reflection level, ducted between two levels, or they are generated in a non-linear interaction between gravity and tidal waves. The E-region electron density is dominated by particle precipitation. After a short burst of more intense precipitation, a sporadic E-layer forms at 105km and then disappears 40min later. Within this time, the layer rises and falls by a few kilometres, following closely the motion of a convergent null in the velocity profile. We suggest that the formation and destruction of this layer is controlled by both the precipitation, which indirectly provides a source of metal ions through charge exchange, and the superposition of gravity waves and the tidal wave.     

Effects of geomagnetic storms in the lower ionosphere,middle atmosphere and troposphere

Geomagnetic storm effects at heights of about 0–100 km are briefly (not comprehensively) reviewed, with emphasis being paid to middle latitudes, particularly to Europe. Effects of galactic cosmic rays, solar particle events, relativistic and highly relativistic electrons, and IMF sector boundary crossings are briefly mentioned as well. Geomagnetic storms disturb the lower ionosphere heavily at high latitudes and very significantly also at middle latitudes. The effect is almost simultaneous at high latitudes, while an after-effect dominates at middle latitudes. The lower thermosphere is disturbed significantly. In the mesosphere and stratosphere, the effects become weaker and eventually non-detectable. There is an effect in total ozone but only under special conditions. Surprisingly enough, correlations with geomagnetic storms seem to reappear in the troposphere, particularly in the Northern Hemisphere. Atmospheric electricity is affected by geomagnetic storms, as well. We essentially understand the effects of geomagnetic storms in the lower ionosphere, but there is a lack of mechanisms to explain correlations found deeper in the atmosphere, particularly in the troposphere. There seem to be two different groups of effects with possibly different mechanisms—those observed in the lower ionosphere, lower thermosphere and mesosphere, and those observed in the troposphere.     

Simultaneous observation of the quasi-two-day variations in the lower and upper ionosphere over Europe

The large scale character of the observed quasi-two-day fluctuations in the whole ionosphere (from D- uptoF-region maximum) over Europe is shown. The study is based on the lower and upper ionospheric data obtained in Sofia (42.9°, 23.4°E), Ebre Observatory (40.9°N, 0.5°E) and El Arenosillo (37.1°N, 6.7°W) during two summer intervals: June–August 1980 and 1983. The obtained prevailing periods for the F-region fluctuations are 52–55 h and the mean amplitude is higher than 1 MHz. It was found that the fluctuations propagate westward with a mean phase velocity between 4.6 and 6° /h. The quasi-two-day variations in the F-region maximum are probably generated by flucutations in the mesospheric, neutral wind. During the time when well developed quasi-two-day fluctuations exist in the mesospheric neutral wind, similar variations are observed in the lower ionosphere also. Possible mechanisms for generating the D- andF-region electron density fluctuations from these oscillations in the neutral wind are proposed.