On the global and regional behaviour of the mid-latitude ionosphere

Data from a chain of seven ionosondes in the range of 56–38 N and 1–38° E geographic coordinates were analysed to illustrate the global and regional behaviour of the mid-latitude F-region for some selected geomagnetic storms that occurred during the solar cycle 21. It was found that there are different spatial scales in the response of the mid-latitude ionosphere to the disturbance in the magnetosphere-ionosphere thermosphere system. The physical mechanisms and processes are discussed in relation to the relevance of various theories in the understanding of the dynamics of ionospheric storms.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Diffusion equations for the major ions in the mid-latitude ionosphere and plasmasphere

Diffusion equations for O⁺ and H⁺ ions for ionosphere-plasmasphere interaction are derived from the transport equations formulated by Schunk. Low speed geomagnetic field aligned flow was assumed and the interaction between different kinds of ions, between ions and electrons and between ions and neutrals taken into account. The appropriate terms of the equations have been derived and the transport coefficients calculated using parameters typical for the mid-latitude ionosphere and the ionospheric main trough. It is found that interaction of ions with neutral particles influences to some extent the ion thermal diffusion. Diffusion equations retaining only terms not smaller than one-tenth of the largest are given in the paper.     

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.     

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.     

Lower ionosphere,lower atmosphere and IMF sector structure in winter

There are two basic types of IMF sector boundary crossing effect. One is observed in the IMF magnitude, geomagnetic activity, the nighttime lower ionosphere in high mid-latitudes and in an inverse form in cosmic rays and is reasonably well explained. The other is observed in the noon lower ionosphere in high mid-latitudes and in the troposphere; its interpretation remains open.     

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.     

On temperature trends of the atmosphere from rocket and radiosonde data

The results of statistical analyses of more than 7000 rocket and radiosonde sounding from five stations between 1964 and 1990 are presented. With the help of special statistical analysis, their regular periodic components and trends between 5 and 75 km are estimated. The main conclusion is that the stratosphere and mesosphere become cooler from 1964 to 1990. The rate of cooling in the mesosphere is about 0.3–1.0 K/yr. According to radiosonde data, there is a positive temperature trend in the lower stratosphere and troposphere of about 0.1–0.2 K/yr at middle and low latitudes.     

Modulation of the auroral proton spectrum in the upper atmosphere

An energetic auroral proton entering the atmosphere will alternate between being a proton and a neutral hydrogen atom by charge-exchange collisions with atmospheric constituents. This study uses a simple procedure to evaluate the energy degradation of the penetrating protons/hydrogen atoms by using semi-empirical range relations in air, and derives the particle energy variation as a function of altitude, starting from proton spectra observed from rockets above the main collision region. The main assumptions are that the geomagnetic field is homogeneous and vertical and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. The calculations show that the incoming particle flux first loses the low energy particles at the highest pitch angles, even if the beam itself widens as it penetrates the atmosphere. The largest energy loss for particles with initial energy between 10 and 1000 keV occurs in the height interval between 100 and 125 km.     

A simple model of gravity-wave momentum and energy fluxes transferred through the middle atmosphere to the upper atmosphere

Vertical fluxes of momentum and energy through the middle atmosphere are calculated by using a simple semi-empirical model of quasi-monochromatic internal gravity waves with dominant vertical wavenumbers. In this model those dominant gravity waves are assumed to saturate and break at each observational altitude by an effective critical-layer mechanism. The dominant value of the vertical wave-number is expressed by an exponential function of altitude, decreasing upward with a scale height of 34 km. This expression gives the momentum and energy flux densities decreasing upward with scale heights of 12 and 18 km, respectively, and typical values at 100 km altitude are estimated as 4 × 10⁻⁵ Pa and 4 × 10⁻³ W/m². A heat flux induced by wavebreaking turbulence also has an order of magnitude similar to that of the wave energy flux. Variabilities around these values and comparisons with other momentum and heat inputs to the upper atmosphere are only briefly discussed.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

Calculation of auroral Bahner volume emission height profiles in the upper atmosphere

Energetic protons entering the atmosphere will either travel as auroral protons or as neutral hydrogen atoms due to charge-exchange and excitation interactions with atmospheric constituents. Our objective is to develop a simple procedure to evaluate the Balmer excitation rates of H_α and H_β, and produce the corresponding volume emission rates vs height, using semi-empirical range relations in air, starting from proton spectra observed from rockets above the main collision region as measured by Reasoneret al. [(1968) J. geophys. Res.73, 4185] and Søbraaset al. [(1974) J. geophys. Res.79, 1851]. The main assumptions are that the geomagnetic field is parallel and vertical, and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. Calculations show that the largest energy losses occur in the height interval between 100 and 125 km, and the corresponding volume emission rate vs height profiles have maximum values in this height interval. The calculted volume emission rate height profile of H_β compares favorably with that measured with a rocket-borne photometer.     

MF Doppler and spaced antenna radar measurements of upper middle atmosphere winds

Observations of upper mesospheric and lower thermospheric wind velocities obtained simultaneously over six days with MF Doppler and Spaced Antenna (SA) radars at Adelaide, Australia in November 1980 are presented. To obtain these measurements, the large (~ 1 km diameter) Buckland Park MF array was run in a dual beam Doppler radar configuration, and a portable SA radar was operated adjacent to the main array. Hourly mean values of wind velocity show considerable consistency, with cross correlation coefficients of about 0.6–0.8 for the entire observational period. However, agreement between the magnitudes of the wind velocities as measured by each technique is found to be significantly improved when the effect of the aspect sensitivity of the backscattering irregularities on the effective beam pointing angle of the Doppler radar beams is taken into account. This is also found to be true for SA and Doppler radar observations obtained in adjacent periods of 2–5 days over two years with the Buckland Park facility operating alternatively as a Doppler and SA radar. Some representative examples of these results are also presented and discussed. A preliminary comparison between MF Doppler and SA radar derived vertical wind velocities is also briefly considered.     

Small scale stratification of the mid-latitude F-region

lonograms recorded over limited frequency and height ranges have been used to reveal systematic structures on both main and range spread traces for a mid-latitude region. This fine structure has apparently not been reported previously. When disturbances are present ionogram traces are often made up of quasi-horizontal segments, suggesting that the F2-layer consists of stratified regions spaced a few kilometres apart vertically. It is proposed that if in fact stratifications are present, they may be produced by instabilities created by the presence of internal gravity waves. Interference effects on ionogram traces are also reported. These are shown to result from the presence of overlapping traces. The existence of these quasi-horizontal trace segments and interference effects is shown to contribute to the diffuseness of a range spread mid-latitude ionogram. Thus a mid-latitude range spread ionogram often appears diffuse even though the spread may consist primarily of a number of duplicate traces produced by specular reflections from a range of off-vertical angles.     

Winter-time disturbances in the mid-latitude D-region

Radio-wave absorption data from sixteen mid-latitude stations distributed in longitude, together with magnetic-field disturbance parameters and satellite measurements of thermal radiances, have been examined for the winter of 1976–1977. It has been demonstrated that D-region disturbances at mid-latitudes in winter can be associated with both the delayed effects of geomagnetic storms and with changes in mesospheric temperature.     

The role of the inversion population in barium cloud ionization in the upper atmosphere

The influence of the population of barium atoms on barium cloud ionization in the upper atmosphere is investigated. It is shown that the experimental values of the effective time for barium cloud ionization is determined by the metastable ³D states' inversion population.     

Characteristics of variations of temperature regime and circulation in the upper atmosphere in middle and high latitudes

Statistical characteristics of the variability of meteorological parameters of the stratosphere and mesosphere are reviewed for various latitude belts in the winter and summer seasons. Rocketsonde data were used to calculate mean and median values, variance of the meteorological parameters, excess and asymmetry of empirical distributions, probability of deviations and auto- and mutual correlation functions, which enabled the variability of atmospheric processes in various latitude belts to be characterized. The resolution of a sequence of vertical profiles of temperature, pressure, density and wind components with the help of natural orthogonal components made possible the estimation of the scale of wave perturbations responsible for the variations of the parameters in the middle atmosphere.