Analytical modelling of the electric field distribution in the high-latitude ionosphere

An analytical approach is implemented for self-consistent modelling of the high-latitude convection electric field. Input parameters are determined as distributions of field-aligned currents and height-integrated conductivity. The high-latitude ionosphere is approximated with an arbitrary number (N) of concentric rings. The height-integrated conductivity (∑) is independent of co-latitude within any ring, but depends on the longitude ~ sin λ. The field-aligned currents flow only along the boundaries of each ring and are presented by Fourier series in longitude. The analytical solution for the potential φ as a function of longitude is also presented as a Fourier series. An analytical solution is obtained for the potential dependencies on co-latitude. For the extreme case, when the integrated conductivity does not depend on longitude, this solution coincides with the analytical results, obtained by other authors. Based on this solution, the potential distribution in the high-latitude ionosphere, an example with N = 5 is shown, the values of conductivity and field-aligned currents being similar to those values used by other authors.     

Electrical structure in the high-latitude middle atmosphere

Middle atmosphere electrodynamics at high latitudes differs significantly from the normally assumed picture of a passive region through which electric fields of external origin couple. Large Vm ⁻¹ electric fields, both horizontal and vertical, have been observed within bounded regions of the upper stratosphere and lower mesosphere. They seem to occur only in regions where the electrical conductivity is a few times 10⁻¹⁰ S m⁻¹ or less and appear to be current limned. While low conductivity is necessary, it is not a sufficient condition for occurrence. The observed large horizontal electric fields were found to be anticorrelated with the local neutral wind. However, a generation mechanism of these electric fields is as yet unknown but must involve space charge separation rather than dynamo effects. Large variations in the conductivity were also observed to occur with fluctuations in magnetic activity, and these were found to be consistent with measured variations in energy deposition during auroral phenomena. Theoretical concepts of mapping of electric fields downward from the thermosphere along equipotential magnetic field lines were shown to hold qualitatively in the D-region at the mV m⁻¹ level. Perturbations affecting such models were determined to be small.     

A survey of simultaneous observations of the high-latitude ionosphere and interplanetary magnetic field with EISCAT and AMPTE-UKS

This paper surveys the results of simultaneous observations by the EISCAT incoherent scatter radar and the AMPTE-UKS satellite, made during three periods in September and October 1984, when AMPTE-UKS was in the solar wind on the dayside of the Earth and the UK-POLAR EISCAT experiment was measuring ionospheric parameters at invariant latitudes 70.8–75.0°. A total of 42 h of EISCAT convection velocity data, with 2.5 min resolution, were obtained, together with 28 h of simultaneous 5 s resolution AMPTE-UKS observations of the solar wind and interplanetary magnetic field (IMF). The general features of the AMPTE-UKS data are described in Section 2 and those of the EISCAT data are described in Sections 3 and 4. The main subjects discussed are the form of the plasma convection patterns and their dependence on all three components of the IMF (Section 5), the ionospheric response to abrupt changes in the IMF (Section 6), in particular a sharp ‘southward turning’ of the IMF on 27 October 1984, and a crossing of an IMF sector boundary. Section 7 describes ‘short lived rapid flow burst’, which are believed to be related to flux transfer events at the magnetopause.     

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.     

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.     

Interplanetary magnetic field structure and the variations of the ELF and VLF emissions in the topside ionosphere

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters [the polarity of the sector structure, the signs and magnitudes of the B_y, and B_z, components of the IMF as defined in the Solar Ecliptic coordinate system (Nishida, 1978)]. The comparison shows that:

• 1.(i) the positive polarity of the IMF sector structure (when IMF vector is directed toward the Earth) involves an enhanced probability of the detection of larger emission field intensities (>25–30dB);
• 2.(ii) the emission median intensity is ~20dB higher at B_y > 0 compared with B_y < 0;
• 3.(iii) the 0.72 kHz emission median intensity in the polar caps and at night-side auroral latitudes is lower when B_z > 0 as compared with B_z < 0;
• 4.(iv) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.

     

A method of determining horizontal structure of the ionosphere from backscatter ionograms

We have developed a technique lor oblique backscatter sounding (OBS) ionogram inversion as a diagnostic tool for the horizontally inhomogeneous structure of the ionosphere. Input data for the method include the leading edge of a backscalter ionogram that is measured through soundings in a given direction, and the vertical electron density profile measured over the sounding station or over some other site lying in the sounding direction. The method may be useful for reconstructing the two-dimensional electron density distribution in a vertical plane aligned with the direction of sounding. The inverse problem has been solved using the Newton Konlorovich method and the Tikhonov regularization method. The algorithm we have developed was tested against model data, that is, OBS ionograms synthesized using geometrical optics calculations for different models of the inhomogeneous ionosphere. Test results demonstrate that our method converges reliably, is stable to measurement errors and provides a good accuracy of reconstruction of inhomogeneous structures with scales of 100 2000 km. This indicates that this method shows promise as an operative remote diagnostic tool for ionospheric irregularities of natural and artificial origin.     

Fine scale structure and turbulence parameters in the equatorial middle atmosphere

Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 70–100 km. The results are found to be different from those at middle and high latitudes.     

A study of the meridional structure of the equatorial red arcs in the night-time ionosphere from the ISIS-II satellite

Equatorial 6300 Å arcs observed by the ISIS—II satellite close to the magnetic equator over the African and Asian zones are studied for night-time conditions from 21:00 h to 02:00 h local time in the summer and spring of 1972–1974 and 1976, respectively. Case studies of the arcs have been made for quiet geomagnetic conditions and for minor storms. Sometimes very intense arcs with intensities of 1–2 kR are observed. Arcs of moderate intensities (300–400 R) are observed during geomagnetically disturbed periods. It is confirmed that these intensities can be fully accounted for theoretically by the dissociative recombination of molecular oxygen ions. Since the emission intensities are found to be sensitive to the geomagnetic activity, the influence of the latter has been taken into account and discussed.Equatorial spread-F (ESF)/bubble conditions are usually present at these local times. The data presented here show a correlation between the 6300 Å emission rate at one of the anomaly crests, the gradient in h (the lowest scaled real height from topside ionosonde trace) and the existence of ESF and gravity waves. This correlation is consistent with the scenario put forward by Maruyama and Matuura that the occurrence of ESF requires a symmetrical electron density distribution around the magnetic equator, so that a transequatorial wind causes an asymmetry and inhibits the formation of ESF.For the ISIS data we conclude that where strong transequatorial winds exist the 6300 Å emission rate at one of the anomaly crests is very large and there is a steep gradient in h. When these winds are weak, the 6300 Å emission is low and the gradient in h is also small. In the latter case, gravity waves of wavelength 200–400 km were present as well, which suggests that ESF is promoted by the existence of gravity waves. However, the magnetic disturbance level was higher during these orbits, which offers another source of gravity waves.     

Topside irregularities in the equatorial ionosphere

This paper is a brief review of ionospheric irregularities in the equatorial topside ionosphere. Results from topside sounders, direct measurement satellites, and the Jicamarca incoherent scatter radar are discussed. Scintillation observations and theories of irregularities are not discussed in detail as these are the subject of other review papers. Many of the phenomena detected in the topside ionosphere are related to bottomside irregularities, commonly known as spread-F. These include aspect-sensitive scattering observed on topside sounders, significant concentrations of Fe⁺, electrostatic turbulence and the topside irregularities detected by the Jicamarca radar. Satellite measurements show that the irregularities in electron concentration have amplitudes which increase almost linearly with wave-length over the range 70m to 3km. Duct irregularities detected by the topside sounders and some wavelike irregularity structures detected occasionally by direct measurement satellites may be separate from the general spread-F phenomenon although this has not definitely been established.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Fine structure and dynamics of the Harang discontinuity: An event study

Two rockets were launched simultaneously from the Andöya Rocket Range in northern Norway into and through a Harang discontinuity region during a small isolated substorm. Each was equipped with an array of instruments for monitoring ionospheric electric fields and auroral particles. Different sized rockets allowed both to traverse a range of 300 km while introducing altitude separations of up to 440 km, and separations between the feet of the magnetic field lines intersecting the payloads of up to 100 km. The data sets, coupled with multi-station ground-based observations, provide information on the structure and dynamics of the Harang discontinuity region. Two boundaries were encountered. The first electric field reversal was observed simultaneously by both payloads in a region of weak precipitation, while the second reversal was associated with an intense auroral band. Since an unambiguous interpretation of these data is not possible, we present two alternative scenarios. In one the discontinuity becomes realigned during the flight following a decay in the activity. The second involves a triple cell convection system, possibly the result of an eddy in the flow.     

The theory of radio windows in the ionosphere and magnetosphere

Radio waves in a stratified plasma can sometimes penetrate through a region where, according to a simple ray theory, they would be evanescent. They emerge on the far side in a different magnetoionic mode. This occurs when the incident wave normal is within a small cone of angles, called a radio window. The best known example is the Ellis window, used to explain the Z-trace in ionosonde records. Other phenomena where windows may be important have recently been studied. Simple approximate formulae are given for the transmission coefficient of a window and for its angular widths. These show the dependence on frequency, electron concentration gradient and direction of the ambient magnetic field. Comparison with more accurate calculations shows that these formulae are likely to be reliable in practical applications. The tracing of rays near a window is discussed, and the properties of a second kind of window are described.     

LHR whistlers and lhr spherics in the outer ionosphere

Special types of VLF signals, which follow whistlers and spherics and have an anomalous dispersion near the lower hybrid resonance (LHR) frequency, have been observed on the low-altitude Intercosmos satellites. These signals have been named LHR whistlers and LHR spherics, respectively. A mechanism is suggested for the formation of their spectra, based on the peculiarities of quasi-resonance wave propagation at frequencies near the LHR frequencies. It is shown that the large dispersion observed may be accounted for by a significant increase in the propagation time of the wave as its frequency approaches the maximum in the LHR frequency profile.     

Parametric and related non-linear wave-wave interactions in the ionosphere

Six papers all dealing with non-linear wave interaction processes excited during ionospheric modification experiments are reviewed. The papers were presented as posters at the URSI Open Symposium 2 on Active Experiments in Space Piasmas, Florence, Italy, 30–31 August 1984.