High-latitude D- and E-region investigations using imaging riometer observations

The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B_Y component of the interplanetary magnetic field. A typical example is examined.     

Tests of an ion-chemical model of the D- and lower E-region

A relatively simple steady-state ion-chemical model for the non-auroral D- and lower E-regions is presented. Solar fluxes and the neutral atmospheric constituents are taken from up-to-date literature. The model results are systematically compared to measurements, namely: ion composition, electron density vs solar zenith angle, solar activity, season and latitude. Although the agreement is not always good, it is believed to exceed that of purely statistical models.     

The effects of the resonance broadening of Farley-Buneman waves on electron dynamics and heating in the auroral E-region

Several theories have now been developed which deal with the saturation mechanism of the Farley-Buneman instability and also the heating effect of Farley-Buneman waves on E-region electrons. Various related phenomena, such as anomalous collisions, anomalous cross-field diffusion and non-zero aspect-angle propagation have been treated. In this paper, it is demonstrated how several of these features may be included in a single resonance broadening formalism. Using this approach, the effects of Farley-Buneman waves on the heating and dynamics of background ionospheric electrons is discussed. In addition the effects of anomalous parallel diffusion in the context of E-region Farley-Buneman waves is treated for the first time.     

The E-region electron density diurnal asymmetry at Saint-Santin: observations and role of nitric oxide

Measurements of the E-region electron density were made with the Saint-Santin incoherent scatter radar during consecutive days in June 1978, March 1979 and December 1980. On the basis of a statistical study, the observations show the presence of a diurnal asymmetry of the electron density, with morning values usually exceeding the afternoon densities by 3–20%. Two possible causes of the dissymmetry are examined: the asymmetry in the diurnal variation of the neutral composition and the effect of nitric oxide. The presence of NO partly converts O₂⁺ into NO⁺ ions and increases the effective recombination rate of the electrons in the afternoon. Numerical simulations assessing the relative importance of the two factors are, in general, in good agreement with the measurements.     

Nonlinear solitary wave in the ionospheric E-region

A nonlinear wave equation for a solitary one-dimensional inhomogeneity is studied. It is similar to the diffusion equation with the diffusion coefficient depending on the phase velocity. The phase velocity depends, in turn, on the electron density. A weak inhomogeneity moves with the velocity close to that of the electric drift. If this velocity exceeds the ion acoustic speed the effective diffusion is negative, and the inhomogeneity grows and contracts. The velocity of the growing inhomogeneity becomes smaller. It absorbs weaker and faster moving inhomogeneities from the back side. In a stationary regime, the ionosphere will be filled with rare but strong inhomogeneities with sharp back sides.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Winds and electric fields in the upper E-region over Malvern

Some preliminary hourly values of total ionic velocity in the upper E-region (122–144 km) are presented and the magnetic meridian component of horizontal neutral wind, averaged from observations on 14 days spread over about two years, is studied. The semi-diurnal variation is found to be predominant at 122 km, almost exactly in phase with the same component at St. Santin (France). Knowledge of the complete ionic velocity allows the rigorous calculation of the meridian perpendicular component of the electric field, some preliminary values of which are presented.     

Electrodynamic effects of metal ions in the noon equatorial E-region

The observations of metal ions in significant concentrations and in layer formation in the lower E-region are briefly reviewed. It is expected that a metal ion layer may alter the electro-conductivity of the ionosphere and modify the distribution of ionospheric dynamo current. The variation in the electroconductivity is theoretically calculated, and it is shown that the Pedersen conductivity is reduced by less than 14%. In the equatorial ionosphere, field line integration of metal ion effects is likely to be large. A metal ion layer, similar to those observed, is numerically modelled in the equatorial region and its electrodynamic effects on the equatorial electrojet (EEJ) are numerically examined using our self-consistent model of the ionospheric dynamo. The effects are found to be significant on the amplitude of the EEJ, but not effectively large on its peak altitude.     

Electron temperature enhancements in the polar E-region measured with EISCAT

We have found electron temperature enhancements up to 1000K at 110 km altitude using the EISCAT multipulse method which allows high spatial resolution measurements within E-region. This electron temperature enhancement which is closely related to the d.c. electric field strength, is in good agreement with theoretical estimates based on wave heating. The results of the measurements are presented together with a discussion of the electron gas heating, its height variations and its difference in the eastward and westward electrojet.     

On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.     

Adiabatic and isothermal ion-acoustic speeds of stabilized Farley-Buneman waves in the auroral E-region

The influence of ion and electron energetics on the propagation speeds of stable Parley Buneman waves which are excited by E × B drifts in the auroral E-region is studied theoretically in the fluid limit, with the effects of anomalous collisions on electron thermal conduction included for the first time. In particular, the ratio of the phase speed of waves, stabilized by enhanced diffusion effects, to the isothermal ion-acoustic speed are calculated for realistically modelled E-region ion and electron temperatures, as functions of altitude, flow velocity and wavelength. It is found that the phase speeds of these stabilized waves begin to increase above isothermal ion-acoustic speeds as wave frequencies increase to values where they are comparable with the electron inelastic collision frequency. However, at still higher frequencies their phase speeds tend to fall back towards their isothermal values due to the increasing effects, with increasing wavenumber, of electron thermal conductivity. It is also found that the phase speeds are not always isotropic with respect to flow angle. The relationship between the predictions of the present fluid theory and a previous kinetic theory calculation is also briefly discussed.     

Direction of arrival of radio waves reflected from the E-region

A simple system for measuring the direction of arrival of continuous high frequency radio waves reflected obliquely from the ionosphere is described. Results obtained over a near vertical path are used to evaluate changes in the electron density gradients (tilts) in the E-region. Wave interference on the transmission due to multiple hop or ground wave propagation is discussed.     

Nocturnal high-latitude E-region in winter during extremely quiet conditions

The quiet night-time E-region at high latitudes has been studied using the EISCAT UHF radar. Data from three subsequent nights during a long period of low magnetic activity are shown and typical features of electron density are described. The background electron density is observed to be 5·10⁹ m⁻³ or smaller. Two types of enhancements above this level are observed ; one is due to charged particle precipitation associated with the F-region trough and the other is composed of sporadic-E layers due to waves in the neutral atmosphere. The sporadic-E is observed to exist almost continuously and to exhibit a regular diurnal behaviour. In addition to the typical afternoon and morning sequential layers, a third major descending layer is formed at night after the passage of the F-region trough The afternoon layer disappears simultaneously with the enhancement of the northward trough-associated electric field and the night-time layer appears at high altitudes after the field has again been reduced to a small value. It is suggested that metal ions from low altitudes are swept by the electric field to the upper E-region where they are again compressed to the night-time layer. A set of steeply descending weaker layers, merging to the main night-time layer are also observed. These layers are most probably caused by atmospheric gravity waves. Theoretical profiles for molecular ions indicate that the strongest layers are necessarily composed of metal ions but, during times when the layers are at their weakest, they may be mainly composed of molecular ions.     

Solar quiet geomagnetic variations and E-region neutral winds at equatorial latitudes

A model of the ionospheric current system valid at zones close to the geomagnetic equator, taking into account the contribution of neutral winds, is proposed. From this, the external magnetic field at ground is calculated. Also, ground records of the geomagnetic field variations at the Peruvian equatorial zone were separated into their external and internal contributions. Using an iterative process a local particular fitting was found by comparing the separated external field to the one calculated with the proposed model.     

Coherent radar (SABRE) studies of dynamical processes in the auroral E-region

In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

E-region nitric oxide concentrations inferred from the 26 February 1979 eclipse expedition

Nitric oxide (NO) concentrations have been determined from the analysis of positive ion composition data obtained by AFGL for eclipse and post-eclipse conditions near Red Lake, Canada. Values of about 3 × 10⁸ cm⁻³ for 105–110 km and about 3 × 107 cm⁻³ for 90 km have been established. A residual ion-pair production rate of about 50 cm⁻³ s⁻¹ is estimated for eclipse totality at 110 km. A factor of two uncertainty is thought to be appropriate for all deduced values. The calculated NO concentrations appear to be within the range of typical variations for this season (late winter) and latitude (51°N).     

Generation of stationary irregularities due to heating of the high-latitude ionospheric E-region

The nonlinear equations describing the generation of artificial irregularities in the E-region of the high-latitude ionosphere due to Joule electron heating under the action of high-power radio waves are derived, including the low-frequency nonlinearities, and investigated in the three-wave approximation. Expressions for the stationary spectra of the short wavelength two-stream and long wavelength gradient-drift and current-convective irregularities are presented. The typical saturation amplitudes of the density fluctuations are obtained.     

Thermodynamics and electrodynamics of the auroral E-region during the D salvo of the MAP/WINE campaign

With the help of incoherent scatter (EISCAT) data the thermodynamics and electrodynamics of the auroral E-region north east of Andøya Rocket Range has been investigated between 1740 UT and 2040 UT on 31 January 1984. This time period covers the D salvo of MAP/WINE and the EISCAT incoherent scatter data comprise a useful supplement to interpret the rocket data. Good agreement has been found between the EISCAT temperatures and those derived from mass spectrometer data. Neutral wind velocity estimates from EISCAT and from a falling sphere rocket experiment are in satisfactory agreement for the zonal wind component, but disagree for the meridional component.