Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

A climatology of quiet/disturbed ionospheric conditions derived from 22 years of Westerbork interferometer observations

Knowledge of the quiet and disturbed conditions in the propagation medium is essential for quality control of transatmospheric radio signals. This holds equally for the troposphere and the ionosphere. This paper describes a climatology of ionospheric irregularities obtained from observations of celestial radio sources by radio interferometry, i.e. by the Westerbork Synthesis Radio Telescope (WSRT) in The Netherlands. This instrument is located at geomagnetic mid-latitude. All WSRT calibrator observations in the 22-year period 26 June 1970–31 December 1991 have been checked for manifestations of ionosopheric effects. Although seasonal effects are clear, the occurrence and ‘strength’ of ionospheric irregularities show no dependence on solar activity. Assuming that the frequency of occurrence of ionospheric disturbances in spring and autumn are similar, it is found that ‘ionospheric’ winter starts on day 348 ± 3 and all seasons last for 3 months. Medium-scale travelling ionospheric disturbances (TIDs) occur most frequently during the daytime in winter periods. The occurrence of non-periodic irregularities is, however, not a function of time in the day. The daily variation in the amplitude and frequency of the occurrence of the TIDs suggests that the solar terminator and Joule heating near the electrojets do not contribute substantially to their generation. Generation of gravity waves may be caused by winds and tides in the lower thermosphere-mesosphere. This has to be investigated further.On the basis of the available data, a ‘disturbance measure’, indicating to what extent the ionosphere is ‘quiet’, is proposed. The output of this project may be of immediate use for different ionospheric investigations, such as ionospheric modelling and the study of excitation mechanisms for ionospheric irregularities.     

Passage of a powerful HF radio wave through the lower ionosphere as a function of initial electron density profiles

The results of numerical modelling of powerful HF radio wave propagation through the ionosphere plasma are presented. Comparison of the heating wave parameters with those of a low powerwave gives the possibility to study the self-action of the powerful HF wave. At low altitudes the ‘translucence’ of the ionosphere plasma takes place. At high altitudes the wave absorption sharply increases. Both self-action effects lead to the reduction of the altitude range of the heated region. The dependence of self-action effects and the resulting electron temperature profiles on the initial electron density profiles are studied.     

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.     

Modelling of the electron density profile in the lowest part of ionosphere D-region on the basis of radiowave absorption data: 2. Seasonal variations

A new model of the lowest part of the D-region is obtained by a trial-and-error inversion method. Its basic feature is a step-like transition between 55 and 70 km which is not depicted in most ionospheric models. The seasonal differences of this are considered to be quite important: the bottom of the ionsphere is found to be lower in summer and spring, the gradient of the profile below the CR-layer is stronger in winter, and a well defined ‘valley’ exists around 70 km in spring. By simulating the ionospheric response to a, solar flare (SID-effect) in summer and in winter, an attempt was made to verify the obtained seasonal peculiarities of the quiet ionosphere.     

Gyro line observations with the EISCAT VHF radar

This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.     

Night-time electron density of the lower ionosphere in the South Atlantic Geomagnetic Anomaly region obtained with a LF/VLF oblique ionosonde

Studies of the reflectivity of the night-time lower ionosphere in the region of the South Atlantic Geomagnetic Anomaly have been carried out for the period August 1980 to July 1981, using data collected by an oblique ionosonde at low/very low frequency, in the southern part of Brazil, near the centre of the Anomaly. From these studies monthly average behaviour of heights and reflection coefficients of the lower ionosphere between 15 and 60 kHz have been deduced. Assuming an exponential model of the electron density distribution in the lower ionosphere the appropriate numerical parameters were calculated using a trial-and-error approach with ‘full wave’ calculations and iterative computational techniques. For the period considered three sets of parameters could satisfactorily represent the lower ionosphere in the anomalous region and in the period analysed: one valid from August to November 1980, one valid from December 1980 to March 1981 and the other valid from May to July 1981. April 1981 seemed to present anomalous behaviour.     

Simultaneous observations of the enhanced plasma line and of the reflected HF wave at Arecibo

A suitably devised pulsing sequence of the powerful HF transmissions radiated towards the ionosphere below the penetration frequency made it possible to separate the attenuation of the reflected HF wave due to thermal and to ponderomotive type parametric instabilities. The separation was possible on account of the different growth times of the thermal and ponderomotive type parametric instabilities. At the same time the power in the 430 MHz enhanced plasma line was recorded.The results show that previously accepted explanations of the overshoot in the 430 MHz enhanced plasma line for 5.3 MHz HF transmissions are invalid. A strong overshoot was observed but the attenuation of the powerful HF wave was too small to be observed. For 3.175 MHz HF transmissions substantial attenuation was observed on both time scales.     

Fine structure of the high-latitude ionosphere in the cleft and cusp

The results of studies of the fine structure and dynamic processes in the high-latitude ionosphere in the cleft and cusp region by the data of complex radiophysical observations on high-latitudinal paths are presented. They are based on experimental material obtained on board the research vessel ‘Professor Vize’ during July-September 1990 when the vessel was at the Greenwich meridian and the latitudes 75–78°N. The distinctive feature of the radiophysical observations on the vessel was the simultaneous observations by the Doppler method at two fixed frequencies in the decameter range and by the method of oblique sounding of the ionosphere at a frequency sweeping the range from 3.5 to 27.5 MHz. From the observations, the typical feature of the cleft and the cusp has been found to be the presence of wave processes of various periods from 30–40 s to 3–8 min. It is suggested that the emergence of typical negative tracks on the dynamic spectra of HF signals is related to the ionospheric manifestations of flux transfer events.     

Changes of the electron concentration profile during local heating of the ionospheric plasma

Using numerical simulation of a non-stationary problem of thermodiffusion and diffusive spreading of the electron component of the dense cold ionospheric plasma, the processes of formation and relaxation of strong disturbances of the electron temperature and concentration in the E- and F-regions of the middle-latitude ionosphere are examined, taking into account the altitudinal distribution of the electron transport coefficients. The cases of local heating and heating at separated altitudes of the ionospheric plasma by powerful radio waves generated from ground-based HF-facilities are numerically investigated. The numerical simulations of the non-stationary problem are compared with the analytical evaluations carried out for the stationary and quasi-stationary heating models. Results obtained from numerical experiments give good explanations of the experimentally observed deformation of the altitudinal ionospheric plasma density profile and the creation of negative cavities in the upper ionosphere and positive cavities in the lower ionosphere during the process of plasma heating.     

Ionospheric modification by high power radio waves

Powerful, high frequency (HF) radio waves can be used to temporarily modify the ionosphere. These controlled, active experiments have proven useful both for studies of the natural upper atmosphere through observations of ionospheric response to HF induced perturbations, and for basic physics investigations exploiting the ionosphere as a large, natural plasma laboratory-without-walls. This experimental diversity has attracted the attention and participation of physicists from a wide range of disciplines. As a result, HF ionospheric modification research continues to be strongly motivated by its many applications in the fields of aeronomy, space physics, plasma physics, and telecommunications science.     

VLF radiation generated by a loop antenna in P2 layer of the ionosphere—2. Interpretation of the measurements

The results of an experiment on radiation and reception of VLF waves with a frequency of 5 kHz in the F2 layer of the ionosphere are compared with theoretical calculations made in the framework of the linear theory of a loop antenna in a plane-stratified plasma. The measurements made outside the Storey cone can be explained in the framework of the linear theory by the proper modification of the ionosphere model. The wave polarization differs from circular polarization on the boundary of caustic cone, due to non-linear effects.     

The morphology of dayside Birkeland currents

Intense (10⁵ A) electric currents flow into and from the Earth's two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west (Y) component of the IMF. The presence of an IMF Y-component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.     

Large-scale irregularities of electron concentration in the southern magnetic anomaly area according to Intercosmos 19 satellite data

Using Intercosmos 19 satellite topside sounding data, a type of complex ionogram for which the lowest frequency of the radio-wave which has passed through the ionosphere is smaller than the greatest frequency of the radio-wave reflected from the ionosphere is considered. (Under normal conditions these frequencies are identically equal.) A mechanism is suggested by which radio-waves transmitted by the satellite propagate over 3000 km in the topside ionosphere in the presence of inclined large-scale plasma structures, which can explain the main features of such ionograms.The space-time distribution of this phenomenon on a global scale is analysed. It is shown that it manifests itself mainly in the local winter, in the daytime and in the Southern Hemisphere. It is hypothesized that these large-scale irregularities are formed in the vicinity of the South Atlantic magnetic anomaly and then move westward.     

Small scale ionospheric irregularities near regions of soft particle precipitation: scintillation and EISCAT observations

Results are presented from a coordinated experiment involving scintillation observations using transmissions from NNSS satellites and simultaneous measurements with the EISCAT ionospheric radar facility. The scintillation was used to indicate the presence of sub-kilometre scale irregularities while the radar yielded information on the larger structures in the background ionosphere. Two examples are discussed in which localised patches of scintillation were observed at L-shells near ‘blob’ like enhancements in F-region ionisation density. Elevated electron temperatures indicated that the enhancements may have had their origins in soft particle precipitation. While structuring of the precipitation on the 100 m scale cannot be completely ruled out as a source of the irregularities, in one case the blob gradient can be shown to be stable to the E λ B mechanism. The most likely cause of the irregularities appears to be shearing of the high velocity plasma flow in a region adjacent to the density enhancement. This region is characterised by a high ion temperature while the resulting scintillation has a shallow spectral slope.     

Theorising other, ‘other childhoods’: Issues emerging from work on HIV in urban and rural Zimbabwe

This paper agrees that universal models of childhood must be unpacked in order to reveal the diversity of ‘other childhoods’ in the global south, but argues that local, culturally specific understandings of childhood also need to be theorised and deconstructed. We attempt to do this by exploring experiences that are ‘other’ to ‘other childhoods’ and so examine the ‘un-childlike’ issues of young peoples' sexual health and child household headship in Zimbabwe. We contend that a century of contestation around the social production of identities in and through space has produced local contemporary understandings of childhood that seriously endanger youngsters in an era of HIV/AIDS. We argue both that other dimensions of ‘other childhoods’ must be recognised locally and that local understandings of childhood require greater international recognition if the pandemic is to be tackled. Finally, our exploration of these ‘margins’ of human experience lead us to believe that children must be understood both as competent and independent agents of social change and as vulnerable social becomings in need of protection.     

Seasonal variations of day-time ionisation flows inferred from a comparison of calculated and observed NmF2

This paper reports on a comparison of calculated and observed monthly mean day-time ionospheric F2-peak density (NmF2) at a chain of stations from Japan to Australia for both solar minimum (1976) and solar maximum (1980). Nm values are calculated using the MSIS model for the observed peak heights (hmF2) and a simplified version of the continuity equation for day-time equilibrium conditions. The observed NmF2 values are always higher than the calculated ones in winter. This implies that a substantial downward flow of ionisation from above into the winter ionosphere is induced by the strongly poleward winter neutral wind which drives the ionisation down the field lines, lowering the peak height hmF2. In summer, winds are smaller, and the fluxes are more upward in comparison to winter. The seasonal variation of the ionisation fluxes and neutral winds are estimated for solar minimum, and compared with results of detailed calculations.     

Mid-latitude spread-F structure

Spread-F has been observed at frequencies of 1.98, 3.84 and 5.80 MHz and multiple angles of arrival have been resolved using an HF radar near Brisbane (27°S, 153°E). The spreading of the ionogram trace has been shown to be due to a spread in angles of arrival of echoes, rather than any ‘vertical’ spreading. The reflection process appears to involve total specular reflection rather than scattering. The previously reported very strong bias for angles of arrival from the north-west at Brisbane is supported. The direction of movement of the reflection points is not radial and therefore, the structure cannot be purely frontal with purely linear movement, as is often supposed. The velocities are much less than for coexisting travelling ionospheric disturbances. The variations of angle of arrival, range and rate of change of range with frequency do not fit previously proposed ideas of the plasma distribution and an alternative is suggested in which the distortions of the isoionic surfaces resemble small, elongated, asymmetrical ‘hills’ or ‘dips’.     

The influence of large-scale TIDs on the bearings of geographically spaced HF transmissions

An investigation is made of the adequacy of the simple corrugated reflector model to simulate the effects of large-scale travelling ionospheric disturbances (TIDs) on the bearings of HF transmissions. Model results are compared with the quasi-periodic variations of bearings measured simultaneously for a number of geographically spaced transmission paths and with results obtained from 3D ray tracing studies incorporating a more realistic TID model (the ‘'wave’ model).Although the bearing error signature is, at some times (particularly during the day), similar to that predicted by the corrugated reflector model, ray tracing calculations with the ‘wave’ model give generally better results and predict bearing error signatures which correspond much better to some of those observed in the experimental data than those predicted by the simpler model. However, the prediction accuracy of both models is found to be limited by temporal variation of both the TID waveform and the ambient ionosphere.