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.     

Three-dimensional ionospheric currents and fields generated by the atmospheric global circuit current

Three-dimensional ionospheric currents and fields generated by atmospheric global circuit currents, using the distribution of air-Earth currents as a lower boundary conditions of the ionosphere, have been studied. The air-Earth currents are obtained taking geomagnetic and orographic effects into account, under the assumption of an ionosphere with infinite conductivity. Three dimensional ionospheric currents due to thunderstorm sources are calculated, considering the conductivity distribution in the ionosphere and the configuration of the magnetic field. The calculated potential difference in the ionosphere is 55 V and according to our model the horizontal electric field is too weak to affect the ionosphere and magnetosphere significantly. Horizontal currents are not distributed uniformly, but preferably in the day-side hemisphere and especially in the equatorial region, and vertical currents and fields do not simply decrease with altitude near the equator because of anisotropy and nonuniformity in the conductivity.     

Production of electromagnetic field disturbances due to the interaction between acoustic gravity waves and the ionospheric plasma

The problem of electromagnetic field disturbances produced by the interaction between winds of acoustic gravity waves (AGW) origin and the ionospheric plasma has been considered. It is shown that, when not allowing the electrostatic approach, electromagnetic field disturbances represent shear Alfvén and compressional modes modified by ionospheric Pedersen and Hall conductivities. It is further shown that the quasielectrostatic Alfvén type disturbances give the main contribution to electric field perturbations. Magnetic field perturbations due to Alfvén and compressional modes have the same order of magnitude. Two numerical models for simulation of the problem under consideration have been developed. The first model is intended for the simulation of Alfvén type disturbance production and transmission into the magnetosphere, taking into account the dipole geometry of the geomagnetic field, but a mutual transformation of Alfvén and compressional modes is ignored. The second model is constructed for the simulation of both electromagnetic field disturbance production and their mutual transformation in the ionosphere. The results of numerical simulations with these models show that there is an opportunity for AGW activity monitoring in the lower thermosphere by ground-and satellite-based recordings of magnetic and electric field variations.     

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.     

Observations and modelling of ionospheric and thermospheric disturbances during major geomagnetic storms: A review

The thermosphere is primarily energised by the combination of three sources of energy and momentum. Solar UV and EUV energy is absorbed globally on the dayside within the middle and upper thermosphere. There is a persistent, but highly variable, inflow of energy and momentum from the magnetosphere. These magnetospheric inputs are usually confined to high latitudes, except at times of very large geomagnetic disturbances. Tides and gravity waves upwell from their sources in the troposphere and stratosphere to deposit energy and momentum at levels from the middle mesosphere to the upper thermosphere. Solar EUV radiation between 120 ran and 250 nm photo-dissociates the molecules which dominate the composition of the lower thermosphere, in particular producing atomic oxygen which dominates the composition of the upper thermosphere. The combination of solar EUV radiation at wavelengths shorter than 120 nm, plus energetic (mainly) charged particles from the magnetosphere, also ionise the neutral constituents of the thermosphere, creating the ionosphere. Particularly at high latitudes, within the geomagnetic polar caps and auroral ovals, the energetic, dynamical and chemical coupling and interactions between the thermosphere and ionosphere dominate the structural and dynamical response of both the thermosphere and ionosphere to solar and geomagnetic inputs of energy and momentum.Comparisons between predictions using global thermosphere-ionosphere coupled models and comparable observational sets have shown encouraging agreement during periods of relatively quiet geomagnetic activity. This indicates that the major energetic, ionisation, chemical and dynamical processes and interactions can be described in models with reasonable accuracy. During periods of high geomagnetic activity, and particularly during major geomagnetic storms, large rapid disturbances of the thermosphere occur with extremely rapid variations. These disturbances are observed as large increases of temperature, density, major changes of neutral composition, and with the development of high speed wind flows and large amplitude waves which may propagate to affect the entire globe. Since the ionosphere is formed from thermospheric constituents and affected by thermospherc dynamics, the gross disturbances of the ionosphere during highly disturbed periods are related to contemporary changes of density, composition and flows of the thermosphere, as well as changes of ionisation sources and electric fields. Observations which describe the nature and scale of disturbances of the thermosphere during geomagnetic storms will be used, in combination with appropriate global numerical simulations, to aid interpretation of storm-time ionospheric phenomena. The role of energetic, dynamical and chemical coupling between the thermosphere and ionosphere is emphasised.     

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.     

What became of Appleton's ionosphere?

Appleton's papers on ionospheric physics spanned a period of forty years and dealt with all the main regions of the ionosphere. They include discussions of the ionosphere at low, middle and high latitudes, and consider many kinds of ionospheric anomalies and disturbances — the seasonal anomaly, the equatorial anomaly, atmospheric tides, eclipse effects, and the effects of solar flares and magnetic storms. A century after his birth, it seems appropriate to consider where some of Appleton's ideas on the ionosphere led to, and how some of the topics that interested him are seen today. The paper discusses some theoretical ideas that Appleton developed or used, reviews his work on the E layer, F layer and other ionospheric topics, and outlines some of the major developments since his day.     

Time-variation of solar influence on tropospheric circulation

The influence of large geomagnetic disturbances and solar magnetic sector structure on Vorticity Area Index (VAI) at 500 mb is studied by subdividing the data of 1946–1978 into 15 blocks, each of five years with three years overlapping in successive blocks. Results clearly show marked time-variation in the amplitude of solar-induced signal in VAI in association with the sector boundary passage as well as with large geomagnetic disturbances. These time-variations bear a close parallelism with those of the absolute value of VAI. It is inferred that the initial intrinsic state of the atmosphere is an important factor in controlling the ensuing sun-weather effect. If the variation of VAI with time is taken into account, varying effects reported by earlier workers can be fairly explained.     

Ionospheric control of polarization of low-latitude geomagnetic micropulsations at sunrise

Continuous observations of low-latitude Pc3 and Pc4 geomagnetic micropulsations were carried out at ASO (22.0°N, 198.0° geomagnetic coordinates) from November 1979 to July 1980 to confirm the ionospheric control of polarization characteristics of low-latitude pulsations presented by Saka etal. (1980). The present study confirms the previous result that D-component amplitude starts to increase with sunrise. From the present study, the following results are obtained : (1) the D-component amplitude, which is much smaller than the H-component amplitude before sunrise, increases as much as that of the H-component after sunrise, and this brings about the tilting of the major axis of the polarization ellipse from north to northwest; (2) the onset-time of the D-component increment (or tilting of the major axis) coincides with the appearance of the E-layer in the ionosphere within an hour, and the time of the coincidence shifts from season to season, in parallel with the change of sunrise ; and (3) the ellipticity of the polarization in the horizontal plane is not affected appreciably by sunrise.It is suggested that the Hall conductivity increment associated with the E-layer sunrise enhancement affects the characteristics of the D-component on the ground.     

Refraction distortions of transionospheric radio signais caused by changes in a regular ionosphere and by travelling ionospheric disturbances

This paper generalizes experimental data on variations of the angles of arrival of transionospheric radio signals caused by changes in a regular ionosphere and by effects of medium-scale travelling ionospheric disturbances (TIDs). The data are based on radio astronomical observations of discrete sources and compact active features on the Sun as well as on angular measurements of signals from artificial Earth satellites with geostationary and circular orbits.The experimental data are interpreted through calculations of refraction corrections using a Gaussian model of a regular ionosphere disturbed by a three-dimensional travelling wave (the TID model) as well as an adaptive model of a regular ionosphere. Some possibilities of correcting refraction distortions with the use of appropriate models and ionospheric diagnostic tools are discussed.     

Ionosphere-thermosphere space weather issues

Weather disturbances in the ionosphere-thermosphere system can have a detrimental effect on both ground-based and space-based systems. Because of this impact and because our field has matured, it is now appropriate to develop specification and forecast models, with the aim of eventually predicting the occurrence, duration, and intensity of weather effects. As part of the new National Space Weather Program, the CEDAR community will focus on science issues concerning space weather, and this tutorial/review is an expanded version of a tutorial presentation given at the recent CEDAR annual meeting. The tutorial/review provides a brief discussion of weather disturbances and features, the causes of weather, and the status of weather modeling. The features and disturbances discussed include plasma patches, boundary and auroral blobs, sun-aligned polar cap arcs, the effects of traveling convection vortices and SAID events, the lifetime of density structures, sporadic E and intermediate layers, spread F and equatorial plasma bubbles, geomagnetic storms and substorms, traveling ionospheric disturbances (TID's), and the effects of tides and gravity waves propagating from the lower atmosphere. The tutorial/review is only intended to provide an overview of some of the important scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. Tutorials on thermospheric and magnetospheric weather issues are given in companion papers.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

Major ionospheric storms during July–September 1982 at lower latitudes in East Asia

The period July–September 1982 was the peak of geomagnetic activity during the 21st solar cycle. There were four very severe storms, which covered the strongest period of geomagnetic disturbances seen in recent years, namely the storms on 13/14 July, 7 August, 6/7 September and 21/22 September 1982. The present paper analyzes the corresponding ionospheric behaviour during these major magnetic storms, along with the storms that occurred around the local summer months during the preceding years within both hemispheres of the East Asia-Oceanian sector. It is shown that during the summer months in the low-latitudes and lower mid-latitudes of this sector the strong geomagnetic storms generally result in depletion of the ionospheric electron content and electron concentration at the F-region peak during both day-time and night-time. The situation in summer is quite different from other seasons and is basically unrelated to SSC time or/and maximum disturbance time and duration.     

Association of whistlers with lightning discharges on the Earth and on Jupiter

The association between whistlers and lightning discharges has been reviewed on the basis of terrestrial ionospheric satellite observations of VLF radio noise. Evidence indicating that the observed low-latitude radio noise is associated with thunderstorms includes (1) amplitude distribution and noise properties, (2) geographical location, (3) diurnal variation in activity, and (4) diurnal variation of frequency spectrum. Corresponding studies on the propagation of sferics in the ionosphere and the excitation of whistlers recently carried out for Jupiter are presented here and compared with the terrestrial studies.     

Experimental results on satellite scintillations due to field-aligned irregularities at mid-latitudes

Experiments using multi-station networks receiving signals from the VHF beacon of a geostationary satellite have been carried out in order to clarify the geometrical factor involved in ionospheric intensity scintillations due to field-aligned irregularities. The characteristics of scintillation observed in the daytime agree with the theoretical value expected for weak diffractive scattering by ionospheric irregularities with an elongation of 10 along the geomagnetic field. However, those in the night-time show much marked enhancement along the field-line due to strong refractive scattering by irregularities having the same elongation. Finally, it is shown that the geometrical factor in scintillation at mid-latitudes can be expressed as a function of the propagation angle between the radio path and the geomagnetic field in the ionosphere. The maximum values of the geometrical factor are respectively about 5 in the daytime and 14 at night.     

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.     

Pfister's space characterisation of ionospheric movements

Pfister's theorem provides a relationship between the velocity of an effective reflecting surface, the Doppler shifts of the reflected radio waves and their angles of arrival. The Bribie Island HF radar measures the Doppler shift, the north-south angle of arrival and the east-west angle of arrival of the ionospheric echo(es). These three dimensions were used to form Pfister's space. The signatures produced by modelled data in this space were used to characterise ionospheric movements. These signatures allowed non-dispersive plane-wave motions, non plane-wave motions which obey Pfister's theorem, dispersive and superposed motions to be distinguished. These different characteristic motions were also identified in a time-series of F-region measurements. Some of the ionospheric motions previously attributed to dispersion may have been produced by superposed disturbances. The day to day variation of how well Pfister's theorem is obeyed may be due to the variability of sources producing disturbances, and therefore superposition effects, and not just dispersion as previously claimed. The individual parameters of superposed disturbances may be difficult to extract using spectral analysis techniques. The wide variety of motions observed at mid-latitudes have important consequences for those techniques which assume a uniform drift of the ionosphere.     

Predictability of ionospheric variations for quiet and disturbed conditions

Empirical models can predict the average monthly behaviour of the ionosphere and allow for day-to-day variability. Significant departures from these predicted conditions can be forecast using a range of solar, magnetic and ionospheric observations. The main improvements needed in the prediction models is that they should be made more precise, by introducing variable prediction limits, and should be improved by harnessing the impressive power of the current physical models so that ionospheric data can be assimilated in near real time. This strategic objective will lead to ionospheric weather forecasting.     

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.     

Estimation of the true ionospheric height profile,with a continuous gradient,from oblique sounding data

A technique to calculate the true height ionospheric profile from multifrequency oblique soundings is proposed. In the classic isotropic statement of the problem, it avoids a discontinuity in the gradient of electron density with height at nodal points and. as shown in test examples, it considerably increases the accuracy of the reconstructed profile as compared with well known, currently employed methods. The influence of the geomagnetic field is taken into account in the technique by constructing the equivalent isotropic ionogram. Both numerical and experimental results are presented and discussed.