Applicability of the two-layer model for simulation of non-linear evolution of a large scale plasma cloud in the ionospheric F-region

The adequacy of the two-layer model of Lloyd and Haerendel for describing the behaviour of an ionospheric irregularity is verified by numerical simulation of large plasma cloud dynamics. The background ionosphere is approximated by a set of conductive layers with ion mobilities and concentrations corresponding to the real ionospheric conditions. Polarization electric field produces positive and negative image clouds, i.e. plasma density enhancements and depletions in each layer. Their intensity, form and orientation turn out to change with height depending on the local conditions. However, the drift and deformation of the released cloud slightly differ from the case when the ionosphere is characterized by constant, height averaged parameters, at least if altitude dependent neutral wind and photochemical processes are ignored.     

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.     

Introduction to ionospheric heating at Tromsø—I. Experimental overview

The HF ionospheric modification (heating) facility at Ramfjordmoen will become a part of the EISCAT association from January 1993. This paper, which is intended for the new user, describes the technical capabilities of the facility and the broad range of geophysical and plasma physical experiments which are possible. An overview is presented of the physical effects that a powerful HF electromagnetic wave incident on the ionosphere can produce on timescales ranging from tens of microseconds to minutes in height regions ranging from 50 to hundreds of km. Emphasis is placed on the practical implementation of ionospheric heating experiments using the EISCAT incoherent scatter radars as the main diagnostic, but other diagnostic techniques using ground-based radars, radio links, radio receivers, photometers, rocket and satellite instrumentation are also described. A companion paper presents in greater depth some of the current scientific issues being addressed in ionospheric modification research.     

Variations of the lower ionosphere parameters measured by the resonance scattering method

This paper presents the results of diagnostics of the lower ionospheric parameters by using the resonance scattering method (RSM) of radio waves on periodic artificial irregularities (PAI). The following ionospheric parameters have been measured by the heating facilities ‘Zimenki’ and ‘Sura’: electron density including the E-F interlayer valley and the lowest height of the ionosphere; the velocity of the vertical wind; the relaxation times of the PAI characterizing coefficients of ambipolar diffusion, and coefficients of attachment and detachment of electrons from negative ions. Variations of these parameters have been considered as a function of height, local time, season, solar activity as well as being induced by passing acoustic-gravity waves and by ionospheric disturbances.     

Diagnostics of the lower ionosphere by the method of resonance scattering of radio waves

Results are presented on measurements of lower ionosphere parameters (electron concentration, coefficient of ambipolar diffusion, etc.) by the method of resonance scattering of radio waves by periodic artificial irregularities. The method of resonance scattering is based on the generation of periodic irregularities in the ionospheric plasma by powerful radio emission and investigation of the characteristics of the back scattering of diagnostic radio waves by these irregularities.     

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.     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

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.     

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.     

Ionospheric tomography: an algorithm enhancement

The Applied Research Laboratories, The University of Texas at Austin (ARL: UT), conducted a computerized ionospheric tomography (CIT) experiment called the “Mid-America CIT Experiment”, or MACE'93, from June through December, 1993. Portions of the data from this experiment have been input into various CIT algorithms to reconstruct a two-dimensional electron density image of the ionosphere. One of the CIT algorithms relies on model ionospheres to incorporate a priori information into basis functions that are used to represent the unknown ionospheric electron density structure. When global ionospheric models are used to generate the basis model ionospheres for this algorithm, the reconstructed ionospheres often underestimate the F₂ peak density and some reconstructions display incorrect latitudinal electron density variation. These erroneous reconstructions are mainly caused by the global ionospheric models incorrectly modeling the ionosphere in terms of the F₂ region thickness and latitudinal variations. This paper identifies an alternate set of basis ionospheres that improves the accuracy of the reconstructed ionospheres. In general, this enhancement has reduced the average per cent difference between actual and CIT reconstructed ionograms to 6% and the average absolute error between measured and CIT reconstructed f₀F₂ to 2.6%.     

Investigations in the U.S.S.R. of non-linear phenomena in the ionosphere

Investigations of non-linear ionospheric phenomena in the U.S.S.R. are reviewed. The facilities available and some non-linear phenomena first observed with these facilities are described. Among these are strong heating and non-linear absorption in lower parts of the ionosphere, self-modulation caused by intense Langmuir oscillations, the modulation of the auroral electrojet at VLF, stimulated ionization in the F-region, self-focusing effects at oblique incidence as well as many theoretical investigations of non-linear phenomena.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

IMF effects in the appearance of slant E conditions (SEC) in the northern and southern polar regions

Vertical sounding data of the polar ionosphere are used to study slant E condition (SEC) related to the development of instabilities in the ionospheric plasma. The measurements from three Arctic and three Antarctic stations located correspondingly in the polar cap, daytime cusp and auroral zone are analyzed. It is shown that the SEC daily variations in these three regions are different. Distinctions between SEC features in the opposite hemispheres are affected by the azimuthal and vertical interplanetary magnetic field (IMF) components.     

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.     

Ionospheric disturbances over Japan due to the 18 May 1980 eruption of Mount St. Helens

The effects of the Mount St. Helens eruption at 1532 UT on 18 May 1980 on the ionosphere over Japan are examined using data on total electron content obtained at three closely-spaced stations and HF (5 and 8 MHz) Doppler recordings together with microbarograph data for the surface pressure perturbation. The results strongly suggest that ionospheric perturbations having a predominant period of about 9 min propagated from north to south approximately along the great circle path with a horizontal velocity of about 300 m s⁻¹. The observed time variations of perturbations can be well explained in terms of Lamb waves propagating through the atmospheric sound channel while launching up-going waves to produce the ionospheric oscillations.     

A numerical model of the ionospheric dynamo—I. Formulation and numerical technique

A new method of numerically solving a suitably formulated ionospheric wind dynamo equation for electrostatic potential and field is developed. Unlike in many other dynamo models, the upper boundary does not exist and the formulation asymptotically approaches the equatorial boundary condition. Therefore, it naturally incorporates the symmetric, asymmetric E- and F-region dynamo actions in any given ionosphere and any given global or local wind field. It also enables the equation to be posed as an initial value problem and solved numerically using an efficient, accurate, stable and fast integration method of ordinary differential equations. The numerical technique can be extended to compute three dimensional dynamo-generated electric currents in the ionosphere.     

The theory of ionospheric focused heating

Ionospheric modification by high power radio waves and by chemical releases are combined in a theoretical study of ionospheric focused heating. The release of materials which promote electron-ion recombination creates a hole in the bottomside ionosphere. The ionospheric hole focuses high power radio waves from a ground-based transmitter to give a 20 dB or greater enhancement in power density. The intense radio beam excites atomic oxygen by collisions with accelerated electrons. Airglow from the excited oxygen provides a visible trace of the focused beam. The large increase in the intensity of the radio beam stimulates new wave-plasma interactions. Numerical simulations show that the threshold for the two-plasmon decay instability is exceeded. The interaction of the pump electromagnetic wave with the backward plasmon produces a scattered electromagnetic wave at 3/2 the pump frequency. The scattered wave provides a unique signature of the two-plasmon decay process for ground-based detection.     

Climatic trends of the mid-latitude upper atmosphere and ionosphere

Long-term variations of electron concentration in the mid-latitude ionosphere, independent of heliogeophysical conditions and the meteorological characteristics of the atmosphere, have been determined. It is suggested that a long-term decrease of atomic and molecular oxygen concentrations in the mid-latitude thermosphere is the most possible reason for the trends found in ionospheric parameters.     

Horizontal transmission of the polar electric field to the equator

A mode which makes possible the instantaneous transmission of the polar ionospheric electric field to the equator is obtained. The problem is solved analytically as an initial-boundary value problem by assuming a plane Earth-ionosphere waveguide system composed of the metallic ionosphere with vertical static magnetic field and the perfectly conducting Earth, The TM₀ (zeroth-order transverse magnetic) waveguide mode excited by the polar electric field propagates instantaneously to the low-latitude accompanying the electric field with the same direction as the polar field. The simultaneous occurrence of the PRI (preliminary reverse impulse) of the SC1 and the DP-2 geomagnetic variations in the high-latitude and equatorial regions are interpreted in terms of the polar originated electric field associated with the TM₀ mode. The geometrical attenuation of the transmitted electric field due to the finite scale of the polar field causes the disappearance of the PRI at low-latitudes, but the PRI appears again in the dayside equatorial region because of the enhanced ionospheric conductivity. The twin current vortices for the high-latitude PRI are interpreted as the Hall current caused by the polar electric field with finite scale. It is suggested that the direct current flows between the magnetosphere and the equatorial ionosphere via the polar ionosphere, when a large scale electric field is generated in the magnetosphere. From the mid-latitude ionospheric part of this current, the electric field may be mapped upward along the magnetic lines of force into the low L-value hydromagnetic region and as a result the penetration of the magnetospheric electric field into the plasmasphere is achieved by way of the Earth-ionosphere waveguide.