Topside and intelhemispheric ion flows in the mid-latitude plasmasphere

A modelling study has been carried out of field-aligned ion flows in the topside ionospheres of conjugate hemispheres under solstice conditions at mid to low latitudes. In the model calculations coupled time-dependent O⁺, H⁺ and electron continuity, momentum and heat balance equations are solved along dipole magnetic field lines at L = 1.5 and 3.0 Sunspot medium and sunspot minimum atmospheric conditions are considered.It has been found that thermal coupling between conjugate hemispheres gives rise to strong flows of O⁺ in the topside ionosphere of the summer hemisphere that are directed upwards at conjugate sunrise and directed downwards at conjugate sunset. At conjugate sunrise in the winter hemisphere there is a small upward-directed signature in the O⁺ field-aligned flux; there is no observable signature in the O⁺ field-aligned flux in the winter hemisphere at conjugate sunset. There are strong upward and downward flows of O⁺ at local sunrise and local sunset, respectively, in both the summer and winter hemispheres.At both L = 1.5 and 3.0 the 24 h time-integrated interhemispheric H⁺ flux is in the direction summer hemisphere to winter hemisphere. At L = 1.5 its magnitude is in good agreement with the magnitude of the 24 h time-integrated plasma (O⁺ + H⁺) field-aligned flux at 1000 km altitude; there are no such agreements at L = 3.0.A study of the roles played by the individual terms of the O⁺ momentum equation has demonstrated the complex structure of momentum balance. Certain of the terms may be orders of magnitude greater than the combined total of the individual terms, i.e. the O⁺ field-aligned flux.     

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.     

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.     

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.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Dynamo-electric field-aligned currents in the plasmasphere

The dynamo-theory has been used to interpret antisymmetric S_q-variations. The following results have been found analytically: 1. The antisymmetric electric field is important only for driving field-aligned coupling currents between both hemispheres. 2. The antisymmetric part of the observed S_q-variations can be explained best by the dynamo-action of seasonally depending (1, 1)-,(1, −2)-, and (2, 2)-tidal modes. 3. At all seasons the field-aligned currents are strongest near noon where they have at north-summer in the northern hemisphere a source near 50° and a sink near 30° latitude.     

Winter-time disturbances in the mid-latitude D-region

Radio-wave absorption data from sixteen mid-latitude stations distributed in longitude, together with magnetic-field disturbance parameters and satellite measurements of thermal radiances, have been examined for the winter of 1976–1977. It has been demonstrated that D-region disturbances at mid-latitudes in winter can be associated with both the delayed effects of geomagnetic storms and with changes in mesospheric temperature.     

Temperatures in the plasmasphere determined from VLF observations

Satellite and ground-based VLF recordings were made at SANAE, Antarctica from 1976 to 1979. In this paper we combine ground and satellite observations to determine temperatures in the plasmasphere. Scale heights in the plasmasphere are determined at high altitudes using a diffusive equilibrium model and measurements of equatorial electron densities and densities at about 3000 km. The temperatures corresponding to these scale heights show a gradual increase with increasing L-value and sharp increases of about 2000 K just inside the plasmapause.     

Refilling process in the plasmasphere and its relation to magnetic activity

When geomagnetic activity is moderate, the geosynchronous orbit crosses the plasmasphere bulge region in which the variations of plasma density from day to day can therefore be detected by geosynchronous satellites. The plasma density was measured by the Relaxation Sounder onboard ESA's GEOS-2 satellite. Variations of plasma density reflect the combined effects of refilling of particles from the ionosphere and loss of plasma by convection. The saturation level of the electron density at the geo-synchronous orbit and the refilling rate under different conditions of geomagnetic activity have been obtained and are found to be 70.5 cm⁻³ and 7–25 cm⁻³ day⁻¹, respectively. In this paper the refilling morphology and the relationship between the refilling process and magnetic activity (Dst index) are analysed. The refilling rate or refilling time constant inferred from the data, either directly on fairly well-defined refilling events, or indirectly through a simple model, are found to compare reasonably well with the refilling time constant expected by theory. The observed correlation of refilling rate with Dst index is interpreted as resulting from the modification of the composition of the topside ionosphere occurring after intense storms.     

Electric field sources in the quiet plasmasphere from whistler observations

Existing evidence for the ionospheric dynamo being the source of quiet time electric fields in the plasmasphere is reviewed. Part of a 24 h set of whistler data recorded continuously at Sanae, Antarctica (L = 4), during quiet magnetic (average K_p = 1) is analysed to obtain westward electric fields in the equatorial plane. These electric fields are examined as a function of L-value in order to infer their source. It is found that for periods of outward flow of plasma during the noon-midnight local time period, the electric fields are consistent with the dominant source being the ionospheric dynamo. There is some evidence that during the evening period of inward flow the electric fields are magnetospheric in origin, although this could also be consistent with a refined dynamo model. The observed whistler duct convection patterns do not fit either of two theoretical models, which invoke a magnetospheric field but not a dynamo field.     

F-region drift velocities in the dusk sector mid-latitude trough

Analysis of 6 months of ground-based ionosonde data from mid/high-latitude Digisonde stations at Millstone Hill, Argentina and Goose Bay, shows the relation between the formation of the mid-latitude trough in the dusk sector and the measured F-region drift velocities. The observed westward drift velocities in the trough are comparable in magnitude with the velocity of the Earth's rotation as required by the stagnation theory of trough formation. Using the Digisonde database of 15 min samples of electron density profiles and F-region drifts, a new trough detection algorithm automatically identifies the occurrence of the trough at any of the three stations. Correlating trough occurrence with the measured drift velocities indicates that troughs develop due to an increase in the horizontal westward velocity component. The extent of the trough formation relates to the magnitude of the horizontal velocity.     

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.     

General formulae for absorption of radio waves in the ionosphere

The relationship between group path, phase path and absorption of radio waves is discussed and new approximations relating these quantities are presented. The new relationships include dependence on the angle between the wave normal and magnetic field directions and so, in contrast to other approximations they are not restricted to quasi-longitudinal or quasi-transverse situations. For deviative absorption it is found that the ordinary mode quasi-transverse approximation introduces errors of less than 5% except in the case of purely longitudinal propagation. For non-deviative absorption, use of the quasi-longitudinal approximation can introduce significant errors which particularly affect the determination of latitudinal variations in absorption.     

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.     

Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations.Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.     

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.     

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.     

Average electron density profiles in the plasmasphere between L = 1.4 and 3.2 deduced from whistlers

985 whistlers recorded at Tihany, Hungary (L = 1.9) between December 1970 and May 1975 have been processed for equatorial crossing radius L, equatorial electron density n_eq and tube electron content N_T. The obtained L values lie in the range L = 1.4-3.2. The median values of n_eq and N_T as a function of L fit well to the density profiles published by Park et al. The results also indicate that at lower L values the whistlers are ducted by stronger density enhancements.     

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.     

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.