A survey of simultaneous observations of the high-latitude ionosphere and interplanetary magnetic field with EISCAT and AMPTE-UKS

This paper surveys the results of simultaneous observations by the EISCAT incoherent scatter radar and the AMPTE-UKS satellite, made during three periods in September and October 1984, when AMPTE-UKS was in the solar wind on the dayside of the Earth and the UK-POLAR EISCAT experiment was measuring ionospheric parameters at invariant latitudes 70.8–75.0°. A total of 42 h of EISCAT convection velocity data, with 2.5 min resolution, were obtained, together with 28 h of simultaneous 5 s resolution AMPTE-UKS observations of the solar wind and interplanetary magnetic field (IMF). The general features of the AMPTE-UKS data are described in Section 2 and those of the EISCAT data are described in Sections 3 and 4. The main subjects discussed are the form of the plasma convection patterns and their dependence on all three components of the IMF (Section 5), the ionospheric response to abrupt changes in the IMF (Section 6), in particular a sharp ‘southward turning’ of the IMF on 27 October 1984, and a crossing of an IMF sector boundary. Section 7 describes ‘short lived rapid flow burst’, which are believed to be related to flux transfer events at the magnetopause.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

Preliminary results from a study of the F-region heating during an intense aurora observed by EISCAT

Observations of large time variations in the ionospheric F-region temperature derived from EISCAT are compared with simultaneous observations of the E- and F-region plasma densities. The observations suggest that the F-region may be heated by current driven instabilities generated during intense precipitation of auroral electrons.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

Polar cap potential distributions during periods of positive IMF By and Bz

We compare the DE-2 electric field measurements used by Heppner and Maynard [(1987) J. geophys. Res.92, 4467] to illustrate strongly distorted, BC convection patterns for IMF B_z > 0 and large |B_y|, with simultaneous detections of particle spectra, plasma drifts and magnetic perturbations. Measured potentials >50 keV, driven by the solar wind speeds exceeding 500 km/s, are greater than published correlation analysis predictions by up to 27%. The potential distributions show only two extrema and thus support the basic conclusion that under these conditions the solar wind/IMF drives two- rather than fourcell convection patterns. However, several aspects of the distorted two-cell convection pattern must be revised. In addition to the strong east-west convection in the vicinity of the cusp, indicated by Heppner and Maynard, we also detect comparable components of sunward (equatorward) plasma flow. Combined equipotential and particle precipitation distributions indicate the presence of a lobe cell embedded within the larger, afternoon reconnection cell. Both types rotate in the same sense, with the lobe cell carrying 20–40% of the total afternoon cell potential. We detected no lobe cell within morning convection cell.     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Small scale sub-auroral F-region irregularities were observed on 6–7 February 1984 by the two HF radars of the EDIA experiment while the EISCAT UHF system was scanning the ionosphere between 57° and 66° invariant latitude at a slightly different longitude. The bistatic EDIA system was mainly designed to detect the F-region irregularities at sub-auroral latitudes and to measure their perpendicular velocities. This paper is devoted to an examination of the morphology of the irregularity regions detected by the HF radars and of their production mechanisms, by comparison with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360–430 km height are not associated with any large scale horizontal density gradients in the F-region (350km). However, a strong north-south gradient observed at lower altitudes (150–200km), which is likely to indicate the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars. The EISCAT electron temperature measurements at 350km height also show horizontal gradients which are well correlated with the small scale F-region irregularities. We discuss implications of these observations on the mechanisms of production of irregularities in the sub-auroral F-region.     

Ionospheric plasma convection in the southern hemisphere

The first ionospheric plasma convection maps ordered by the y- and z-components of the IMF using only data from the southern hemisphere are presented. These patterns are determined from line-of-sight velocity measurements of the Polar Anglo-American Conjugate Experiment (PACE) located at Halley, Antarctica, with the majority of the observations coming from 65°–75° magnetic latitude. For IMF Bz positive and negative conditions, the observed plasma motions are consistent with a standard two cell pattern. For the periods from dusk through midnight to dawn, flow speeds are at least twice as large for Bz negative component compared with Bz positive. The observations about noon are significantly different from each other. For Bz positive, little ordered plasma motion is observed. For Bz negative, there are large anti-sunward flows the orientation of which is ordered by IMF By. These By orientated flows are consistent with theoretical predictions, and are anti-symmetric to those reported from the northern hemisphere. The two most significant differences from previous observations are that the convection reversal in the late morning sector for By negative conditions occurs at about a 4° lower latitude than the Heppner and Maynard (1987) model. This may be due to a seasonal bias in the PACE dataset. Also, the separatrix between eastward and westward flow near midnight has a very different shape dependent upon the orientation of IMF By. For positive By conditions, the separatrix is observed at progressively lower latitudes at later local times, but for By negative conditions, the separatrix appears at increasingly higher latitudes at later times.     

A unified view on convection and field-aligned current patterns in the polar cap

During the last two decades measurements of polar cap ionospheric electric fields and currents, field-aligned currents, and global auroral forms have been made from ground-based and space-based platforms. An attempt is made to unify these observations into a large-scale view of polar phenomena. In this view, plasma convection patterns and the corresponding electrodynamics in the polar region can consistently be ordered by the orientation of the interplanetary magnetic field (IMF). The different patterns of the electric potential and of field-aligned currents depend on where the main interaction between the terrestrial and interplanetary fields occurs, on the morning or evening side of the central polar cap, or on the dayside portion of the ‘closed’ cusp region, or on the nightside portion of the ‘open’ cusp region. One of the essential elements of this unified view is that it is possible to account for various convection patterns ranging from the four-cell pattern (during periods of strong northward IMF and B_y ~ 0), to the three-cell pattern (B_z > 0 and |B_y| 2> 0), to the conventional two-cell pattern (B_z < 0) with its possible deformation into a convection throat near the dayside cusp (during southward IMF). We also discuss the way in which the complicated field-aligned current systems can consistently be accounted for in terms of these convection patterns.     

Co-ordinated EISCAT-MICADO interferometer measurements of neutral winds and temperatures in E- and F-regions

The MICADO instrument has been built to measure temperature and wind in the E- and F-regions. It employs a thermally stable field-compensated Michelson interferometer to allow wind measurements. During the winter of 1988–1989, the MICADO instrument was operated at Sodankylä (67°22′N, Finland). Measurements were made by observing the O¹S (low thermosphere) and the O₁D lines (high thermosphere) emission. Two co-ordinated campaigns were organized with the EISCAT radar, which operated in special modes. Neutral wind and temperature are derived from EISCAT data. Results of the two instruments are shown. The differences between the two sets of results are discussed and show that most of the discrepancy is due to the presence of vertical winds during the observations where the magnetic activity was high.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

EISCAT observations of large scale electron temperture and electron density perturbations caused by high power HF radio waves

In this paper EISCAT observations of the effect of artificial modification on the F-region electron temperature and electron density during several heating experiments at Tromsø are reported. During O-mode heating at full power (ERP = 240 MW) the electron temperature is increased by up to 55% of its ambient value at altitudes close to the heater interaction height. Measurements of the electron density have revealed both enhancements and depletions in the vicinity of the heater reflection height. These differences are indicative of variations in the balance between the transport and chemical effects. These results are compared with a time dependent numerical model developed from the perturbation equations of Vas'kov and Gurevich [(1975) Geomagn. Aeron.15, 51]. The results of numerical modelling of the electron temperature are in good agreement with the EISCAT observations, whereas there is less good agreement with regard to electron density.     

EISCAT incoherent scatter radar observations and model studies of day to twilight variations in the D-region during the PCA event of August 1989

An intense solar proton event causing enhanced ionization in the ionospheric D-region occurred on 12 August 1989. The event was partially observed during three successive nights by the EISCAT UHF incoherent scatter radar at Ramfjordmoen near Tromsa, Norway. Ion production rates calculated from GOES-7 satellite measurements of proton flux and a detailed ion chemistry model of the D-region are used together with the radar data to deduce electron concentration, negative ion to electron concentration ratio, mean ion mass and neutral temperature in the height region from 70 to 90 km, at selected times which correspond to the maximum and minimum solar elevations occurring during the radar observations. The quantitative interpretation of EISCAT data as physical parameters is discussed. The obtained temperature values are compared with nearly simultaneous temperature measurements at Andøya based on lidar technique.     

Dynamical coupling of the auroral F-region ionosphere and thermosphere: case studies

During two 24 h periods of EISCAT observations in the summer of 1982, the F-region ion temperature and density responded differently before and after midnight to large ion convective flows. Such observations were recently reported at Chatanika (Alaska), however, the mechanism invoked to interpret these measurements (large day-to-night variation in electron density affecting the coupling between ions and neutrals) appears insufficient, for summer conditions, to account for the EISCAT observations. Hence, it is proposed, with the support of Fabry-Perot observations and numerical models, that in addition to the electron density asymmetry, the presence of a large southward neutral wind around midnight induces, through Coriolis coupling, a zonal neutral wind of an opposite direction to the convective flow. This enhances considerably the frictional energy and momentum transfer between ions and neutrals in the post-midnight sector.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Thermospheric wind measurements with EISCAT

Thermospheric wind measurements with the EISCAT UHF radar around the evening Harang discontinuity are presented both in the E- and F-layers. Within the E-layer auroral oval the Lorentz and Coriolis force are shown to be more or less in balance. The neutral velocity is a factor of the order of two smaller than the ion velocity and is on average advanced 90° in a clockwise direction compared to the ion velocity. In the low electron density region just before the Harang discontinuity and outside the auroral oval a large (~250 m s⁻¹), thermally dominated neutral wind is closely followed by the ion wind in the antisolar direction. There is also a large downward flow present just before the Harang discontinuity. In the F-layer the neutral wind approximately follows the ion convection pattern, except for a couple of hours after the sudden change in the ion convection just after the passage of the evening Harang discontinuity. The close resemblance between the equilibrium ion and neutral flow when the neutral-ion collision frequency is close to twice the Earth's angular velocity may be connected to back pressures created by Joule heating in the case of an appreciable ion-neutral velocity difference.     

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.     

The dynamic F2-layer over Arecibo

The idealized ‘servo’ model of the ionospheric F2-layer, developed by Rishbeth, ganguly and Walker (1978), is used to simulate the observed behaviour of the daytime F2-peak at Arecibo for sunspot minimum. Taking the east-west electric field to be given by the observed plasma drift velocity perpendicular to the magnetic field, the theoretical equations are integrated using a trial-and-error approach to match the observed values of field-parallel plasma velocity, and the height and electron density of the F2-peak. From the calculation is determined empirically the meridional pressure-gradient force associated with the meridional neutral-air wind. The local time variation during the day is found to be consistent with the semidiurnal variation given by the MSIS atmospheric model of Hedinet al. (1977a, b), though with a phase shift that varies with season; on some days in the fall the pressure-gradient force displays a strong equatorward ‘surge’ in the evening. The values of F2-layer loss and diffusion coefficients needed to match the data are broadly consistent with the MSIS model. The analysis thus validates the MSIS model by way of ionospheric parameters quite independent of the data from which MSIS was originally derived.     

Observation and theoretical modelling of a region of downward field-aligned flow of O+ in the winter dayside polar cap

Combined optical and radar measurements of ion drift at high latitudes near the terminator show that large downward field-aligned ion flows occur below the F-peak. At an invariant latitude of 72° and in the local time period from 1100 to 1500, downward velocities of 400 m s ⁻¹ have been observed. At the same time, the poleward component of field-perpendicular ion velocity was only 100 m s ⁻¹. The high latitude ionospheric model of Queganet al. (1982), as modified by Allenet al. (1984), predicts downward field-aligned velocities with the same sign morphology as the observations, but with only one fifth of the magnitude. However, the existence of downward neutral winds might lead to non-linear amplification of the downward ion motion. Using the vertical wind measurements of Reeset al. (1984), a possible explanation of the fast ion flow is suggested.