UHF Radar Research Articles

Deriving the normalised ion-neutral collision frequency from EISCAT observations

Abstract. Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s–1 corresponding to a perpendicular electric field of some 100 mV m–1. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric field. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, first-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.

The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off‐the‐shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State‐of‐the‐art exciter and receiver hardware has been developed in‐house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron‐induced spurious Doppler effects from the data. Intermediate‐frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground‐based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

Retrieval of stratified atmospheric reflectivity and wind velocity using inverse methods: application to a VHF ST mini-radar

Abstract The growing interest of measuring wind and turbulence in the lower atmosphere has led to the development of new radar systems. UHF radars, though being an interesting solution from a technical and logistical point of view, have some disadvantages due to their high sensitivity to Rayleigh scattering and interference from precipitations, birds and insects. The standard VHF ST radars were originally designed for high altitude investigations and are consequently not suited for low atmosphere soundings. This context makes it necessary to develop the new concept of a VHF ‘mini-radar’. But the simultaneous use of a small antenna in the VHF band, combined with a beam having a more grazing angle, results in an important mixing of the altitude contributions for each range, a problem which is non existent with previous ST radars. Consequently, the atmospheric reflectivity and wind velocity profiles cannot be directly obtained and have to be treated by other methods. In this context, the aim of the present work consists in the development of an appropriate inverse method. Two different classic methods are considered, the least squares method and the maximum entropy method. The ‘direct problem’ is first addressed, resulting in an integral description of the zeroth and first moments of the Doppler spectra. In order to perform various simulations to test the validity of the two proposed inverse methods in the particular case of the VHF miniradar, a model is built for the radar which includes a set of reference atmospheric profiles. The simulations give evidence for the validity of the inversion processes. The high robustness of the least squares method always leads to significant results. But its over-determined nature results in a poor vertical resolution for the inverted profiles. Consequently this method is not suited to retrieving strong gradients. The maximum entropy method is intrinsically much more appropriate in terms of vertical resolution and consequently leads to valuable results, but its high sensitivity to the data noise requires some additional constraints. The practical efficiency of the methods is tested with real data from the mini-radar, and the resulting retrieved profiles are compared to those obtained simultaneously using a conventional ST radar (the ‘Provence’ radar). As a result of a poor vertical resolution, the least squares method cannot provide a valid retrieval of the atmospheric profiles under real experimental conditions. Nevertheless, a preliminary inversion using the least squares method can be used as a constraint for initializing the maximum entropy inversion process. Although this processing appears to be very efficient for the reflectivity, the retrieved profiles reveal a smoothing effect which seems to be linked to a faulty radar antenna radiation model. In contrast, the retrieval of wind velocity seems to be more difficult and requires additional investigations.

Average received signal power after two‐way propagation through ionized turbulence

In this paper we consider propagation through a structured ionosphere separating a monostatic or bistatic radar and its target. A new analytic calculation of the correlation function of the received, demodulated signal is presented. This result is a function of the locations of the transmit and receive apertures and of time during the duration of the received pulse. Strong scattering is assumed where the one‐way scintillation index is unity; under this assumption the parabolic wave equation can be solved using the quadratic approximation for the phase structure‐function; this facilitates tractable expressions for the mutual coherence function (MCF) for both one‐ and two‐way propagation. A new expression is given for the two‐position, two‐time, two‐frequency MCF for spherical wave propagation that allows simultaneous description of all effects of the ionospheric propagation channel (or transmission medium) on a propagating radar signal. To obtain useful and tractable results, the transmitted signal is assumed to be a chirp (linear FM) pulse, a waveform used by most long‐range defense radars. The result for the correlation function of the received signal is a function of the basic parameters that describe wave propagation through the ionospheric structure, including the channel decorrelation distance, decorrelation time, and coherence bandwidth. Both the two‐way enhancement in average power and aperture averaging effects are explicitly included in this derivation, and new results are presented that relate these effects to the characteristics of the propagation channel. The results are useful for propagation under conditions of saturated scintillation, for example HF through UHF radar in the natural equatorial environment and HF through gigahertz‐band radars that are required to operate in a disturbed propagation environment (i.e., in chemical releases or nuclear detonations).

Heating of the high-latitude ionospheric plasma by electric fields

Electric fields in the high-latitude ionosphere, perpendicular to the magnetic field, drive plasma flow in the F-region at speeds which regularly exceed 2000 m s −1. Relative motion between the ions and the neutral atmosphere heats the ion population through collisions with neutral particles. Ion frictional heating constitutes a systematic heating effect on the F-region ions in the presence of ionospheric electric fields. In the E-region, however, the ions are coupled to the neutrals by frequent collisions and, since the electrons are still magnetised, these electric fields set up currents. E-region electric currents are responsible for the generation of instabilities which turbulently heat the electrons. This paper reports a statistical study of both ion frictional heating and electron turbulent heating based on around 900 hours of common programme observations by the European incoherent scatter (EISCAT) UHF radar. Although limited to a far narrower altitude regime, electron turbulent heating is demonstrated to constitute as systematic an effect as ion frictional heating.

Deriving the normalised ion-neutral collision frequency from EISCAT observations

Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s–1 corresponding to a perpendicular electric field of some 100 mV m–1. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric field. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, first-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

Abstract. We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.

Numerical modelling of the thermospheric and ionospheric effects of magnetospheric processes in the cusp region

Abstract. The thermospheric and ionospheric effects of the precipitating electron flux and field-aligned-current variations in the cusp have been modelled by the use of a new version of the global numerical model of the Earth's upper atmosphere developed for studies of polar phenomena. The responses of the electron concentration, ion, electron and neutral temperature, thermospheric wind velocity and electric-field potential to the variations of the precipitating 0.23-keV electron flux intensity and field-aligned current density in the cusp have been calculated by solving the corresponding continuity, momentum and heat balance equations. Features of the atmospheric gravity wave generation and propagation from the cusp region after the electron precipitation and field-aligned current-density increases have been found for the cases of the motionless and moving cusp region. The magnitudes of the disturbances are noticeably larger in the case of the moving region of the precipitation. The thermospheric disturbances are generated mainly by the thermospheric heating due to the soft electron precipitation and propagate to lower latitudes as large-scale atmospheric gravity waves with the mean horizontal velocity of about 690 m s–1. They reveal appreciable magnitudes at significant distances from the cusp region. The meridional-wind-velocity disturbance at 65° geomagnetic latitude is of the same order (100 m s–1) as the background wind due to the solar heating, but is oppositely directed. The ionospheric disturbances have appreciable magnitudes at the geomagnetic latitudes 70°–85°. The electron-concentration and -temperature disturbances are caused mainly by the ionization and heating processes due to the precipitation, whereas the ion-temperature disturbances are influence strongly by Joule heating of the ion gas due to the electric-field disturbances in the cusp. The latter strongly influence the zonal- and meridional-wind disturbances as well via the effects of ion drag in the cusp region. The results obtained are of interest because of the location of the EISCAT Svalbard Radar in the cusp region and the associated observations at lower latitudes that will be possible using the existing EISCAT UHF and VHF radars. The paper makes predictions for both these regions, and these predictions will be tested by joint observations by ESR, EISCAT UHF/VHF and other ground-based ionosphere/thermosphere observations.

Observations of nightside auroral plasma upflows in the F-region and topside ionosphere

Abstract. Observations from the special UK EISCAT program UFIS are presented. UFIS is a joint UHF-VHF experiment, designed to make simultaneous measurements of enhanced vertical plasma flows in the F-region and topside ionospheres. Three distinct intervals of upward ion flow were observed. During the first event, upward ion fluxes in excess of 1013 m–2 s–1 were detected, with vertical ion velocities reaching 300 m s–1 at 800 km. The upflow was associated with the passage of an auroral arc through the radar field of view. In the F-region, an enhanced and sheared convection electric field on the leading edge of the arc resulted in heating of the ions, whilst at higher altitudes, above the precipitation region, strongly enhanced electron temperatures were observed; such features are commonly associated with the generation of plasma upflows. These observations demonstrate some of the acceleration mechanisms which can exist within the small-scale structure of an auroral arc. A later upflow event was associated with enhanced electron temperatures and only a moderate convection electric field, with no indication of significantly elevated ion tem- peratures. There was again some evidence of F-region particle precipitation at the time of the upflow, which exhibited vertical ion velocities of similar magnitude to the earlier upflow, suggesting that the behaviour of the electrons might be the dominant factor in this type of event. A third upflow was detected at altitudes above the observing range of the UHF radar, but which was evident in the VHF data from 600 km upwards. Smaller vertical velocities were observed in this event, which was apparently uncorrelated with any features observed at lower altitudes. Limitations imposed by the experimental conditions inhibit the interpretation of this event, although the upflow was again likely related to topside plasma heating.

Auroral-arc splitting by intrusion of a new convection channel

Abstract. During a run of the Common Programme Three of the EISCAT radar the splitting of an auroral arc was observed by high time-resolution, ground-based cameras when the UHF radar beam was close to the arc. The evening eastward electrojet situation with a large-scale northward ionospheric electric field was disturbed by the intrusion of a convection channel with southward electric field from the east. The interaction of the new convection channel with the auroral arc caused changes in arc brightness and arc splitting, i.e. the creation of a new arc parallel to the pre-existing auroral arc. The event is described as one possibility for the creation of parallel arcs during slightly disturbed magnetic conditions far from the Harang discontinuity.

Comparison of F-region electron density observations by satellite radio tomography and incoherent scatter methods

Abstract. In November 1995 a campaign of satellite radiotomography supported by the EISCAT incoherent scatter radar and several other instruments was arranged in Scandinavia. A chain of four satellite receivers extending from the north of Norway to the south of Finland was installed approximately along a geomagnetic meridian. The receivers carried out difference Doppler measurements using signals from satellites flying along the chain. The EISCAT UHF radar was simultaneously operational with its beam swinging either in geomagnetic or in geographic meridional plane. With this experimental set-up latitudinal scans of F-region electron density are obtained both from the radar observations and by tomographic inversion of the phase observations given by the difference Doppler experiment. This paper shows the first results of the campaign and compares the electron densities given by the two methods.

Enhanced incoherent scatter plasma lines

Abstract. Detailed model calculations of auroral secondary and photoelectron distributions for varying conditions have been used to calculate the theoretical enhancement of incoherent scatter plasma lines. These calculations are compared with EISCAT UHF radar measurements of enhanced plasma lines from both the E and F regions, and published EISCAT VHF radar measurements. The agreement between the calculated and observed plasma line enhancements is good. The enhancement from the superthermal distribution can explain even the very strong enhancements observed in the auroral E region during aurora, as previously shown by Kirkwood et al. The model calculations are used to predict the range of conditions when enhanced plasma lines will be seen with the existing high-latitude incoherent scatter radars, including the new EISCAT Svalbard radar. It is found that the detailed structure, i.e. the gradients in the suprathermal distribution, are most important for the plasma line enhancement. The level of superthermal flux affects the enhancement only in the region of low phase energy where the number of thermal electrons is comparable to the number of suprathermal electrons and in the region of high phase energy where the suprathermal fluxes fall to such low levels that their effect becomes small compared to the collision term. To facilitate the use of the predictions for the different radars, the expected signal- to-noise ratios (SNRs) for typical plasma line enhancements have been calculated. It is found that the high-frequency radars (Søndre Strømfjord, EISCAT UHF) should observe the highest SNR, but only for rather high plasma frequencies. The VHF radars (EISCAT VHF and Svalbard) will detect enhanced plasma lines over a wider range of frequencies, but with lower SNR.

An ambiguity‐function‐based method for analysis of Doppler decompressed radar signals applied to EISCAT measurements of oblique UHF‐VHF meteor echoes

Binary phase modulation using Barker codes as the modulating sequences is often used as a pulse compression technique in incoherent scatter radar measurements of the ionospheric E region. Demodulation of the compressed signal is usually accomplished by matched filtering techniques. If the target has appreciable velocity, the resulting Doppler shift detunes the scattered signal relative to the filter, and the target response spreads over a large range interval. The paper describes the development of a new data analysis method, based on an ambiguity function description of radar total‐power measurements, which uses the characteristic Doppler decompression signatures to identify, extract, and analyze high‐velocity events from conventional Barker‐coded power profile type incoherent scatter (IS) measurements. Data sets recorded with the European Incoherent Scatter UHF (931 MHz) and VHF (224 MHz) radars during the 1990‐1991 Geminid and 1993 Perseid showers are shown to contain many events which are associated with individual meteors crossing the radar beams over a wide range of aspect angles, including a few near‐radial cases. Line‐of‐sight velocities and effective cross sections are derived for some of the best dual radar events. The cross sections are very small and increase with decreasing wavelength, something not observed before. It is suggested that this is indicative of a scattering mechanism different from the one operating at near‐normal incidence at VHF, and Rayleigh scatter from compact “balls” of plasma contained within the meteor coma is proposed as a possible candidate, in qualitative agreement with the observations. Finally, some signal‐processing related aspects of the method are discussed. It is shown that most IS radars should be able to apply it to their existing programs with very little effort and without having to compromise the performance in normal E region applications. The method could therefore become a powerful tool for studies of meteor statistics at UHF frequencies.

Lower thermosphere mean temperatures, densities, and winds measured by EISCAT: Seasonal and solar cycle effects

The European Incoherent Scatter (EISCAT) UHF radar in northern Scandinavia has now been making regular measurements of lower thermosphère temperatures, densities, and meridonal winds for 10 years, covering almost a complete solar cycle. Daily averages from a total of 110 days of operations have been used to study the seasonal, solar flux and magnetic activity dependence of these parameters. In the case of densities, lower values occur in summer than in winter, but no significant solar flux or magnetic activity dependence can be resolved. The daily mean meridional winds are only a few meters per second and show large day‐to‐day variability. Between 100‐ and 120‐km altitudes, there is a suggestion of a reversal from equatorward to poleward prevailing wind from winter to summer, particularly at solar minimum. As regards temperature, the seasonal variations are as expected, with lower values in summer than in winter below 98‐km altitude but with a summer maximum at higher altitudes. A clear dependence on magnetic activity is seen above 110‐km altitude in all seasons and amounts to about 1 K/Ap unit between 110‐ and 120‐km altitudes. In winter, magnetic activity effects seem to penetrate to lower altitudes. No clear dependence on solar flux is seen below 100‐km altitude. At higher altitudes, a positive dependence on solar flux is seen during summer, increasing with altitude and reaching 0.25 K/10.7 cm flux unit. In winter, however, at altitudes 100–115 km, temperatures are significantly lower (by about 10 K) at solar maximum than at solar minimum.

Tidal variations in the high-latitude E- and F-region observed by EISCAT

Abstract. During the MLTCS (Mesosphere-Lower Thermosphere Coupling Study) campaign the EISCAT UHF radar was continuously operated over 7 days (30 July–5 August 1992) in the CP-1 mode. The long time series obtained of the fundamental ionospheric parameters field-aligned ion velocity (Vi), ion and electron temperature (T and Te), and electron density (Ne) are useful in investigating tidal variations in the E- and F-region since the geomagnetic activity was particularly low during the time of measurement. Maximum entropy spectra of the parameters were calculated for the relatively quiet interval from 1 August to 4 August 1992 and indicated dominant variations with harmonics of 24 hours. In the electron density spectrum especially, harmonics up to the sixth order (4-h period) are clearly visible. The phase and amplitude height profiles (100–450 km) of the diurnal, semidiurnal, and terdiurnal variations were determined by Fourier transform for a 24-h data set beginning at 12:00 UT on 3 August 1992 when the contaminating influences of electric fields were negligible. The tidal variations of the ion temperatures are compared with the corresponding variations of the neutral temperature predicted by the MSISE-90 model. A remarkable result is the dominance of terdiurnal temperature oscillations at E-region heights on 3–4 August 1992, while the measured diurnal and semidiurnal variations were negligible. The finding was confirmed by the analysis of further EISCAT data (2–3 August 1989, 2–3 July 1990, 31 March–1 April 1992) which also showed a dominant terdiurnal temperature tide in the E-region. This is different from numerous observations of tides in the E-region at mid-latitudes where the diurnal and especially the semidiurnal temperature oscillations were dominant.

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.

Numerical modelling of intermittent ion outflow events above EISCAT

EISCAT observations with the UHF and VHF radars of the dynamics of the upper ionosphere have revealed the occurrence of intermittent ion outflows with velocities reaching several hundred m s −1. It was previously shown that, during such events, the topside downward electron heat flux, inferred from the analysis of the vertical (field-aligned) structure of the electron temperature profiles, increases drastically up to values of about 10 μW m −2. The numerical models described in a companion paper are used here to simulate the effects of energy inputs driven by the magnetosphere. Three main effects are simulated separately: effects of frictional heating related to E × B drifts, effects due to topside heat flux perturbations and effects of upward field-aligned currents of a few tens of μA m −2. It is shown that field-aligned currents and topside heat flux perturbations produce very similar effects and that combining field-aligned currents with frictional heating allows us to model the overall characteristics of the perturbations of the electron density, of the electron and ion temperatures, and of the ion vertical velocity. A good agreement with observations is found.

Coherent echoes during EISCAT UHF Common Programmes

Coherent echoes, whatever their origin, appear as narrow spectra superimposed on the normal incoherent scatter spectra recorded by the EISCAT UHF and VHF radars. Their narrow width, higher than normal amplitude and the induced spectral asymmetry in the overall spectrum enable them to be easily found. by a simple computer algorithm which has routinely been used to pick out such spectra from Common-Programme (CP) data, usually in real time. The UHF CP-1 data have been searched back to 1986 for such echoes. Most of the echoes found in this way are caused by noise spikes, but radio interference, satellite echoes, and, of most interest, enhanced ion acoustic echoes caused by plasma processes are all found. We present a summary of the findings of this search, with special emphasis on the CP-1 data set. Examples of remote site enhanced ion acoustic shoulders are presented, as well as some unexplained echoes from Tromsø.

Variability of dayside high latitude convection associated with a sequence of auroral transients

10 second resolution ionospheric convection data covering the invariant latitude range from 71° to 76°, obtained by using the EISCAT UHF and VHF radars, are combined with optical data from Ny Ålesund during a sequence of auroral transients in the post-noon sector (∼ 15 MLT). Satellite observations of polar cap convection patterns suggest negative B Z and B Y components of the interplanetary magnetic field. Burst-like enhancements of westward (sunward) post-noon convection were accompanied by eastward moving auroral forms at higher latitudes, above the convection reversal boundary. In this case the background convection was weak, whereas the integrated potential drop across the radar field-of-view associated with the westward flow bursts was typically ∼ 20-35 kV. The auroral phenomenon consists of a series of similar events with a mean repetition period of 8 min. A close correlation between the auroral activity and convection enhancements in the cleft ionosphere is demonstrated.

Investigation of the upper mesospheric dynamics under late polar summer conditions by EISCAT and lidar

In August 1989, a strong polar cap absorption event allowed extended measurements of both horizontal and vertical wind components in the upper mesosphere and lower thermosphere by means of the EISCAT UHF radar. A distance (130 km) away from the radar site, a Na lidar instrument simultaneously measured temperature profiles over several hours in the altitude range of 85–100 km. This combination of measurements gives detailed information about the behaviour of the polar mesopause region in the transition period from summer to autumn conditions. Below 87 km, the wind measurements reveal typical summer profiles of the mean horizontal components and a significant long-period wave which we interpret as the semi-diurnal tide. Between 87 and 90 km, we find a strong wind shear; there, we observe a strong damping of the tide. Higher up, the mean horizontal wind is nearly zero, which represents autumn rather than summer conditions. In particular the vertical wind, but also the horizontal components reveal short-period oscillations with a small or vanishing phase progression, mainly in the 75–85 km altitude region. Most probably these are gravity waves with very long vertical wavelengths as predicted by theory to be prominent in the vertical wind. The temperature measurements confirm the typical polar summer conditions with a marked temperature minimum of 120-130 K at around 87 km and partially very large positive temperature gradients higher up, especially before local midnight. The measurements suggest that this peculiar thermal and dynamical structure in late summer acts as a major barrier for tidal waves and short-period waves leading to strong turbulence slightly above the mesopause temperature minimum.