The EISCAT mesospheric measurements during the CAMP campaign

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10¹⁰ m⁻³ in the upper D-region. The measured meridional winds were lower than 10 m s⁻¹. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.     

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.     

Electric field measurements during the MAC/EPSILON campaign

As part of the MAC/EPSILON campaign in northern Norway during October and November 1987, five rocket-borne payloads made three-axis electric field measurements of the middle atmosphere. Flights 31.066 and 31.067 consisted of large multi-experiment packages, while the other three flights (30.036, 30.037 and 30.038) were devoted primarily to electrical measurements. Simultaneous measurements of the horizontal electric field made by flights 31.066 and 30.036 were in general agreement in their limited altitude region of overlap. A simultaneous small temporal feature was observed in both datasets. The relatively more extensive horizontal E-field datasets from 31.066 and 31.067 both exhibited a decreasing mapping function with decreasing altitude, which is an indication of the observation of fields from a local auroral patch. Small-scale variations in the horizontal fields of the flights were similar to observed wave-like variations in the neutral wind field. No unusual features were observed at high altitudes in the measured E₁ field. Two of the payloads observed small vertical layer structures between 40 and 50 km. No electric field structure was observed in association with the presence of a sudden sodium layer.     

Incoherent scatter spectral measurements of the summertime high-latitude D-region with the EISCAT UHF radar

Measurements of incoherent scatter spectra from the auroral D-region were obtained during the summer of 1985 using a sophisticated pulse-to-pulse correlation technique with the EISCAT UHF radar. The spectral width variations with altitude are interpreted in terms of ion-neutral collision frequency, neutral temperature, mean positive ion mass and negative ion number density. Close agreement with predictions of currently available atmospheric models is obtained, except for a narrow layer around 86 km altitude. This layer showed evidence of increased positive ion mass for most of the experiment, and for short intervals indicated a mean ion mass close to 200 a.m.u. It is suggested that the layer is composed of proton hydrates in the vicinity of a structured noctilucent cloud, and that the index of hydration is occasionally large.     

A comparison of PMSE and other ground-based observations during the NLC-91 campaign

During the period July–August 1991, observations were made of Polar Mesospheric Summer Echoes (PMSE) at 46.9 MHz and 224 MHz by the CUPRI and EISCAT radars, respectively, at two sites in northern Scandinavia. Those observations are compared here with observations of noctilucent clouds, energetic particle precipitation and magnetic disturbances. The appearance and morphology of PMSE are found to be closely correlated at the two frequencies and the two sites, 200 km apart. No correlation is found between PMSE and noctilucent clouds or magnetic disturbance. No correlation is found between energetic particle precipitation and the appearance of PMSE at 46.9 MHz for the whole time period. At 224 MHz, there is no evidence for a correlation before the beginning of August and only one event suggesting a possible correlation after the beginning of August. A minimum in occurrence frequency for PMSE is found between 16 and 21 UT (17–22 LST) which may be related to an expected minimum in background wind strength in that time interval.     

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.     

In situ ionospheric current measurements during ROSE rocket flights: evidence for tilted current layers

Data recorded by the DC magnetometer on board the ROSE payloads launched into regions of moderate substorm activity have been interpreted in terms of current densities. Adopting generally accepted assumptions on the geometry of the currents, vertical profiles of the Hall current density could be computed. Systematic differences between the results of the upleg and downleg suggest that the assumption of a horizontal current sheet might not be applicable. Reasonable agreement between the observations of both legs is achieved if the Hall current layer is considered to be perpendicular to the ambient geomagnetic field lines.     

Data comparison of EISCAT and the energetic particle instrument on GEOS 2

Flux variations of high energy (E > 16 keV) electrons, measured by the energetic particle spectrometer onboard the geosynchronous satellite GEOS 2, have been compared to simultaneously observed electron density structures in the lower ionosphere, measured with the incoherent scatter facility EISCAT. The very localized E-region electron density enhancements caused by the precipitation allow a preliminary estimate of the location of the footpoint of the magnetic field line through GEOS 2. Various other results of the intercomparison are discussed.     

Neutral air density and temperature measurements by the TOTAL instrument aboard the ROSE payloads

Four ROSE payloads, launched from November 1988 to February 1989 from northern Scandinavia, carried ionization gauges (‘TOTAL’ instruments) for neutral air density measurements in the altitude range 90–105 km. Temperature profiles are derived by integrating the number density profiles. Density and temperature data are presented. The limitations of the measurement technique as well as instrumental errors are discussed. In one of the flights (F1) a significant temperature enhancement was observed at an altitude where plasma instabilities were detected by independent measurements.     

Comparisons of EISCAT and dynasonde ionospheric measurements: simple to moderately structured plasma densities

Plasma densities obtained from EISCAT's UHF incoherent scatter system are compared with profiles inverted from the digital ionograms of a co-located dynasonde. Excellent agreement is found for the bottomside ionosphere when conditions of horizontal stratification and classical photochemical equilibrium prevail. However, departures from such conditions are frequent and intense at Tromsø. Compensating errors of EISCAT calibration and long pulse convolution are resolved by analysis of power profile data. Good agreement is recovered for tilted and more complex ionospheric structure, provided that accurate echo location data are used to confirm a common volume. Monotonic inversion of the ionograms is inadequate. Dynasonde recordings are analysed to show characteristic structure in vertical and horizontal planes as a context for EISCAT measurements along a fixed (magnetic field) direction. Incoherent scatter and modern total reflection sounding, used together and coordinated in one consistent data reduction system, could produce a far more powerful ionospheric diagnostic program than either technique seems capable of providing alone.     

Gyro line observations with the EISCAT VHF radar

This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.     

Measurements of electron density fluctuations during the ROSE rocket flights

Electron density fluctuations have been measured with a modified retarding potential analyzer during the four ROSE rocket flights. The instrument is described and the results obtained are explained and discussed. They are generally in agreement with similar earlier results, but also show new features.     

Mesospheric studies using the EISCAT UHF and VHF radars: a review of principles and experimental results

Studies of the mesosphere and ionospheric D-region are carried out with the incoherent scatter as well as with the MST radar technique. We briefly describe the principal differences between these techniques and the corresponding scattering mechanisms. Particular emphasis is placed on the observation of the coherent echoes from the summer mesosphere, which the EISCAT VHF and UHF radars have recently also detected. We present an overview of the historical development of EISCAT observations and results in these areas, and suggest possible directions of progress which would make the EISCAT radar systems even more suitable for mesosphere measurements.     

A comparison of plasma densities by EISCAT and the Dynasonde from auroral altitudes: evidence of intense structure

Under conditions of moderately-energetic particle precipitation typical of the equatorward side of the auroral oval, plasma densities obtained from routine analysis of EISCAT Common Program data are often a factor 2 to 5 smaller than those suggested by co-located digital ionograms. We consider the reasons for this disagreement, and in particular we reject the implications of diffractive and multiplyrefractive scatter as alternatives to the usual plasma-frequency interpretation of ionogram echoes. We examine the effects of the (5 min and shorter) temporal averaging applied to the EISCAT data and conclude that together with the evidently small size (perhaps as little as 20 km) and high velocity of these structures, this accounts for much, if not all, of the disagreement. We point out the significance of the higher plasma densities in the 100–150 km height range for estimates of Joule and particle heating.     

Electron temperature enhancements in the polar E-region measured with EISCAT

We have found electron temperature enhancements up to 1000K at 110 km altitude using the EISCAT multipulse method which allows high spatial resolution measurements within E-region. This electron temperature enhancement which is closely related to the d.c. electric field strength, is in good agreement with theoretical estimates based on wave heating. The results of the measurements are presented together with a discussion of the electron gas heating, its height variations and its difference in the eastward and westward electrojet.     

Observations of ion layer motions during the AIDA campaign

The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.     

A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.     

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.     

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.