EISCAT Measurements Research Articles

The generation and propagation of atmospheric gravity waves from activity in the auroral electrojet

EISCAT measurements of the electric field in the auroral electrojet are compared with the signature of TIDs propagating equatorward as observed by an HF-Doppler network. At night-time the onset of auroral activity is usually followed by the arrival of a TID at lower latitude. Cross-correlation of the time variations of the electric field measured by EISCAT and the frequency offset recorded by the HF-Doppler system confirms a relationship between the auroral activity and the gravity wave, indicating both the travel time and the periodicity of the wave. The relationship is especially close under quiet conditions when the cross-correlation coefficient is typically 60%, significant at 0.1%. When the observed electric field is used as input to a thermosphere-ionosphere coupled global model it predicts the time signature of the observed HF-Doppler variation reasonably well but seriously underestimates the amplitude of the disturbance. Examination of this discrepancy may lead to a better understanding of the mechanisms involved in the generation and propagation of atmospheric gravity waves.

Neutral dynamics of the high latitude E region from EISCAT measurements: a new approach

We have analysed the EISCAT data from two long campaigns, in October 1992 and in January 1993, in order to derive the three components (N-S, E-W, vertical) of the neutral wind in the E region. The winds are derived using the simultaneously measured ion-neutral collision frequency. We calculate the tidal parameters of the measured winds and compare them with models. In addition the CP2 experiment allows us to determine the vertical wind in two independent ways. During the disturbed day of the October campaign we derived a very large vertical wind. Such large values are not reproduced in any model. In this article we discuss the validity of our results.

A comparison of EISCAT and Dynasonde measurements of the auroral ionosphere

A comparison of EISCAT and Dynasonde measurements of the auroral ionosphere

EISCAT measurements of the solar wind

EISCAT measurements of the solar wind

A new approach to the determination of the E-region drift velocity using radar aurora doppler spectral shape

In radar auroral research, Doppler spectra have been and still are being studied intensively. This is because the spectra can provide valuable information about auroral plasma irregularities. A simple model of the laminar ionospheric current is not able to explain the measured shapes and widths of the radar aurora Doppler spectra. In the present paper, a turbulent current model is used as a basis for the theoretical determination of the radar aurora Doppler spectra. The results of the theoretical calculations of the Doppler spectra are compared with some experimental data. It is found that the theoretical and experimental spectra coincide very well. The method that has been developed allows us to calculate all known basic; types of Doppler spectra which correspond to the Farley-Buneman plasma instability. This approach provides a way of developing a procedure for determination of electron drift velocities and electric fields in the auroral E-region using only a single coherent radar. As an example we use some STARE spectra to determine the electron drift velocity vectors. The values of the drift velocity vectors derived from the spectra are higher than the values determined by the STARE technique. To find out which of these results are closer to reality is possible only by analysing the simultaneous spectral measurements by the STARE radars and the EISCAT measurements of the drift velocity vectors.

Ion Composition in the Lower-F Region Inferred from Residuals of Ion Temperature Profile Measured with the EISCAT CP1 Experiments.

Ion composition in the lower-F region for the daytime period of summer season has been obtained by correcting the EISCAT composition model with the ratio between the measured and modeled ion temperatures. The model profile of ion temperature was estimated with a fitting procedure of tan-1 function on the EISCAT measurements under an assumption that the temperature is almost free from any additional heating. In order to warrant this assumption, the data periods during which the effect of frictional heating was negligibly small have been selected under the criterion that the F-region d.c. electric field remained below 4 mV/m in magnitude. The result reveals that the transition altitude between molecular and oxygen ions has a clear diurnal variation with a minimum altitude of about 187 km around 13 LT.

Morphology of the D- and E-regions at high latitudes in the northern hemisphere during the DYANA campaign in 1990

The morphology of the D- and E-regions in the northern hemisphere at latitudes above 60°N during the DYANA campaign from 11 January to 16 March 1990 is described on the basis of riometer, rocket and EISCAT measurements. The campaign was carried out during the sunspot maximum year of solar cycle 22 and was aimed at studies of the middle atmosphere. During the winter anomaly days of the campaign period an increased ionospheric absorption measured by a riometer was seen on only one winter anomaly day, and only at the southernmost station of the riometer network at latitude 63°N. The ionospheric absorption did not increase during a stratospheric warming in early February. Both winter and autumn anomalies in the ionospheric absorption occurred during 1990. The substorm events observed during the campaign period are discussed and electron density profiles in the D- and E-regions are presented for different substorm phases.

Worldwide atmospheric gravity-wave study in the European sector 1985–1990

Four campaigns of the Worldwide Atmospheric Gravity-wave Study (WAGS) have taken place in the European sector. On many occasions the onset of auroral activity in the evening and midnight sector, as indicated by EISCAT measurements of the electric field, was associated after a suitable delay with the detection of periodic ionospheric disturbance travelling southward over the U.K. at speeds between 500 and 1000 m s −1. The velocity and wavelength of the TIDs corresponded to large-scale atmospheric gravity-waves. The characteristic periods of the travelling disturbances were similar to the intrinsic time scales of the auroral activity for periods of 40 min or more, but variations on a time scale of 20 min or less were strongly attenuated. The r.m.s. amplitude of the auroral electric field was proportional to the r.m.s. amplitude of the HF Doppler-shift associated with the gravity-wave. The time-lag between the onset of strong auroral activity and the arrival of the travelling disturbance over the U.K. was usually about an hour, suggesting a source region about 2000 km north. Similar levels of activity in the afternoon did not appear to produce strong waves in the far field. This is possibly due to ion-drag in the daytime ionosphere although the effects of the lower sensitivity of the HF Doppler-network during daytime must also be considered.

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.

Worldwide atmospheric gravity-wave study 1985–1989

Three campaigns of the Worldwide Atmospheric Gravity-wave Study have taken place in the European sector. On many occasions the onset of auroral activity in the evening and midnight sector, as indicated by EISCAT measurements, was followed after an hour or so by the arrival of a large-scale atmospheric gravity wave over the UK. Similar levels of activity in the early afternoon did not appear to produce waves in the far field, possibly as a result of ion-drag in the daytime ionosphere. The intrinsic time variation of the auroral activity determined the original time variation of the travelling disturbance, but waves with a period of about 30 min were strongly attenuated whereas waves with a period of about 60 min propagated to very great distances.

Large ionospheric plasma flows along magnetic field lines: EISCAT observations and their interpretation

EISCAT measurements of plasma velocities in the auroral ionosphere have indicated very large plasma flows along magnetic field lines. Velocities exceeding 500 ms −1 have been observed on several occasions, always in association with large electric fields. Observations with the EISCAT radar pointing along its local field line have provided height profiles of the various plasma parameters during periods when large field-aligned flows occur. The most important factor in driving the plasma is found to be the gradient in plasma pressure. Electric fields up to 50 mVm −1 cause frictional heating of the ion population, but little change in recombination coefficient so that increased plasma pressure in the F-region drives plasma upwards into the topside. However, when the electric field is even larger then the rate of recombination increases sharply, the electron concentration is depleted, F-region plasma pressure decreases and plasma is driven downwards.

The incoherent scattering of radio waves in a non‐Maxwellian plasma: The effects of Coulomb collisions

In this paper the formulas for the ion distribution as well as the spectrum of radio waves scattered in a magnetized plasma with a strong electric field are derived. It is shown that the presence of the electric field in the ionosphere leads to an anisotropic ion velocity distribution and, therefore, to untypical incoherent scatter spectra for the F region of the polar ionosphere which are caused by ion‐neutral together with ion‐ion collisions. The effect of ion‐ion collisions, which has not been taken into account so far, is to reduce the anisotropy of the ion velocity distribution. Estimates of the ion‐ion collision frequency derived from EISCAT measurements show that this may happen above about 300 km.

On the identification and occurrence of ion frictional heating events in the high-latitude ionosphere

Enhancements in the F-region ion temperature, caused by frictional heating, are a notable manifestation of the interaction between the ionized and neutral atmospheres. Heating of this type arises whenever the ion and neutral populations move with different velocities. A velocity difference of order 1 km s −1 provides sufficient heat input to double the unperturbed ion temperature. This paper presents a synoptic study of ion heating events observed by the EISCAT UHF facility during the 2-year period from January 1985 to December 1986. The data set consists of observations obtained during the ‘Common Programme’ operation of EISCAT in the CP-1 and CP-2 modes. Only ion temperatures measured in the field-aligned direction have been considered, since in this direction the line-of-sight ion temperature can be obtained by assuming that the ion thermal velocity distribution remains Maxwellian. This assumption is known to break down progressively as the difference between the ion and neutral velocities increases, but the resulting discrepancies in the field-parallel ion temperature are smaller than those for other observing directions and do not significantly affect the conclusions presented here. A criterion previously applied to the examination of satellite data has been adopted to identify intervals of F-region frictional heating and employed to derive their diurnal distribution in 1000 h of EISCAT measurements. Almost equal numbers of heating events were encountered in both the dawn and dusk sectors. While this result contrasts with some earlier findings, the present study makes use of a much larger data set than previous investigations and is concerned with somewhat lower latitudes. The magnitudes of the ion temperature enhancements are similar for both the pre-midnight and post-midnight sectors, although the longest intervals of ion heating tended to be concentrated in the early morning hours. Many of the ion heating events occurred in association with intervals of intense and structured particle precipitation, with F-region ion temperatures and velocities being enhanced at the boundaries of auroral forms, but depressed above the regions of precipitation themselves. It is suggested that this behaviour can be explained by the presence of a polarization electric field within the auroral form, whose direction opposes that of the convection electric field. Similar observations of reduced electric fields above regions of particle precipitation have been reported by previous workers, who have classified such events as ‘anti-correlation’ or ‘asymmetric’ auroral arcs.

Simulation of the polar cap thermal plasma and comparison with EISCAT measurements

Four differential continuity equations with momentum equations are solved for O +, N 2 +, O 2 +, and NO + ions. Because of the high value of the convection electric field it is necessary to take into account a stress tensor for the transport of the ionization along the magnetic field lines and to calculate an effective temperature for recombination processes. The horizontal distribution of the thermal plasma density depends on the convection electric field pattern and its intensity. Deduced from 24 h of EISCAT observations in the invariant latitude (λ) range from 60 to 72°, the equipotentials of the electric field are represented by an analytical expression depending on λ and on the magnetic local time (MLT). The equipotentials are extrapolated toward higher latitudes by making reasonable hypotheses, which are generally accepted. The expression reproduces the ion velocities observed over the polar cap with more or less accuracy depending on the number of coefficients in the expansion. The influence of the neutral wind model is studied. It is concluded that the depression of the electron density observed in the morning sector is due entirely to chemical recombination.

STARE and EISCAT measurements: Evidence for the limitation of STARE Doppler velocity observations by the ion acoustic velocity

Measurements of the phase velocity of ionospheric plasma waves using the STARE radar system are compared with the electron drift velocity and the ion acoustic velocity calculated from data measured by EISCAT. This comparison shows that for those flow angles that are smaller than the largest flow angle (θs) for which the component of the electron drift velocity along the radar line of sight equals the ion acoustic velocity, the phase velocity observed by STARE tends to be limited by, or lower than, the ion acoustic velocity. Using observations with flow angles larger than θs, one can infer the electron drift from STARE measurements by the well‐known cosine relation.

Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.

An attempt to derive electron energy spectra for auroral daytime precipitation events

A set of quiet daytime electron density profiles is established on the basis of EISCAT measurements. This set is used as a background correction in the CARD program [ Brekke A., Hall C. and Hansen T.L. (1989) Ann. Geophysicae 7, 269] in order to derive the energy spectra of precipitating electrons at daytime. For disturbed daytime events on 25–26 June 1985, we find that the particle precipitation typically consists of small fluxes (10 5 el/cm 2 sster keV) of high energetic particles (20–30 keV). The Hall: Pedersen conductance ratios for such events are found to be meaningless as an indicator of the energy of the particles, in contrast to nighttime precipitation.

Ground-based observations coordinated with Viking satellite measurements

The instrumentation and the orbit of the Viking satellite made this first Swedish satellite mission ideally suited for coordinated observations with the dense network of ground-based stations in northern Scandinavia. Several arrays of complementing instruments such as magnetometers, all-sky cameras, riometers and doppler radars monitored on a routine basis the ionosphere under the magnetospheric region passed by Viking . For a large number of orbits the Viking passages close to Scandinavia were covered by the operation of specially designed programmes at the European incoherent-scatter facility (EISCAT). First results of coordinated observations on the ground and aboard Viking have shed new light on the most spectacular feature of substorm expansion, the westward-travelling surge. The end of a substorm and the associated decay of a westward-travelling surge have been analysed, eiscat measurements of high spatial and temporal resolution indicate that the conductivities and electric fields associated with westward-travelling surges are not represented correctly by the existing models.

Observations of tidal modes in the lower thermosphere using EISCAT

Measurements of plasma velocity in the height range 101–133 km and at 279 km have been made by EISCAT as part of the Common Programme CP1. Correcting for the effect of the electric field as determined at the F-region peak, using estimates of the ion-neutral collision frequency based on previous EISCAT measurements, it is possible to calculate the neutral wind velocity in the E region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal variations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height is consistent with a dominant (2,4) mode below 120 km. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2,4) mode is attenuated and the (2,2) mode is more important.

On the determination of ion temperature in the auroral F-region ionosphere

Assessment is made of the effect of the assumed form for the ion velocity distribution function on estimates of three-dimensional ion temperature from one-dimensional observations. Incoherent scatter observations by the EISCAT radar at a variety of aspect angles are used to demonstrate features of ion temperature determination and to study the ion velocity distribution function. One form of the distribution function which has recently been widely used In the interpretation of EISCAT measurements, is found to be consistent with the data presented here, in that no deviation from a Maxwellian can be detected for observations along the magnetic field line and that the ion temperature and its anisotropy are accurately predicted. It is shown that theoretical predictions of the anisotropy by Monte Carlo computations are very accurate, the observed value being greater by only a few pecent. It is also demonstrated for the case studied that errors of up to 93% are introduced into the ion temperature estimate if the anisotropy is neglected. Observations at an aspect angle of 54.7°, which are not subject to this error, have a much smaller uncertainty (less than 1%) due to the adopted form of the distribution of line-of-sight velocity.