European Incoherent Scatter Facility Research Articles

The polarization characteristics of ELF/VLF waves generated via HF heating experiments of the ionosphere by EISCAT

This paper presents the experimental results of modulation and heating of the ionosphere using the European Incoherent Scatter facility in Troms ϕ, Norway, heating facility. The frequency characteristics of the excited extremely low frequency (ELF)/very low frequency (VLF) radiation sources in the amplitude modulated (AM) and dual-beam beat-wave (BW) modes are compared, and the polarization of the horizontal ELF/VLF magnetic field received at 15 km from the heating facility is studied. The results show that in the AM mode, the amplitude of the ELF/VLF signal decreases as the radiation frequency increases when the amplitude of the ELF/VLF signal reaches the maximum at 2017 Hz. In the BW mode, the maximum appears at 2017 Hz, but the overall amplitude of the ELF/VLF signal increases as the radiation frequency increases. The AM and BW modes have different frequency characteristics on ELF/VLF radiation sources formed by modulation and heating of the ionosphere. When the frequency is low, the amplitude of the ELF/VLF signal excited by the AM mode is greater than that excited by the BW mode, but as the frequency increases, the BW mode performs better than the AM mode. Under the same background conditions, the polarization characteristics of the ELF/VLF radiation signals excited by the AM and BW modes are different. The former tends to produce circularly polarized waves, while the latter produces linearly polarized waves.

IONONEST—A Bayesian approach to modeling the lower ionosphere

AbstractObtaining high‐resolution electron density height profiles for the D region of the ionosphere as a well‐sampled function of time is difficult for most methods of ionospheric measurement. Here we present a new method of using multifrequency riometry data for producing D region height profiles via inverse methods. To obtain these profiles, we use the nested sampling technique, implemented through our code, IONONEST. We demonstrate this approach using new data from the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) instrument and consider two electron density models. We compare the recovered height profiles from the KAIRA data with those from incoherent scatter radar using data from the European Incoherent Scatter Facility (EISCAT) instrument and find that there is good agreement between the two techniques, allowing for instrumental differences.

First modulation of high‐frequency polar mesospheric summer echoes by radio heating of the ionosphere

AbstractThe first high‐frequency (HF, 8 MHz) observations of the modulation of polar mesospheric summer echoes (PMSE) by artificial radio heating of the ionosphere are presented and compared to observations at 224 MHz and model predictions. The experiments were performed at the European Incoherent Scatter facility in northern Norway. It is shown that model results are in qualitative and partial quantitative agreement with the observations, supporting the prediction that with certain ranges of ice particle radii and concentration, PMSE at HF radar wavelengths can be enhanced by heating due to the dominance of dust charging over plasma diffusion.

Results from a new auroral lower ionosphere model

This paper presents first results from the development of a new auroral latitude lower ionospheric model. The major difference between this new model and other models for the same region is that the technique of neural networks (NNs) was employed in the development. Data from the European Incoherent Scatter facility combined with rocket measurements were used to provide a database of reliable lower ionosphere data from approximately 70° geomagnetic. NN modelling requires a substantial database of reliable data with which the NN is trained to learn the relationship between the input space and the output parameter. Combinations of various input parameters known to produce a response in the lower ionosphere were investigated as potential contributors to the input space. The final input space consisted of local magnetic time, riometer absorption, a local magnetic K index, the inverse Chapman function corresponding to the solar zenith angle, the F 10.7 cm solar radio flux, and the pressure surface. The pressure surface is a representation of the seasonal variation and the altitude. The output was the electron density for the given set of input parameters at a particular pressure surface. Therefore, an average electron density profile describing the behaviour of the auroral lower ionosphere can be determined for a particular instance of the input space. A process of minimisation of mean squared errors was used to optimise the NNs. It is shown that this model is capable of predicting realistic average electron density profiles for the high latitude lower ionosphere. In addition, the ability of this model to simulate the total absorption to within the resolution of a riometer will be demonstrated. It is the intention of the authors to provide this model in a suitable form for incorporation into the International Reference Ionosphere model and, therefore, these results may be of interest to the ionospheric community.

A new approach to modelling the daytime lower ionosphere at auroral latitudes

In this paper the results of an attempt to model the auroral daytime lower ionosphere using the procedure of Neural Networks(NNs) are presented. Data from the European Incoherent Scatter facility (EISCAT) combined with rocket borne measurements were used to provide a database of reliable lower ionosphere data for NN modelling. Combinations of various input parameters known to produce a response in the daytime lower ionosphere were investigated as potential contributors to the input space. These parameters consisted of day number, hour, solar activity, riometer absorption, magnetic activity and an indication of the altitude. The output was the electron density for a given set of input parameters. Therefore, an average electron density profile describing the shape and location of the D- and E-regions at high latitudes can be determined for a particular set of inputs. A process of minimisation of mean squared errors was used to optimise the NNs. Predicted profiles are compared with actual data as well as an existing ionospheric model built from the same data. Results show that the NN can predict reasonable average electron density profiles for the high latitude lower ionosphere within the limits of the input space.

Meteor fluxes and visual magnitudes from EISCAT radar event rates: a comparison with cross-section based magnitude estimates and optical data

Abstract. Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earth's atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids ("meteor head echoes") are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range.Key words. Ionosphere (ionization mechanisms). Radio science (ionospheric physics). Space plasma physics (ionization processes)

Meteor fluxes and visual magnitudes from EISCAT radar event rates: a comparison with cross-section based magnitude estimates and optical data

Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earth's atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids ("meteor head echoes") are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range.Key words. Ionosphere (ionization mechanisms). Radio science (ionospheric physics). Space plasma physics (ionization processes)

Ionospheric Small-Scale Structures and Turbulence Inferred from EISCAT Observations: a Review.

Measurements of the European incoherent scatter facility (EISCAT) occasionally indicate the presence for brief periods of ionospheric small-scale structures and turbulence at scales smaller than the typical space and time resolution of the radar. Such events violate the assumption of space and time uniformity used in the analysis of the received signal. This results in an erroneous estimate of the derived ionospheric parameters, or even in the failure of the analysis when the measured spectrum departs too much from its usual shape. We review recently published observations from EISCAT data of two types of such phenomena, namely the small-scale auroral structures of intense flow bursts, and the ion-acoustic turbulence. The large differences in their spectral signature and origin illustrate both the variety of the small-scale phenomena occuring in the ionosphere, and the capability of the incoherent scattering technique to detect them and to infer original informations on the mechanisms responsible for them.

Electric field and electron density thresholds for coherent auroral echo onset

Simultaneous observations of the Polar Geophysical Institute 83‐MHz auroral radar and the European Incoherent Scatter facility were used to study the coherent echo threshold phenomenon. It is shown that, for electric fields not far from the Farley‐Buneman instability threshold of ∼20 mV/m, the onset of echo occurrence was due to an electron density increase above some threshold value. This value was estimated to be ∼2.5 × 1011 m−3. For smaller electric fields of 5‐10 mV/m, echoes were not observed even for electron densities double the above threshold value. The importance of both factors, electric field and electron density, and some other effects are discussed in light of recent studies of coherent echo intensities and their dependence on plasma conditions at electrojet altitudes.

The quiet sunrise E region: Enhancements at high latitudes in winter due to increased nitric oxide

E region electron concentrations derived from European Incoherent Scatter Facility observations at 70°N were found to be much larger than normal for high solar zenith angles during 23 of 32 days in winter. Electron concentrations computed with the Keneshea code are compatible with the observations if nitric oxide concentrations are about 109 cm−3 at 100‐110 km. For these altitudes near ground sunrise, it is shown that numerically, the nitric oxide concentration approximately equals the square of the electron concentration.

Comparison of solar wind velocity measurements with a theoretical acceleration model

Interplanetary radio scintillation (IPS) measurements of the solar wind velocity were made using the receiving antennas of the European Incoherent Scatter Facility (EISCAT) radar system in northern Scandinavia from June through October 1990. The observations, which cover the distance range from. 11 to 90 Rs from Sun center, were taken with sufficient density to measure the same stream at two (or more) different distances. The deduced velocities are in the range 100 ≲ u ≲ 540 km s−1. We selected from 192 observations, 16 examples of streams observed with good radial alignment, of which 12 were observed unchanged for several days. The measured velocities are compared with calculations based on a two‐fluid solar wind model with Alfvén waves. In eight cases the measurements are in good agreement with the model when a moderate amount of wave energy is added to the flow. In four cases the observed streams show low or moderate velocities below, say, 20 Rs but then accelerate fast at larger distances from the Sun. This delayed acceleration is much steeper than the acceleration in the model at these distances. In the remaining four cases the streams seem to reach their final velocities much closer to the base than in other cases, and they are not observed to accelerate much between 10 and 90 Rs. At these distances all related solar wind models we have seen give the same results; they all fit half the data, and none can fit the other half. We do not see an obvious modification to these models which will allow us to fit either the delayed acceleration or the early acceleration cases.

Lower thermospheric neutral densities determined from Søndre Strømfjord Incoherent Scatter Radar during LTCS 1

Ion‐neutral collision frequencies determined from measurements obtained by the incoherent scatter radar located at Søndre Strømfjord, Greenland, have been used to derive lower thermospheric neutral densities during the first Lower Thermosphere Coupling Study (LTCS 1), September 21–26, 1987. Periods of Joule and particle heating which might disturb the E region thermal equilibrium were systematically eliminated. The mean profile of neutral density for the period is in good agreement with the mass spectrometer incoherent scatter 1986 (MSIS‐86) model between 92 and 104 km. A tendency to overestimate collision frequencies above 105 km may arise from range‐smearing effects. The results of a tidal analysis performed on the neutral density between 92 and 109 km show that the amplitudes of the diurnal and semidiurnal components of the tides are approximately equivalent. The observations are generally in better agreement with the MSIS‐86 predictions than with the thermosphere‐ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval. The observed phase of the diurnal component is approximately constant with height above 98 km and is in close agreement with the MSIS‐86 model phases; however, the TIGCM diurnal phases are shifted by 6–8 hours to later local times. The phase of the semidiurnal tide is in good agreement with predictions of the MSIS‐86 model and the TIGCM simulation of this interval, except near 98 km. The observed semidiurnal phase is also consistent with previous high‐latitude results (Kirkwood, 1986). The relative amplitude of the observed semidiurnal oscillation is up to 15% larger than that previously observed at the European Incoherent Scatter facility but is consistent with the amplitudes presented in an earlier study of Millstone Hill measurements (Salah, 1974).

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.

Substorm processes in the geomagnetic tail and their effect in the nightside auroral zone ionosphere as observed by eiscat

Current understanding of magnetospheric substorms is reviewed with special emphasis on the relation between space-based and ground-based observations. It is pointed out that the traditional means of monitoring substorms from the ground (by using magnetometers, riometers and auroral observations) give only a selective picture of the whole phenomenon, related to the precipitation of electrons with energies above 1 keV. Measurements by incoherent scatter radar, such as the European incoherent scatter facility (eiscat), give a more complete and continuous picture. The ‘neutral line’ model of substorms provides a natural, physical basis on which relevant data can be interpreted. In this picture, two sources of flow are anticipated in the nightside auroral zones, one ‘directly driven ’ (with a delay of 15-20 min) by the interplanetary magnetic field (imf) B z component and associated with dayside reconnection, and the other appearing typically an hour after southward turnings of the imf and associated with rapid tail reconnection during substorms. Evidence for the influence of both sources of flow is found in nightside eiscat data. These data also reveal that, overall, the nightside ionospheric flow and plasma param eters often vary in a quasi-periodic way with a period of ca. 1 h. In two cases in which concurrent interplanetary data are available it appears that the periodicity is inherent in imf B z , but this is not expressed unmodified in the auroral zone because of the presence of the two sources of flow which depend on imf B z in different ways.

The influence of convection on the structure of the high-latitude ionosphere

Fundamental to the understanding of high-latitude ionospheric behaviour is an appreciation of the convection electric field and its consequences. This electric field drives a number of processes which directly influence the plasma, including vertical ion drifts, Joule heating and enhanced neutral air velocities. Particle precipitation regions are closely connected with convection boundaries. Universal time variations can, in many cases, be directly related to the diurnal variation of the convection electric field in the geographical frame. Convection also has a profound influence on the interpretation of observations, because understanding why plasma densities have their observed characteristics requires knowledge of the past history of the observed plasma. This past history is determined by the convection electric field, as it transports plasma between regions of differing dominant plasma processes. Where reasonable estimates of the convection electric field exist, numerical models of the ionosphere seem capable of reproducing the characteristics of observed plasma behaviour. This is demonstrated by using the European incoherent-scatter facility’s observations for days for which such estimates are available.

Observations of nonthermal plasmas at different aspect angles

Data are presented from the EISCAT (European Incoherent Scatter (Facility)) CP‐3‐E experiment which show large increases in the auroral zone convection velocities (>2 km s−1) over a wide range of latitudes. These are larger than the estimated neutral thermal speed and allow a study of the plasma in a nonthermal state over a range of observing angles. Spectra are presented which show a well‐defined central peak, consistent with an ion velocity distribution function which significantly departs from a Maxwellian form. As the aspect angle decreases, the central peak becomes less obvious. Simulated spectra, derived using theoretical expressions for the O+ ion velocity distribution function based on the generalized relaxation collision model, are compared with the observations and show good first‐order, qualitative agreement. It is shown that ion temperatures derived from the observations, with the assumption of a Maxwellian distribution function, are an overestimate of the true ion temperature at large aspect angles and an underestimate at low aspect angles. The theoretical distribution functions have been included in the “standard” incoherent scatter radar analysis procedure, and attempts have been made to derive realistic ionospheric parameters from nonthermal plasma observations. If the expressions for the distribution function are extended to include mixed ion composition, a significant improvement is found in fitting some of the observed spectra, and estimates of the ion composition can be made. The non‐Maxwellian analysis of the data revealed that the spectral shape distortion parameter, D*, was significantly higher in this case for molecular ions than for atomic ions in a thin height slab roughly 40 km thick. This would seem unlikely if the main molecular ions present were NO+. We therefore suggest that N2+ formed a significant proportion of the molecular ions present during these observations.

Ion composition in sporadic E layers measured by the EISCAT UHF radar

We present ion composition results for several sporadic E layers observed during a Norwegian special program carried out by the European Incoherent Scatter Facility (EISCAT) UHF radar in August 1984. A range resolution of 600 m was used. Eight sporadic E layers at altitudes 95–114 km were studied using various fitting strategies for obtaining the ion composition. A fixed temperature was assumed in some of the strategies; in others the temperature was fitted together with the composition. Heavy ions, most probably Fe+, were found to be present in most layers with relative abundances of 30–80%. In one case, the composition analysis shows that essentially no heavy ions are present. This layer is supposed to be composed of light metal ions, such as Mg+. In one particular event a layer is seen to separate into two sheets with a height difference of 2–3 km, and the ion compositions in the two parts are found to be different. Possible explanations of this phenomenon are discussed.

Ionospheric ion heating by ULF Pc 5 magnetic pulsations

Frictional heating of the ions resulting from dc ionospheric electric fields is experimentally and theoretically well known. We extend these results to ion heating due to ULF magnetic pulsations of periods as low as 3 min. Ion temperature fluctuations as measured by the European incoherent scatter facility are very well correlated to magnetic Pc 5 pulsations. We present a method which estimates these ion temperature enhancements from ion velocity measurements.

Convection electric fields and electrostatic potential over 61° < Λ < 72° invariant latitude observed with the European Incoherent Scatter Facility: 1. Initial results

Convection electric fields are observed with the European Incoherent Scatter Common Program CP‐3 which probes the F2 region of the auroral ionosphere between 61° and 72° invariant latitude for 24‐hour runs. Several examples of convection fields are discussed, corresponding to various levels of magnetospheric activity. The principal characteristics of the convection drifts are discussed in the light of indices of magnetospheric activity level. It is shown that depending on the level of magnetospheric activity as revealed by the Kp index, the two‐cell convection pattern is more or less present in the field of view of the radar and that the convection velocities around dawn and dusk increase with increasing activity. A method is described which assumes that the 24‐hour convection pattern is due to a steady state potential viewed by the radar rotating with the earth under its ionospheric induced structures; as this assumption can be wrong, in some extreme circumstances of very high and unsteady magnetospheric activity, a test is proposed to ascertain the validity of the method. Inferred potential distributions are then discussed; it is shown in particular that the potential drop from dawn to dusk increases with the magnetospheric activity index Kp. A preliminary analysis of the shapes and positions of the two convection cells is then presented.

Pc 3–4 ULF magnetic variations measured in the ionosphere by Eiscat

ULF pulsations of Pc 3–4 type were recorded in the ionosphere at several longitudes with the European Incoherent Scatter facility. The analysis of these pulsations needs sophisticated signal processing. We found that a detailed description of these pulsations must be made burst by burst. The characteristic parameters are time, frequency, and space extent. Comparison with ground recordings indicates that only the H component is well related to the ionospheric ion velocity. The ionosphere‐ground relation is discussed in terms of variability of the auroral conductivity and spatial extent.