Magnetometer and incoherent scatter observations of an intense Ps 6 pulsation event

In the morning sector of 21 April 1985, during the recovery phase of a geomagnetic storm, a Ps 6 pulsation event was recorded by the EISCAT magnetometer cross in northern Scandinavia. Simultaneously, the EISCAT incoherent scatter radar measured E- and F-region plasma parameters with a latitudinal scanning program. Electric fields and height-integrated Hall and Pedersen conductivities are derived. Two-dimensional patterns of these quantities are constructed for one Ps 6 period. The conductance patterns closely resemble the typical auroral forms of eastward drifting Ω bands with low and high conductances at the northern and southern edges of the scanned area, respectively. From the equatorward region a tongue of high ionization extends poleward into the dark area. The location of the maximum southward current is slightly displaced towards the west from the centre of the conductance tongue. The east-west disturbance electric field points towards the tongue; the north-south fields are enhanced outside and reduced inside the high conductance region. As has been previously suggested, the observations can be explained with a model which superposes currents caused by conductance variations and electric fields. Both effects need to be taken into account for this event. The current structures move within a few degrees in the direction of the background E×B drift, but their speed is about 15% lower than the average F-region plasma drift.     

Tidal wind observations with incoherent scatter radar and meteorological rockets during MAP/WINE

Measurements of winds in the mesosphere and lower thermosphere were carried out during the main phase of the MAP/WINE project in January and February 1984 with the EISCAT UHF incoherent scatter radar near Tromsö, Norway, and with meteorological rockets launched from the Andøya Rocket Range, Norway. The radar measurements yield wind profiles between the altitudes of about 80 km and 105 km and the rockets between about 60 km and 90 km. Results from both techniques are combined to yield mean profiles which are particularly evaluated in terms of tidal variations. It is found that the semidiurnal tide constitutes an essential wind contribution between 85 km and 105 km. Whereas the tidal amplitudes are below 5 m s⁻¹ at about 80 km, they increase to 20–30 m s⁻¹ at 100 km. The average vertical wavelength of 35 km points to the S₄² mode, but coupling and superposition of different modes cannot be excluded.     

Statistics of incoherent scatter multiparameter fits

A new statistical inversion method to estimate the errors in incoherent scatter measurements for a given set of ionospheric parameters is presented. The possibilities to determine the ion composition in a multiparameter fit are considered. The method can also be applied to estimate the requirements on the measurement of autocorrelation functions to find the minimum possible lag resolution and lag extent for a stable inversion solution of the plasma parameters, and the temperature interval for which a stable solution exists, with or without the zero lag data. The results are illustrated in the case of a multipulse code.     

Incoherent scatter observations of thin ionization layers at Sondrestrom

High resolution incoherent-scatter observations of E-region thin (1–3 km) metallic ion layers are presented. Data were collected during three different periods from August 1990 to August 1991, in three different experimental modes. First, the antenna was directed vertically and the entire duty cycle was devoted to Barker coded multi-pulse [Zamlutti (1980) J. atmos. terr. Phys.42, 975–982] measurements to determine the densities and temperatures in the E-region with 300 m resolution. The second experiment measured the F-region electric field as well as the high resolution E-region densities. For the third experiment the antenna was scanned magnetic north-south while only the E-region densities were measured. The experiments were carried out on 16 different nights for a period of 4 h each night at a time near magnetic midnight. Thin ionization layers were observed on 12 of the 16 nights. The first experiment demonstrated that the thin layers are composed of a significant fraction of heavy metallic ions; assuming the layers are composed of a mixture of Fe⁺ and Mg⁺ a composition estimate of 63% Fe⁺ was obtained in one example. The second experiment investigated the relationship between the direction of the electric field and the presence of the thin layers. In these observations thin layers were only present when the electric field was pointed in the magnetic north-west or south-west quadrants, most frequently when the field was near magnetic west. Correlation between layer altitude and field direction was also observed, layers occurring at higher altitudes for fields directed in the north-west, and lower altitudes for fields directed to the south-west. The observations are compatible with the electric field mechanism for thin ionization layer formation. The scanning experiment showed that the layers were of a limited latitudinal extent, typically about 100 km up to a maximum of about 200 km.     

Comparison of incoherent scatter observations of electron density,and electron and ion temperature at Millstone Hill with the International Reference Ionosphere

Millstone Hill incoherent scatter (IS) observations of electron density (N_e, electron temperature (T_e) and ion temperature (T_i) are compared with the International Reference Ionosphere (IRI-86) for both noon and midnight, for summer, equinox and winter, at both solar maximum (1979–1980) and solar minimum (1985–1986). The largest difference inN_e is found in the topside, where values of N_e given by IRI-86 are generally larger than those obtained from IS measurements, by a factor which increases with increasing height, and which has a mean value near two at 600 km. Apart from the bottom of the profile, which is tied to the CIRA neutral temperature, the IRI-86 T_e model has no solar cycle variation. However, the IS measurements during the summer reveal larger T_e at solar maximum than at solar minimum. At other seasons higher T_e at solar maximum occurs only during the daytime at the greater heights. Nighttime T_e is shown by the IS radar to be generally larger in winter than in summer, an effect not included in the IRI. This is apparently due to photoelectron heating during winter from the sunlit ionosphere conjugate to Millstone Hill. The day-night difference in T_i given by IRI-86 above 600km is not as large in the IS measurements.     

A nonlinear maximum entropy method for spectral estimation applied to incoherent scatter radar measurements: application to synthetic incoherent scatter data

The EISCAT measurements are based on the autocorrelation function technique. An alternative approach is to derive the ionospheric parameters from an estimate of the power spectrum for the received radar signal. We have used a nonlinear maximum entropy method to deduce parameters from the power spectrum of a generated signal. A comparison has been made with parameters obtained by the ACF method. This preliminary study suggests that the spectrum method could become useful, especially for determining the position of peaks in the incoherent scatter spectrum.     

Observational technique and parameter estimation in plasma line spectrum observations of the ionosphere by chirped incoherent scatter radar

With the 430 MHz incoherent scatter radar facility at the Arecibo Observatory, the plasma line spectrum has been measured with a linearly frequency modulated or ‘chirped’ pulse. On reception the signal is demodulated (‘dechirped’). The paper describes the experimental set-up and the parameter estimation for day-time ionosphere observations. Using a non-linear least squares fit it is possible to determine the linear and quadratic coefficients of a locally parabolic plasma line frequency versus height profile. The strength of the plasma line and the point of tangency of the locally parabolic plasma line frequency profile can be determined very accurately. In measurements of the day-time ionosphere the plasma line was found to be enhanced by about 70 times over the thermal equilibrium level.     

Asymmetric incoherent scatter spectra from a relaxation collision model

A relaxation collision model for ion flow through a stationary neutral gas has been used to obtain ion velocity distributions and line-of-sight incoherent scatter spectra for a range of values of collision frequency and electric field. The mean velocity of the line-of-sight ion velocity distribution has been compared with the Doppler shift of the corresponding spectra. The latter is not always a good estimate of the former, because the ion velocity distribution in the plane perpendicular to the magnetic field direction is highly distorted. For ion-neutral collision frequency to ion gyrofrequency ratios 0.1 ≤V_i,/Ω_i≤0.5, the greatest inaccuracies in mean velocity estimation take place along the electric field direction, while for 0.5 ≤ V_i/Ω_i ≤ 1.0 the greatest inaccuracies occur across the electric field direction. These inaccuracies would be reduced but not eliminated in a more realistic model. At F-region altitudes, other processes must be invoked to explain observed asymmetrical spectra, but the comparison of mean line-of-sight ion velocity and spectrum Doppler shift may still have relevance.     

An incoherent scatter experiment for the measurement of particle collisions

An incoherent scatter experiment is suggested where a VHF-and an UHF-radar simultaneously measure in an identical scatter volume. This paper discusses possible applications and, in particular, the estimation of collision frequencies.     

Morning sector electron precipitation events observed by incoherent scatter radar

Two auroral zone electron precipitation events in the morning sector have been studied in detail using the UHF incoherent scatter radar in northern Scandinavia. The electron density profiles are interpreted in terms of the incoming spectrum of energetic electrons, and it is shown that the spectrum is most energetic at the maximum of the event and softens subsequently. The observations cannot be explained by simple gradient-curvature drift of trapped electrons. It is shown, further, that events appearing to be fresh substorms in magnetometer and riometer data may be no more than intensifications of continuing activity. During a pulsation event the incoming electron spectrum was modulated in energy as well as in intensity. The height and thickness of the resulting radio-absorption layers are derived.     

F-region temperatures measured by AEROS-A satellite compared with incoherent scatter radar

Carefully designed probes of the retarding potential analyzer (RPA) type allow electron temperatures to be correctly measured. When compared with incoherent scatter stations longitudinal structures sometimes appear.     

Range ambiguity effects in Barker-coded multipulse experiments with incoherent scatter radars

A Barker-coded multipulse is the best of the classical modulations when high spatial resolution is needed in incoherent scatter measurements. Unfortunately, this type of modulation generates range ambiguities which reduce the quality of data. In this paper, range ambiguity functions for an EISCAT experiment, ESLA-T4, are displayed. A method of eliminating the ambiguities in data analysis is developed and its applicability is demonstrated. It is found that the method can remove the effects of the range ambiguities in the lower part of the E-region, but above 120 km altitude the correction is not sufficient. The residual of the fit is reduced by an order of magnitude. The changes in the temperatures are a few Kelvin at the lowest altitudes and a few tens of Kelvin between 110 and 120 km.     

Parameter mixing errors within a measuring volume with applications to incoherent scatter

The effect on parameter error estimates resulting from parameter variations within the measuring volume under consideration is studied in the framework of linear statistical inversion theory. It is shown that using estimates for the parameter averages is equivalent to having the theory corrected by the covariances of the variables coupled with the second derivatives of the theory function. If the parameter distributions were known exactly, this would only introduce a bias in the linear theory and hence a systematic error in the parameter centre point estimates. When the distributions are not known exactly, there is another source of error consisting of the uncertainties in the parameter variation estimates. This leads to new error bounds on the allowed parameter variability within the volume under consideration if some prescribed accuracy in the parameter average estimates is required. These considerations are applied to incoherent scatter (IS) radar measurements, where it is important to be able to estimate the effect of integrating both in space and time over a volume with varying parameters in order to obtain spectra or autocorrelation functions. Numerical examples are given in the case of the O⁺ content estimates in measurements with the EISCAT UHF radar, when for example the ion temperature varies over the integration ranges. The results obtained may be used in the design of experiments when high resolution composition measurements are required.     

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

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

A comparison of ionization densities determined from spacecraft and incoherent scatter radar data

By comparing direct measurements taken from onboard Atmosphere Explorer spacecraft (AE), in eccentric orbit, with incoherent scatter radar (ISR) measurements taken from the ground, we illustrate both the merits and the difficulties involved in such comparisons. Five altitude profiles of ionization determined from AE, in near coincidence with ground stations making ISR measurements, compared favorably with the ISR data so long as the AE measurements were properly analyzed for the effects of variations in latitude and solar zenith angle along the spacecraft orbit.     

Detecting travelling ionospheric disturbances in incoherent scatter data using the maximum entropy method

Two different methods of determining autoregressive coefficients in Maximum Entropy Method (MEM) were compared, investigating power spectra of synthetic signals consisting of three sinusoids and white noise of relatively high level. Spectral line shifts due to the initial phases changes were evaluated. Burg's method was chosen for determining power spectra of experimental signals. Simulating the incoherent scatter data obtained with EISCAT, it was shown that the increasing noise and wave attenuation effects are difficult to separate. Integrating the ME spectral density it was found that, in many cases, it is impossible to reproduce the true power of each harmonic of the signal.