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.     

Incoherent scatter radar contributions to high latitude D-region aeronomy

Recent aeronomical work on the high latitude D-region is reviewed, restricting the discussion to observations of the D-region by the incoherent scatter technique. Emphasis is given to chemical aeronomy, which governs part of the coupling between the neutral atmosphere and the ionosphere, and forms the basis for the global role of the high latitude D-region. Details of the dynamics of the high latitude D-region, and thus the actual coupling with regions below and above, are, however, not discussed in this context. The aeronomical consequences of special high-latitude phenomena are discussed. These include the effects of the polar summer, precipitation of high energy electrons during auroral substorms and high ionization of the D-region during solar proton events. A detailed discussion is given on selected studies concerning the series of solar proton events that occurred in 1989. Problems of ion and neutral chemistry are readily accessible with incoherent scatter measurements through chemical modelling of the D-region. In this way the continuous nature of incoherent scatter measurements can be utilized to expand our knowledge of the D-region, which earlier was mainly based on momentarily sounding rocket experiments. However, it is pointed out how the interpretation of incoherent scatter data from the D-region strongly benefits from multi-instrument configurations. An outline is given of a possible new development based on the combined use of the Tromsø heating facility and the EISCAT incoherent scatter radars.     

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.     

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.     

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 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.     

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.     

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.     

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.     

Studies of the cusp and auroral zone with incoherent scatter radar: the scientific and technical case for a polar-cap radar

A combined scientific and technical case is presented for the establishment of a new incoherent scatter radar facility on the archipelago of Svalbard. The scientific case rests principally on the ability of such a system to contribute significantly to the elucidation of the chain of physical processes involved in solar wind-magnetosphere-ionosphere-thermosphere coupling, particularly those processes associated with the dayside cusp and auroral zone. These latter regions map magnetically to the vicinity of the dayside magnetopause, and the consequent prospect of conducting co-ordinated observations with the ESA Cluster spacecraft at high altitudes provides strong motivation for ensuring that the radar facility becomes operational no later than 1995. Important features of the Svalbard site include its relatively high geographic latitude, which allows cusp aurorae to be observed under winter solstice conditions, and its proximity to the existing EISCAT incoherent scatter system, with the possibility of joint operations. The latter possibility is not only important for studies of the structure and motion of the high-latitude ionosphere, but is also particularly significant for plasma-physics investigations, which form another major topic of study. It is possible that the facility will be able to contribute significantly to polar stratospheric-tropospheric circulation studies relevant to the ozone-depletion problem. To accomplish these objectives, a tristatic radar system, capable of making full velocity vector measurements, would be ideal. However, the realization of such a system on the islands of Svalbard would present formidable logistic difficulties and an adequate alternative would be a system with three co-located fully steerable parabolic antennae, which could be operated either independently or together, in any combination. This configuration lends itself to a construction scheme that would allow significant observations to be made at an early stage with a partial radar system. The proposed construction scheme could be implemented by 1995 and would have sufficient flexibility to incorporate possible enhancements to the radar system in the future.     

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.     

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.     

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.     

Further effects of charged aerosols on summer mesospheric radar scatter

In an earlier paper, we showed that charged aerosols play a crucial role in enhancing radar echoes from the summer polar mesosphere through reduced diffusion turbulent scatter and dressed aerosol scatter (Cho et al., 1992a). Here, we explore the effects of charged aerosols on radar scatter through ‘fossil’ turbulence and electron density depletion layers. We find that the former can produce radar scatter even after the decay of neutral gas turbulence, while the latter, which are probably produced by the scavenging of free electrons by ice particles, are a candidate for causing partial reflection or Fresnel scatter. Furthermore, we examine the mutual aerosol interaction restriction on dressed aerosol scatter more closely. We find that a high ambient electron density and low aerosol number density are needed for effective dressed aerosol scatter to occur. We then show that very small (less than 1 nm radii), negatively charged aerosols enhance electron diffusivity, and thus inhibit radar scatter. Also, ice aerosol sedimentation, in the light of the reduced diffusion theory, leads us to conclude that the statistical peak in Polar Mesospheric Summer Echoes (PMSE) power should be located between the mean mesopause and the average noctilucent cloud (NLC) height, which agrees with observations. Finally, we invoke time lags in the ice particle formation cycle to account for the observed non-correlation between PMSE and NLC occurrence.     

High latitude quiet summer ion composition profiles derived from a combined ion line/plasma line incoherent scatter experiment

High latitude quiet summer ion composition values in the altitude range from 200 to 245 km have been derived from a combined ion line/plasma line experiment in a full five-parameter fit. The EISCAT UHF radar was used with a 5 × 14 μs multipulse scheme for the ion line measurements, giving a range resolution of 3 km. Plasma line signals from the same altitudes were measured with a 70 μs pulse using a spectrum analyzer. Significant deviations from the standard EISCAT composition model were found, mainly at the upper altitudes. The O⁺ content was generally lower than predicted by the model. For the largest composition deviations, significant effects were seen in the temperatures, particularly in the electron temperature. The electron temperatures derived by a standard ion line fit applying the model were underestimated by up to 15%.     

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.     

The effect of a priori knowledge on parameter estimation errors with applications to incoherent scatter

The dependence of the width of the a posteriori distribution (i.e. parameter estimation error) in a multiparameter fit on the a priori width of another parameter is studied in the framework of linear statistical inversion theory. An exact formula is given which makes it possible to estimate how accurate the a priori information on a given parameter must be in order to obtain the other parameter with a prescribed accuracy. These estimates are useful in those cases where some errors are strongly correlated, like the ion temperature and the O⁺ content in incoherent scatter measurements with the EISCAT UHF radar. The dependence of the solution on the accuracy of the power profile estimates is given. Likewise, the behaviour of the error when a linear combination of the parameters is known exactly is given. The formulae given have a very simple geometrical meaning, and together they give a complete solution to the problem of estimating the accuracies in a multiparameter fit when a linear combination of the variables is known with a prescribed accuracy. It is shown that under the conditions considered in this paper, oxygen content can be determined to an accuracy of 10 % when the ion temperature is known to within 7 %, if we can assume that the collision frequency is zero.     

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.     

Gravity wave detection in the lower thermosphere with the French incoherent scatter facility

Very few gravity-wave measurements are available in the 100 to 120 km altitude range of the atmosphere. This paper illustrates the capability of the French multistatic incoherent scatter facility to fill this altitude gap in medium scale gravity-wave studies. We have been able to detect a particular gravity-wave and to compute its horizontal characteristics: horizontal wave-length: 51km, azimuth: 294°, period: 15 min. Its associated vertical energy flux at 105 km was equal to 0.4 erg cm⁻² s⁻¹. This flux shows the importance of medium scale gravity-waves in the lower thermosphere equilibrium.