Incoherent Scatter Experiment Research Articles

Lag profile inversion method for EISCAT data analysis

Abstract. The present standard EISCAT incoherent scatter experiments are based on alternating codes that are decoded in power domain by simple summation and subtraction operations. The signal is first digitised and then different lagged products are calculated and decoded in real time. Only the decoded lagged products are saved for further analysis so that both the original data samples and the undecoded lagged products are lost. A fit of plasma parameters can be later performed using the recorded lagged products. In this paper we describe a different analysis method, which makes use of statistical inversion in removing range ambiguities from the lag profiles. An analysis program carrying out both the lag profile inversion and the fit of the plasma parameters has been constructed. Because recording the received signal itself instead of the lagged products allows very flexible data analysis, the program is constructed to use raw data, i.e. IQ-sampled signal recorded from an IF stage of the radar. The program is now capable of analysing standard alternating-coded EISCAT experiments as well as experiments with any other kind of radar modulation if raw data is available. The program calculates the ambiguous lag profiles and is capable of inverting them as such but, for analysis in real time, time integration is needed before inversion. We demonstrate the method using alternating code experiments in the EISCAT UHF radar and specific hardware connected to the second IF stage of the receiver. This method produces a data stream of complex samples, which are stored for later processing. The raw data is analysed with lag profile inversion and the results are compared to those given by the standard method.

F-region electron density and <i>T<sub>e</sub> / T<sub>i</sub></i> measurements using incoherent scatter power data collected at ALTAIR

Abstract. The ALTAIR UHF radar was used in an incoherent scatter experiment to observe the low-latitude ionosphere during the Equis 2 rocket campaign. The measurements provided the first high-resolution electron density maps of the low-latitude D- and E-region in the Pacific sector and also extended into the F-region and topside ionosphere. Although the sampling frequency was well below the Nyquist frequency of F-region returns, we were able to estimate Te / Ti ratio and infer unbiased electron density estimates using a regularized inversion technique described here. The technique exploits magnetic aspect angle dependence of ISR cross-section for Te>Ti.

Comparison of the first long-duration IS experiment measurements over Millstone Hill and EISCAT Svalbard radar with IRI2001

The first long-duration incoherent scatter (IS) radar observations over Millstone Hill (42.6°N, 288.5°E) and EISCAT Svalbard radar (ESR, 78.15°N, 16.05°E) from October 4 to November 4, 2002 are compared with the newly updated version of the IRI model (IRI2001). The present study showed that: (1) For the peak parameters h mF 2 and f oF 2, the IRI results are in good agreement with the observations over Millstone Hill, but there are large discrepancies over ESR. For the B parameters, the table option of IRI produces closer values to the observed ones with respect to the Gulyaeva’s option. (2) When the observed F 2 peak parameters are used as input of IRI, the IRI model produces the reasonably results for the bottomside profiles during daytime over Millstone Hill, while it gives a lower bottomside density during nighttime over Millstone Hill and the whole day over ESR than what is observed experimentally. Moreover, IRI tends to overestimate the topside N e profiles at both locations. (3) The T i profiles of IRI can generally reproduce the observed values, whereas the IRI-produced T e profiles show large discrepancies with the observations. Overall comparative studies reveal that the agreement between the IRI predictions and experimental values is better over Millstone Hill than that over ESR.

Decoding of Barker-coded incoherent scatter measurements by means ofmathematical inversion

Abstract. The standard analysis of Barker-coded incoherent scatter experiments is based on a matched filter with an impulse response which is a mirror image of the code itself. The method produces small sidelobes which cause contamination from regions outside the nominal range gate. A corresponding effect is also encountered in the lag direction, where individual lag estimates are biased by the variation of the plasma autocorrelation function around the nominal lag value. The present paper introduces a new method of analysing Barker-coded experiments by means of stochastic inversion. Since it does not apply a decoding filter, it does not suffer from drawbacks caused by the sidelobes of the range ambiguity function. The method combines the profile of each full lag and a number of surrounding fractional lags into a single inversion problem. Error analysis also indicates that the statistical accuracy given by inversion is better than that obtained by means of standard decoding. Furthermore, the inversion method gives a possibility to reduce the bias due to the variation of the autocorrelation around the nominal lag. In this paper the method is described and applied to data obtained by means of the EISCAT Svalbard radar. In addition, it is shown that mathematical inversion can be used instead of the the conventional height integration. Key words. Radio science (ionospheric physics; signal processing; instruments and techniques)

A new incoherent scatter technique in the EISCAT Svalbard Radar

A special incoherent scatter technique in the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) was tested in November 1999. After the 70‐MHz IF stage the standard ESR receiver was replaced by a spectrum analyzer acting as a combination of a down‐converter, an AD converter, and a quadrature detector and giving complex digital samples at a rate of 4 MHz. The samples were fed into a PCI‐bus‐based programmable digital I/O card, which performed a four‐sample summing operation to give an effective sampling rate of 1 MHz, large enough to span all the frequency channels used in the experiment. Finally, the resulting samples were stored on hard disk. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits; for example, the ground clutter can be eliminated with only a small loss in statistical accuracy, and the true phase of the transmitter waveform in phase‐coded experiments can be measured better than in the present ESR system. In this paper the technical arrangements of this setup are presented. A new incoherent scatter experiment is also described, and the first results obtained using the hardware are shown.

Incoherent scatter experiments at Jicamarca using alternating codes

An incoherent scatter experiment using alternating coded pulses has been implemented at Jicamarca. The experiment is intended to facilitate aeronomy research in the topside and protonosphere. By virtue of utilizing the full duty cycle capabilities of the radar transmitter, the experiment has improved sensitivity over the conventional double pulse experiment and permits multiple parameter estimation at protonospheric altitudes. Error analysis, including the estimation of the complete error covariance matrix for the alternating coded pulses, is discussed. Example density, temperature, and composition data illustrate the capabilities of the new experiment.

Simulating the ionospheric dynamo — II. Equatorial electric fields

Different wind models are used as input to the equivalent circuit model and their contributions to the ionospheric electric fields in the equatorial region are discussed. The dependencies of the electric field variations on altitude, season and solar activity are presented. The separate contributions of the southward and eastward components of the winds are derived and it is found that the global southward winds are the main contributors to the electric fields and currents in the equatorial ionosphere. It is confirmed that the prereversal enhancement of the eastward electric field is produced by the F region dynamo. The solar cycle variations in the strength of this enhancement seem to be controlled by the F region ion density. The tidal winds of Forbes and Gillette (1982 [A compendium of theoretical atmospheric tidal structure, part I, model description and explicit structure due to realistic thermal and gravitational excitation, AFGL-TR-82-0173(I)]) reproduce most features of the Jicamarca incoherent scatter experiment and of the AE-E and DE-2 satellite observations of the electric fields. The HWM90 empirical wind model failed to produce the observed electric field and it seems the semidiurnal wind is too strong in that model.

Alternating-coded multipulse codes for incoherent scatter experiments

A possibility of using strong alternating codes as a phase modulation pattern in multipulse incoherent scatter experiments is investigated. It is shown that such a modulation leads to a high range resolution much in the same way as the use of Barker codes does. The suggested modulation gives two types of data, one with a range resolution corresponding to the pulse length in the pulse code and the other with that corresponding to the bit length in the alternating code. Most of the efficiency of the experiment goes to high resolution data; when long alternating codes are used, the proportion of low resolution data is negligible. Practical examples using this idea are constructed and their speeds are compared with those obtained using Barker codes. The comparison is made using the GUISDAP package; hence this paper also demonstrates the capabilities of GUISDAP in planning the experiments. It is found that an alternating-coded pulse code is approximately as fast as a Barker-coded pulse code and its speed is about one third of that of a Barker-coded alternating code. Barker-coded pulse codes have small range ambiguities within a wide altitude range, which make the fit residual depart from the theoretical value of unity in the least squares parameter fits and put an upper limit to the working range of the experiment. These limitations can only partly be removed by the range ambiguity correction. The suggested modulation has the benefit that it has no range ambiguities. As a result, no systematic errors should occur and the experiment is expected to work at greater heights. Although Barker-coded alternating codes also have range ambiguities, they are always situated close to the main peak of the range ambiguity function and are therefore much less harmful. The suggested modulation is not expected to surpass Barker-coded alternating codes, except perhaps in the presence of intense sporadic E layers.

The ability of the EISCAT radar in measuring fast variations and low values of E region electron density

A special EISCAT incoherent scatter experiment designed for measuring the Erregion electron density with a very high temporal resolution or in extremely quiet conditions is briefly described. The range resolution is 1050 km and the maximum temporal resolution 200 ms. Two examples of the use of the experiments are shown. In the first one the E region effective recombination rate is determined from rapid plasma decay following impulsive precipitation bursts and, in the second one, thin layers at electron densities of the order of 5·10 9 m −3 are observed with a time resolution of 3 min.

Tristatic EISCAT‐UHF measurements of the HF modified ionosphere for low background electron temperatures

The effects of a powerful high‐frequency electromagnetic wave transmitted in the ordinary mode from the European Incoherent Scatter (EISCAT) ionospheric modification facility up to the daytime ionospheric F region have been studied with the EISCAT‐UHF radar system. This is one of the first tristatic EISCAT‐UHF incoherent scatter experiments, in which the excited Langmuir and ion acoustic fluctuations are measured simultaneously at scattering angles near parallel (Tromsø site), ∼22° (Kiruna site), and ∼31° (Sodankylä site) to the geomagnetic field. We present measurements of the spectral structure and temporal evolution with a maximum resolution of 13 ms, at the different scattering angles. For the first time, up to five cascade lines in addition to the decay line in the Tromsø HF‐enhanced plasma line and well‐developed cascade type plasma lines in Sodankylä were observed. In spite of low background electron temperature, and hence negligible electron Landau damping parallel to the geomagnetic field, the excited plasma turbulence varied considerably.

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.

Application of alternating codes for EISCAT observations during the ERRRIS campaign for E-region plasma irregularities

The first extensive use of alternating codes in incoherent scatter experiments took place during the ERRRIS campaign in March and April 1988. Alternating codes provide a very powerful technique which in theory offers time resolution a factor of 5 shorter than is possible with the commonly used multipulse codes for low signal-to-noise ratios. For quiet conditions during the ERRRIS campaign this was confirmed by the remarkable consistency of measured parameters which were adjacent in time or height, indicating a low level of random error even for integration times as short as 45 s. This gave confidence in the value of the parameters measured during disturbed periods when the electron temperature at 110km reached values of several thousand kelvin for short periods, with simultaneous increases in ion temperature at greater heights.

Derivation of ion‐neutral collision frequencies from a Combined Ion Line/Plasma Line Incoherent Scatter Experiment

Data from a combined ion line/plasma line incoherent scatter experiment with the European Incoherent Scatter (EISCAT) UHF radar are used to derive the ion‐neutral collision frequency in the lower E region (101–114 km). The ion line measurement is based on a 5×14 µs multipulse scheme giving a range resolution of 3 km. A spectrum analyzer is used to measure plasma line signals from the same altitudes. A 1‐hour period at local noon where stable photoelectron enhanced plasma lines were detected is chosen for the analysis. The temperature/collision frequency ambiguity in the ion line autocorrelation function is resolved by incorporating the observed plasma line offset in the least squares fitting procedure. No assumption on the electron‐to‐ion temperature ratio is needed. The fitted collision frequencies compare well with a profile based on the mass spectrometer/incoherent scatter (MSIS) 1983 neutral atmosphere model.

Derivation of ion composition from a combined ion line/plasma line incoherent scatter experiment

A method is established for deriving ion composition from a combined ion line/plasma line incoherent scatter experiment in the ionospheric F1 region. The new method resolves the temperature/composition ambiguity in the ion line autocorrelation function by incorporating the measured plasma line offset in the least squares fitting procedure. The method has the advantage that rather short pulse lengths and integration times can be used, giving reasonably good height and time resolution. We apply the method to data from an European incoherent scatter UHF experiment carried out in July 1985 which involves an event of soft particle precipitation. The result indicates that soft energy particles depositing their energies in the F1 region tend to increase the percentage of atomic ions at these heights.

A Safari‐Eiscat comparison between the velocity of F region small‐scale irregularities and the ion drift

Simultaneous data were obtained between the SAFARI HF coherent radars and the EISCAT incoherent scatter experiment in December 1983. The SAFARI experiment (Scandinavian And French Auroral Radar Investigation) is a set of two HF coherent radars located at Lycksele (Sweden) and Oulu (Finland). These radars are able to detect λ = 10 m F region ionospheric irregularities in the polar and auroral zone. The phase velocity of the irregularities is deduced from the Doppler spectra. During the joint measurements, EISCAT was operated in a meridian scan mode. In this mode, the ion drift velocity is measured at 325‐km altitude for each different position of the radar beam, and after completion of the scan, a two‐dimensional altitude ‐ latitude map of electron density is obtained. This electron density map is used together with a ray tracing program to accurately localize the HF radar echoes. Then, the phase velocity of small‐scale F region irregularities is compared with plasma drift velocity from simultaneous observations of the same volume of the ionosphere. For the set of measurements these drifts are found to be approximately equal. Implications in the existing theories for the generation of F region plasma irregularities at high‐latitude are presented, as well as the potentialities of HF radars to measure the plasma convection at very high latitude.

Requirements for space shuttle incoherent scatter experiments

The feasibility of carrying out incoherent scatter experiments on the space shuttle has been analyzed. Design criteria considered were the required average transmitter power, frequency resolution, spatial resolution, and statistical accuracy. Experiments analyzed were measurement of the naturally enhanced plasma line and the ion component of the incoherent scatter spectrum, and the plasma line artificially enhanced by an intense HF radio wave. The ion component measurement does not appear feasible, while the other two appear reasonable for short ranges only.

Atmospheric wave observations at Arecibo

Multiple pulse incoherent scatter experiments, observing E-region altitudes with height resolution better than 2.4 km, have been carried out at Arecibo from August 1972 to January 1973. Neutral temperature, ion-neutral collision frequency and drift velocities were obtained. The results showed an oscillatory behavior of the neutral atmosphere. Evidences of non-linearities of the atmospheric oscillations are explored. They help to determine the period of the tidal mode consistent with the semi-diurnal tide predicted by the theory.

Semi-diurnal wind and temperature oscillations in the E-region observed by the Nancay incoherent scatter experiment

A correlation method applied to the wind and temperature data obtained between 95 km and 140 km from incoherent scatter experiment shows a prevailing semi-diurnal oscillation whose phase propagates downwards. It can be interpreted as semi-diurnal tide, but with quite different wavelength from that at lower altitudes.

Diurnal variations of the atomic oxygen density and temperature determined from incoherent scatter measurements in the ionosphericFregion

Recent studies have shown that exospheric temperatures can be obtained from incoherent scatter experiments. A method proposed by Carru et al. (1967) is extended to yield the atomic oxygen density and a temperature shape parameter. The method involves the determination, by means of a nonlinear regression analysis, of the values of neutral temperature and oxygen density that best reproduce the measured ion temperature profile, given measured profiles of electron density and electron temperature. By this method, it is possible to obtain a continuous daytime variation of the neutral parameters with a relative accuracy of 15–20%. The first results exhibit a definite day-to-night variation of the atomic oxygen density, which is compared to the variation given by the Jacchia and Cira models.