<p>Diss. (sammanfattning) Umeå : Umeå universitet, 1995, härtill 5 uppsatser.</p> / digitalisering@umu

Abstract This dissertation presents new incoherent scatter radar measurement techniques and data analysis methods. The measurements used in the study were collected by connecting a computer-based receiver to the EISCAT (European Incoherent SCATter) radar on Svalbard. This hardware consists of a spectrum analyzer, a PCI-bus-based programmable digital I/O card and a desktop computer with a large-capacity hard disk. It takes in the 70-MHz signal from the ESR (Eiscat Svalbard Radar) signal path and carries out down-conversion, AD conversion, quadrature detection, and finally stores the output samples effective sampling rate is 1 MHz, large enough to span all the frequency channels used in the experiment. 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 including elimination of ground clutter with only a small loss in statistical accuracy. The capability of our hardware in storing the incoherent scatter radar signals directly allows us to use very flexible and versatile signal processing methods, which include clutter suppression, filtering, decoding, lag prole calculation, inversion and optimal height integration. The performance of these incoherent scatter radar measurement techniques and data analysis methods are demonstrated by employing an incoherent scatter experiment that applies a new binary phase code. Each bit of this code has been further coded by a 5-bit Barker code. In the analysis, stochastic inversion has been used for the first time in decoding Barker-coded incoherent scatter measurements, and this method takes care of the ambiguity problems associated with the measurements. Finally, we present new binary phase codes with corresponding sidelobe-free decoding filters that maximize the signal-to-noise ratio (SNR) and at the same time eliminate unwanted sidelobes completely. / Original papers The original papers are not included in the electronic version of the dissertation. Lehtinen, M., Markkanen, J., Väänänen, A., Huuskonen, A., Damtie, B., Nygrén, T., &amp; Rahkola, J. (2002). A new incoherent scatter technique in the EISCAT Svalbard Radar. Radio Science, 37(4), 3-1-3–14. https://doi.org/10.1029/2001rs002518 Damtie, B., Nygrén, T., Lehtinen, M. S., &amp; Huuskonen, A. (2002). High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment. Annales Geophysicae, 20(9), 1429–1438. https://doi.org/10.5194/angeo-20-1429-2002 Damtie, B., Lehtinen, M. S., &amp; Nygrén, T. (2004). Decoding of Barker-coded incoherent scatter measurements by means of mathematical inversion. Annales Geophysicae, 22(1), 3–13. https://doi.org/10.5194/angeo-22-3-2004 Lehtinen, M. S., Damtie, B., &amp; Nygrén, T. (2004). Optimal binary phase codes and sidelobe-free decoding filters with application to incoherent scatter radar. Annales Geophysicae, 22(5), 1623–1632. https://doi.org/10.5194/angeo-22-1623-2004

Abstract From the very beginning of modern ionospheric science, different radar applications have been utilised in ionospheric measurements. The most sophisticated ionospheric radars are the incoherent scatter radars, which detect the extremely weak scattering of radio waves from thermal fluctuations in the ionospheric plasma. Besides the low signal level, the stochastic nature of the scattering process causes further complications to the measurements. The scattering produces a zero-mean random signal, whose autocorrelation function contains the information of the ionospheric plasma parameters. Incoherent scatter radars have been used for about half a century, but the demanding task of developing transmission modulation and data analysis is still in progress. In this thesis, a statistical inversion based method for removing range ambiguities from the autocorrelation functions, lag profile inversion, is applied to incoherent scatter radar data. The data have been recorded with the EISCAT incoherent scatter radars, located in Northern Fennoscandia. The method is first applied to standard EISCAT experiments, the results giving strong evidence that the method is applicable for the purpose, and it provides results of at least equal quality with the present standard methods. In subsequent studies, new radar modulation methods are developed, which may provide significant improvements to the present incoherent scatter radar experiments. All the methods have been tested with a real radar, and lag profile inversion has been successfully applied to the recorded data. The methods are also put to use in order to measure the predicted effects of artificial heating of the free electrons in the D-region of the ionosphere.

No description available.

Incoherent scatter radar (ISR) systems allow researchers to peer into the ionosphere via remote sensing of intrinsic plasma parameters. ISR sensors have been used since the 1950s and until the past decade were mainly equipped with a single mechanically steerable antenna. As such, the ability to develop a two or three dimensional picture of the plasma parameters in the ionosphere has been constrained by the relatively slow mechanical steering of the antennas. A newer class of systems using electronically steerable array (ESA) antennas have broken the chains of this constraint, allowing researchers to create 3-D reconstructions of plasma parameters. There have been many studies associated with reconstructing 3-D fields of plasma parameters, but there has not been a systematic analysis into the sampling issues that arise. Also, there has not been a systematic study as to how to reconstruct these plasma parameters in an optimum sense as opposed to just using different forms of interpolation. The research presented here forms a framework that scientists and engineers can use to plan experiments with ESA ISR capabilities and to better analyze the resulting data. This framework attacks the problem of space-time sampling by ESA ISR systems from the point of view of signal processing, simulation and inverse theoretic image reconstruction. We first describe a physics based model of incoherent scatter from the ionospheric plasma, along with processing methods needed to create the plasma parameter measurements. Our approach leads to development of the space-time ambiguity function, forming a theoretical foundation of the forward model for ISR. This forward model is novel in that it takes into account the shape of the antenna beam and scanning method along with integration time to develop the proper statistics for a desired measurement precision. Once the forward model is developed, we present the simulation method behind the Simulator for ISR (SimISR). SimISR uses input plasma parameters over space and time and creates complex voltage samples in a form similar to that produced by a real ISR system. SimISR allows researchers to evaluate different experiment configurations in order to efficiently and accurately sample specific phenomena. We present example simulations using input conditions derived from a multi-fluid ionosphere model and reconstructions using standard interpolation techniques. Lastly, methods are presented to invert the space-time ambiguity function using techniques from image reconstruction literature. These methods are tested using SimISR to quantify accurate plasma parameter reconstruction over a simulated ionospheric region.

Ionospheric scale height is a measure of the topside altitude dependence of electron density and is a key ionospheric parameter due to its intrinsic connection to ionospheric dynamics, plasma temperature, and composition. A longtime problem has been that information on the bottomside ionospheric profile is readily available, but the observation of the topside ionosphere is still challenging. Despite numerous data techniques to characterize the topside ionosphere, the knowledge of the behavior of the topside ionosphere and its subsequent scale heights remains insufficient. The goal of this study is to evaluate whether or not we can characterize the topside ionospheric density and temperature profiles in the event that neither temperature nor electron density are measured by using a cost-effective method. In a simple model, the electron density in the F-region topside decreases exponentially with height. This exponential decay is mainly driven by thermal diffusive equilibrium, but also dependent on the dominant ion species, as well as other drivers during nondiffusive conditions. A scale height based on observations of the temperature can generate topside electron density profiles. While a measure of the electron density profile enables a scale height to be inferred, hence yielding temperature information. We found a new way to represent how much total electron content (TEC) is allotted for the topside ionosphere. We then used this information to successfully determine TEC using ionosonde data containing only bottomside electron density information. For the first time, slab thickness, which is directly proportional to scale height, was found to be correlated to the peak density height and introduced as a new index, k. Ultimately, k relates electron density parameters and can be a very useful tool for describing the topside ionosphere shape and subsequently, scale height. The methodology of using cost-effective, readily available ionosonde bottomside electron density data combined with GPS TEC was discovered to be capable of inferring the topside ionosphere. This was verified by incoherent scatter radar (ISR) data, though major issues surrounding the availability of ionogram data during nighttime hours greatly limited our study, especially during diffusive equilibrium conditions. Also, significant differences were found between ISR and ionosonde-determined peak density parameters, NmF2 and hmF2, and raised concerns in how the instruments were calibrated.

Incoherent scatter radar (ISR) measurements were used in conjunction with plasma simulations to study two micro-scale plasma processes that commonly occur in the auroral ionosphere. These are 1) ion acoustic turbulence and 2) Langmuir turbulence. Through an ISR experiment we investigated the dependence of ion acoustic turbulence on magnetic aspect angle. The results showed a very strong aspect angle sensitivity which could be utilized to classify the turbulence according to allowable generation mechanisms and sources of free energy. In addition, this work presents results that led to the discovery of a new type of ISR echo, explained as a signature of cavitating Langmuir turbulence. A number of incoherent scatter radar experiments, exploiting a variety of beam and pulse patterns, were designed or revisited to investigate the Langmuir turbulence underlying the radar echoes. The experimental results revealed that Langmuir turbulence is a common feature of the auroral ionosphere. The experimental efforts also led to uncovering a relationship between Langmuir turbulence and one type of natural electromagnetic emission that is sometimes detected on the ground, so-called “medium frequency burst”, providing an explanation for the generation mechanism of these emissions. In an attempt to gain insights into the source mechanism underlying Langmuir turbulence, 1-dimensional Zakharov simulations were employed to study the interactions of ionospheric electron beams with a broad range of parameters with the background plasma at the F region peak. A variety of processes were observed, ranging from a cascade of parametric decays, to formation of stationary wave packets and density cavities in the condensate region, and to direct nucleation and collapse at the initial stage of the turbulence. The simulation results were then compared with the ISR measurements where inconsistencies were found in the spectral details and intensity of the simulated and measured Langmuir turbulence echoes, suggesting the possibility that the direct energy for the turbulence was provided by unstable low-energy (5 − 20 eV) electron populations produced locally in the F region of the ionosphere rather than by electron beams originating from the magnetosphere.

There was a large radar campaign during 2007 and 2008, the International Polar Year (IPY),and at that time the EISCAT Svalbard Radar was operated and measured the ionosphere continuouslyat most times. This report presents statistical results from an electron enhancementpoint of view. Until now there has been some research into the field and results based on theions in the ionosphere, and the enhancements we refer to as Naturally enhanced ion acousticlines (NEIALs). Plasma line data from May 2007 to February 2008 has been analysed inorder to find and classify enhancements as NEIALs have been classified but with respect tothe electron distribution instead of the ion distribution. A method of detection was developedin order to differentiate the enhancements from the background with a relation between theminimum and maximum power of each measured dump. Results show that there is a largedifference between the downshifted plasma lines and the upshifted plasma lines, both has arange distribution peak at 180 km and the upshifted plasma line has another peak at 230 kmwhich the downshifted plasma line does not. The occurrence rate of the enhancements was1.64 % for the downshifted plasma line and 4.69 % for the upshifted plasma line. Threedifferent types of enhancements are classified using the variance distribution for the peakfrequency of that detected dump, Single, Profile, and Diffuse. The Single enhancements havea bit different spectral, range, and time of day distributions than of the Profile and Diffusedistributions. The Diffuse classifications are mostly wrong classifications and aliasing and itis very similar to Profile enhancements as seen by its distribution.

No description available.

No description available.