Description: Quasi-thermal noise (QTN) spectroscopy is a plasma diagnostic technique which enables precise measurements of local electron velocity distribution function (VDF) moments. This technique is based on measurements and analysis of voltage fluctuations at the antenna terminals, induced by thermal motion of charged particles. In this work, we accommodate, for the first time, this technique to weakly ionized collisional plasmas. It turns out that the QTN spectrum is modified both at low frequencies, increasing the level of power spectrum, and around the plasma frequency, where collisions damp the plasma oscillations and therefore broaden and reduce the amplitude of so called ‘plasma peak‘, while the spectrum at high frequencies is nearly unmodified compared to the collisionless case. Based on these results, we show that QTN spectroscopy enables independent measurements of the collision frequency, electron density and temperature, provided the ratio of collision frequency to plasma frequency is ν / ω p ∼0.1. The method presented here can be used for precise estimation of plasma parameters in laboratory devices and unmagnetized ionospheres, while application in the ionosphere of Earth is possible but limited to small, low frequency range due to magnetic field influence.

Description: In this paper a new approximate fast method of calculating the bistatic-scattering coefficients of a multilayer structure with random rough interfaces was presented based on the Kirchhoff Approximation(KA) and the electromagnetic theory of stratified media. Firstly, the electromagnetic scattering from a Gauss rough metal or dielectric surface was calculated by KA method and method of moment(MOM), and the effectiveness of KA method was confirmed and verified. Secondly, a new approximate fast method was presented to calculate electromagnetic scattering from a multilayer random rough surface based on electromagnetic reflection from multilayer parallel surfaces and KA. The calculated results by the new method were in good agreements with those by MOM, especially near the specular point. Finally, a comparison of the new method and MOM was carried out in consuming computing time, memory resources and complexity. The comparison indicated that the new approximate method was faster about 30-150 times than MOM. The new approximate fast method could avoid a large matrix inversion, and greatly reduce the computation time and memory resources, and thus improve the computational efficiency. It was an effective approximation fast analyzing method of electromagnetic scattering from multilayer rough surfaces.

Description: In this paper, the amplitude scintillation index (s4) derived from COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Radio Occultation (RO) technique and ground-based Ionospheric Scintillation Monitor (ISM) at Haikou station (Geo. Lat.: 20.0°N, Geo. Lon.: 110.3°E, Mag. Lat.: 10.02°N) is used to investigate the morphology of F-region irregularities in the low latitudes of China. The RO events of tangent point within the range of 10-30°N latitude, 70-160°E longitude, and 150-500 km altitude are adopted to analyze the ionospheric scintillation characteristics. The percentage of ionospheric scintillation occurrence is computed to obtain its diurnal variations, seasonal trends and the dependence on solar and geomagnetic activities. Based on a statistical analysis of a long-term period dataset (year 2007 to 2013), we found that the ionospheric scintillation occurrence from both techniques show similar variations. After sunset (18 LT), the scintillation occurrence increases rapidly and reaches the maximum 3 hours later. Then it decreases rapidly till 04 LT and remains low level during the daytime. The ionospheric scintillation tends to occur more frequently during vernal and autumnal equinoxes, especially in March-April and September-October. The equinoctial asymmetry could be seen clearly from the ground-based ISM observations. The peak ionospheric scintillation occurrence time varies with seasons. It is reached latest in summer while in spring it is very close to that in autumn. The nighttime ionospheric scintillation occurrence tends to increase with increasing solar activities. The increasing tendency is more prominent in vernal and autumnal equinox than that in summer and winter. In general, the control of geomagnetic activities is apt to inhibit ionospheric scintillation at equinox nighttime. In summer and winter, the geomagnetic activities could either trigger or inhibit the generation of ionospheric irregularities in a much more complicated way. Thus it can be concluded that the tangent point location does accurately represent the scattering region, at least in an average sense. The RO technique is demonstrated to be a useful tool for remotely sensing the terrestrial ionosphere on a global scale down to the regional scale in terms of scintillation occurrence.

Description: The electric field amplitude of Very Low Frequency (VLF) transmitter from Hawaii (NPM) has been continuously recorded at Chofu (CHF), Tokyo, Japan. The VLF amplitude variability indicates lower ionospheric perturbation in the D-region (60-90 km altitude range) around the NPM-CHF propagation path. We carried out the prediction of daily nighttime mean VLF amplitude by using Nonlinear Autoregressive with Exogenous Input Neural Network (NARX NN). The NARX NN model, which was built based on the daily input variables of various physical parameters such as stratospheric temperature, total column ozone, cosmic rays, Dst and Kp indices possess good accuracy during the model building. The fitted model was constructed within the training period from 1 January 2011 to 4 February 2013 by using three algorithms namely BRANN, LMANN and SCG. The LMANN has the largest Pearson correlation coefficient ( r ) of 0.94 and smallest Root Mean Squared Error (RMSE) of 1.19 dB. The constructed models by using LMANN were applied to predict the VLF amplitude from 5 February 2013 to 31 December 2013. As a result the one step (1 day) ahead predicted nighttime VLF amplitude has the r of 0.93 and RMSE of 2.25 dB. We conclude the model built according to the proposed methodology provides good predictions of the electric field amplitude of VLF waves for NPM-CHF (mid-latitude) propagation path.

Description: The aim of this paper is the introduction of a new analytically regularizing procedure, based on Helmholtz decomposition and Galerkin method, successfully employed to analyze the electromagnetic scattering by zero-thickness perfectly electrically conducting (PEC) circular disk. After expanding the fields in cylindrical harmonics, the problem is formulated as an electric field integral equation (EFIE) in the vector Hankel transform (VHT) domain. Assuming as unknowns the surface curl-free and divergence-free contributions of the surface current density, a second-kind Fredholm infinite matrix-operator equation is obtained by means of Galerkin method with expansion functions reconstructing the expected physical behavior of the surface current density and with closed-form spectral domain counterparts, which form a complete set of orthogonal eigenfunctions of the most singular part of the integral operator. The coefficients of the scattering matrix are single improper integrals which can be quickly computed by means of analytical asymptotic acceleration technique. Comparisons with the literature have been provided in order to show the accuracy and efficiency of the presented technique.

Description: A diagnostic method for the detection, identification and characterization of precursors of faults due to partial insertion of pin-socket contacts within electrical connectors commonly used in avionics systems is presented. It is demonstrated that a miniaturized ultra-wideband (UWB) source and a mini spectrum analyzer can be employed to measure resonant frequency shifts in connector S parameters as a small and low cost alternative to a large and expensive network analyzer. The transfer function of an electrical connector is represented as a ratio of the spectra measured using the spectrum analyzer with and without the connector. Alternatively, the transfer function is derived in terms of the connector S parameters and the reflection coefficients at both ports of the connector. The transfer function data obtained using this derivation agreed well with its representation as a measured spectral ratio. The derivation enabled the extraction of the connector S parameters from the measured transfer function data as a function of the insertion depth of a pin-socket contact within the connector. In comparison with the S parameters measured directly using a network analyzer at multiple insertion depths, the S parameters extracted from the measured transfer function showed consistent and reliable representation of the electrical connector fault. The results demonstrate the potential of integrating a low cost miniaturized UWB device into a connector harness for real-time detection of precursors to partially inserted connector faults.

Description: No abstract is available for this article.

Description: The first results from coordinated experiments between the Radio Receiver Instrument (RRI) on the Enhanced Polar Outflow Probe (e-POP) and the Super Dual Auroral Radar Network (SuperDARN) Saskatoon High Frequency (HF) radar are examined for a conjunction on July 8, 2014. e-POP, a payload on the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) spacecraft, was located at 380 km altitude, approximately 10 ∘ north (geographic) and 2 ∘ west of Saskatoon, Canada, moving in a southeast direction. We use a matched filter technique to extract individual received SuperDARN pulses from the RRI data stream. The pulses show characteristics of propagation through the F-region ionosphere: they are heavily they show significant pulse-to-pulse variability in magnitude, and there is clear evidence that they experienced multipath propagation. We calculate the polarization parameters of the pulses and use them to identify magnetoionic phenomena such as mode-splitting and single-mode fading. These first RRI results provide compelling insight into HF radio wave propagation, and show RRI's potential to significantly advance radio science.

Description: In this paper, we show how to successfully deal with the problem of focusing a vector field in a 3D scenario, by properly extending two recently introduced procedures developed with respect to 2D fields. In particular, to address this problem relevant in many applications, we describe an improved version of the well known Time Reversal technique, the Optimized Time Reversal, and a procedure able to provide an optimal focusing in the presence of arbitrary upper bounds for the field intensity, the Optimal Constrained Power Focusing. The two approaches are presented, discussed, and comparatively assessed in two different scenarios representative of realistic working conditions. The results confirm the possibility of tackling the problem of focusing a 3D vector field in an efficient way and that the Optimal Constrained Power Focusing, while computationally heavier, provides the best possible focusing performance.

Description: This paper describes a set of experiments which examine the feasibility of applying polarization diversity to a 12760 km ionospheric link from Livingston Island (62.6 o S, 60.4 o W), South Shetlands, to Cambrils (41.0 o N, 1.0 o E), Spain. A transmitting monopole and both a monopole and a dipole (inverted-V) at reception are the antennas used in this measurement campaign. Receiving antennas are orthogonal polarized to check whether wave polarization rotation causes differences between simultaneous signals. The results gathered in the paper indicate that there is a marked difference between simultaneous signals at vertical and horizontal polarized antennas, which depends on the hour of the day and the signal frequency. In this low throughput HF data link, polarization diversity could be used to increase SNR and performance through smart combination of receiving signals, due to the low cross-correlation factor between them.