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The rough structure may affect the flow field and electromagnetic (EM) scattering characteristics of a hypersonic vehicle. In this study, the EM scattering characteristics of the hypersonic vehicle RAM C-II with a microrough surface were investigated in the millimeter wave band. We first simulated the flow field of a smooth RAM C-II vehicle and calculated the radar cross section (RCS) in the wideband using the physical optics method. The calculation results show that many factors contribute to the variation in the RCS; however, it is occasionally difficult to predict. We then used a one-dimensional Gaussian random rough surface to generate a RAM C-II with a microrough surface through a point-coordinate transformation and simulated its flow field. The electron density flow field of the rough surfaced aircraft fluctuates greatly on the wall. Finally, we calculated the RCS of RAM C-II with different values of roughness in the millimeter wave band. The change of surface structure can be reflected in millimeter waves, and the RCS is determined by both plasma and rough structure. [ABSTRACT FROM AUTHOR]

Electron density is one of the most important parameters for characterizing plasma properties, so obtaining accurate electron density is a prerequisite for studying the interaction between plasma and the electromagnetic waves. This paper presents the effects of different electron densities on the electric field distribution of a microstrip antenna with a center frequency of 2.45 GHz. Then, on the basis of the integrated model of plasma and the microstrip antenna, the Bayesian regularization neural network (BRNN) is used to retrieve the electron density of inhomogeneous plasma. Furthermore, the performance of the proposed approach is evaluated and analyzed by comparison with Levenberg–Marquardt (LM) and Scaled Conjugate Gradient (SCG) neural networks. The results show that the BRNN provides better performance than LM and SCG neural networks to retrieve plasma electron density based on the electric field intensity at fewer spatial positions. The accurate distribution of the electron density of inhomogeneous plasma can be obtained using BRNN. In addition, the greater the range variation of electron density, the greater the relative inversion error. This study provides an important theoretical basis for the diagnosis of electron density for inhomogeneous plasma in experiments. [ABSTRACT FROM AUTHOR]

The rocket exhaust, as a source of the disturbance, provides a way to examine physical and chemical phenomena in the ionosphere. Since the disturbance morphological characteristics are closely related to rocket orbit, the ionospheric perturbation produced by the rocket exhaust considering the trajectory effect is simulated. The three-stage booster rocket trajectory is calculated, considering multi-stage thrust, aerodynamic drag, gravity and apparent force. Based on trajectory calculation and the electron density depletion theory of neutral gas injected into ionosphere, the exhaust is divided into a series of mass elements released at different time and different positions along the trajectory. The theory considers the charge exchange between the ambient O+ ions and the high-speed exhaust molecules (primarily H 2 O) in the exhaust plume. The simulation results show that the depletion structure restricted by the trajectory are dramatically different from the structure produced by a point source release, meaning that it no longer appears as an isotropic quasi-symmetric distribution, but the overall orientation of the depletion structure is consistent with the trajectory direction. Furthermore, the ray tracing technique is used to simulate the propagation of radio waves passing through the depletion structures. Due to the steepness of the depletion boundary and the difference in the shape of the boundary, the "focusing" effect of the rays which though the disturbance structure restricted by the trajectory disappear. These characteristics results can be an important reference for rocket detection and identification. [ABSTRACT FROM AUTHOR]

The attenuation characteristics of obliquely incident electromagnetic (EM) wave in L-Ka frequency band in weakly ionized dusty plasma are analyzed based on the modified Bhatnagar–Gross–Krook (BGK) collision model. According to the kinetic equation and the charging theory, the total complex dielectric constant of the weakly ionized dusty plasma is derived by considering that the minimum velocity of the electron accessible to the dust particle surface is non-zero and the second potential part of the collision cross-section contributes to the charging. The attenuation characteristics within the modified model are compared with those within the traditional model. The influence of the dusty plasma parameters and the incident angle of EM waves on the attenuation in weakly ionized dusty plasma is further analyzed. Finally, the influence of different reentry heights on the attenuation characteristics of the obliquely incident EM wave is discussed. The results show that the effect of the minimum electron velocity and the second term of the collision cross-section on the attenuation characteristics of EM waves cannot be ignored. When the dust density and dust radius are changed, the trends of the attenuation of obliquely incident EM waves are consistent, but the influence of dust density is weaker than that of dust radius due to the constraint of orbit-limited motion (OLM) theory. The plasma thickness, electron density, and incident angle are proportional to the attenuation amplitude of EM waves. The effect of different reentry heights on the attenuation obliquely incident EM waves is related to the electron density and plasma thickness. [ABSTRACT FROM AUTHOR]

A new electromagnetic (EM) calculation model, the inhomogeneity zonal medium model (IZMM) (based on the physical optics (PO) method), is proposed to calculate and analyze the EM scattering characteristics of hypersonic vehicle covered with a plasma sheath. First, the flow field of the plasma sheath is divided into nonuniform calculation subregions with different spacing sizes. The shape of the external flow field of the aircraft is modeled by fitting with a paraboloid and circular platform with different radii. Then, the electron density, temperature, and pressure distribution are obtained by collecting data from different points in the flow field of the sheath. Finally, a piecewise PO method is used to calculate the backscattering radar cross section (BRCS) of the hypersonic vehicle covered with plasma sheath from $L$ -band to $X$ -band. The numerical results show that the EM scattering characteristics of a target spacecraft covered with a plasma sheath can be described more accurately by using zoned medium model simulation. Under different flying conditions, the BRCS of hypersonic vehicle covered with a plasma sheath appears to be reduced to different degrees in the $S$ -band and the band range of 7–11 GHz, and a large attenuation of BRCS occurs in the head area of the aircraft. The research results enrich the EM modeling method of hypersonic vehicles, overcome some shortcomings of the existing PO method, and provide technical support and innovative ideas for the accurate analysis of the EM scattering characteristics of hypersonic targets. [ABSTRACT FROM AUTHOR]

Based on a proposed high-resolution numerical simulation, a qualified development of ionospheric instability triggered by chemical release is first presented in this paper. First, the upwelling, pinching, penetrating, and bifurcating processes of artificial instability are overall produced with electron density patterns generated by using this model. This high-resolution simulation method is first used in artificial instability rather than similar previous work involving natural cases. The numerical method provides a clearer prospect on the development of artificially induced instability. Second, aside from the morphology, statistical characteristics of the electron density fluctuations are obtained and used to verify the effect of instability. The variance and the power spectral density (PSD) of artificial instability are first analyzed in detail by using data extracted from simulations. The empirical PSD of the generated natural irregularities yields Shkarofsky's spectrum, which is deemed to be the general spectrum of the ionospheric plasma. It is interesting that the PSD of artificially initiated electron density fluctuation is found to be in good agreement with Shkarofsky's spectrum. This consistency indicates the occurrence of irregularities in nonlinear simulation and applicability of the simulations to extended studies. Finally, as the electron density fluctuation shown in simulation varies along the altitude, the multiple phase screen (MPS) method is ready for scintillation calculation of the diagnostic radio propagation. Validity of the scintillation calculation by using the MPS reveals that the electron density predicted by instability simulation could be useful for further studies involving both natural and artificial instabilities in the ionosphere. It is shown that the high-resolution simulation model proposed could be useful for further studies involving both natural and artificial instabilities in the ionosphere. [ABSTRACT FROM AUTHOR]

In this paper, we present an improved model associated with a three‐dimensional heating model with a stratified DC approach model considering inhomogeneity of ionosphere to study the distributions of characteristic parameters of lower ionosphere during long‐period modulation heating in the polar region. The distributions of Pederson conductivity, Hall conductivity, and excitation current are discussed in detail. The distributions of Pedersen conductivity reveal that the electron temperature plays more important role in spatial distribution of Pedersen conductivity, and the electron density has a small effect on the distribution of Pedersen conductivity. The distributions of Hall conductivity exhibit that the main contribution of the negative disturbance of Hall conductivity comes from the temperature‐dependent electron collision frequency and the positive disturbance comes from electron density. The distributions of excitation current indicate that the amplitude variation of excitation current is dominated by the Pedersen conductivity and Hall conductivity, and the shape variation (the rotation of long axis) of excitation current is dominated by Hall conductivity. Above all, these findings suggest that the difference between steady‐state time scale of electron temperature (milliseconds) and that of electron density (minutes) will greatly administrate the distributions of Pedersen conductivity and Hall conductivity. Excitation current, which leads to the generation of ultralow frequency/extremely‐low frequency/very low frequency, will be further affected. Finally, the estimated amplitude of the magnetic field in model is consistent with the previous experimental results in the long period modulation heating, which proves the accuracy of the model. The amplitude of magnetic field can reach several hundreds of pT at the modulation period of 50 s. Key Points: We present an improved model associated with a three‐dimensional heating model with a stratified DC approach modelThe distributions of Pederson conductivity, Hall conductivity, and excitation current are discussed in detailThe difference between steady‐state time scale of electron temperature and density will administrate the distributions of conductivities [ABSTRACT FROM AUTHOR]

A space vehicle traveling in the atmosphere at hypersonic speed is enveloped by a time-varying plasma sheath, and the performance of onboard radar reconnaissance system will be significantly interrupted. Plasma sheath would generate parasitic modulations of the amplitude and phase of radar signals. Therefore, based on the transmission coefficient of radar wave in time-varying plasma sheath, the Wigner–Hough transform and autocorrelation time-domain signal detection method are used to estimate the frequency- and time-domain parameters of the linear frequency modulation (LFM) pulse signal statistically. A variation rule of the estimation errors of signal parameters is established under various plasma characteristics parameter and frequency and bandwidth of LFM waves. The critical signal-to-noise ratio that is the threshold value for failure of signal parameter estimation has been accurately obtained for different electron densities and radar frequencies. The calculation results could provide useful information for onboard electronic reconnaissance system in further applications. [ABSTRACT FROM AUTHOR]

Dusty plasma differs from ordinary plasma, and its unique properties have always been an active research branch of plasma science. In this article, a finite-difference time-domain (FDTD) method is used to study the propagation characteristics of terahertz (THz) waves in spatially inhomogeneous and time-varying weakly ionized and weakly colliding dusty plasma. The reflection and transmission coefficients are analyzed for the concentration and radius of dust particles, the time-varying rate of electron density, and the effective collision frequency. Results show that increasing the concentration and radius of dust particles will enhance the adsorption ability of the dust particles toward electrons, thereby weakening the electron density in dusty plasma, which will increase the penetration ability of THz waves into the dusty plasma layer. The increase in the effective collision frequency in the dusty plasma will enhance the collisional absorption of dusty plasma and restrain the ability of electromagnetic (EM) waves to penetrate the plasma layer. However, the ability of EM waves to penetrate the dusty plasma layer can be effectively improved by increasing incident wave frequency. Therefore, using THz technology to penetrate spatially inhomogeneous and time-varying dusty plasma is feasible. [ABSTRACT FROM AUTHOR]

A two-dimensional model coupling Maxwell's equations with plasma fluid equations is used to simulate nitrogen breakdown in a microwave field radiated from a circular waveguide. The balance equations of an excited molecule (ion) density, taking into account the production of excited states as well as quenching in radiation and collisional processes, are included in this model to predict the brightness of light produced in the breakdown. The electron energy distribution function (EEDF) obtained from the analytic expression is adopted to compute the rate coefficients in the model, and its effects on the light brightness are considered. The light brightness in the high electric field initially increases sharply over time. After the occurrence of the breakdown, the electron density saturates at a high level, but the local electric field collapses, leading to the decrease in the mean electron energy. In this case, the light brightness decreases in time since the production of excited molecules (ion) is less than its quenching. The plasma produced in nitrogen breakdown modifies the electric field distribution in space, and the light brightness in the enhanced electric field increases because of the enhancement of the electron density and mean electron energy. The difference in the evolutions of light brightness of different wavelengths is discussed. When the gas pressure increases, the critical electron density above which the incident wave is disturbed increases, and the corresponding light brightness for several wavelengths in the range of visible light first increases and then decreases, showing the similar trend as the experiment. The breakdown formation times predicted by the model also show the similar trend as the experiment. [ABSTRACT FROM AUTHOR]

A model coupling Maxwell’s equations with electron fluid equations is used to simulate air and SF6 breakdown at high pressure by microwaves radiated from a circular waveguide. The modeling for microwaves radiated from a circular waveguide is validated by comparing its results with those from the commercial software FEKO. Under a fixed initial growth rate of electron density in the maximum radiated field, the space and time evolution of electron density in SF6 breakdown is similar to that in air breakdown. However, SF6 plasma causes less change in the radiated field than air plasma. This is because the saturation electron density playing an important role in absorption and scattering by plasma is lower in SF6 than in air. The ratio of critical radiated power in SF6 to that in air increases as pulsewidth or gas pressure increases. The breakdown threshold from simulations as a function of pressure shows the same trend as the experiment. [ABSTRACT FROM AUTHOR]

3-D time-varying and spatially nonuniform plasma is applied using the finite-difference time-domain algorithm in investigating the effects of electromagnetic (EM) wave scattering in the plasma sheath. The plasma sheath is stratified in space, and three types of electron density in time-varying forms are assumed. The characteristics of different time-varying forms are analyzed using radar cross section (RCS). In addition, the effects of RCS change with time at S - and L -bands are investigated using electron density as the time variable. In the condition of time-varying and spatially nonuniform plasma sheath, the attenuation of EM wave is affected by different time-varying regularities of electron density ( ne ). The RCS varying with time is related to the change rate of ne . [ABSTRACT FROM PUBLISHER]

Spread-F is known as the electron density inhomogeneous structures in F layer of ionosphere and can usually be classified as frequency spread-F (FSF) and range spread-F (RSF). Few studies have reported on the statistical characteristics of spread-F occurrences at midlatitudes in Eastern Asia, particularly the comparison of spread-F occurrences between China and Japan. In this paper, we used spread-F data recorded by ten ionosondes located between 25°N and 45°N from 1997 to 2016, to investigate the longitudinal differences in the statistical characteristics of spread-F occurrence and the probable mechanism for its occurrence at midlatitudes in Eastern Asia. Variations in the spread-F occurrences with the solar and geomagnetic activities, season and local time are presented. The main conclusions are as follows: (a) the occurrence percentage of FSF is higher than that of RSF, of which the former is anti-correlated with the solar or geomagnetic activities; (b) higher FSF occurrence percentages mostly appeared during summer, while RSF occurred more frequently in winter near 45°N latitude such as Urumqi, Changchun and Wakkanai; (c) the maximum of the FSF occurrence percentages mostly appeared between 01:00 and 02:00 LT approximately, whereas that of RSF appeared near 00:00 LT; (d) the spread-F occurrence percentages in the coastal or marine areas are higher than those in the inland region between 35°N and 45°N latitudes; however, this phenomenon is not obvious at lower latitudes; and (e) both the mean occurrences of FSF and RSF reach the minimum around 31°N latitude. These above results are helpful for understanding variations in spread-F occurrence at midlatitudes in Eastern Asia. [ABSTRACT FROM AUTHOR]

High frequency ionosphere vertical sounding, as an important and representative application for detecting the ionosphere and studying the characteristics of radio propagation, can be utilized to monitor the ionosphere continuously variation and to acquire the ionosphere asymmetrical features of diverse scale above the ionosphere vertical sounding stations. This is a first article on real time application of numerical methods to obtain the parameters of traveling ionosphere disturbances (TIDs) using vertical incident ionograms. In this paper, the distribution of ionosphere electron density with TIDs is constructed using a background ionosphere model superimposed a perturbation theory model. The background ionosphere electron density is modelled by the inversion of vertical incident ionograms which are observed before the appearance of the disturbance. Based on the fourth order Adams-Bashforth-Moulton (the so-called ABM) predictor corrector method, instead of Runge-Kutta method, the fast digital ray tracing method is established. According to process of the disturbed trace simulation and parameters inversion, the characteristic parameters of ionosphere disturbance at different detection time can be obtained in real time. The numerical analysis of TIDs is then captured completely. [ABSTRACT FROM AUTHOR]

When a hypersonic vehicle flies in near-space, the electron density of the plasma surrounding the aircraft is inhomogeneous and varies with time. Ablation occurs around the vehicle as the temperature rises. Therefore, investigating the propagation characteristics of electromagnetic (EM) waves in a space/time inhomogeneous and fully ionised dusty plasma sheath is important. In this article, a space/time inhomogeneous dusty plasma sheath model is first established. The expression of the dielectric coefficients of weakly collisional and fully ionised dusty plasma sheath is adopted. The shift-operator finite difference time-domain method is used to calculate the reflection and transmission coefficients of different dusty particle concentrations and radii, changes in electron density with time, electron temperatures and ion charges in the terahertz (THz) band. Finally, the propagation characteristics of THz waves with different electron density distributions at eight re-entry heights are analysed. Results show that increasing the incident wave frequency can effectively improve the effects of dusty particle concentration and radius, electron temperature and ion charge on the transmission of the EM wave. Different electron density change forms and electron density distributions significantly influence the propagation properties of THz waves. [ABSTRACT FROM AUTHOR]