Absorption Of Radio Waves Research Articles

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Collisions between electrons and neutral molecules are of special interest for the physics of the Earth’s ionosphere, in particular, for determining the ionospheric conductivity and current systems in the lower ionosphere of the planet, as well as elucidating the role they play in attenuating radio waves propagating inside the D and E regions of the ionosphere. The effective collision frequency of electrons can be estimated from laboratory studies of electron mobility in atmospheric gases in combination with rocket measurements of temperature and particle density in the Earth’s upper atmosphere, or it can be determined independently from analysis of radio occultation data. We have developed a method for reconstructing the vertical profiles of the absorption coefficient of decimeter (wavelength ~19 cm) radio waves by solving the inverse problem of signal absorption in the D and E regions of the Earth’s ionosphere. Based on the analysis of radio occultation data from the FORMOSAT-3/COSMIC satellites, the altitude profiles of the absorption coefficient of decimeter (DM) radio waves in the planet’s ionosphere during the geomagnetic storm on June 22–23, 2015, were determined. It is known that the absorption coefficient at a given fixed frequency is directly proportional to both the electron density and the collision frequency of electrons with ions and neutrals. Using the data on the vertical profiles of the absorption coefficient of DM radio waves and the electron density reconstructed from the analysis of FORMOSAT-3/COSMIC radio occultations, we estimated the effective collision frequency of electrons in the D and E regions of the Earth’s high-latitude ionosphere. The practical significance of studying the frequency of electron collisions and the effects of radio wave absorption in the D and E regions of the planet’s ionosphere is associated with maintaining the uninterrupted operation of space radio communication and navigation systems.

Влияние осциллирующего поляризационного тока на поглощение ОНЧ-радиоволн, распространяющихся вдоль линии геомагнитного поля

Taking into account the oscillating polarization current in Maxwell's equations made it possible to determine the “transparency band” of the ionosphere by the angle between the direction of the magnetic field and the wave vector for VLF radio waves. Waves with both linear polarization of the electric field vector and an extraordinary wave can penetrate to heights above the maximum of the ionosphere layer. The absorption of radio waves is largely determined by the magnitude of the angle between the geomagnetic field vector and the direction of propagation of the radio wave. When radio waves propagate strictly along the geomagnetic field line, an extraordinary wave is not absorbed, but a wave with linear polarization is absorbed. Calculations make it possible to explain the maxima of the energy spectrum of radio waves recorded by the DEMETER satellite in the area of the transmitter location and in the area of the magnetically conjugate point.

Folded-core radar-absorbing sandwich composite with sendust particle-added Ni-plated glass/polyether ether ketone thermoplastic resin in the ultrahigh-frequency band

A novel lightweight folded-core radar-absorbing sandwich composite composed of Ni-plated glass fiber/polyether ether ketone (PEEK) and sendust-particle-filled polyurethane (PU) foam thermoplastic resin is presented. It is designed as a V-pattern folded-core unit cell, which can lead to gradual loss of the incident radar signal intensity and enhanced radio wave absorption performance in the ultra-high frequency (UHF) band (0.3–3.0 GHz). Ni-plated glass fiber was prepared by an electroless plating technique, and a folded-core radar-absorbing sandwich composite by a stamp-forming process. The folded-core was decorated with sendust-particle-filled PU foam to enhance radio wave absorption performance. The radio wave absorption performance of the folded-core sandwich composite was measured using a proposed UHF band single-port free space measurement system. The proposed sandwich composite with a thickness of 81.6 mm achieves a reflection loss below −10 dB (90% absorption) and bandwidth covers in the frequency range of 0.9–3 GHz while maintaining a low density of 0.142 g/cm3. This study presents the new radar-absorbing sandwich composite with a UHF absorber as a low-frequency band, a low density, and excellent radio wave absorption performance.

Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods

Increased solar radiation during solar flare events can cause additional ionization and enhanced absorption of the electromagnetic (EM) waves in the ionosphere leading to partial or even total radio fade-outs. In this study, the ionospheric response to large solar flares has been investigated using the ionosonde data from Juliusruh (54.63° N, 13.37° E), Průhonice (49.98° N, 14.55° E) and San Vito (40.6° N, 17.8° E) Digisonde (DPS-4D) stations. We studied the effect of 13 intense (>C4.8) solar flares that occurred between 06:00 and 16:30 (UT, daytime LT = UT+1 h) from 04 to 10 September 2017 using three different methods. A novel method based on the amplitude data of the measured EM waves is used to calculate and investigate the relative absorption changes (compared to quiet period) occurring during the flares. The amplitude data are compared with the variation of the fmin parameter (fmin, the minimum measured frequency, it is considered as a qualitative proxy for the “non-deviative” radio wave absorption). Furthermore, the signal-to-noise ratio (SNR) measured by the Digisondes was used as well to quantify and characterize the fade-out events and the ionospheric absorption. In order to compare the three different methods, residuals have been defined for all parameters, which provide the percentage changes compared to the selected reference periods. Total and partial radio fade-outs, increased values (+0.4%–318%) of the fmin parameter, and +20%–1400% amplitude changes (measured at 2.5 and 4 MHz) were experienced during and after the investigated flares. Generally, the observed changes depended on the intensity, solar zenith angle and duration of the flare events. Although the three different methods have their own advantages/disadvantages and their limitations, the combination of them seems to be an efficient approach to monitor the ionospheric response to solar flares.

Global Simulations of Multi‐Frequency HF Signal Absorption for Direct Observation of Middle Atmosphere Temperature and Composition

AbstractThis paper presents the first numerical study on a new concept for the direct measurement of D‐region absorption in the high‐frequency (HF) band. Numerical simulations based on the Appleton‐Hartree and Garrett equations of refractive index are presented. Electron temperature as a result of HF radio pumping of the ionosphere is included in the calculations using proper numerical formulation. Both O‐ and X‐mode radio wave polarizations are taken into consideration. A global map of HF absorption in the northern hemisphere is calculated. Detailed calculations of HF radio wave absorption as it propagates through the lower atmosphere are presented. The effect of several parameters on the amount of absorption is calculated. The best frequencies to be used for the purpose of this study are discussed. A machine learning model is developed and the capability of the model in estimation of D and E‐region constituents includes N2, O, O2, as well as T and Ne is examined. Such a technique can also lead to global mapping of HF absorption and improve OTHR (over‐the‐horizon‐radar) performance.

Impact of Various Disturbance Sources on the Atmospheric Electric Field and Thunderstorm Activity of the Northern Tien-Shan

Features of the manifestations of various sources of disturbances in the atmospheric electric field at the Tien-Shan high-mountain station have been studied, and the statistical relationship between solar and thunderstorm activity has been established. Air currents arising over the ocean, having overcome the mountain gorges of the Himalayas, carry moisture to the Northern Tien-Shan and determine the weather features and thunderstorm activity at the Tien-Shan high-mountain station. It was established that the maximum and minimum thunderstorm activity at the Tien-Shan high-mountain station corresponds to the distribution curve of daily thunderstorm activity for the Asia–Australia region. The investigation showed that coronal mass ejections (CMEs) affect the atmospheric electric field by increasing or decreasing its level or causing its fluctuations, depending on the features of manifestations in the near-Earth space and in the Earth’s magnetosphere. In at least 70% of cases, thunderstorm activity was observed with a 1–2-day delay after impact on the Earth’s magnetosphere by CMEs and HSSs. A number of cases were studied when thunderstorm activity was observed in a quiet geomagnetic field. In these cases, an increase in the values of fmin (the minimum frequency of reflection from the ionosphere) and foEs (the critical frequency of the sporadic E layer) was observed, which indicated an increase in the level of radio wave absorption in the ionospheric D-region and the level of electron density at altitudes of 100–120 km.

АНАЛИТИЧЕСКОЕ ИССЛЕДОВАНИЕ ВЛИЯНИЯ ИОНОСФЕРЫ НА ЭНЕРГЕТИЧЕСКИЕ СООТНОШЕНИЯ В СПУТНИКОВЫХ СИСТЕМАХ СВЯЗИ МЕТОДАМИ ФИЗИЧЕСКОЙ КИНЕТИКИ ПЛАЗМЫ

In civil aviation, an urgent and important task is to provide air traffic participants with uninterrupted radio communication. Satellite communication is a global mobile network covering the entire surface of the Earth, including polar regions, which provides a two-way data and voice communication exchange. An analysis of the principles of functioning of satellite communication systems (SCS) showed that the determination of energy characteristics is of great importance. Energy characteristics in satellite communication channels and the level of interference determine SCS operational capabilities and quality indicators. One of the critical factors that determine the effectiveness of the SCS functioning is the conditions for the propagation and absorption of radio waves, including ionospheric phenomena, which are not yet fully understood, and this determines the research relevance. This article is devoted to theoretical studies of the ionospheric plasma by the methods of physical kinetics to increase the energy potential of the radio links of a satellite communication system.

A New Method of Reconstructing Galactic 3D Structures Using Ultralong-wavelength Radio Observations

The free–free absorption of low-frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at ≲10 MHz (ultralong wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest Wavelength and Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets will produce high-resolution multifrequency sky maps at the ultralong wavelength, providing a new window to observe the universe. In this Paper we propose that from these ultralong-wavelength multifrequency maps, the 3D distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the H ii regions around massive stars.

Robert J. Gould (1935–2009)

Gould conducted fundamental theoretical research on the emission and absorption of radio waves, the cosmic microwave background, infrared line emission from planetary nebulae, and excitation conditions in HII regions.

On the determination of ionospheric electron density profiles using multi-frequency riometry

Abstract. Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a “complete” measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.

Geomagnetic Pulsations, Blackout and Low-Latitude Aurorae Associated with Power Magnetic Storm

The research analyzed the regularities of the dynamics of geomagnetic pulsation regimes in the frequency range 0.002–5 Hz, the generation of which reflects the interaction with the Earth's magnetosphere of the solar filament ejected by a powerful solar flare of 3B. We compared the dynamics of the change in the types and modes of geomagnetic pulsations with the dynamics of the atmosphere glow in two spectral lines and the total ionospheric absorption of radio waves. The study developed a possible model of the observed phenomenon.

Modelling the influence of meteoric smoke particles on artificial heating in the D-region

Abstract. We investigate if the presence of meteoric smoke particles (MSPs) influences the electron temperature during artificial heating in the D-region. By transferring the energy of powerful high-frequency radio waves into thermal energy of electrons, artificial heating increases the electron temperature. Artificial heating depends on the height variation of electron density. The presence of MSPs can influence the electron density through charging of MSPs by electrons, which can reduce the number of free electrons and even result in height regions with strongly reduced electron density, so-called electron bite-outs. We simulate the influence of the artificial heating by calculating the intensity of the upward-propagating radio wave. The electron temperature at each height is derived from the balance of radio wave absorption and cooling through elastic and inelastic collisions with neutral species. The influence of MSPs is investigated by including results from a one-dimensional height-dependent ionospheric model that includes electrons, positively and negatively charged ions, neutral MSPs, singly positively and singly negatively charged MSPs, and photochemistry such as photoionization and photodetachment. We apply typical ionospheric conditions and find that MSPs can influence both the magnitude and the height profile of the heated electron temperature above 80 km; however, this depends on ionospheric conditions. During night, the presence of MSPs leads to more efficient heating and thus a higher electron temperature above altitudes of 80 km. We found differences of up to 1000 K in electron temperature for calculations with and without MSPs. When MSPs are present, the heated electron temperature decreases more slowly. The presence of MSPs does not much affect the heating below 80 km for night conditions. For day conditions, the difference between the heated electron temperature with MSPs and without MSPs is less than 25 K. We also investigate model runs using MSP number density profiles for autumn, summer and winter. The night-time electron temperature is expected to be 280 K hotter in autumn than during winter conditions, while the sunlit D-region is 8 K cooler for autumn MSP conditions than for the summer case, depending on altitude. Finally, an investigation of the electron attachment efficiency to MSPs shows a significant impact on the amount of chargeable dust and consequently on the electron temperature.

Verification of the Radio Wave Absorption Effect in the Millimeter Wave Band of SWCNTs and Conventional Carbon-Based Materials

Using a sample coated with three types of carbon-based paints, namely single-wall carbon nanotube (SWCNTs), carbon black, and graphite, the amount of radio wave absorption for each was measured. SWCNTs proved to have the superior radio wave absorption effect in the millimeter band. Considering the change in the amount of radio wave absorption depending on the coating amount, three different coating thicknesses were prepared for each test material. The measurement frequency was set to two frequency bands of 28 GHz and 75 GHz, and the measurement method was carried out based on Japanese Industrial Standard (JIS) R1679 “Radio wave absorption characteristic measurement method in the millimeter wave band of the radio wave absorber.” As for the amount of radio wave absorption in the 28 GHz band, a maximum amount of radio wave absorption of about 6 dB was obtained when 35 m of CNT spray paint was applied. It was confirmed that the carbon black paint came to about 60% that of the SWCNT, and the graphite paint did not obtain much radio wave absorption even when the coating thickness was changed. Furthermore, even in the 75 GHz band, the radio wave absorption was about 7 dB when 16 μm of CNT spray paint was applied, showing the maximum value. Within these experimental results, the CNT spray paint has a higher amount of radio wave absorption in the millimeter wave band than paints using general carbon materials. Its effectiveness could be confirmed even with a very thin coating thickness of 35 μm or less. It was also confirmed that even with the same paint, the radio wave absorption effect changes depending on the difference in coating thickness and the condition of the coated surface.

Influence of the geomagnetic field on absorption of radiowaves

Exact solutions of Maxwell's equations and the equation of motion of charged particles under the action of a radio wave (for frequencies exceeding the gyro-frequency) and the Earth's magnetic field shows the occurrence of an oscillating polarization current. Using the example of estimates of the HF radio wave amplitude during the operation of the EISCAT high power transmitter, it is shown that the oscillating polarization current can lead to significant losses of radio wave energy. This current allows us to explain the reasons for the efficiency of the high power transmitter on November 6, 2010 (a strong perturbation of the electron density and temperature was observed) and the absence of the effect on March 4, 2011. In the first case, most of the HF radio wave energy reached the height of the reflection point in the F-layer region, and led to noticeable changes in the electron density and temperature. In the second case, most of the HF radio wave energy losses could occur at altitudes below the F-layer due to the appearance of a significant oscillating polarization current, and therefore the HF radio wave could not have a noticeable effect on the F-layer.

Disturbances of Ozone Layer and Radio Wave Absorption in D-Region of Ionosphere of the Earth During Solar Proton Event: Simulations with СHARM-I Model

As it is well known [1], the high-energy solar particles (mainly solar protons), with an energy of several to 500 MeV, enter into the Earth’s stratosphere and mesosphere only in polar regions. For the first time the sharp decrease of the ozone and ion content in e stratosphere was detected on board the American Nimbus-4 satellite in course of one of the strongest flares on the Sun (August 4, 1972) [2].

Simple synthesis of conductive poly aniline/cobalt ferrite magnetic nanocomposite: its radio waves absorption and photo catalyst ability

Photo-catalytic activity of cobalt ferrite/polyaniline magnetic nanocomposite was investigated for degradation of four Azo dyes (acid black, acid brown, acid violet and Congo red). For this purpose at the first step CoFe2O4 nanoparticles was synthesized via a fast and simple precipitation method. CoFe2O4/polyaniline nanocomposites were then prepared using simple polymerization method of aniline monomer. The absorption property of radio waves in the microwaves band by cobalt ferrite–polyaniline nanocomposites was studied as well. The results introduce the cobalt ferrite–polyaniline nanocomposites as a radar absorbing coatings. The crystal structure was studied using X-ray diffraction and particle size was determined by scanning electron microscopy. Vibrating sample magnetometer was used for magnetic analysis and by applying Fourier transform infrared spectrometer, the purity of the material was also determined. Photocatalytic activity of nanocomposites was investigated by analyzing UV–Vis spectrograph. In order to examine the radar absorption properties of cobalt ferrite/polyaniline nanocomposites, the absorption of radio waves in the range of 8–12 GHz was measured. These samples have absorption peaks around bandwidth 12–12.5 GHz and absorption amounts were about 70 percent and decrease reflecting was around − 6.4 dB frequency ranges.

Impact of the Ionosphere Disturbed by Rocket Plume on OTHR Radio Wave Propagation

AbstractThe main chemicals of rocket exhaust plume are released into the ionosphere, which cause three‐dimensional (3‐D) disturbances. By simulating the disturbance, we investigate the influence of ionospheric disturbance by the chemical release on the propagation of over‐the‐horizon radar (OTHR) waves. Based on the absorption simulation model, the absorption loss of OTHR waves caused by rocket plume was simulated. Using 3‐D digital ray tracing, we simulated the propagation path of radio waves in the “ionospheric hole” region. The simulation results show that the absorption loss of short waves decreases with an increase in the frequency and elevation angle of the ray. The absorption loss in the year of high solar activity is significantly higher than that in the year of low solar activity. The absorption value is largest at noon and smallest at night. In terms of seasonal variation, the absorption of radio waves is strongest in summer and weakest in winter. Additionally, the ionospheric hole area increases with the release time. The 6 MHz high‐frequency waves cannot reach the disturbance height and are completely reflected to the ground. Most of the 8 MHz radio waves are reflected to the ground, and some of them pass through the ionosphere. The radio waves at 10, 15, and 20 MHz can pass through the hole area of the ionosphere. Moreover, the ionospheric disturbance generated by the plume has a focusing effect on the propagation of OTHR shortwaves. With the increase in radio frequency, the focusing height gradually increases, and the focusing effect gradually weakens.

Radio-wave absorption by aluminum and its dependence on the absorption distance

The absorption of radio wave is relevant to electromagnetic interference (EMI) shielding and low observability. The effect of the absorption distance much above the calculated skin depth on the absorption of radio wave (600–2000 MHz) is studied by determining the absorption loss per unit thickness for aluminum sheets of thickness ranging from 0.528 to 2.053 mm. Aluminum is dominantly used for EMI shielding. The absorption loss SEA increases with increasing thickness, such that when the absorption distance x exceeds 1.3 mm, the increase is slight. The distance 1.3 mm corresponds to ~ 500 times the calculated skin depth for x = 0. Furthermore, when x exceeds 1.3 mm, the linear absorption coefficient α obtained from SEA/thickness decreases linearly with increasing x. At x below 1.3 mm, α decreases with increasing x in a nonlinear non-exponential manner that is not far from linearity and corresponds to much less decrease for the same x than the exponential case. The non-exponential relationship indicates that the skin effect alone cannot explain the observed relationship. The observed high α at large x, at which the electric field is low, indicates nonlinear dielectric behavior. The factor β that relates α2/α1 and x1/x2 (where α1 is the α value at x1, and α2 is the α value at x2) increases with x and levels off at ~ 1.5 when x exceeds 1.3 mm. The effect of x on α is large compared to the effect of the frequency on α.

Statistical analysis of short-wave fadeout for extreme space weather event estimation

Solar flares trigger an increase in plasma density in the ionosphere including the D region, and cause the absorption of radio waves, especially in high-frequency (HF) ranges, called short-wave fadeout (SWF). To evaluate the SWF duration and absorption statistically, we analyze long-term (36 years) ionosonde data observed by the National Institute of Information and Communications Technology (NICT). The minimum reflection frequency, fmin, is used to detect SWFs from 15-min-resolution ionosonde observations at Kokubunji, Tokyo, from 1981 to 2016. Since fmin varies with local time (LT) and season, we refer to dfmin, which is defined as fmin subtracted by its 27-day running median at the same LT. We find that the occurrence of SWFs detected by three criteria, (i) dfmin ≥ 2.5 MHz, (ii) dfmin ≥ 3.5 MHz, and (iii) blackout, during daytime associated with any flare(s) greater than the C1 class is maximized at local noon and decreases with increasing solar zenith angle. We confirm that the dfmin and duration of SWFs increase with the solar flare class. We estimate the absorption intensity from observations, which is comparable to an empirical relationship obtained from sudden cosmic noise absorption. A generalized empirical relationship for absorption from long-distance circuits shows quantitatively different dependences on solar flare flux, solar zenith angle, and frequency caused by different signal passes compared with that obtained from cosmic noise absorption. From our analysis and the empirical relationships, we estimate the duration of extreme events with occurrence probabilities of once per 10, 100, and 1000 years to be 1.8–3.6, 4.0–6.8, and 7.4–11.9 h, respectively. The longest duration of SWFs of about 12 h is comparable to the solar flare duration derived from an empirical relationship between the solar flare duration and the solar active area for the largest solar active region observed so far.

Study of ionospheric D region changes during solar flares using MF radar measurements

During solar flares, the X-ray radiation suddenly increases, resulting in an increase in the electron density of the atmospheric D region and a strong absorption of short-wave radio waves. Based on Langfang medium frequency (MF) radar, this paper analyzed the variation characteristics of D region in the lower ionosphere from 62 km to 82 km. The analysis focused on multiple C-level and M-level solar flare events before and after the large-scale flare event at 11:53 (UT) on September 6, 2017. The results show that it is difficult to detect the electron density over 70 km in Langfang during solar flares, but the electron density value can be obtained as low as 62 km, and the stronger the flare intensity, the lower the detectable electron density height. Besides, the equal electron density height, the received power of X and O waves will also be significantly reduced during the flares, and the reduction of equal electron density height has a weak linear relationship with flare intensity.