Amplitude Of Radio Waves Research Articles

Why Does the October Effect Not Occur at Night?

AbstractThe October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency (VLF), reflected at the lowest edge of the ionosphere. This strong decrease can be observed only during the daytime. Although the October effect is long known, it is hardly investigated and its mechanism is still unknown. To get closer to a mechanism, we answer why the October effect does not occur during nighttime. Therefore, average characteristics of the October effect are obtained from different VLF transmitter‐receiver combinations. The occurrence of the October effect is then compared with characteristics of the neutral atmosphere temperature at VLF reflection heights as it seems to act as a proxy for the unknown mechanism. The temperature shows an asymmetric seasonal behavior at daytime VLF reflection heights poleward of 50°N but not during the nighttime, resulting in the October effect.

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.

Analysis of acoustic-gravity waves in the mesosphere using VLF radio signal measurements

A simple method has been proposed to determine acoustic-gravity wave (AGW) characteristics at lower ionosphere (mesosphere) heights by measuring VLF radio signal amplitudes. For relatively short radio paths (less than ∼ 1500 km long) we obtained the simple relations for estimation of AGW neutral density fluctuation and for vertical displacement of an elementary volume of atmospheric gas. The proposed method has been verified by using VLF radio wave amplitude measurements on several radio paths. AGW properties based on these measurements are approximately calculated at heights of radio signal reflection. The relative fluctuation of 3%–6% of neutral particle concentration and elementary volume displacement of 0.6–1.2 km have been obtained. These results are in good agreement with AGW theory in the Earth atmosphere.

Modeling of the Lower Ionosphere During Solar X‐Ray Flares of Different Classes

AbstractThis study presents the results obtained from modeling the lower ionosphere response to C‐, M‐ and X‐class solar X‐ray flares. This model is based on a 5‐component scheme for the ionization‐recombination cycle of the ionospheric D‐region. Input parameters for the plasma‐chemical model under different heliogeophysical conditions corresponding to selected X‐ray flares were determined by using data received from the AURA, SDO, and GOES satellites. Verification of the obtained results was carried out with use of ground‐based radiophysical measurements taken at the geophysical observatory Mikhnevo. The results obtained from a comparison of the calculated and experimental radio wave amplitude variations along six European very low frequency (VLF) paths show that the average normalized root mean square error is ∼7%, 14%, and 18% for C‐, M‐, and X‐class flares, respectively. Qualitative and quantitative analysis of the verification results for the VLF signal amplitude show good predictive capability of the model built for describing weak and moderate ionospheric disturbances.

Analysis of Acoustic-Gravity Waves in the Mesosphere Using VLF Radio Signal Measurements

A simple method has been proposed to determine acoustic-gravity wave (AGW) characteristics at lower ionosphere (mesosphere) heights by measuring VLF radio signal amplitudes. For relatively short radio paths (less than ~ 2000 km long) it has been obtained the simple relations for estimation of AGW neutral density fluctuation and for vertical displacement of an elementary volume of atmospheric gas. The proposed method has been verified by using VLF radio wave amplitude measurements on several radio paths. AGW properties based on these measurements are approximately calculated at heights of radio signal reflection. The relative fluctuation of 3—6 % of neutral particle concentration and elementary volume displacement of 0.6--1.2 km have been obtained. These results are in good agreement with AGW theory in the Earth atmosphere.

Disturbance in the lower ionosphere that accompanied the reentry of the Chelyabinsk cosmic body

The paper describes quasi-periodic and aperiodic variations in the phase and amplitude of radio waves of LF and VLF ranges, which accompanied the flight and explosion of the Chelyabinsk meteoroid. Quasi-periodic variations in the phase have been explained by the generation of acoustic-gravity waves in the atmosphere, which modulate the electron density in the ionosphere and the phase of radio waves. Aperiodic variations in the phase and amplitude of radio waves are associated with an increase in the electron density in the lower ionosphere (at altitudes of 65–70 km). This increase was most likely caused by the interactions of subsystems in the Earth–atmosphere–ionosphere–magnetosphere system or, more correctly, by the precipitation of high-energy electrons from the magnetosphere into the lower ionosphere, which was stimulated by the flight and explosion of a cosmic body. According to the estimates, the density of the flux of electrons with energies of 100 KeV should be on the order of 106 m–2 s–1.

Sporadic Structures in the Equatorial Ionosphere from the GPS—Formosat-3 Radio-Occultation Experiments

We analyze the results of the radio-occultation experiments which were carried out for six years in two equatorial regions of the lower ionosphere. The features of the decimeter radio-wave amplitude variations are considered for the case of remote sensing of the different sporadic structures along the radio paths between two satellites. The diurnal dependences of the height and thickness of the Es layer are presented for different time intervals. It is shown that in different days in the nighttime the Es-layer plasma structures can exist in the form of a single layer or in the form of two layers simultaneously. The first layer is located at an average altitude of 116 km, while the second one, at 96 km. It was found that in the daytime the height of the first layer decreases, while in the evening, irregular structures are formed at altitudes distributed randomly between 90 and 100 km. The amplitude variations of the sounding signals are discussed for cases where the distance (in the perigee) of the radio ray path from the Earth’s surface is between 70 and 85 km in the mesosphere. Statistics of the sporadic structure occurrence is given for the regions located in Africa and Indonesia.

Radio Wave Propagation

This thesis deals with Radio Wave Propagation in environments where the propagation is severely influenced by surrounding objects, mainly walls and other building structures. As an introduction, a brief description is given on propagation terms like loss, delay and dispersion along with system functions relevant for the topic. The focus is then placed on propagation from an outdoor transmitter to an indoor receiver at frequency bands aimed at personal communications and wireless local area networks. Penetration loss is measured at 1.8 and 5.8 GHz for various small scale wall and window structures. Empirical models have been evaluated for the dependence on incidence angle in order to provide a simple yet accurate model for the loss. I. Introduction : The advent of the Marconi Wireless in 1895 made possible transmission of messages in the form of Radio Waves. The frequency falling between 3 KHz to 300 GHz are called Radio Frequencies since they are commonly used in Radio Communication. We know that the Radio Frequency spectrum divided into different frequency bands. The basic principles that enable Radio Waves to be propagated through space are the same today as they were 100 years ago. Some people view Radio Wave Propagation confusing as it lacks the senses of sight and touch. Now talking about the mechanism of Radio Wave Propagation, I want to include that Radio Wave Propagation related with the phenomena that occur in the medium between transmitting antenna and receiving antenna.When a signal is transmitted from transmitter antenna, the Radio Wave which is actually radiated from antenna is spread in all directions. As the distance between transmitting antenna and receiving antenna is increased, the amplitude of Radio Wave is decreased. The electromagnetic waves in the frequency spectrum of 0.001 to 1000000 Hz are called to be Radio Waves. As we know that in the surrounding environment in which the Radio Wave is propagating may have obstacles, discontinuities and propagation medium variations. The region far from earth's surface realized as a Free Space Propagation. Or now we can say that Radio Propagation is the behavior of Radio Waves, when they are transmitted

Modeling of ionospheric scintillation at low-latitude

The presence and movement of plasma density fluctuations in the F-region of the ionosphere are studied by monitoring phase and amplitude of radio waves propagating through the region. In this paper, we have used weak scattering theory and assumed the plasma density fluctuations to behave like phase changing diffraction screen. Appropriate relations for scintillation index S 4, and phase variance δϕ are derived and computed for different parameters of the plasma density irregularities of the ionosphere. SROSS-C2 satellite in situ measurements of plasma density fluctuations, which provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves, were used first time to develop a scintillation model for low latitude. It is observed that the scintillation index S 4 and phase variance δϕ depends on the strength of the plasma turbulence. Finally, the results obtained from modeling are compared and discussed with the available recent results.

Effect of the solar-wind shock wave on the polar ionosphere according to the radio occultation data on satellite-to-satellite paths

The polar lower ionosphere is distinguished by its variability due to various effects of solar activity. They result in the appearance of sporadic E S structures, in excitation of the broad spectrum of plasma inhomogeneities, and in variations of the electron concentration. The goal of the present study is to reveal regularities in the variations of the night-side polar lower ionosphere on the basis of radio wave measurements on links from navigation GPS satellites to CHAMP satellite. These ionosphere variations are stipulated by the effect of the flare solar activity. To this end, it was necessary to determine the altitude profiles of the electron concentration N e ( h ) and to find the characteristics of both sporadic E S structures and plasma inhomogeneities. We used the data of 327 radio occultation events of the night side polar ionosphere in regions of latitudes higher than 65° N for the time period from October 25 to November 9, 2003, when particularly strong solar activity was observed. In the radio occultation events of the ionosphere, we have registered the amplitude E and the increments of the phase path ψ for two coherent signals with the frequencies of f 1 = 1.5 GHz and f 2 = 1.2 GHz [1‐3]. In order to determine the excitation degree for small-scale inhomogeneity of the ionosphere plasma, we analyzed random fluctuations of the radio-wave amplitude δ E . These fluctuations were subjected to the standard procedure of statistical processing in order to obtain the root-mean-square amplitude deviation σ . The fluctuations δ E are caused mostly by plasma inhomogeneities located in the ionosphere F region at altitudes h ≈ 200–300 km. The analysis of these fluctuations has shown that the quantity σ is subjected to variations with the minimum values being σ min ≈ 1 .5% and with the maximum fluctuation level attaining 20%. In Fig. 1, dots indicate values of root-mean-square deviations σ for the fluctuations of the amplitude E , which were registered within the period of October 26 to November 9, 2003 (measurement dates are indicated at the horizontal axis). In the beginning of this period, the fluctuation intensity attained σ ≈ 2%, whereas later, on October 27, an increase in the fluctuations was observed. The maximum of the field-intensity fluctuations took place on October 29, 30, and 31 when σ attained 10‐14%. Starting from November 1, the fluctuation intensity began to decrease, and on November 4, elevated values of σ were recorded again. In order to determine the amplitude fluctuations σ A more objectively, we have excluded the slight effect of continually present small fluctuations with σ min = 1.5%. We have used here the relationship = σ 2 – and have realized smoothing the dependence as a function of time by the sliding mean technique with time interval 6-hour.

The effects of hard‐spectra solar proton events on the middle atmosphere

The stratospheric and mesospheric impacts of the solar proton events of January 2005 are studied here using ion and neutral chemistry modeling and subionospheric radio wave propagation observations and modeling. This period includes three SPEs, among them an extraordinary solar proton storm on 20 January, during which the >100 MeV proton fluxes were unusually high, making this event the hardest in solar cycle 23. The radio wave results show a significant impact to the lower ionosphere/middle atmosphere from the hard spectrum event of 20 January with a sudden radio wave amplitude decrease of about 10 dB. Results from the Sodankylä Ion and Neutral Chemistry model predict large impacts on the mesospheric NOx (400–500%) and ozone (−30 to −40% NH, −15% SH) in both the northern (winter) and the southern (summer) polar regions. The direct stratospheric effects, however, are only about 10–20% enhancement in NOx, which result in −1% change in O3. Imposing a much larger extreme SPE lasting 24 hours rather than just 1 hour produced only about 5% ozone depletion in the stratosphere. Only a massive hard‐spectra SPE with high‐energy fluxes over ten times larger than observed here (>30 MeV fluence of 1.0 × 109 protons/cm2), as, e.g., the Carrington event of 1859 (>30 MeV fluence of 1.9 × 1010 protons/cm2), could presumably produce significant in situ impacts on stratospheric ozone.

Radio remote sensing of the corona and the solar wind

AbstractModern radio telescopes are extremely sensitive to plasma on the line of sight from a radio source to the antenna. Plasmas in the corona and solar wind produce measurable changes in the radio wave amplitude and phase, and the phase difference between wave fields of opposite circular polarization. Such measurements can be made of radio waves from spacecraft transmitters and extragalactic radio sources, using radio telescopes and spacecraft tracking antennas. Data have been taken at frequencies from about 80 MHz to 8000 MHz. Lower frequencies probe plasma at greater heliocentric distances. Analysis of these data yields information on the plasma density, density fluctuations, and plasma flow speeds in the corona and solar wind, and on the magnetic field in the solar corona. This paper will concentrate on the information that can be obtained from measurements of Faraday rotation through the corona and inner solar wind. The magnitude of Faraday rotation is proportional to the line of sight integral of the plasma density and the line-of-sight component of the magnetic field. Faraday rotation provides an almost unique means of estimating the magnetic field in this part of space. This technique has contributed to measurement of the large scale coronal magnetic field, the properties of electromagnetic turbulence in the corona, possible detection of electrical currents in the corona, and probing of the internal structure of coronal mass ejections (CMEs). This paper concentrates on the search for small-scale coronal turbulence and remote sensing of the structure of CMEs. Future investigations with the Expanded Very Large Array (EVLA) or Murchison Widefield Array (MWA) could provide unique observational input on the astrophysics of CMEs.

Detection of layering in the upper cloud layer of Venus northern polar atmosphere observed from radio occultation data

Observations of radio wave scintillations represent an important tool for measuring of small‐scale irregularities in the atmosphere of Venus. Prominent features of enhanced scintillation located in the 60‐km region were observed in Mariners 5 and 10, Venera 9, and Pioneer Venus occultations. It is possible that the enhanced scintillations are due to the random turbulence in the upper region which is caused by trapped small‐scale gravity waves. However, other interpretations are possible. Thin stable layers, which are commonly observed in the Earth stratosphere under cloud‐free conditions, could also contribute to scattering in the Venus stratosphere. If scintillations observed in different occultations are correlated, then these scintillations may be attributed to the persistent layers. Cross correlations of 32‐cm radio wave amplitude fluctuations have been determined for seven radio occultation measurements of Venus's northern polar atmosphere using Venera 15 and 16. Significant cross correlations were found between 59.0 and 61.5 km in four different radio occultations. Layering is revealed in the upper layer of the Venus clouds at altitudes of 59.0–61.5 km, which is specified by enhanced turbulence of the atmosphere. It is found that the lifetime of the small‐scale layered irregularities is 2 d or more and that their horizontal extension in the meridional direction can exceed 180 km. A possible cause of emergence of the layered structures inside the upper layer of polar clouds of Venus is discussed.

Potentialities of radio-occultation monitoring of the lower ionosphere on satellite-to-satellite paths

The data on the radio-occultation probing of the daytime ionosphere at altitudes of 70–130 km that are measured on satellite-to-satellite paths in subequatorial regions at latitudes of ±30° are analyzed. The dependence of regular variations in the integral electron concentration on the height of the ray line is discussed. It is shown that, at altitudes of 85–110 km, the height profile of electron concentration can be determined from the occultation data. The specific features of the variations in the radio-wave amplitude and in the integral electron concentration that are observed during the probing of sporadic ionospheric formations Es are analyzed. Data on the frequency of occurrence of the Es formations, their height, and the thickness of the layered structures are presented. The interfering effect of the ionospheric F region during determination of the parameters of the lower ionosphere is analyzed.

Localization of plasma layers in the ionosphere based on observing variations in the phase and amplitude of radiowaves along the satellite-to-satellite path

We propose a method for localization of layered structures in the ionosphere, which is based on simultaneous observations of time variations in the intensity and phase of radio waves along transionospheric paths. The method determines the position of the turning point, where the gradient of the refraction index is perpendicular to the ray trajectory and the influence of the layered structure on the parameters of radio waves is maximal. The layer position is estimated by analyzing the variations in the phase and intensity of radio waves in combination. The method was used to analyze the experimental data obtained by the radio occulation mission CHAMP. The position for inclined plasma layers was determined and the electron density distribution was found for the considered radio occultation sessions. The method was verified by measuring the turning point on the ray trajectory in a neutral gas in the atmosphere.

ELF Attenuation Factor Derived from Distance Dependence of Radio Wave Amplitude Propagating from an Artificial Source

The distance dependence was used of the artificial extremely low frequency (ELF) radio signal measured in the Atlantic Ocean for establishing the attenuation rate at a particular frequency. We demonstrate that value obtained is in accord with the literature data, as well as with the model based on the Schumann resonance measurements. Deviations do not exceed the standard inaccuracy conditioned by the finite signal to noise ratio. The attenuation factor thus obtained might be used for testing models of the Earth - ionosphere cavity. Absolute calibrations of measurements provided the effective current moment of the field source.

A Study of Precipitation Effect on Tropospheric Electromagnetic Wave Propagation at Frequency 19.5 GHz

In this article, we take advantage of a terrestrial propagation link at Ka-band 19.5 GHz located on the campus of the National Central University in Chung-Li City to carry out a tropospheric radio propagation experiment. Data collected in 2004 were used in data analysis to provide a deeper understanding of the precipitation effect on tropospheric electromagnetic wave propagation at Ka-band. The results indicate that the relation between specific attenuation ĸe of 19.5 GHz radio wave amplitude and the measured rain fall rate R (in unit of mm hr -1 ) is of the form: ĸe = aR b (in unit dB km -1 ) with a = 0.0699 and b = 1.0984, which is fairly close to that obtained by assuming Laws-Parsons drop size distribution. In order to study the overall attenuation characteristics of Ka-band radio wave influenced by precipitation particles, we used a ground-based 2D disdrometer to measure the terminal velocities of raindrops and their size distributions. The result shows that the relation between raindrop diameter and terminal velocity in terms of the Power-Law relation, i.e., V(D) = AD B , and the drop size distribution can be expressed in the form of a Gamma distribution function, i.e., N(D) = N0D μ exp(-δD). With these results, the corresponding rain attenuation of the electromagnetic wave at Ka-band is estimated and discussed in this article.

FORMOSAT-3/COSMIC GPS Radio Occultation Mission: Preliminary Results

The Formosa Satellite-3 and Constellation Observing System for the Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) radio occultation (RO) mission has been successfully launched on April 14, 2006. The FORMOSAT-3/COSMIC mission uses global positioning system (GPS) signals to study the atmosphere and the ionosphere with global coverage. Receivers that are installed onboard of the six small FORMOSAT-3/COSMIC satellites register the phase and the amplitude of radio waves at two GPS frequencies. We give a preliminary analysis of the first RO measurements that are provided by the FORMOSAT-3/COSMIC mission. The geographical distribution of the first FORMOSAT-3/COSMIC RO experiments is shown. We demonstrate that the performance of the first measurements allows obtaining the vertical profiles of the refractivity, temperature, and pressure for the considered FORMOSAT-3/COSMIC RO events with expected accuracy, which is quite similar to the accuracy of the previous Challenging Mini-Satellite Payload and Gravity Recovery and Climate Experiment RO missions. New elements in the RO technology are suggested for further improving the accuracy and broadening the application range of the RO method. We emphasize new directions in applying the RO method to measure the vertical gradients of the refractivity in the atmosphere, to determine the temperature regime in the upper stratosphere, and to investigate the internal wave activity in the atmosphere. We find a significant correlation between the phase acceleration and the intensity variations in the RO signals that are emitted by GPS satellites and registered by the FORMOSAT-3/COSMIC satellites. This correlation opens a way to locate the layered structures in the propagation medium based on simultaneous observations of the radio wave intensity and the phase variations in trans-ionospheric satellite-to-satellite links.

Low frequency modulation of transionospheric radio wave amplitude at low-latitudes: possible role of field line oscillations

Abstract. Ionospheric scintillations of radio waves at low-latitudes are associated with electron density irregularities. These irregularities are field-aligned and can provide excitation energy all along the field line to non-local field-aligned oscillations, such as the local field line oscillations. Eigen-periods of toroidal field line oscillations at low-latitudes, computed by using the dipole magnetic field and ion distributions obtained from the International Reference Ionosphere (IRI) for typical nighttime conditions, fall in the range of 20–25 s. When subjected to spectral analysis, signal strength of the radio waves recorded on the 250 MHz beacon at Pondicherry (4.5° N dip), Mumbai (13.4° N dip) and Ujjain (18.6° N dip) exhibit periodicities in the same range. For the single event for which simultaneous ground magnetic data were available, the geomagnetic field also oscillated at the same periodicity. The systematic presence of a significant peak in the 20–25 s range during periods of strong radio wave scintillations, and its absence otherwise suggests the possibility that field line oscillations are endogenously excited by the irregularities, and the oscillations associated with the excited field line generate the modulation characteristics of the radio waves received on the ground. The frequency of modulation is found to be much lower than the characteristic frequencies that define the main body of scintillations, and they probably correspond to scales that are much larger than the typical Fresnel scale. It is possible that the refractive mechanism associated with larger scale long-lived irregularities could be responsible for the observed phenomenon. Results of a preliminary numerical experiment that uses a sinusoidal phase irregularity in the ionosphere as a refracting media are presented. The results show that phase variations which are large enough to produce a focal plane close to the ground can reproduce features that are not inconsistent with our observations.Key words. Magnetospheric physics (magnetosphere – ionosphere interactions) Ionosphere (ionosphere – magnetoshere interactions; ionospheric irregularities)

Inversions of radio occultation amplitude data

Radio occultation remote sensing of the Earth's atmosphere consists of satellite‐to‐satellite observations of phase and amplitude of radio waves that propagate through the atmosphere. The observed excess phase along with the positions and velocities of the satellites are inverted into bending angle as a function of impact parameter and then into vertical profiles of refractivity, pressure, and temperature in the neutral atmosphere, or into electron density in the ionosphere. The retrieved profiles are assigned to the perigee points of the sounding rays. Amplitude data are normally not used, except when solving diffraction back propagation problems. In this paper a simple method to utilize amplitude radio occultation data is discussed. Equations based on geometric optics are considered for the inversion of an amplitude into bending angle. These inversions do not require high coherence of radio waves or precise orbit determination, as with phase inversions, but they do require precise calibration of the amplitude. Even though amplitude inversions are not so precise as phase inversions, they may still be useful for a number of applications. When compared to phase inversions they allow the optimization of the filter bandwidth for phase inversions, the detection of multipath propagation, and the localization of electron density irregularities in the ionosphere. These applications are demonstrated by processing of the Global Positioning System/Meteorology (GPS/MET) radio‐occultation data collected onboard the satellite Microlab‐1.