PROPAGATION AND GENERATION OF LOW-FREQUENCY ELECTROMAGNETIC WAVES IN THE UPPER ATMOSPHERE

The goal of this project is to conduct the first geographically distributed, low-frequency skywave propagation measurements during a solar eclipse. There is a lack of knowledge about how radio waves below frequencies of 500 kHz are affected by a total eclipse and a lack of experimental data reflecting these low-frequency radio wave transmissions at geographically diverse locations during an eclipse. A low-frequency band receiver system for people across the United States to assemble and use is designed, allowing for a crowd-sourced collection of measurements of relative signal strength of the WWVB and Dixon low-frequency station signals during the eclipse over North America on August 21, 2017.

Lightning produces an extremely low frequency (ELF) radio wave that propagates along magnetic field lines to higher altitudes in the ionosphere. Venus lacks an intrinsic magnetic dipole, so the interplanetary magnetic field (IMF) drapes around the planet forming a comet-like tail. The IMF induces currents in the ionosphere that generate an opposing field. The field lines tend to be nearly horizontal to the surface around much of the planet, except in the tail where it is more radial. There must be a dip to the field in order for waves to be guided to higher altitudes on the dayside. Therefore, a wave on the dayside is less likely to enter the ionosphere at the zenith of its source and more likely to enter at angles towards the horizon, where the field lines and wave path are more aligned.The dual fluxgate magnetometer onboard Venus Express (VEX) was able to detect ELF signals up to 64 Hz at various altitudes throughout the mission. We searched all available data within the ionosphere for lightning-generated whistler-mode waves. These waves are right-handed, circularly-polarized waves and propagate along the magnetic field. With a complete set of whistler observations, we can then calculate the Poynting flux of the waves. The Poynting flux requires the three components of both the wave electric field and magnetic field. Unfortunately, VEX did not have a means of measuring the electric field, but we can infer it if we know the phase velocity of the wave. In order to calculate the phase velocity, we need to employ the Venus International Reference Atmosphere model of electron density since VEX did not have any measurements coincident with whistler observations .The mission was in orbit from 2006-2014 and in that time there were nearly 7 cumulative hours of whistler observations below 400 km. In some cases, there was continuous activity for over a minute, implying a connection to an electrical storm below. These signals were most frequently seen when the spacecraft was at ~250 km altitude. Most signals were observed within 200-350 km altitude with a rate of ~3% of the time the spacecraft spent at these altitudes. It should be noted that due to the polar orbit of Venus Express, the lowest latitude of a detection was ~50°.The VEX mission spanned almost one solar cycle, so we can compare observations during the solar minimum and maximum periods. Because the ionosphere becomes strongly magnetized during solar minimum, detection rates are about twice as high compared to solar maximum. The Poynting flux during solar maximum shows a decrease with increasing altitude, providing further evidence that the waves were generated below the ionosphere. This conclusion is less clear during solar minimum. A large sample of case studies are left for future work to highlight features that might be lost to statistical averaging.Pioneer Venus (PVO) was able to detect the electric component of lightning-generated waves at 100 and 700 Hz, but on the nightside and at lower latitudes in contrast to the North polar orbit of VEX. The improved capability of VEX over PVO has greatly increased our knowledge of Venus lightning. The Indian Space Agency (IRSO) has announced plans for a future Venus orbiter at low latitudes. If a lightning oriented investigation were included, their data would be very complementary to previous studies.

The study of the Earth’s atmosphere and the space environment is important because of the role played by the medium in the activities that affect the Earth and its inhabitants directly or indirectly. A robust capability to monitor, model and predict the happenings in the atmospheric space through deployment of both space- and ground-based observational systems for data acquisition, is key to result-oriented scientific research in atmospheric and space sciences. In this paper, we highlight the importance of regional deployment of observational facilities for data acquisition to complement current observational tools. We briefly review the capabilities of very low frequency (VLF) radio waves (in monitoring and studying changes in the atmosphere and ionosphere), and also present data obtained from our newly deployed VLF radio waves receiver at Anchor University, Lagos (AUL). We show that the diurnal signature characterised by VLF radio signal reflected in the data of three of four propagation paths (AUL-HWU, AUL-JJI, AUL-NWC and AUL-VTX) received at AUL. This outcome shows that operational condition of the VLF radio wave receiver is good. We anticipate interesting findings as we exploit this institution-based dataset to probe ionospheric irregularities in our region for the first time. In the light of some identified ionospheric changes in the equatorial region, we anticipate that our research effort using this regional VLF dataset will yield new results that will be resourceful for better understanding and characterisation of the dynamics of the equatorial D-layer ionosphere, since our region of reception (Lagos) lies close to the equator.

Study of RF waves on plasma

Plasma bubbles are large‐scale irregular structures that often occur at equatorial latitudes, equatorial plasma bubbles (EPBs) affect the propagation of high‐frequency (HF) radio waves. Based on their main observational characteristics, a three‐dimensional (3D) model of EPBs was established, and a preliminary simulation of EPBs was performed. Three‐dimensional ray tracing can be used to simulate HF radio wave propagation. In this study, we consider EPBs at different scales, appearing at different times and different heights, and investigate the influence of EPBs on the propagation path of radio waves at low latitudes. The simulation results show that when they propagate through EPBs of different scales, the propagation paths of radio waves are completely different. Radio waves at 6, 8, and 10 MHz show obvious refraction; With an increase in EPB scale, the focusing effect of radio waves will diminish, and the focusing height will increase. The propagation of HF radio waves through EPB change with time as follows: partial penetration, partial reflection, and total penetration, the refraction effect of EPB on radio waves gradually decreases over time. When radio waves propagate through EPB at different heights, the simulation results show that EPB at an altitude of 200 km makes little impact on the radio waves. As the height of EPB increases, the radio waves will exhibit significant refraction, showing an obvious focusing effect.

Investigations on Lanthanide polymers for radiation shielding purpose

Very low frequency (VLF) radio waves have very effective long-distance propagation characteristics. Therefore, they have a wide application prospect in underwater and underground communication. To effectively suppress the interference of channel noise and improve the performance of the VLF communication system, a low-density parity check (LDPC) coded orthogonal frequency division multiplexing-minimum frequency shift keying (OFDM-MSK) communication scheme is designed according to the characteristics of the VLF channel. Considering that the harmonic interference in the VLF channel can be regarded as frequency selective fading, the OFDM technology is applied to the VLF channel. In addition, the performance of VLF communication system is mainly affected by strong impulse atmospheric noise, which can be successfully simulated by symmetrical alpha stable (SαS) distribution. To reduce the impact of SαS noise on the OFDM system and data loss in frequency selective fading channel, LDPC channel coding technology is proposed. Finally, the robustness of the LDPC coded OFDM-MSK scheme is tested and evaluated from the perspective of the bit error rate (BER). The results show that the LDPC coded OFDM-MSK communication scheme has better performance than the independent OFDM-MSK communication scheme.

There are several sources of high energy radiations in the Universe. Each of them has a totally different emission mechanism. In this review, I concentrate on the study of the general properties of radiations emitted by matter which are accreted towards black holes and neutron stars. We also study the nature of X-ray radiation coming from the Sun in the form of energetic solar flares. These radiations are constantly bombarding the Earth’s ionosphere changing its electron content and affecting the wave propagation through the Earth and the lower-ionosphere waveguide. We monitor Very Low Frequency (VLF) radio waves and correlate our observations with the presence or absence of the high energy radiations from space. Our results during solar flares, show how the number density of the ionosphere is enhanced by these celestial sources. The terrestrial high energy phenomena such as earthquakes are believed to affect the ionospheric properties and we have monitored VLF signals during earthquakes and found possible effects of seismic events on the VLF signals.

Ball lightning caused by a semi-relativistic runaway electron avalanche

Abstract. Measurements of reflection heights of low-frequency (LF) radio waves at oblique incidence and estimates of mesosphere/lower thermosphere (MLT) region horizontal winds applying the D1 spaced receiver method on LF field strength registrations are analyzed with respect to possible long-term trends and interdecadal variability in the time interval from ~1980 to date. While no clear signal of mesospheric height trend is registered during the last two decades, significant trends of MLT horizontal winds are found. These trends are non-linear, in particular a change of trends around 1990 is found, which is probably connected with changes in tropospheric and stratospheric conditions at that time.

Low radio frequencies are favourable for the identification of emission from\nnon-thermal processes such as synchrotron emission. The massive protostellar\njet associated with IRAS 18162-2048 (also known as the HH80-81 system) has been\nimaged at low radio frequencies: 325, 610 and 1300 MHz, using the Giant\nMetrewave Radio Telescope, India. This is the first instance of detection of\nnon-thermal emission from a massive protostellar jet at such low radio\nfrequencies. The central region displays an elongated structure characteristic\nof the jet. In addition, the associated Herbig-Haro objects such as HH80, HH81,\nHH80N, and other condensations along the inner regions of the jet exhibit\nnegative spectral indices. The spectral indices of most condensations are\n~-0.7, higher than the value of -0.3 determined earlier using high frequency\nmeasurements. The magnetic field values derived using radio flux densities in\nthe present work, under the assumption of equipartition near minimum energy\ncondition, lie in the range 116-180 microgauss. We probe into the hard X-ray\nnature of a source that has been attributed to HH80, in an attempt to reconcile\nthe non-thermal characteristics of radio and X-ray measurements. The flux\ndensities of condensations at 610 MHz, measured nearly 11 yrs apart, display\nvariability that could be ascribed to the cooling of condensations, and\nemphasize the importance of coeval or nearly-coeval measurements for estimation\nof spectral indices.\n

Radiowave propagation model is an empirical mathematical formulation for characterization of radiowave propagation as a function of frequency, distance and other conditions. Much like 4G or 3G before it, the radio waves used in 5G are low frequency and non-ionizing radiation. These are on the opposite end of the electromagnetic spectrum to ionizing radiation sources like X-rays, gamma rays, and ultraviolet rays.This study explains the various attenuating factors prevalent in radiowave propagation. It highlights the various types of radiowave propagation; its classification based on their propagation paths; its layers in the atmosphere, its frequency bands and propagation mechanism. The study also entails the various radiowave propagation models and their application in VHF and UHF band.

Connection between winter mesopause region temperatures and diurnal LF reflection height variations measured at Collm

The next generation of radio telescopes will enter what is virtually uncharted territory, observing the Universe at low radio frequencies with unprecedented resolution and sensitivity. A glimpse of what they might find is provided by the discovery of a new class of low frequency galaxy clusters. Deep observations with the Giant Metrewave Radio Telescope, located just north of Pune in India, reveal a radio halo at low frequencies associated with the merging cluster Abell 521. The halo has an extremely steep radio spectrum that makes it difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been studied). The spectrum supports a turbulent acceleration mechanism, which predicts that many radio haloes in the Universe should emit mainly at low frequencies. Clusters of galaxies are the largest gravitationally bound objects in the Universe. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes'. A radio halo at low frequencies associated with the merging cluster Abell 521 is reported. This halo has an extremely steep radio spectrum that makes it difficult to detect with observations at 1.4 GHz. The spectrum supports a turbulent acceleration mechanism, which predicts that many radio haloes in the Universe should emit mainly at low frequencies. Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 1015 solar masses of hot (108 K) gas, galaxies and dark matter in a typical volume of 10 Mpc3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant ‘radio haloes’, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster centre1,2. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence)3,4,5,6, or are injected (as secondary particles) by proton collisions into the intergalactic medium7,8,9,10. They are found only in a fraction of massive clusters that have complex dynamics11,12,13,14, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

The basic purpose of this work is to investigate experimentally the phase characteristics of radio waves propagated along the surface of the earth as proposed by K.A. Norton (reference 1). Norton's theory indicates that the ground wave phase at distances of the order of tens of miles is greater than the corresponding space wave phase by not more than 180 degrees. From a radio surveying point of view, range errors of tens of feet may result. To avoid difficulties inherent in the direct and absolute approach, that of comparison of optical surveying with radio surveying over actual terrain at normally used frequencies (200 kilocycles per second to 15 megacycles per second), a new technique is developed. It is proposed that the propagation of radio waves may be studied by use of appropriate scale models, permitting experiments otherwise impossible because of the gigantic size of the equivalent true scale systems. Further, relative measurements may be obtained directly in a model system by proper choice of paths as contrasted with the absolute measurements usually made in a full-scale system. The technique is demonstrated by a preliminary experimental confirmation of Norton's theory. Suggestions for technique improvement and for further investigation possibilities are given.