HF Radio Wave Propagation Research Articles

Application of the theoretical reference ionosphere model for calculating HF-radiowave propagation characteristics

We study the possibilities of the Theoretical Ionosphere Model (TIM) developed at the Institute of Solar-Terrestrial Physics, Siberian Branch, Russian Academy of Sciences, for calculating the HF-radiowave propagation characteristics. The results of simulation based on the TIM are compared with calculations based on the IRI model and data from experimental observations. Analysis of the results of calculations for the maximum usable frequency (MUF) have shown that with the same input data (coordinates of the receipt and transmission points, the route length, date, and time), the differences in the calculated MUFs (using two different models supplying radio routes with ionospheric information) amount to ∼1% in the daytime and reach 10% at night.

Effects in the ionosphere and HF radio-wave propagation during an intense substorm

We present the results of combined radiophysical studies during the period of an intense magnetospheric substorm which occurred from 00:00 to 02:00 UT in April 12, 1999. Measurements of the ionospheric parameters by a chain of European ionosondes for this period were compared with the variations in ionospheric parameters averaged over more than 70 substorms. The latter variations were obtained by data from the ionosondes of Europe, Central Siberia, and North America in 1993-1999. Data from the CUTLASS radar as well as the DMSP and POES satellites were used for the analysis of the April 11-12 substorm. Numerical calculations of HF radio-wave propagation on the St. Petersburg—Longyearbyen (Svalbard) high-latitude path were carried out by the ray tracing technique. Two simultaneous effects have been revealed in the ionosphere. One occurs immediately during the substorm and another is associated with the end of the magnetic storm in April 10, 1999. According to the CUTLASS radar data, the number of backscattering irregularities in the ionospheric F layer notably decreased during the substorm expansion phase. Satellite data showed an increase in the “soft” (hundreds of eV) particle precipitation before and after the substorm. Numerical calculations of HF radio-wave propagation on the St. Petersburg—Longyearbyen path have demonstrated an essential change of propagation mechanisms during the substorm and a tangible change in the wave arrival angles before and after the substorm.

The new AIS-INGV digital ionosonde

A new digital ionosonde called AIS-INGV (Advanced Ionospheric Sounder) was designed both for research and for routine service of HF radio wave propagation forecast. Nearly the entire system was developed in the Laboratorio di Geofisica Ambientale at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome. It exploits advanced techniques for signal analysis, recent technological devices and PC resources. This paper describes design concepts and performance of the new ionosonde.

Propagation of HF radio waves over northerly paths: measurements,simulation and systems aspects

Large deviations in the direction of arrival of ionospherically propagating radio signals from the Great Circle Path (GCP) have serious implications for the planning and operation of communications and radiolocation systems operating within the HF-band. Very large deviations are particularly prevalent in the polar and sub-auroral regions where signals often arrive at the receiver with bearings displaced from the great circle direction by up to ±100° or more. Measurements made over several paths are presented in this paper, and the principle causes of off-great circle propagation outlined. Significant progress has been made in modelling the propagation effects and work is now in hand to incorporate the results into tools to aid the planning and operation of HF radio systems operating at northerly latitudes.

Ionospheric effects on terrestrial communications :Working Group 3 overview

Telecommunications via ionospheric reflection of radio signals of ground-based transmitters are a traditional area. However, this technique is still in use in telecommunications, broadcasting, etc. Various problems have not yet been solved and some of them were studied in Working Group 3 (WG3). Structure of WG 3 and the terms of reference of its four working packages are described in the introductory paper by Zolesi and Cander (2004). Here we describe the main results achieved in COST 271 in the following areas: i) large-scale fluctuations of planetary and gravity waves; ii) development of a new type of HF channel simulator; iii) geomagnetic storm effects on the F1-region ionosphere; iv) the sporadic E-layer and spread-F phenomena; v) the HF radio wave propagation over northerly paths; vi) how to increase the bit rate in ionospheric radio links. In general, substantial progress was achieved but some problems remain open for future investigations.

Ionospheric disturbances in the years of solar activity minimum and their influence on HF radiowave propagation

The oblique sounding data at the Magadan-Irkutsk and Norilsk-Irkutsk paths together with the vertical sounding at stations located in northeastern Russia were used to analyze ionospheric disturbances in September 2005 and during geophysically active period in December 2006. It is found that during the main phase of magnetic storms, wave disturbances with a period of 2-4 h are registered. These disturbances cause variations in the layer maximum height up to 40-100 km and in the critical frequency up to 1.5-2 MHz. Those variations change substantially values of the maximum observed frequencies (MOF) of the ionospheric radio channel at the paths considered. Such wave disturbances can be caused by generation of AGWs in the auroral zone and their propagation to equatorial latitudes.

Physical and model interpretation of HF radio propagation on the St. Petersburg–Longyearbyen (Svalbard) path

HF radio wave observations have been carried out with an oblique ionospheric sounding (OIS) method on the radio path from St. Petersburg to Longyearbyen (Svalbard), and experimental ionograms were obtained for December 2001. These ionograms have been analysed to investigate the impact of the main ionospheric trough (MIT) and magnetic disturbances on the signals on this path. The observations during weakly disturbed ( K р = 2) magnetic conditions on 14–15 December 2001 were compared with predictions from ray-tracing through a numerical model of the ionosphere. The ray-tracing computer program synthesizes the OIS ionograms by means of the “shooting method”. This method calculates trajectories of HF radio waves for different values of elevation angle and transmission frequency. There was a variety of calculated trajectories, from which we choose those which reach the receiver, and the selected paths provide a synthesis of the oblique ionograms. To simulate HF radio wave propagation, we apply a three-dimensional distribution of the electron density calculated with the mathematical model of the high-latitude ionosphere developed in the Polar Geophysical Institute (PGI). These numerical simulations permit us to interpret specific peculiarities of the OIS data such as abnormal propagation modes, increased delays of signals, enhanced MOF (maximum observed frequency) values etc. New results of the study are summarised as follows. (1) An unusual feature of the propagation along the path is the change of propagation mechanism during substorms on entering a path midpoint (or 1-hop reflection point) to the MIT. (2) Even weak substorms, having the distinguished intensities, lead to the appearance of different types of irregularities observed by the CUTLASS radar and therefore to the different propagation modes and F2MOF values. (3) The PGI model of the ionosphere was first used for ray-tracing at high latitudes. The model results are basically in a good qualitative agreement with experimental observations. This model provides the satisfactory agreement between the calculated and experimental F2MOF values while not correctly representing the fine structure of the experimental OIS ionograms at night. An agreement between the calculated and experimental data is better for day and evening hours than at night.

Observations of small- to large-scale ionospheric irregularities associated with plasma bubbles with a transequatorial HF propagation experiment and spaced GPS receivers

The results from simultaneous observations of the nighttime transequatorial propagation (TEP) of HF radio waves between Australia and Japan and the GPS scintillation measurements in south China and Vietnam are presented in this paper. The results showed that there was good correspondence between the nighttime eastward traveling off-great circle propagation (OGCP) of broadcasting waves of Radio Australia from Shepparton, Australia, measured at Oarai, Japan, and the scintillations in GPS radio waves at Hainan, China. This shows that the nighttime eastward traveling OGCP in HF TEP is caused by a large- scale ionospheric structure associated with a plasma bubble. The zonal drift velocities of the large- scale ionospheric structure estimated by the change in the direction of arrival of the OGCP were similar to those of the small- scale irregularities associated with plasma bubbles measured by the GPS scintillation spaced- receiver technique. Our results show that the HF TEP measurement is quite useful for monitoring the plasma bubble occurrence over a wide area and for forecasting the arrival of the plasma bubble at places located to the east of it.

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.

Heights of SuperDARN F region echoes estimated from the analysis of HF radio wave propagation

Abstract. Tomographic estimates of the electron density altitudinal and latitudinal distribution within the Hankasalmi HF radar field of view are used to predict the expected heights of F region coherent echoes by ray tracing and finding ranges of radar wave orthogonality with the Earth magnetic field lines. The predicted ranges of echoes are compared with radar observations concurrent with the tomographic measurements. Only those events are considered for which the electron density distributions were smooth, the band of F region HF echoes existed at ranges 700–1500 km, and there was a reasonable match between the expected and measured slant ranges of echoes. For a data set comprising of 82 events, the typical height of echoes was found to be 275 km.

Model study of the effect of the main ionospheric trough on oblique HF radiowave propagation

The purpose of this paper is to study the effect of the main ionospheric trough location on the form of oblique sounding ionograms on the Murmansk-St. Petersburg subauroral radio path. Using a mathematical model of the high-latitude ionosphere, we have calculated four different distributions of electron density along the radio path. One of the distributions has been obtained when the trough is absent, and the remaining three distributions contain troughs of approximately identical depth and width but located at different distances from the ends of the radio path. Using the program of two-dimensional ray tracing, we numerically synthesized oblique-incidence ionograms for each of the four obtained distributions of electron density. The calculations have shown that the location of the main ionospheric trough affects considerably the shape of oblique-incidence ionograms.

Generation and evolution of density irregularities due to self‐focusing in ionospheric modifications

Simulations of the generation and evolution of ionospheric density irregularities due to thermal self‐focusing are presented. A nonlinear, self‐consistent development of the self‐focusing instabilities (SFI) is studied with a two‐dimensional model of HF radio wave propagation in inhomogeneous magnetized plasmas. The full‐wave model is coupled with the electron temperature and plasma density equations. In these studies, the pump beam is incident vertically or tilted to the magnetic zenith direction. Our results demonstrate that irregularities generated and developed due to a local heating of the anisotropic ionosphere plasma are density depletions/striations with a strong enhancement of electron temperature inside depletions elongated along the magnetic field lines. For the ordinary mode (O mode) heating the reflection point is shifted upward with decreasing electron concentration. During the nonlinear evolution of the SFI instability the bunching of striations is observed. A number of the bunch‐scale irregularities increase when the beam is tilted to the magnetic zenith direction. The presented results are consistent with the experiments carried out at the midlatitude SURA HF heating facility.

Ionospheric structure effects on HF radio wave propagation for the Enhanced Polar Outflow Probe (e‐POP) satellite mission

The Enhanced Polar Outflow Probe (e‐POP) payload will be launched on a small satellite in 2007 for exploring plasma and atmospheric outflow process in the polar region. The subject of this paper is whether one can determine the properties of large‐scale ionospheric structures by studying the perturbations that they cause on HF radio waves received at the e‐POP satellite from ground‐based transmitters such as the Canadian Advanced Digital Ionosonde (CADI). The perturbations on the received waves have been investigated using numerical ray‐tracing methods. These simulation results show that ionospheric irregular structures lead to a complex pattern of amplitude, propagation time delay, Doppler frequency, and direction‐of‐arrival (DOA) effects on the HF radio waves received at the satellite. The simulations also show that ionospheric density structure cannot be measured unambiguously using a single wave property. Therefore a “catalog” of HF signatures of typical ionospheric irregularities has been established in order to be able to interpret the e‐POP HF measurements in terms of ionospheric structures.

Evolution of near vertical incidence skywave communications and the Battle of Arnhem

The use of near vertical incidence skywaves (NVIS) in battlefield communications is now commonplace. Though not referred to as such until recently, the propagation of HF radio waves over short distances without the intervention of the skip zone is a natural consequence of the use of appropriate frequencies plus transmitting and receiving antennas that favour high angles of radiation. It has occasionally been suggested that the first dedicated use of NVIS techniques took place during conflicts in the 1960s, whereas evidence exists of its use during the D-Day landings of June 1944. However, wartime documents have recently come to light which show that the British Army Operational Research Group carried out dedicated research into this method of short-range HF communication at least a year earlier and released its reports containing operational recommendations in 1943, prior to the Battle of Arnhem.

Electron-Density Distribution in the Upper Ionosphere under Conditions of a Mid-Latitude F-Spread

We present the results of the last experiments aimed at studying the phenomenon of mid-latitude F-spread by radio-raying of the ionosphere using the signal from the SURA facility received onboard the NASA spacecraft WIND. A generalized model of spatio-temporal distribution of the electron density in the mid-latitude ionosphere under F-spread conditions is proposed. Based on this model describing large-scale ionospheric irregularities, we calculate the ray trajectories of HF radio waves. We also discuss the known results of comparative measurements of the variances of relative electron-density fluctuations in large-scale irregularities of the mid-latitude ionosphere, carried out by the OGO-6 spacecraft, and the frequency broadening of the ionograms of ground-based stations for the vertical sounding. It is shown that the proposed model of electron-density disturbances in the mid-latitude ionosphere in the presence of F-spread is able not only to describe well the main qualitative feature of HF radiowave propagation under disturbed geophysical conditions, but also to reproduce quite accurately the quantitative parameters of the frequency broadening of reflected HF signals under conditions of standard and developed F-spread.

HF radio wave attenuation due to a meteoric layer in the atmosphere of Mars

In the atmosphere of Mars, the effect of an ionospheric layer of meteoric origin on an HF radio wave propagation is investigated. We consider the putative magnesium ion layer which results from the ablation of sporadic meteors from a work recently published. To account for day‐night variability, two electron profiles are considered in an altitude range around 80 km. First we argue that there may be some observational evidence of this magnesium layer in early Mars observations that were not noticed before. Then we study the effect of this ionospheric layer on the attenuation of HF waves. For a propagation path through the ionosphere as envisaged for HF subsurface sounding on future Mars missions, the one‐way attenuation may range from up to 360 dB's at 1.8 MHz and up to 18 dB's at 9 MHz.

Propagation of HF radio waves over sloping coastal land

The field of a TM wave at a reception point over sloping coastal land, farther than a wavelength from the sea, has been calculated for a transmitting element located infinitely far away over a flat sea. The field obtained is normalized to that of a wave approaching the water/land boundary from infinity. A transmitting element placed both at some elevation angle and on the surface of the sea has been considered. In both instances, simple formulas providing fast computation of the field have been obtained by employing the integral equation approach formulated in terms of the normalized surface impedance. It has been found that the sloping land to some extent compensates for power losses in the ground and may even enhance the power density of high-frequency waves coming from low elevation angles. The expressions obtained remain valid for flat ground in place of sea. In this instance, the permittivity and conductivity of the sea should simply be substituted by those of the ground.

Ambiguity of the Reconstruction of Plasma Frequency Profiles from a Given Height-Frequency Characteristic and Their Discernibility for Oblique Propagation of HF Radio Waves in an Isotropic Ionosphere

We specify the formulation of the problem of reconstructing the plasma frequency height distribution from a given height-frequency characteristic (HFC) for a spherically symmetric isotropic ionosphere. We propose a numerical algorithm for solving the problem of correctly processing the region of near-critical frequencies of ionospheric layers. On this basis, we study the discernibility of plasma frequency profiles (PFPs) satisfying a given HFC when the properties of an obliquely propagating HF signal are analyzed.

Features of HF-Radiowave Propagation on Midlatitude and Subauroral Long-Distance Paths

We present experimental results on HF-radiowave propagation features for the midlatitude Khabarovsk--Nizhny Novgorod and subauroral Magadan--Nizhny Novgorod paths using chirp ionospheric sounding. The spatial correlation of the maximum observed frequency (MOF) is measured. It is found that at 07:00-14:00 MSK in quiet days with a magnetic-activity index Kp≤ 2 the correlation coefficient amounts to 0.75-0.95. With increase in disturbance for Kp≥ 3, the correlation coefficient decreases to about 0.65-0.8. In the evening and night hours (20:00-02:00 MSK), the MOF spatial correlation decreases considerably, and reverses sign in some cases, which can be an indication of the different mechanisms of HF-signal field formation on the paths considered. Signal characteristics as functions of the path orientation relative to the ionospheric trough are analyzed. It is shown that at 20:00-02:00 MSK, the Magadan-Nizhny Novgorod path passes near the northern wall and the Khabarovsk-Nizhny Novgorod path, near the southern wall of the trough. At that time, the HF-radiation propagation was implemented through standard hop modes on the mid-latitude path and through the high-angle ray (Pedersen mode) on the subauroral path. The signal diffusivity on the Magadan-Nizhny Novgorod path exceeds by about two or three times the signal diffusivity on the Khabarovsk-Nizhny Novgorod path. The experimental data are compared with the results of simulation of the signal diffusivity due to HF-radiowave scattering by the magnetic-field-aligned ionospheric irregularities located in the vicinity of the southern boundary of the auroral oval.

Ionosphere plasma holes-modeling and diagnostic

The influence of ionosphere plasma holes on HF radio wave propagation in approximation of geometrical optics is studied. The results of HF radio wave propagation modeling on routes crossing ionosphere holes are presented. The natural and modeling ionosphere sections are used (we use experimental radiotomography sections on routes: Moscow-Arkhangelsk, Moscow-Murmansk (Russia) and Block Iland-Rabiwhol (USA)). It is shown that characteristics of HF radiowave trajectory (in coordinate: height-latitude) depend on geometrical sizes of hole and the direction, point, angle and frequencies of sounding. It is shown that the asymmetry of electron density distribution results in qualitative and quantitative distinctions of waves trajectories characteristics in dependencies on direction propagation. The characteristic modes of HF radio wave propagation through hole are emphasized — one hop, captured by channel E-F, captured by hole, trapezifrom and so on. The higher interest presents the family of trajectories captured by hole. On estimations, the longest time-delay for different conditions makes up to 23 ms (or range up to 7000 km!). It is possible to say, that the hole in ionosphere can be the trap of HF radiowave. It is discussed conditions of capture and keeping of radio wave by ionospheric hole. The fact, that holes in top ionosphere are long-living and large-scale formations, indicates that disregard of radiowave propagation effects, connected with holes, will result in data distortion or misunderstanding of ionosphere monitoring and control by radiosounding in HF radiowave range.