High Frequency Radio Wave Propagation Research Articles

High‐Latitude Off‐Great Circle Propagation Associated With the Solar Terminator

AbstractLarge‐scale ionospheric gradients associated with the solar terminator can deflect high frequency (HF) radio waves to off‐great circle paths during the morning and evening, negatively impacting technologies reliant on HF radio wave propagation. For example, geolocation algorithms used by scientific and military over‐the‐horizon radars (OTHRs) generally assume on‐great circle propagation, and thus lateral deviations from the great‐circle path can lead to positioning errors. In this study, radio wave propagation is simulated via 3D numerical ray traces though an empirical, high‐latitude model ionosphere initialized for a variety of times of the day and year to explore and quantify high‐latitude off‐great circle propagation associated with the solar terminator. Analysis of these simulations show large scale east‐west ionospheric gradients due to the solar terminator can cause lateral deviations in north‐directed propagation paths exceeding 20° at sunrise and sunset depending on radio wave frequency, though the largest portion of received signal power tends to experience maximum deflections of 5°. An exploration of the dependence of propagation direction on deflection shows that propagation paths parallel to the solar terminator tend to experience the largest deflections. Since the solar terminator at high latitudes is at an angle with respect to north in the winter and summer, propagation paths oriented west or east of north can experience larger deflections than north oriented paths at sunrise and sunset during these times of year. Impacts of these diurnal deflections on the operation of OTHR and scientific radar are discussed, as well as possible strategies for mitigating them.

Duration and extent of solar X-ray flares and shortwave fadeouts likely to impact high frequency radio wave propagation based on an evaluation of absorption at 30 MHz

High frequency (HF; 3–30 MHz) radio wave propagation can be impacted by absorption that results from enhanced photoionization in the dayside D-region following a solar X-ray flare. A database of >25,000 solar X-ray flares was evaluated to characterize the relationship between flare duration and the peak of the 0.1–0.8 nm solar X-ray flux. Expressions describing the mean and 90th percentile duration were developed. Based on these models, mean durations of 13, 18, 27, and 39 min and 90th percentile durations of 30, 48, 77, and 123 min are expected for C1, M1, X1 and X10 solar X-ray flares, respectively. A probability distribution of flare durations was developed to describe the probability of flare duration lasting 0–15, 15–30, 30–45, 45–60, 60–90, >90 min. In addition to flare duration, the duration of the expected impact to HF radio waves was evaluated. By considering examples where HF radio wave propagation in the 5–15 MHz range was impacted by space weather, a 0.5 dB threshold at 30 MHz was observed in samples of riometer data. Absorption modelled at 1-min increments from 1986 to 2017 was evaluated to create a probability distribution of impact duration, defined as the length of time the modelled 30 MHz absorption exceeded 0.5 dB during a single event. Modelled absorption was further evaluated to demonstrate the geographic extent of enhanced absorption, and to determine the minimum solar X-ray flux required to exceed the 0.5 dB impact threshold at a given latitude as a function of solar zenith angle and time of year. The results of this paper provide a better understanding of the impact of solar X-ray flares on high frequency radio wave propagation and aid in the development of tools and services for mitigating space weather impacts to systems that rely on HF radio wave propagation.

Simulation Research on the Influence of Plasma Bubbles on Radio Wave Propagation

AbstractPlasma 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.

Evaluation of the effect of sporadic-E on high frequency radio wave propagation in the Arctic

High Frequency (HF) radio propagation, and applications such as Over-The-Horizon Radar (OTHR), is sensitive to ionospheric disturbances caused by space weather. Improved ionospheric modelling and monitoring techniques for the high-latitude and polar regions supports high quality OTHR long-range surveillance. One such ionospheric disturbance is Sporadic-E, a phenomenon in which a thin enhancement in E-region (approximately 90–150 km altitude) electron density acts as a strong reflector of HF radio waves. In this study, we perform a case study of the effect a sporadic-E layer has on HF radio propagation for a layer that was detected over Eureka on July 11, 2012. We study this event using HF radio receiver measurements for a path intersecting the layer, simultaneous ionosonde measurements of the layer, and a series of ray traces through a model ionosphere containing a model of the sporadic-E layer. Utilizing these measurements and simulations, we show how sporadic-E can aid HF radio propagation in some cases, and show that a simple Gaussian sporadic-E model can replicate real HF radio measurements. We also comment on how sporadic-E could affect OTHR operation.

Characterization of high latitude radio wave propagation over Canada

High frequency (HF) radio wave propagation is sensitive to ionospheric disturbances caused by space weather. Changes in propagation conditions can be directly measured with an HF transmitter/receiver link. This paper presents data from one such link, consisting of a transmitter located in Ottawa, ON, Canada, and a directional receiver located in Alert, NU, Canada. The transmitter emits signals at 6 distinct frequencies between 5.4 and 14.4 MHz on a set schedule that are detected and processed by the receiver. Based on 644 days of archived data, a statistical analysis characterizing HF radio wave propagation conditions over the course of the day and over the year are presented. HF propagation is found to follow the regular diurnal variation in ionospheric density, modulated by seasonal trends. A new method for distinguishing space weather effects from diurnal changes to HF propagation is presented, which utilizes quiet day baselines, similar to riometer quiet day curves. This method allows for easy analysis of the impacts of space weather on HF radio propagation between two points. This method is applied to an auroral absorption event that occurred on 06 June 2016, and is used to successfully distinguish auroral absorption related dropouts from dropouts related to daily ionospheric variation.

Features of High-Frequency Radio Wave Propagation on Auroral Paths at Different Geomagnetic Activity

Variations in the parameters of the propagation of decameter waves on two auroral radio paths, Dikson–Pevek (D = 2800 km) and Lovozero–Pevek (D = 4200 km), were studied via oblique ionospheric sounding for two days, one weakly disturbed and one strongly disturbed. The following parameters were analyzed: F2 MOF and Es MOF (the maximum observed frequencies when the signal is reflected from the F2 layer and the Es layer), F2Spread (diffuse reflection), and lateral reflections. The following new results have been obtained: the F2 MOF level for the strongly disturbed day is higher than that for the weakly disturbed day on both paths; however, the F2 MOF values during substorms are either small or absent due to increased absorption. The Es MOF values on the Dikson–Pevek path increase during the period of disturbances. As for the Lovozero–Pevek path, the probability of Es reflections decreases during the disturbed day. The diffusion in the F2 layer on the Dikson–Pevek path is mostly typical for a strongly disturbed period, while that on the Lovozero–Pevek path, conversely, is mostly typical for weakly disturbed conditions. Lateral reflections on the Dikson–Pevek path occur rather rarely. On the Lovozero–Pevek path, there are very few lateral reflections on the weakly disturbed day and no lateral reflections on the strongly disturbed day.

A regional model for the prediction of M(3000)F2 over East Asia

Research on empirical or physical models of ionospheric parameters is one of the important topics in the field of space weather and communication support services. To improve the accuracy of predicting the monthly median ionospheric propagating factor at 3000 km of the F2 layer (identified as M(3000)F2) for high frequency radio wave propagation, a model based on modified orthogonal temporal–spatial functions is proposed. The proposed model has three new characteristics: (1) The solar activity parameters of sunspot number and the 10.7-cm solar radio flux are together introduced into temporal reconstruction. (2) Both the geomagnetic dip and its modified value are chosen as features of the geographical spatial variation for spatial reconstruction. (3) A series of harmonic functions are used to represent the M(3000)F2, which reflects seasonal and solar cycle variations. The proposed model is established by combining nonlinear regression for three characteristics with harmonic analysis by using vertical sounding data over East Asia. Statistical results reveal that M(3000)F2 calculated by the proposed model is consistent with the trend of the monthly median observations. The proposed model is better than the International Reference Ionosphere (IRI) model by comparison between predictions and observations of six station, which illustrates that the proposed model outperforms the IRI model over East Asia. The proposed method can be further expanded for potentially providing more accurate predictions for other ionospheric parameters on the global scale.

Studying the Parameters of Frequency Dispersion for Radio Links of Different Length Using Software‐Defined Radio Based Sounding System

AbstractMethods and relevant software were developed to study frequency dispersion parameters of wideband high‐frequency (HF) radio channels over propagation paths of different lengths: near vertical incidence skywave paths (up to 500 km) and long‐distance radio paths (from 1,000 up to 3,500 km). We developed a universal ionosonde (in terms of methods and signals) to do experimental research into HF radio wave propagation and dispersion using software‐defined radio technology and platform Universal Software Radio Peripheral N210. The multipurposeness of the ionosonde is manifested in the sounding methods used (oblique incidence and vertical incidence) as well as in the spread spectrum signals used (binary phase shift keying, frequency modulated continuous wave, and frequency modulated interrupted continuous wave). The characteristics of the megahertz‐bandwidth HF channels were studied. The dependences of the variation of the third‐order dispersion parameter on the time of day, the radio path length, and the channel carrier frequency were experimentally found.

The 6 September 2017 X-Class Solar Flares and Their Impacts on the Ionosphere, GNSS, and HF Radio Wave Propagation.

On 6 September 2017, the Sun emitted two significant solar flares (SFs). The first SF, classified X2.2, peaked at 09:10 UT. The second one, X9.3, which is the most intensive SF in the current solar cycle, peaked at 12:02 UT and was accompanied by solar radio emission. In this work, we study ionospheric response to the two X‐class SFs and their impact on the Global Navigation Satellite Systems and high‐frequency (HF) propagation. In the ionospheric absolute vertical total electron content (TEC), the X2.2 SF caused an overall increase of 2–4 TECU on the dayside. The X9.3 SF produced a sudden increase of ~8–10 TECU at midlatitudes and of ~15–16 TECU enhancement at low latitudes. These vertical TEC enhancements lasted longer than the duration of the EUV emission. In TEC variations within 2–20 min range, the two SFs provoked sudden increases of ~0.2 TECU and 1.3 TECU. Variations in TEC from geostationary and GPS/GLONASS satellites show similar results with TEC derivative of ~1.3–1.7 TECU/min for X9.3 and 0.18–0.24 TECU/min for X2.2 in the subsolar region. Further, analysis of the impact of the two SFs on the Global Navigation Satellite Systems‐based navigation showed that the SF did not cause losses‐of‐lock in the GPS, GLONASS, or Galileo systems, while the positioning error increased by ~3 times in GPS precise point positioning solution. The two X‐class SFs had an impact on HF radio wave propagation causing blackouts at <30 MHz in the subsolar region and <15 MHz in the postmidday sector.

Extension of Hybrid Scintillation Propagation Model to the case of field propagation in the ionosphere with highly anisotropic irregularities

AbstractThe detailed effects of the high‐frequency radio wave propagation in a stochastic transionospheric propagation channel are discussed. This is a special case of propagation along or almost along the lines of the Earth's magnetic field. In this case, the ionospheric random irregularities of the electron density of the ionosphere may have very high values of the aspect ratio, which stands beyond the range of validity of the traditional analytical approaches utilized to treat the appropriate transionospheric propagation problems. In the consideration presented here, new analytical results are obtained for treating the problem under consideration. Based on the analytical results obtained, the previously developed Hybrid Scintillation Propagation Model is further extended, which also includes the software simulator of the signals propagating on the transionospheric paths with the random electron density irregularities highly elongated in the direction of the Earth's magnetic field.

First results of HF radio science with e‐POP RRI and SuperDARN

AbstractThe first results from coordinated experiments between the Radio Receiver Instrument (RRI) on the Enhanced Polar Outflow Probe (e‐POP) and the Super Dual Auroral Radar Network (SuperDARN) Saskatoon high frequency (HF) radar are examined for a conjunction on 8 July 2014. e‐POP, a payload on the CAScade, Smallsat and IOnospheric Polar Explorer spacecraft, was located at 380 km altitude, approximately 10° north (geographic) and 2° west of Saskatoon, Canada, moving in a southeast direction. We use a matched filter technique to extract individual received SuperDARN pulses from the RRI data stream. The pulses show characteristics of propagation through the F region ionosphere: they are heavily dispersed, they show significant pulse‐to‐pulse variability in magnitude, and there is clear evidence that they experienced multipath propagation. We calculate the polarization parameters of the pulses and use them to identify magnetoionic phenomena such as mode‐splitting and single‐mode fading. These first RRI results provide compelling insight into HF radio wave propagation and show RRI's potential to significantly advance radio science.

Influence of X-ray and polar cap absorptions on vertical and oblique sounding ionograms on different latitudes

High frequency (HF) radio band is important for the long-range communications and over-the-horizon surveillance, particularly in the polar cap region where the ground infrastructure may be limited. However, the space weather events drastically affect the high frequency radio wave propagation so that the ability to provide now-casting and forecasting of HF radio wave absorption is important for users relying on the HF communications. During the space weather events such as solar proton events and X-ray flares the high-latitude ionosphere becomes a particularly efficient absorber of HF radio waves. There is therefore a need to develop accurate HF propagation prediction services. Absorption of the HF field caused by the X-ray flux, Solar Ultra-Violet flux and proton precipitations is investigated in this paper for the event of the solar flare observed on 11 April 2013. The effects of the X-ray flux and proton precipitations on the structure of the vertical and oblique ionograms for different latitudes are estimated. The simulation of the vertical and oblique ionograms was performed on the basis of the computational model of the ionosphere oriented to applications into the high frequency wave propagation problems. The absorption effects induced by the proton precipitations and X-ray flux are calculated according to the algorithm elaborated by Sauer and Wilkinson and D-region Absorption Model (D-RAP) available from the NOAA Space Weather Prediction Center. The simulated vertical and oblique ionograms with the absorption effects taken into account and the measured ionograms exhibit a fairly good similarity.

Nathaniel C. Gerson (1915–2002)

Nathaniel C. (“Nate”) Gerson, an ionospheric physicist well known internationally for his expertise in high frequency radio wave propagation, died at his home in Millersville, Maryland, 11 January 2002. Gerson served on the AGU Special Committee for AGU/IGY Relationships in 1957, and was also active in AGU efforts at that time to improve the content and usefulness of the Transactions. He had been a member since 1946 (Space Physics and Aeronomy). He was 86 years old.

Studies of High-Frequency Radio Wave Propagation

Studies of multiple signals of high frequency have been made upon a quantitative basis with reference not only to the round-the-world signals (sometimes called echo signals) but to nearby echoes which have a very much shorter time of arrival. A method of predicting in advance the the likelihood of round-the-world echoes occurring between any two different stations has been worked out. Further studies of nearby echoes show a remarkable retardation on very high-frequency signals coming from the Rocky Point stations to Washington, these signals traveling an actual distance varying from 2900 kms. to over 10,000 kms., although the great circle distance is only 420 kms. The apparent violation of the skip distance law by these stations as observed in Washington has been explained. The nearby echo signals have been tentatively assumed to be due either to reflections from a heavily ionized region in the neighborhood of the magnetic poles or more likely to be due to scattered reflections thrown backwards from the first and second zones of reception, which follow the skip distance region. This throwing back of the signal thus permits under certain conditions the reception of the signal on very high frequency within what is really the skipped zone, the signal having entered this zone by a very indirect route and with a considerable time retardation as compared with the direct route. Influence of both nearby and round-the-world echo signals upon various types of radio communication have been briefly discussed.