High-latitude Paths Research Articles

Current frequency variation trends of dangerous winds in the Eastern sector of the Russian Arctic

The article aim is to identify mean values and change trends in the frequency of winds that pose a danger to navigation on shipping routes of the Laptev and East Siberian Seas which became evident in 2010–2021 period. In the period under review the winds were considered dangerous if their average hourly speed over the water surface at 10 m altitude exceeded 15 m/s. The factual material is based on information from the ERA5 global reanalysis. The research methodology involves the use of standard methods of mathematical statistics. A comparative analysis of the last two 12-year periods (1998–2009 and 2010–2021) has shown that the climate changes that happened in 2010–2021 significantly affected the wind regime of the studied region, which had consequences for shipping. In August, a significant decrease in the frequency of dangerous winds was recorded in the region, which generally contributed to the improvement of navigation safety by reducing storm risks. Opposite trends manifested themselves from September to December, which led to increased storm surge and a rise in the likelihood of ships icing, especially on the high-latitude paths of the Northern Sea Route and approaches to the Long Strait. The decrease in the frequency of winds, revealed in January and April, contributed to the preservation of a stable ice cover and stable difficult navigation conditions in these months. The persistence of the identified trends in interannual changes in the wind regime in the region under consideration in the future is not guaranteed. Therefore, the development of the existing network of Arctic weather stations remains an urgent problem for shipping.

Anomalous phenomena on HF radio paths during geomagnetic disturbances

We analyze ionospheric oblique sounding data on three high-latitude and one high-latitude–midlatitude HF radio paths for February 15 and 16, 2014, when two substorms and one magnetic storm occurred. We investigate cases of anomalous propagation of signals: their reflection from sporadic layer Es, lateral reflections, type “M” or “N” modes, the presence of traveling ionospheric disturbances, and the diffusivity of signals and triplets. The most significant results are the following. In geomagnetically undisturbed times, sporadic Es-layers with reduced maximum observed frequencies (MOFEs) on three high-latitude paths were observed in both days. The values of MOFEs during disturbances are large, which leads to the screening of other oblique sounding signals reflected from the ionosphere. On all four paths, the most frequently traveling ionospheric disturbances due to the terminator were observed in quiet hours from 03:00 to 15:00 UT on the first day and from 06:00 to 13:00 UT on the second day of the experiment. In addition, both the sunset terminator and the magnetic storm on the high-latitude–mid-latitude path were found to generate traveling ionospheric disturbances jointly. No such phenomenon was found on high-latitude paths.

Radial Component of the Super-Low-Frequency Magnetic Field of a Horizontal Electric Dipole at Low Conductivity of the Ground Along the Propagation Path

We study the super-low-frequency field of a grounded horizontal electric dipole on a high-latitude submeridional path having a length of up to 1000 km. Unlike most of the earlier works, this paper considers the case of wave propagation along the dipole axis, where the main component of the magnetic field is radial. The frequency dispersion of the ground conductivity under the source, which determines the coefficient of excitation of the Earth—ionosphere waveguide, is found from the measurements of the field at a distance of three heights of the ionosphere. Typical parameters of propagation of super-low-frequency waves are refined allowing for specific features of the path used in the experiment. The significance of allowing for ground conductivity as the radiation propagates along low-conductivity regions is demonstrated. The regions where one of the magnetic-field components is predominant are calculated in a polar coordinate system linked to the source, as well as the regions where the both components should be allowed for. The theoretical possibility of determining the phase velocity of the wave based on the ratios of the magnetic-field components measured at longer lengths is noted.

Влияние вспышек на Солнце в марте 2012 г. на профили проводимости высокоширотной нижней ионосферы

The ionospheric D-layer affects the electromagnetic waves propagated in the Earth – ionosphere waveguide. It is known that the propagation velocity of atmospherics – electromagnetic pulses from lightning discharge depends on the conductivity profile of the lower ionosphere. In this paper the authors have considered the influence of solar flares in March 2012 on the propagation velocity of atmospherics and thus the state of the high-latitude lower ionosphere. The possibility to estimate the conductivity profiles of the daytime ionosphere under disturbed and undisturbed geomagnetic conditions using the measurements of the propagation velocity of atmospherics along the high-latitude path has been demonstrated. Ключевые слова: волновод Земля – ионосфера, D-слой, скорость распространения, атмосферик, солнечная вспышка.

Modeling of HF propagation at high latitudes on the basis of IRI

The paper presents the results of comparison between the modeling calculations and ionograms of oblique sounding for high-latitude HF radio paths of Arctic and Antarctic Research Institute (AARI), which was fulfilled for February 13–14, 2014 (quiet conditions). The International Reference Ionosphere 2012 model of the ionosphere (IRI-2012) was used for the study. The comparison results prove that without adaptation to current diagnostics the IRI model does not reflect the real state of high latitude ionosphere even for quiet conditions. It was found that in general the observed maximum usable frequency values (MUF) exceeded the same values obtained from the model. The adaptation of the model to current diagnostics makes the simulated MUF values significantly closer to the observed MUF. The following parameters were used for the study: critical frequencies foF2 measured by ionosondes located near the considered paths, frequencies calculated on the basis of observed TEC values and median values of the equivalent slab thickness of the ionosphere.The relative error of calculation of MUF values averaged for all the cases for one hop was 23.6% by the initial IRI model. This error was decreased by 4% for the calculations on the basis of observed ТЕС and by 6% for the adaptation to foF2. The higher the latitude of the studied radio path, the more the difference between the observed and simulated MUF values. The conclusion was made that a principal cause of this difference was the deviation of calculated maximum ionospheric height values (hmF2) from the observed hmF2. The additional model update using hmF2 values obtained from Tromso station let to better match between the calculated MUF values and the observed MUF values for all radio paths. The analysis of experimental data showed that the non-predicted events (like traveling ionospheric disturbances, M- and N-modes, lateral modes, triplets, unusual scatter effects, etc.) sometimes took place at high latitude paths even during the quiet conditions.

Sub-ionospheric VLF signal anomaly due to geomagnetic storms: a statistical study

Abstract. We investigate quantitatively the effect of geomagnetic storms on the sub-ionospheric VLF/LF (Very Low Frequency/Low Frequency) propagations for different latitudes based on 2-year nighttime data from Japanese VLF/LF observation network. Three statistical parameters such as average signal amplitude, variability of the signal amplitude, and nighttime fluctuation were calculated daily for 2 years for 16–21 independent VLF/LF transmitter–receiver propagation paths consisting of three transmitters and seven receiving stations. These propagation paths are suitable to simultaneously study high-latitude, low-mid-latitude and mid-latitude D/E-region ionospheric properties. We found that these three statistical parameters indicate significant anomalies exceeding at least 2 times of their standard deviation from the mean value during the geomagnetic storm time period in the high-latitude paths with an occurrence rate of anomaly between 40 and 50 % presumably due to the auroral energetic electron precipitation. The mid-latitude and low-mid-latitude paths have a smaller influence from the geomagnetic activity because of a lower occurrence rate of anomalies even during the geomagnetically active time period (from 20 to 30 %). The anomalies except geomagnetic storm periods may be caused by atmospheric and/or lithospheric origins. The statistical occurrence rates of ionospheric anomalies for different latitudinal paths during geomagnetic storm and non-storm time periods are basic and important information not only to identify the space weather effects toward the lower ionosphere depending on the latitudes but also to separate various external physical causes of lower ionospheric disturbances.

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.

Propagation of HF radiowaves on high-latitude radio paths during the magnetically disturbed period of October 24–28, 2003

The behavior of the HF signal parameters during magnetic storms and substorms has bee experi� mentally studied simultaneously on the Kiruna-Kirkenes auroral path, Kiruna-Longyearbyen polar path, and Murmansk-St. Petersburg subauroral path. The first two paths are equipped with the instruments making it possible to measure the values of the signaltonois e ratio, Doppler frequency shift, and elevation angle. The method of oblique sounding of the ionosphere (OSI) was used on the Murmansk-St. Petersburg path. Two substantial substorms, a moderate storm, and an intense storm occurred during the studied period. Some new regularities have been revealed. On the Kiruna-Kirkenes and Kiruna-Longyearbyen paths, the signal� tonoise ratio increased (due to the transition from the F2 signal reflections to the Es reflections), the elevation angle increased (due to an increase in the ionospheric F2 layer height and a decrease in the critical frequency), and the Doppler shift increased (due to the variations in ionization and the appearance of ionospheric irreg� ularities during a substorm) when the signal was reflected from the F 2 layer close to the moment of the sub� storm or storm beginning T0. It is possible to control the socall ed "main effect" in the ionosphere on the Murmansk-St. Petersburg path.

Effects of a “day-time” substorm on the ionosphere and radio propagation

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.

Non-great-circle propagation modes on the high-latitude HF radio path

We consider characteristics of the non-great-circle signals on the Murmansk—St. Petersburg oblique sounding path with a distance of 1050 km. The path crosses the main ionospheric trough and is highly influenced by the trough’s polar wall and the boundary of diffuse precipitations. Field-aligned ionospheric irregularities and ionization gradients are often observed in this area. The data analysis was performed for quiet, moderately disturbed, and strongly disturbed conditions and for different local times. The main results are as follows. Non-great-circle signals in the form of scattered waves mostly occur in the nighttime hours. The nighttime non-great-circle signals always appear under both quiet and disturbed conditions, while the signals reflected from the ionization gradients appear at any local time during the disturbed periods (storms, substorms). Non-great-circle signals with intense reflections from the ionization gradients are most often observed during a moderate disturbance, especially in the nighttime hours. The ray-tracing model of non-great-circle propagation shows that the calculated oblique sounding ionograms generally coincide with the experimental ones.

Teleconnection between NAO and Climate Downstream of the Tibetan Plateau

Abstract Analysis of the monthly NCEP–NCAR reanalysis data and station data reveals a teleconnection pattern (NAULEA) that links climate changes over the North Atlantic and Eurasia. The NAULEA pattern has five action centers. It extends from the North Atlantic to northwestern Europe, and then stretches eastward to the Urals, with its eastern end over North China. Certain climate changes over East Asia, such as the cooling changes in both the upper troposphere and surface in the last few decades of the twentieth century, can be attributed to the NAULEA pattern anomalies and traced upstream to the North Atlantic. The NAULEA pattern is suggested to be another NAO-related teleconnection pattern. Compared with the pattern with the Asian jet waveguide path, which leads to temperature anomalies over northeastern Asia, the NAULEA pattern with a high-latitude path exerts stronger influences on the climate of southwestern China.

Space weather effects on radio propagation: study of the CEDAR, GEM and ISTP storm events

Abstract. The impact of 14 geomagnetic storms from a list of CEDAR, GEM and ISTP storms, that occurred during 1997–1999, on radio propagation conditions has been investigated. The propagation conditions were estimated through variations of the MOF and LOF (the maximum and lowest operation frequencies) on three high-latitude HF radio paths in north-west Russia. Geophysical data of Dst, Bz, AE as well as some riometer data from Sodankyla observatory, Finland, were used for the analysis. It was shown that the storm impact on the ionosphere and radio propagation for each storm has an individual character. Nevertheless, there are common patterns in variation of the propagation parameters for all storms. Thus, the frequency range Δ=MOF−LOF increases several hours before a storm, then it narrows sharply during the storm, and expands again several hours after the end of the storm. This regular behaviour should be useful for the HF radio propagation predictions and frequency management at high latitudes. On the trans-auroral radio path, the time interval when the signal is lost through a storm (tdes) depends on the local time. For the day-time storms an average value tdes is 6 h, but for night storms tdes is only 2 h. The ionization increase in the F2 layer before storm onset is 3.5 h during the day-time and 2.4 h at night. Mechanisms to explain the observed variations are discussed including some novel possibilities involving energy input through the cusp.

Interpretation of experimental data on HF propagation on the St. Petersburg-Spitsbergen path

The experimental ionograms of the oblique-incidence sounding of the ionosphere, obtained on the St. Petersburg-Spitsbergen high-latitude HF radio path during the magnetically quiet period December 14–15, 2001, are compared with the model calculations of radiowave trajectories. For this purpose, the corresponding oblique-incidence ionograms are numerically synthesized using the technique based on the shooting method and the computer program for constructing HF radiowave trajectories. The three-dimensional electron density distribution, calculated using the mathematical model of the high-latitude ionosphere previously developed at the Polar Geophysical Institute (PGI), is used to model radio propagation. The numerical calculations make it possible mainly to explain the specific features of the experimental data on the oblique-incidence sounding of the ionosphere.

Observations by the CUTLASS radar, HF Doppler, oblique ionospheric sounding, and TEC from GPS during a magnetic storm

Abstract. Multi-diagnostic observations, covering a significant area of northwest Europe, were made during the magnetic storm interval (28–29 April 2001) that occurred during the High Rate SolarMax IGS/GPS-campaign. HF radio observations were made with vertical sounders (St. Petersburg and Sodankyla), oblique incidence sounders (OIS), on paths from Murmansk to St. Petersburg, 1050 km, and Inskip to Leicester, 170 km, Doppler sounders, on paths from Cyprus to St. Petersburg, 2800 km, and Murmansk to St. Petersburg, and a coherent scatter radar (CUTLASS, Hankasalmi, Finland). These, together with total electron content (TEC) measurements made at GPS stations from the Euref network in northwest Europe, are presented in this paper. A broad comparison of radio propagation data with ionospheric data at high and mid latitudes, under quiet and disturbed conditions, was undertaken. This analysis, together with a geophysical interpretation, allow us to better understand the nature of the ionospheric processes which occur during geomagnetic storms. The peculiarity of the storm was that it comprised of three individual substorms, the first of which appears to have been triggered by a compression of the magnetosphere. Besides the storm effects, we have also studied substorm effects in the observations separately, providing an improved understanding of the storm/substorm relationship. The main results of the investigations are the following. A narrow trough is formed some 10h after the storm onset in the TEC which is most likely a result of enhanced ionospheric convection. An enhancement in TEC some 2–3 h after the storm onset is most likely a result of heating and upwelling of the auroral ionosphere caused by enhanced currents. The so-called main effect on ionospheric propagation was observed at mid-latitudes during the first two substorms, but only during the first substorm at high latitudes. Ionospheric irregularities observed by CUTLASS were clearly related to the gradient in TEC associated with the trough. The oblique sounder and Doppler observations also demonstrate differences between the mid-latitude and high-latitude paths during this particular storm. Keywords. Ionosphere (Ionospheric disturbances) – Magnetospheric physics (Storms and substorms) – Radio science (Ionospheric propagation)

Magnetic storm effects on a HF transauroral radio path

[1] Geophysical data obtained from data bases of the Internet are collated with radio propagation parameters on the high-latitude HF path Heiss Island–St. Petersburg (D = 2500 km) for the intensive magnetic storm occurred on 14–17 May 1997. Main attention was given to variations of the operating frequency range of the maximum and the lowest observed frequencies (MOF–LOF). This range was shown to bring some valuable information about ionospheric dynamic. Acquisition of diagnostic data such as the MOF and the LOF values is actually the method of the oblique ionospheric sounding that works properly during magnetic storm periods as opposed to the method of vertical sounding. Specific regularities are revealed for frequency range variations that are substantiated from the physical point of view. Some recommendations are given for radio communications, for example, on operating frequencies selection during a disturbed period in particular

Directional diversity of HF signals received over high latitude paths and the possibility of improved data throughput by means of spatial filtering

The high latitude ionosphere is a dynamic propagation medium in which HF radio signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and in elevation. Furthermore, ionospheric movements at the reflection points impose Doppler shifts and Doppler spreads onto the signals. Large Doppler spreads and multipath propagation in which the delay spread is large are particularly prevalent at these northerly latitudes and can result in a significant degradation in the performance of digital communication systems. This investigation suggests that the application of spatial filtering from simple antenna arrays comprising pairs or triplets of antennas can significantly reduce the level of apparent Doppler spreading, frequently to levels at which modern, high speed modems are expected to operate satisfactorily. The investigation concentrated on Doppler spread since this was the most significant factor in the data employed in this study; however, it is noted that reductions in effective multipath delay spread are also possible.

Behaviour of super-resolution direction finding algorithms for HF signals propagating through the high latitude ionosphere

The performance of high frequency direction finding systems is known to be related to the frequency of operation and to the propagation mode content of the incoming signal, which is dependent upon the electron density distribution in the ionosphere. In addition to large scale tilts that cause gross deviations of the signal from the great circle direction, irregularities in the ionospheric electron density distribution may be considered as providing a rough reflecting surface for HF radio waves. As a result, signals associated with each propagation mode arrive at the receiver over a range of angles in both azimuth and elevation. Measurements are presented of HF radio signals received over a disturbed, high latitude path. Multiple, closely separated traces were apparent in the time history of the calculated bearings, the precise nature of which depends upon the array geometry and algorithm employed. The presence of these multiple traces is attributed to the behaviour of the DF algorithms when the signals arrive over a range of azimuth and elevation angles due to the rough reflector nature of this disturbed region of the ionosphere.

Observations of the reduction in the available HF band on four high latitude paths during periods of geomagnetic disturbance

Abstract Four high frequency propagation paths were provided by a transmitter located within the polar cap and four receivers located variously within the polar cap and at sub-auroral latitudes. Of these paths, one was contained entirely within the polar cap at all times, two were trans-auroral at all times, and one varied from trans-auroral during the day to polar cap during the night. Fourteen frequencies within the HF band were transmitted each hour for the duration of two 24 day experimental campaigns during the summer of 1988 and the winter of 1989. The received signals were analysed to determine signal recognition and signal strength. The mean quiet-time behaviour of the propagation was determined and compared with four periods during which the ionosphere was perturbed by (relatively mild) geomagnetic storm conditions. The daily occurrence of correctly recognised signals decreased during the disturbed periods, both on polar cap and trans-auroral paths. Several geophysical causes for this decrease are identified, including a decrease in F-region electron density, decreasing the MOF, as a consequence of negative storm effects and an extended mid latitude trough. On polar cap paths, an increase in the LOF caused by mild PCA events also decreased the available HF band. Storm-time auroral-E makes propagation possible on frequencies higher than expected during the night on trans-auroral paths, mitigating to some extent the degradation in propagation. The storm-time decrease in the available HF band is quantified, falling to 0.3–0.5 of undisturbed levels even during relatively small geomagnetic storms. During the two experimental campaigns, 50% of days are identified as being storm-disturbed.

Observations of the directional characteristics of ionospherically propagated HF radio channel sounding signals over two high latitude paths

The author presents measurements of the directional characteristics of the Doppler and time delay components of a pulsed channel sounding signal received over two high latitude paths. A variation in bearing with Doppler frequency was frequently evident, an effect attributed to Doppler shifts imposed on the signal when scattered from ionospheric irregularities drifting across the reflection points. For a transauroral path, for which the ionospheric reflection points were subauroral, standard deviations of up to ~ 2.5° were observed in the azimuthal power distribution of the received energy. Much greater disturbances with azimuthal standard deviations of up to ~35° were recorded on signals received over a path contained entirely within the polar cap path.

Observations of Doppler spreading on HF signals propagating over high latitude paths

The high latitude ionosphere forms a highly dynamic and disturbed medium which can severely degrade the performance of HF radio systems when the signals propagate through these regions. One important parameter is the Doppler frequency spreading imposed on the signal by the movement of electron density irregularities in the ionosphere at the reflection points. The magnitude of the Doppler spreading has been quantified in a series of experiments undertaken on a range of frequencies across the HF band over several high latitude paths. Two of these paths are considered in this paper, one of which (Clyde River – Alert, 1345 km) was contained entirely within the polar cap whilst the second (Clyde River – Prudhoe Bay, 2955 km) was sometimes entirely within the polar cap and at other times crossed the auroral oval. Results obtained over two one-month periods, one in summer (July/August 1988) and one in winter (January/February 1989), are presented.