Maximum Observable Frequency Research Articles

Comparison and validation of MOF observations and MUF predictions from seven different models

The prediction method of MUF (Maximum Usable Frequency) has been extensively deliberated as a pivotal technology in HF (High Frequency) communication. To verify the reliability of MUF prediction models commonly used in HF communications, we evaluated the prediction accuracy of seven typical prediction models based on maximum observed frequency (MOFs) and predicted MUFs. The oblique-sounding (OS) circuit was from Changchun to Jingyang, with the vertical-sounding (VS) station situated in Beijing which is, approximately at the midpoint of this oblique-sounding (OS) circuit. The RMSE (root mean square error) and RRMSE (relative root mean square error) between the MOFs and predicted MUFs by the model were used to evaluate the accuracy of the models. The results of statistical analysis show that: (a) the prediction results of the seven models are consistent with the observed results in trend, among which the prediction results of the Lockwood model (LWM), the nonuniform ionospheric conditions MUF calculation model (NYM), and the model proposed by Istituto Nazionale di Geofisica e Vulcanologia (INGV) are in good agreement with the observed results; (b) In the four seasons, the seasonal distribution of the seven models is significantly different, and the INGV model has a better prediction effect; (c) The INGV model is superior to other models in relatively high solar activity epochs and relatively low solar activity epochs, followed by the LWM model, with little difference between the two models and close prediction accuracy; (d) The INGV model has a better prediction result in four different periods of a day, and the improved simplified MUF calculation model (MSPM) has a great prediction result in the periods of nighttime.

Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication Based on Oblique- and Vertical-Incidence Sounding Data

As one of the key technologies of HF communication, the maximum usable frequency (MUF) prediction method has been widely discussed. To experimentally confirm the reliability of commonly used MUFs prediction models for high-frequency communication, we have compared maximum observed frequencies (MOFs) and predicted MUFs to assess the accuracy of two typical prediction models. The root-mean-square error (RMSE) and relative RMSE (RRMSE) between oblique sounding MOFs and the predicted MUFs were used to assess the model’s accuracy. The oblique sounding path was from Changchun to Jinyang, and the vertical-sounding ionosonde was located in Beijing, which was approximately the midpoint of the oblique sounding circuit. The statistical analysis results show that: (a) the trend of prediction results from the Lockwood and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) model are in good agreement with the observations: the mean RMSE and RRMSE of the INGV model are less than those of the Lockwood model; (b) in the four different periods (sunrise, daytime, sunset, and nighttime) of the whole day, the maximum difference of RMSE between the Lockwood and INGV model is 0.14 MHz (the INGV performs better than the LWM), with the corresponding differences of RRMSE being 0.31% at sunrise and 0.68% at daytime; (c) in the four seasons of spring, summer, autumn, and winter, the minimum RMSE values of the Lockwood and INGV models are 1.51 MHz and 1.37 MHz, respectively, which are obtained in winter, and the corresponding RRMSEs are 11.47% and 11.79%, respectively; (d) in the high and low solar activity epochs, the mean RMSEs of the Lockwood and INGV models are 1.63 MHz, and 1.54 MHz, with corresponding mean RRMSE values of 11.47% and 11.55%. In conclusion, the INGV model is more suitable for MUF prediction over Beijing and its adjacent mid-latitude regions from the RMSE comparison of the two models.

A Method for Automatic Inversion of Oblique Ionograms

In this study, a method is proposed to carry out automatic inversion of oblique ionograms to extract the parameters and electron density profile of the ionosphere. The proposed method adopts the quasi-parabolic segments (QPS) model to represent the ionosphere. Firstly, numerous candidate electron density profiles and corresponding vertical traces were, respectively, calculated and synthesized by adjusting the parameters of the QPS model. Then, the candidate vertical traces were transformed to oblique traces by the secant theorem and Martyn’s equivalent path theorem. On the other hand, image processing technology and characteristics of oblique echoes were adopted to automatically scale the key parameters (the maximum observable frequency and minimum group path, etc.) from oblique ionograms. The synthesized oblique traces, whose parameters were close to autoscaled parameters, were selected as the candidate traces to produce a correlation with measured oblique ionograms. Lastly, the proposed algorithm searched the best-fit synthesized oblique trace by comparing the synthesized traces with oblique ionograms. To test its feasibility, oblique ionograms were automatically scaled by the proposed method and these autoscaled parameters were compared with manual scaling results. The preliminary results show that the accuracy of autoscaled maximum observable frequency and minimum group path of the ordinary trace of the F2 layer is, respectively, about 91.98% and 86.41%, which might be accurate enough for space weather specifications. It inspires us to improve the proposed method in future studies.

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 Simplified Sounding System for Finding NVIS Channel Availability to Support Government Radio Networks in Indonesia

Mostly natural disasters in Java Island such as landslides are within the vicinity of not more than 200 Km from the district capital. Cellular communications require complex systems and rather vulnerable  to cope with disasters. NVIS mode is considered as a simple radio link during disaster mitigation initiation process. It needs a valid estimation to figure out the condition of the ionosphere. There are two purposes of this study, the first of which is an attempt to find out a fact the existences of authorized HF users who still work in the band of 3 MHz – 10 MHz. The second is to integrate low cost HF radio communication, commonly available small single board computer hardware, and opensource software, to build a sounding system to evaluate the quality of NVIS channels. Prediction system such VOACAP give hourly prediction data, however it has an inherent limitation because of  nature the underlying databases is monthly average based, therefore, the estimation could not be made in a daily bases. However, a real-time channel evaluation (RTCE) able to purify maximum observed frequency (MOF) estimation, and consequently, its able to select the best available frequency for short term and real time operation. In this study, we used WSPR to perform a simple RTCE technique. Furthermore, we also reviewed the current regulatory status regarding the availability of sub-10 MHz band for NVIS radio operation. The results show that discrepancies between simulation and measurement are occurred mainly because of sporadic data in the band of 60m and 80m. However, all of the measurement results and simulations almost have the same agreement regarding the quiet period between local midnight and local sunrise. The results of measurements show that 60m band is the most reliable NVIS channel between local sunrise and local midnight. Furthermore, 100 watts is a proper transmitter power to reach the required SNR for reliable voice communication.Â

Observed and Calculated Ionograms of Oblique Ionospheric Sounding on HF Radio Paths During a Magnetic Storm of September 7–8, 2017

Ionograms of the oblique ionospheric sounding radio paths Cyprus—Lovozero and Lovozero—Gor’kovskaya (near St. Petersburg) are analyzed using magnetometers and riometers. The behavior of the maximum observed frequencies (MOF) of the F2 and Es layers during a magnetic storm of September 7–8, 2017 is considered. The storm has a specific, not classical form and can be divided into two parts, namely, the first and the second storms. The main results are as follows. 1) The manifestation of the first storm is accompanied by a significant increase in absorption and frequent absence of signals on the paths. The second storm is characterized by increased values of MOF-Es and the presence of signals. 2) The signals on the midlatitude Cyprus—Lovozero path propagate both as 1F2, 2F2, and 3F2 modes and as 2Es and 3Es modes. 3) On the subpolar Lovozero—Gor’kovskaya path, during the disturbance, the signals mainly propagate via reflection from the sporadic Es layer of very high intensity. 4) Modeling of the propagation of HF radio waves using the IRI-2016 model adapted to the vertical sounding data made it possible to match the model ionograms with the real oblique sounding ionograms, including the cases of presence of the M and N modes, as well as triplets, in both quiet and disturbed conditions, which is a new result.

Impact of a Strong Magnetic Storm and Two X-Ray Flares on the Ionospheric HF Channel in the Summer Solstice of 2015 According to Oblique Sounding in the Eurasian Region

We present the results of observations of the impact a strong magnetic storm and two X-ray flares in the summer solstice of 2015 on the HF signal characteristics during oblique sounding of the ionosphere in the Eurasian region. It was found that the negative phase of the magnetic storm led to a strong degradation of the ionospheric channel, up to a long blackout on the paths adjacent to the subauroral latitudes. On the midlatitude paths, a decrease in the maximum observable frequency of the F layer reached 50% with respect to the average values for an undisturbed ionosphere. The propagation velocity of the negative phase of a disturbance from the subauroral to the midlatitude ionosphere is determined (it is equal to about 100 m/s). It is shown that during a magnetic storm the least observable frequency and the average signal-to-noise ratio for the propagation mode via the sporadic E s layer correlate well with the auroral AE index. Anomalous signals were detected in the main phase of the magnetic storm on the Cyprus—Rostov-on-Don path when a chirp ionosonde–radio direction finder was operated in the over-the-horizon HF radar mode. On the basis of modeling and comparison with experimental data, it is shown that the anomalous signals are due to scattering of radio waves by small-scale irregularities located in the subauroral ionospheric F region.

Ionospheric Effects of a Solar Eclipse of March 20, 2015 on Oblique Sounding Paths in the Eurasian Longitudinal Sector

The results of measuring HF signals on oblique chirp sounding paths in the Eurasian region during a solar eclipse of March 20, 2015 and the neighboring days are presented. The solar eclipse took place against the background of a strong magnetic storm. It was established that during the solar eclipse on oblique sounding paths of different length and orientation the decrease in the maximum observable frequency for the F mode (MOF-F) and the lowest observable frequency for the F mode (LOF-F) was 8–14% and 22–33%, respectively. During the eclipse, the signal amplitude increased by 3–5 dB. On the Lovozero—Nizhny Novgorod path in the maximum phase of the solar eclipse, the electron density decrease in the ionospheric E and F2 layers at the midpoint of the path reached 37% and 22%, respectively. According to the MOF and LOF variation measurements for various modes, the group delay time of radio signals, and the results of spectral analysis, it was found that in the eclipse there were wave disturbances with a period of 25 to 50 min. However, quasi-periodic variations of MOF-F and LOF-F having a a period of 50 to 80 min but which started before the eclipse were detected on some paths. Probably, in these cases, the variations were a result of the superposition of disturbances from two sources, namely, the magnetic storm and the solar eclipse.

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.

The investigation of ionospheric response to total eclipses on 29th March, 2006 and on 20th March, 2015 based on HF oblique sounding data

The investigation of ionosphere response to solar eclipses was carried out. Maximum observable frequencies were analyzed during two eclipses on 29th March, 2006 and on 20th March, 2015 on several oblique sounding paths which were within the range of solar flux obscuration. The model describing local changes in the ionosphere, caused by the obscuration of solar flux during eclipse, is suggested. The computer simulation of HF radiowave propagation during the eclipses was carried out on the basis of this model, while quiet ionosphere was described by IRI-2012 model. It is shown that this approach gives adequate description of HF channel during eclipses for all propagation paths under consideration while the parameters of the model were the same for all paths. As the result of computer simulation time delays of ionosperic responses during eclipses were obtained (~1800–2000s). It was found that maximum depletion of electron concentration reached 85% in D-region for both eclipses. The electron density depletions at height of F2-peak were 48% and 34% for eclipse on 29th March, 2006 and on 20th March, 2015 respectively.

Extracting Electrical Network Frequency From Digital Recordings Using Frequency Demodulation

Electric power system instantaneous frequency, also referred to as electrical network frequency (ENF), pollutes digital recordings due to a variety of factors (e.g., transformer hum). This unwanted interference has been used as a forensic tool in conjunction with a database of ground truth ENF measurements to determine the time at which a recording was made, as well as to detect tampering. Recent work has suggested that that ENF could be used to geolocate a digital recording under the assumption that the low-energy location-unique power system dynamics are observable, which is not always the case in the presence of noise and interference. This letter aims to take advantage of the intrinsic nature of power system instantaneous frequency in order to frame the ENF extraction problem as one of FM demodulation. By considering the ENF as a sinusoid of nominal power system frequency that is frequency modulated by a weak stochastic signal, a 0-Hz Intermediate Frequency (IF) signal can be created and analyzed rather than direct measurements. In doing so, significant data reduction can be achieved through extensive downsampling, FM demodulation algorithms can be applied to extract the ENF, and the spectrum of the IF signal provides a straightforward way of estimating the maximum observable frequency content of the ENF buried in a digital recording. These findings are supported through the analysis of audio recordings made in the presence of ENF interference in the U.S. Eastern Interconnection power system.

Spread-F Radar Observations in the Midlatitude Ionosphere Using an Ionosonde–Radiodirection Finder

We present the results of experimental studies of the spread-F phenomenon in the midlatitude ionosphere, which were obtained using a broadband chirp HF radar of bistatic configuration on the Cyprus—Rostov-on-Don oblique sounding path. In the evening, night, and early morning hours of the winter and spring months, in a quiet geomagnetic environment, scattered signals were detected at frequencies exceeding the maximum observable frequency (MOF) of the directsignal 1F mode with the following parameters: delays of 6‐7.5 to 8.5‐11 ms, frequencies of 10‐14 to 13‐19 MHz, vertical angles of arrival of 20 ◦ to 45 ◦ , azimuthal angles of arrival of −30 ◦ to 50 ◦ , and amplitude of scattered signal by 70‐80 dB less than the direct-signal amplitude. Based on modeling of HF propagation and scattering and on comparison with the experimental data, it is established that the observable anormalous signal is due to the backscattering of radio waves by small-scale irregularities with sizes about 8‐10 m, which occupy an extended area of the midlatitude ionosphere (50 ◦ ‐55 ◦ N, 35 ◦ ‐48 ◦ E) at altitudes both below and above the F-layer maximum (250‐450 km). Relative fluctuations of the electron number density of the scattering irregularities are estimated as δN ≈ 7.8 · 10 −3 , which is about a factor of 3 to 5 greater than δN for natural fluctuations of the electron number density under typical conditions in the midlatitude ionospheric F region. Possible relation between spread F and the traveling ionospheric disturbances is discussed.

Reflection of the dynamics of interplanetary space parameters in the maximum observable frequencies on different ionospheric oblique-sounding paths

We study the correlation between variations in the solar–geophysical parameters and variations in the maximum observable frequencies (MOF) on several ionospheric short-wave oblique-sounding paths. The basic interplanetary space parameters, whose variations have the greatest impact on the MOF variations on specified paths, are determined. Variations in the Bx and Bz components of the interplanetary magnetic field (IMF) affect the MOFs on the Norilsk—Rostov-on-Don subauroral path. At mid-latitudes, the influence of the IMF components Bx, By, and Bz is significant on the Inskip—Rostov-on-Don and Irkutsk—Rostov-on-Don paths. On the Cyprus—Rostov-on-Don low-latitude path, the modulus of the IMF has the most pronounced effect. The density and velocity of solar wind manifest themselves in MOF variations on all paths. The technique of MOF retrieval by a neural network on the Cyprus—Rostov-on-Don and Irkutsk—Rostov-on-Don paths using the MOF data of the Inskip—Rostov-on-Don path is developed with allowance for the influence of the interplanetary space parameters. The retrieval efficiency of this technique is no less than 80%.

Wave disturbance influence on HF-propagation characteristics

This work studies the variations of HF characteristics and ionospheric parameters recorded over mid-latitude paths in the Russian East-Siberian region during magnetic storms on May 15, 1997, and September 24, 2006. The sharp wave-like changes in maximum observed frequencies (MOF) were recorded during the main phase of the investigated storms. Assuming that observed MOF variations can be produced by ionospheric disturbances propagating from the northern to the southern latitudes, a simulation of HF propagation conditions was carried out.

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.

Detection of magneto-gravity waves in the ionosphere by analysis of the maximum observable frequencies on oblique-sounding paths

The existence of magneto-gravity waves stipulated by the substorm activity can lead to the occurrence of traveling ionospheric disturbances, whose velocity exceeds the sound speed. For detection of magneto-gravity waves, we used experimental data on propagation of decameter radio waves on the midlatitude and subauroral oblique sounding paths Inskip–Rostov-on-Don, Cyprus–Rostovon-Don, Irkutsk–Rostov-on-Don, and Norilsk–Rostov-on-Don in December 2006 and March 2007 under conditions of weak geomagnetic disturbance. Time delays between the AE indices of polar electrojets and the maximum observable frequencies for the considered paths were established by calculation of linear correlations. These delays correspond to the times required for transport of gravity disturbances from the auroral region to the reflection points of radio waves on respective paths. Among the obtained time shifts, we mention the 5–10-min ones which correspond to increased velocity of the disturbances compared with the usual velocities of acoustic-gravity waves for the paths under study. Such cases can be related to the transport of magneto-gravity waves. Study of the spectral composition of the AE-index disturbances and recorded maximum observable frequencies shows consistency of their spectral features in the cases of increased correlation for small time delays. It is also found that the spectral features of the AE-index disturbances coincide with the spectral features of the disturbances of the horizontal component of the geomagnetic field on ground-based magnetic stations. According to calculated dispersion curves, the frequencies of magneto-gravity waves were estimated, and they turned out to be equal to ω ≈ (1–2) ・ 10−4 Hz.

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.

Sounding of an artificially perturbed ionosphere by means of an ionosonde/position finder with chirp modulation of the signal

We describe the operation of an ionosonde/position finder with chirp modulation of the signal. The first results of measuring the characteristics of short-wave radio signals scattered by artificial small-scale inhomogeneities, which were obtained by means of an ionosonde/position finder on the IZMIRAN—“SURA”—Rostov-on-Don path are presented. It was found that under certain ionospheric conditions, the angular and frequency selection of the scattered signals take place, in which case the signals are observed simultaneously in several frequency intervals (mainly, in three, namely, 6–9.5 MHz, 10–12 MHz, and 15–18 MHz) with different angles of incidence of radio waves in the vertical plane. In this case, the incidence angles were 20◦–35◦, 18◦–32◦, and 10◦–20◦ from the horizon for the first, second, and third frequency interval, respectively. Ionograms of oblique sounding were modeled allowing for the scattering of radio waves by artificial small-scale inhomogeneities. It is shown that at frequencies from 10 to 12 MHz, aspect conditions are fulfilled for the signals ducting along the high-angle beam (Pedersen mode). At frequencies 15–18 MHz (higher than the maximum observable frequency of the forward signal on the path IZMIRAN—Rostov-on-Don), aspect scattering conditions are fulfilled for the signals incident on a scattering area in the ascending part of the trajectory. At low frequencies 6–9.5 MHz (below the maximum observed frequency of the forward signal on the IZMIRAN—Rostov-on-Don path), the observable additional signals are caused by the scattering of radio waves by artificial inhomogeneities with subsequent relfection of the scattered signal from the Earth on the “SURA”—Rostov-on-Don path.

Evaluation of the IRI model for the European region

The aim of this paper is to investigate various aspects of the International Reference Ionosphere (IRI) performance in European area and to evaluate its accuracy and efficiency for: long term prediction of the critical frequencies foF2 and the maximum usable frequencies (MUF); using storm-time correction option (ST); the total electron content (TEC) and the maximum observable frequency (MOF) updating. Data of foF2, TEC, MOF are related to 2005. It is obtained that median values of foF2 can be predicted with the mean error σ(med)∼ 0.49 MHz. For median values of MUF absolute σ was 1.39 MHz and relative σ r was 8.8%. For instanteneous values estimates are increased to 1.58 σ(med) MHz for foF2 and could reach 3.84 MHz for MUF. Using correction ST-option and TEC values provided ∼30% improvement but TEC seems to be more preferable. However, from considered parameters of the IRI updating (ST-factor, TEC, MOF) the best results were demonstrated by MOF. Using the IRI2007 to calculate TEC gives 20–50% improvement of TEC correspondence to experimental values but this improvement is not enough to treat TEC without the IRI model adaptation.

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.