Features of propagation of HF signals on mid-latitude paths under conditions of geomagnetic disturbances

We present the results of experimental studies of the features of HF-signal propagation on oblique-sounding paths in the Eurasian longitude sector between England and Magadan during geomagnetic disturbances. The joint analysis of the satellite data and the data of vertical and oblique sounding of the ionosphere shows that the appearance of additional signals during magnetic disturbances can be stipulated by refraction of radio waves in the region of the auroral oval and the main ionospheric trough as well as by scattering from small-scale magnetic-field-aligned irregularities near the equatorial boundary of the auroral oval. Based on calculations and comparison with experimental data of oblique sounding of the mid-latitude ionosphere, we identify the lateral spreading signals registered on the Magadan — Irkutsk and Inskip (England) — Rostov-on-Don paths as signals scattered by small-scale field-aligned irregularities whose location coincides with the southern boundary of the auroral oval. We show that the Russian and global chirp-ionosonde networks are promising for studying the dynamics of the main ionospheric trough and the auroral oval during geomagnetic disturbances as a manifestation of space weather in the near-Earth environment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 12, pp. 1041–1056, December, 2004.     

Variations in the Characteristics of a HF Signal over an Oblique Sounding Path during the Solar Eclipse on August 11, 1999

We present the results of measurements of the HF-signal characteristics over the path Praha - Nizhny Novgorod during the solar eclipse on August 11, 1999. A 10-dB increase in the HF-signal amplitude and an increase in the frequency of quasiperiodic fading were observed during daytime in the course of the eclipse with phase about 68-85%. An interpretation of the effects observed is given. On the basis of simulations, we show that the increase in the amplitude is related to a decrease in absorption losses which is caused by the corresponding decrease in the electron density in D- and E-layers by about 30-45%. The increase in the fading frequency results from beating of the modes that have different Doppler shifts due to phase-path variations during the eclipse. An estimate for the rate of electron-density variation during the eclipse was obtained: dN/dt (2-2.5) ·10² cm^-3s^-1.     

Structure and dynamics of the ionospheric region with artificial small-scale irregularities according to complex measurements of the scattered radio-signal characteristics

We present the results of complex measurements of characteristics of HF radio signals scattered by artificial small-scale irregularities by means of oblique sounding using a chirp signal and by direction finding. On the basis of comparison of the experimental data and the calculations of aspect scattering of radio waves, the conclusion on the patch structure of an artificially disturbed ionosphere filled with small-scale field-aligned irregularities is drawn. The sizes of the patches amount to several ten kilometers, and they are separated by 100 or more kilometers. The drift speed of irregularities in the direction of the scattering vector in the plane perpendicular to the magnetic field is determined for several patches. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 12, pp. 1011–1025, December 2008.     

Modelling of HF wave propagation in the vicinity of quasicritical rays in the disturbed ionosphere

The results of experimental studies of the fine structure of a signal in the vicinity of the maximum observed frequency (MOF) on the Khabarovsk—Nizhny Novgorod path of oblique chirp sounding (OCS) are presented. Additional tracks were observed in the region between the high-angle and low-angle rays during magneto-ionospheric disturbances. Under strong disturbances the ionograms were of a spreading type in the vicinity of the MOF. The observed effect was modelled in the presence of travelling ionospheric disturbances (TID) with different parameters. It is shown that the stratification of the high-angle ray into several additional tracks is a maximum for TID with vertical scales $\ell _z \sim {\text{ 20 - 40}}$ km whose wave fronts make angles about 0–10° with the horizontal line. The possibilities of using the Pedersen mode as a probing wave for diagnostics of the fine structure of the ionosphere in the vicinity of the F-layer maximum are discussed.     

Convergent instability in the ionosphere

A linear theory of the convergent instability (CI) of ionospheric plasma associated with the nonuniform nature of its regular motion is examined. The conditions under which CI appears in the E- and F-layers for vertical ion motion caused by various physical factors are analyzed. The possibility of small-scale strongly geomagnetic-field-aligned nonuniformities of electron concentration (l_min 10–30 m) is demonstrated. The altitude dependence of collision frequency is shown to play a large role in CI.Scientific-Research Radio-Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 10, pp. 921–928, October, 1993.     

Experimental Studies of Variations in the Maximum Usable Frequency on Oblique Sounding Paths

We present the results of experimental studies of variations in the maximum usable frequency (MUF) on the latitudinal (England–Moscow and Khabarovsk–Nizhny Novgorod) and meridional (Cyprus–Moscow) paths. It is found that the quasi-period of MUF variations ranges from 20 min to several hours in spring 2002. The absolute value of the MUF varied in the range from 0.2 to 2 MHz. We show that variations in distance–frequency characteristics of HF signals propagated over the long-distance latitudinal path are strongly affected by quasi-periodic disturbances being the ionospheric response to acoustic-gravity waves excited by the terminator.     

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 K_p 2 the correlation coefficient amounts to 0.75-0.95. With increase in disturbance for K_p 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.     

Oblique chirp sounding of the modified ionosphere. Experiment and simulation

We present the results of experimental studies of the influence of artificial ionospheric disturbances on HF signals used for oblique sounding of the disturbed volume. The measurements have been performed by a chirp ionosonde over the path Yoshkar-Ola-“Sura”-niznhy Novgorod with length 234 km. We found the 2F2 mode to disappear (attenuation up to 20 dB) when the ionosphere is influenced by a vertical powerful radiation in the ordinary mode with long (15 min each) heating and pause intervals. Modeling of the observed effect was carried out. The calculations agree well with experimental data if the traveling ionospheric disturbances (TID) with vertical and horizontal scales l_z∼20 km and l_x∼50 km, respectively, and the relative disturbance of the electron density δN∼0.2–0.3 are amplified (generated) during the ionosphere heating. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 4, pp. 303–313, April 1999.     

Vertical motions in the lower ionosphere and a sporadicE-layer

We propose a calculation technique for the main characteristics of a sporadic E-layer, including the effective recombination coefficient, the relative content of meteor and atmospheric ions in the layer, and the time of its evolution. This technique is based on measurements of vertical plasma velocities by the method of resonance scattering of radio waves by artificial periodic inhomogeneities of the electron density. The contribution of internal gravity waves and turbulent motions to the formation of sporadic layers is estimated. The characteristic values of the turbulent velocity measured by this method at the heights of the turbopause are presented. The possible mechanisms of mid-latitude sporadic E-layer formation at heights of 90 to 120 km are considered. Experimental studies confirmed the main statements of the wind shear theory concerning the determining role of the redistribution of ionization under the action of atmospheric winds with vertical gradient of velocity in the formation of E_s. Deceased. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 1, pp. 26–35, January, 1999.     

Features of High-Angle Ray Propagation on Paths Crossing the Ionospheric Trough

We present the results of experimental study of the features of high-angle ray propagation on subauroral (Magadan — N. Novgorod) and mid-latitude (Khabarovsk — N. Novgorod) chirp-sounding paths crossing the ionospheric trough near its northern and southern boundaries, respectively. The seasonal and diurnal occurrence of a high-angle ray and its frequency range were studied in 1998–2000. It is found that the rate of occurrence of a high-angle ray on the mid-latitude path is maximum in spring and summer. This rate is also maximum in the evening and nighttime hours (19:00–03:00 LT) during all seasons excluding the summer solstice. The diurnal distribution of the rate of occurrence in June is almost uniform. The observed seasonal-diurnal behavior of the rate of occurrence of the high-angle ray is smoother on the subauroral path. The frequency ranges of the high-angle ray on both paths are similar in June, while in winter and spring the frequency range of the high-angle ray on the subauroral path is larger than on the mid-latitude one. We put forward a hypothesis that the different seasonal and diurnal behavior of the occurrence of a high-angle ray and the frequency range of this ray on both paths are stipulated by the features of the formation of a HF-signal field under the influence of two factors, the photoionization and the small-scale stratification of the electron density in the vicinity of the ionospheric trough, which can form a fluctuational waveguide.