Joint Ionosonde Studies of F2 Layer Critical Frequency Variations in the Ionosphere Over Kharkiv and Tromsø During Fall Equinox in Quiet and Disturbed Conditions

According to the results of joint ionosonde studies of variations in the ionospheric F2 layer critical frequency over Kharkiv and Tromsø during low solar activity for fall equinox on September 22 – 24, 2020, the features of foF2 variations in middle and low latitudes were investigated for magnetically quiet and magnetically disturbed conditions. On the magnetically quiet day of September 22, 2020, the foF2 values over Kharkiv were found to exceed the foF2 values over Tromsø for the entire time interval of joint observations 02:45 - 16:45 UT. Both over Tromsø and over Kharkiv, a rapid increase in foF2 to its local maximum value was observed after the sunrise. Quasi-periodic variations in foF2 were revealed at high latitudes, which had lower amplitude compared to variations in foF2 over Kharkiv. Over both measuring sites, a pre-sunset local maximum in foF2 was observed. During magnetically disturbed conditions over Tromsø and Kharkiv, quasi-periodic fluctuations in foF2 were observed after the sunrise. Oscillations over Tromsø had lower amplitude than over Kharkiv, and were almost completely suppressed after the onset of a strong magnetic disturbance at high latitudes on September 23, 2020. The foF2 values over Tromsø exceeded its values over Kharkiv in a time interval of 10:45 – 12:15 UT. Comparison of the time variation of foF2 over Tromso on a magnetically quiet day, September 22, 2020, and on a magnetically disturbed day, September 23, 2020, showed that the foF2 value for September 23, 2020 from 10:15 to 15:00 UT exceeded the foF2 values for the same period on September 22, 2020. Comparison of the temporal variations in foF2 over Kharkiv on a magnetically quiet day, September 22, 2020, and on a magnetically disturbed day, September 24, 2020, showed that the foF2 value for September 24, 2020 exceeded its value for September 22, 2020 from 03:00 to 04:45 UT and from 07:00 to 13:00 UT. Magnetic disturbances were found to cause a rapid increase in foF2 values both over Kharkiv and Tromsø, which exceeded foF2 values under magnetically quiet conditions, and also led to a significant increase in the relative amplitudes of traveling ionospheric disturbances over Kharkiv.

Dynamics of the Thermal Uplifting in the Atmosphere Under a Continuous Supply of Heat: Practical Application Examples

The author has earlier considered the dynamics of an isolated thermic arising from an instant heat release. The rigorous analytical, as well as simplified, solutions describing the dynamics of the uplifting of a spherical thermic have been obtained. Such a thermic appears during a short-term release of heat, e.g., during an explosion. The uplifting of a meteoroid thermic has also been studied. The theory of the thermic has found applications in the magnetic precursors of earthquakes. At the same time, the heat can be supplied during many hours or even days when big forest fires occur, peat fires burn, volcano eruptions occur for a long time, and during the release of heat before earthquakes. The dynamics of the uplifting of a thermal under these circumstances is considerably different from an instantaneous energy release. Employing the cylindrical model of a thermic, the dynamics of the thermic has been studied in the case of the continuous supply of heat. Within the model, the analytical solutions to the set of equations governing the temporal dependences of the velocity of a parcel of the heated air and the position of the upper bound of the thermic, as well as the excess temperature in the heated parcel have been obtained. The upper thermal boundary speed and location has been shown to increase with uplifting, while the excess temperature to gradually decrease. The numerical estimation has been performed for characteristic situations. The ecological consequences of large-scale fires, as well as the mechanisms for generating gravity waves by the thermals, are discussed. The physics-based mechanisms for generating acoustic wide-band emissions by the thermals have been analyzed; the wave periods have been estimated to be 1–103 s. The energy of acoustic emissions from a big fire has been estimated to be approximately 1014 J. At the same time, the energy of acoustic emissions from all fires that occurred in the Russian Federation in 2020 amounts to 7∙1016 J, while in Ukraine it is three orders of magnitude lower.

Calculation of the Amplitude and Initial Phase of the Signal from Its Most "Reliable" Fragments

The idea of improving the methods of processing the received radio signals used in radio communication and radiolocation systems is considered. It is proposed to consider the radio signal as a noise-resistant code with repetition of its periods (symbols), each of which carries the same information about the value of its amplitude and initial phase. The possibility of realization of coherent reception of radio signals with amplitude or biphasic discrete modulation under the influence of powerful noise on them by their most "reliable" (suitable for calculations) fragments, determined by the values of samples, received at moments when the amplitude of a received signal shall be equal to zero (samples correspond to the value of the noise component – point A) or be maximum – point E (the phase of signals shifted with respect to the phase of noise, by π/2 or 3π/4). The considered approach on processing of a mix of a signal and noise allows to define some varieties of "reliable" fragments (at difference of phases of noise and a signal on π/2 or 3π/4, at a match of zero values of signals and noise, at difference of phases of noise and a signal on the value smaller, than ±π/24) on values of samples in points A and E, and also to exclude from consideration fragments, decision on which often is erroneous. The possibility of obtaining an additional estimation ∆ of the quality of the decision on a fragment of a discrete signal (BPSK), with already calculated its average value SСР on the set of its fragments, which allows to select the best or several best fragments for the decision on the received signal is shown. Fragments with a difference of noise and signal phases on the value smaller than ±π/24 occur most often from all considered variants and allow to distinguish with high accuracy the initial phases of opposite signals (BPSK). To solve the problem, we used all possible temporal representations of fragments of signal, noise and a mixture of signal and noise, mathematical models and formulas of the theory of circuits and signals.

Development of the Analysis Procedure of the Efficiency of Methods for Digital Filtering of the Incoherent Scatter Signal

The article considers the methodological features of creating procedures that help in the implementation of effective methods of recognition and suppression in the input signal, which are recorded by the incoherent scatter radar, noise and impulse interference. It is known that the frequency band of the input signal depends on the observation period of the ionosphere, as well as on its height above the Earth’s surface. Therefore, approaches to using analog filters with time-controlled characteristics in a radar receiver are being practiced. These filters, however, require a priori information about of the ionosphere state. At the present time, it is possible to obtain digital samples of a signal with a very small sampling step (tens of thousands of samples during a radar range sweep) using high-speed analog-to-digital conversion. It is also possible to record these readings in the computer memory relative to each sweep. This gives the prospect, based on the results of the experiment, to carry out auxiliary adaptive digital filtering of the obtained data. In this case, filter properties are selected for each altitude range in order to obtain the maximum possible signal-to-noise ratio. In order to introduce high-quality digital filtering methods, this development is aimed at creating a special software-algorithmic procedure, which allows you to control the effectiveness of the proposed filtration methods. The essence of this procedure is that according to the amplitude-frequency and phase-frequency spectra set by the researcher, the corresponding model of the scattering signal is synthesized and its autocorrelation function is calculated. These characteristics in the following steps as reference are used for comparison with similar characteristics, but obtained from the distorted scattering signal. The experimenter has the ability to apply noise and impulse interference to the scattered signal and to test the digital filtering method he proposed. All these steps in software implementation are accompanied by a clear graphical visualization of the results obtained.

Simultaneous ionosonde investigations of the ionospheric F2 layer critical frequency and peak height at both ends of the geomagnetic tube

Based on the results of simultaneous ionosonde observations during low solar and weak magnetic activities, a coupling was found between diurnal and quasi-periodic variations in ionospheric parameters over magnetically conjugated regions, where the Ukrainian Antarctic Station (UAS) and Millstone Hill Observatory are located. A significant impact of the summer hemisphere on the nighttime variations of the F2 layer critical frequency foF2 in the magnetically conjugated region in the winter hemisphere was found. The most characteristic manifestation of this impact is the control of foF2 variations over the UAS not by the local sunset (sunrise), but by the sunset (sunrise) over Millstone Hill. It was found that the sunset over Millstone Hill leads to an increase in foF2 over the UAS, while the sunrise leads to a decrease in foF2 with a subsequent sharp increase. Both phenomena are associated with changes in the photoelectron flux from the northern hemisphere, corresponding changes in the electron temperature in the ionosphere above the UAS and the effect of these changes on the compression or rarefaction of the ionospheric plasma and changes in the plasmaspheric fluxes of H + ions. It was shown that the transition from nighttime to daytime conditions over both observation points was characterized by a significant decrease in the F2 layer peak height, and the difference in the values of this ionospheric parameter over Millstone Hill and UAS at night is due to seasonal differences in the thermospheric circulation and the difference in the behavior of the ionospheric parameters in the Northern and Southern hemispheres. Manifestations of atmospheric gravity waves, caused by the passage of local sunrise terminators, as traveling ionospheric disturbances with periods of about 90 and 75 – 120 mins over Millstone Hill and UAS, respectively, were found. These waves were most likely generated in the region located between the ionospheric F1 and F2 layers, where the sharp gradients in the electron and ion densities occur during changes in the intensity of solar radiation. It is confirmed that wave disturbances in atmospheric and ionospheric parameters can be transferred between magnetically conjugated regions by slow magnetohydrodynamic waves generated both at the heights of the ionospheric dynamo region due to the modulation of atmospheric and ionospheric parameters by atmospheric waves and the occurrence of external currents, and at the top of the plasmaspheric tube, where sharp plasma compression and heating or rarefaction and cooling occur during the passage of the solar terminator. Keywords: the ionosphere, F2 region, ionosonde measurements, geomagnetic field tube, magnetoconjugate region coupling, atmospheric gravity waves, traveling ionospheric disturbances, generation of slow magnetohydrodynamic waves

Investigation of the ionosphere over Antarctica under quiet space weather conditions: results of vertical sounding of the ionosphere September 14–24, 2020

We present observational results of variations in the ionospheric parameters hmF2 and NmF2 over the Ukrainian Antarctic station “Akademik Vernadsky” for magnetically quiet conditions. The results of comparative analysis of observational data and the International Reference Ionosphere-2016 model predictions are presented. The main objective of this study is to investigate the temporal variations of two key ionospheric parameters – the F2 layer peak height and electron density – during very quiet space weather conditions using data of vertical sounding of the ionosphere obtained over the Ukrainian Antarctic station “Akademik Vernadsky” and comparison the observation results with model values. Methods: The temporal variations of the F2 layer peak height and electron density were calculated from ionograms obtained with ionosonde installed at the Ukrainian Antarctic station “Akademik Vernadsky” with subsequent electron density profile inversion. Diurnal variations of hmF2 and NmF2 were calculated using a set of sub-models of the IRI-2016 model for comparison with results of observational studies. Results: We found that for the Antarctic region option of IRI-2016 model for the F2 layer peak height SHU-2015 provides a better fit for hmF2 through the investigated period compare to the AMTB-2013 model predictions. Electron density models (URSI, CCIR) generally well reproduce the observed variations of NmF2 during periods of absence non-standard manifestations of space weather, which are possible for quiet conditions too. Hypotheses regarding the possible reasons for experimental and model differences in variations of NmF2 are discussed. The analysis of effect of geomagnetic storm on September 24, 2020 on NmF2 variations was carried out. Conclusions: The obtained results demonstrate peculiarities of the state of the ionosphere-plasmasphere system over Antarctica under very quiet space weather conditions and provide evaluation of predictive capabilities of modern international reference ionosphere models. New knowledge about the features of electron density variations in the ionosphere for magnetically quiet conditions over the Antarctic region has practical value for specialists which are engaged in the study of the near-Earth space environment, in particular, at high latitudes, and also work on correction of global ionospheric models. Keywords: electron density, F2 layer peak height, ionosonde, quiet space weather, models of the ionosphere, downward plasma flux

Active power filter for reactive power compensation for the powerful sounding pulses shapers

The expediency of using a power active filter in the mode of reactive power compensation of the supply network for the powerful sounding pulses shaper power supply system of the Institute of ionosphere NAS and MES of Ukraine is shown. An analysis of the literature has been carried out, which shows the effectiveness of solving the problem of reactive power compensation, filtering higher harmonics of the power supply network using active filtering of higher harmonics of current or voltage using an additional energy source to obtain a compensating signal in the form of current or voltage. The choice of the power circuit for constructing a power active filter has been made. A voltage inverter on IGBT transistors is used as a power active filter. The basic principles of operation of the power active filter control system are described, which are based on the p-q theory of power and provide for the calculation of instantaneous values ​​of the task currents for each phase of a three-phase power supply system. The work of the Matlab-model of the power supply system of the shaper is shown, the oscillograms of the main energy characteristics of the shaper are given. As a result of using a power active filter, the current of the supply network becomes close to sinusoidal, and the power factor tends to 1. A positive effect on the efficiency of the shaper when the power active filter is included in the supply network is noted, which is due to the efficiency of compensation of the reactive power consumed by the shaper from the supply network. The simulation results are presented, in particular, graphs of reactive power change at the point of connecting the power active filter to the supply network, at different levels of reactive power consumption by the shaper. The results obtained confirmed the possibility of using a power active filter in the mode of reactive power compensation of the supply network when solving the problems of improving the electromagnetic compatibility of powerful sounding pulses shapers with the supply network, reducing losses and increasing the reliability of the shapers. Keywords: active power filter, reactive power control, control systems

The speed of movement of piece space objects along the Earth's surface and its determination using incoherent scatter radar

Methodological features of registration and separation of coherent radar reflections from space objects and elements of “space debris” operating in orbit are considered. Registration occurs against the background of signals that are scattering of the probe radio wave on particles of the ionospheric plasma. Methods of how to obtain information about the components of the velocity vector of these objects in near-earth space with the help of specialized ground-based radar facilities are analyzed. Their disadvantage is the unreliable control of weak reflections from the elements of “space debris” if they have a small (up to centimeters) scattering cross section. The authors proposed to use the existing high-energy radar installations. Using the signals after the analog-to-digital conversion generated in quadrature, it is proposed to calculate the phase characteristics of the coherent reflection. The radial velocity of the objects along the radar beam is calculated by isolating the Doppler phase difference and statistically averaging these values ​​in the time of reflection. Similarly, by analyzing the time spent in the radar beam, the velocity component associated with the horizontal movement along the Earth’s surface is calculated. Real examples are given, when in one of the observation sessions on the reflection of a signal from a space object, the phase shift in each of its periods is calculated, and then, using the formula, proposed by the authors, the vertical component of the velocity of this object is calculated. Analyzing the observation time of this object in the beam of the transmitter antenna, an example of the calculation and the component of its horizontal velocity is shown. The block diagram of the radar used to calculate the specified parameters of the movement of space objects is presented. The developed approach is an effective solution of many practical problems in those industries that ensure the operation of spacecraft, ensuring the safety of space stations, optimal placement of objects in orbit, etc. Keywords: Incoherent scatter radar, space objects, coherent reflection, signal phase characteristics, radial and horizontal speed

Parameters of the Infrasonic Signal Generated in the Atmosphere by a Powerful Volcano Explosion

The purpose of this work is to represent the results of performing regression analysis to fit the distance and the amplitude of the infrasonic signal generated by the explosion of St. Helens volcano, and to estimate a few signal and atmospheric parameters. The pressure amplitude in the explosion wave generated at the beginning of St. Helens volcano eruption was measured at 13 stations in the 0.9 – 39-Mm distance range; based on these data, an attempt has been made to perform a regression analysis to fit amplitude and distance. The regression based on the assumption that the infrasound propagation takes place in a waveguide where it is subject to attenuation is determined to be the most preferable regression. Based on the observations of the shock from the St. Helens volcano eruption, the shock wave energy and mean power have been estimated to be ~1016 J and ~2.3 TW, respectively. Based on the observations of the amplitude and duration of the trains of the infrasonic wave generated by the St. Helens volcano eruption, the infrasonic wave energy and mean power have been estimated to be ~1016 J and ~2 TW, respectively. Both estimates are in good agreement, but they are significantly different from those found in the literature; the latter seem to be overestimated. From the regression expression obtained, the penetration depth of the infrasonic wave is obtained to be about 33 Mm, whereas at other stations this scale length is estimated to be close to 24 Mm. Based on the theoretical dependence of the attenuation coefficient due to atmospheric turbulence, the attenuation length of the infrasound wave has been estimated for infrasound with 10–300-s periods. For 20–300-s periods, this value has been shown to be significantly larger than the values determined from the observations. Other mechanisms for attenuating the infrasonic signal are discussed (the partial radiation of the infrasonic energy through and losses due to the reflection from the waveguide walls). At the same time, the wave attenuation due to their scattering by turbulent fluctuations can be significant for the periods smaller than 20–50 s, depending on the turbulence intensity. Comparison of the regression functions obtained with the corresponding regression expressions for other sources of infrasound waves propagating in the atmosphere has been made. Keywords: volcano eruption, infrasonic wave, shock wave, signal amplitude, regression, signal attenuation