On the group velocity of whistling atmospherics

The dynamic spectrum of a whistling atmospheric is a signal of falling tone, and the group delay time of the signal as a function of frequency is formed as a result of propagation of a broadband pulse in a medium (magnetospheric plasma) with a quadratic dispersion law. In this paper, we show that for quadratic dispersion the group velocity is invariant under Galilean transformations. This means that, contrary to expectations, the group velocity is paradoxically independent of the velocity of the medium relative to the observer. A general invariance condition is found in the form of a differential equation. To explain the paradox, we introduce the concept of the dynamic spectrum of Green’s function of the path of propagation of electromagnetic waves from a pulse source (lightning discharge in the case of a whistling atmospheric) in a dispersive medium. We emphasize the importance of taking into account the motion of plasma in the experimental and theoretical study of electromagnetic wave phenomena in near-Earth space.

Calibration of Siberian Radioheliograph antenna gains using redundancy

The paper describes application of standard gain calibration using redundancy for a 48-antenna prototype of Siberian Radioheliograph. Traditionally, for calibration, the visibilities were measured only between adjacent antennas since they have the highest signal-to-noise ratio and are sufficient for phase calibration. We have shown that this limited set of visibilities did not allow using the antenna array redundancy potential and obtaining images with a high dynamic range on a permanent basis. Images without amplitude calibration contain many artifacts and require special care when analyzed. The inclusion of visibility measurement between antennas with a double step made it possible to significantly increase the accuracy of solving the system of equations for amplitudes. Images constructed using both phase and amplitude calibrations do not have visible artifacts and are more reliable.

Measurement of Siberian Radioheliograph cable delays

To achieve the maximum dynamic range of solar radio images obtained using aperture synthesis in relatively wide frequency bands 0.1−0.5 % of the operating frequency, it is necessary to compensate the signal propagation delays in the antenna receive path before calculating visibility functions (hereinafter visibilities). When visibilities are corrected without delay compensation, the signal-to-noise ratio decreases due to residual phase slopes in the receiving system bandwidth. In addition to enhancing dynamic range, preliminary compensation for delays simplifies real-time imaging — no antenna gain calibration is required to get a first approximation image. The requirements for the accuracy of antenna placement are also reduced — in contrast to the measurement of the phase visibility error, the measurement of the delay is actually not so critical to the antenna position errors that are larger than the operating wavelength. The instantaneous frequency band of the Siberian Radioheliograph, which determines the minimum step for measuring the phase slope, and hence the accuracy of determining the delay, is 10 MHz. At the speed of light in an optical fiber of ~0.7c, a step of 10 MHz makes it possible to unambiguously measure the difference between electrical lengths of cables up to 20 m and to correct antenna positions by radio observations, even if the error in the position of the antennas exceeds the operating wavelength. Correction of the band phase slopes during the observation time adapts the radio telescope to the temperature drift of delays and decreases antenna gain phase spread. This, in turn, leads to more stable solutions to systems of equations containing antenna gains as unknowns.

Comparison of ground-based and satellite data on spatiotemporal distribution of lightning discharges under solar minimum

Worldwide maps of lightning activity have been obtained from the ground-based World Wide Lightning Location Network (WWLLN) for 2007–2009. We have compiled these maps separately for different seasons and UT periods, using WWLLN data on the time and coordinates of each of the recorded lightning. The total number of flashes of lightning in WWLLN data is by an order of magnitude smaller than in satellite data from Optical Transient Detector and the Lightning Imaging Sensor satellites. However, the key features of the spatial distribution and seasonal trends coincide well. The main difference observed is the absence of diurnal variation (similar to Carnegie curve) in WWLLN data against the satellite one. This concerns the global lightning number as well as its density in major thunderstorm regions. The solar local time dependence is also weak in WWLLN data. We show that in 2007–2009 the mean latitude of lightning observation is shifted to the summer hemisphere up to 10° from the annual mean value. From the beginning of 2007 to the end of 2009, the global monthly average number of flashes of lightning increased threefold. We attribute this fact primarily to improved processing techniques in WWLLN. The constructed maps are necessary for numerical simulation of the Global Electric Circuit.

Variations of flow direction in solar wind streams of different types

Studying the direction of the solar wind flow is a topical problem of space weather forecasting. As a rule, the quiet and uniform solar wind propagates radially, but significant changes in the solar wind flow direction can be observed, for example, in compression regions before the interplanetary coronal mass ejections (Sheath) and Corotating Interaction Regions (CIR) that precede high-speed streams from coronal holes. In this study, we perform a statistical analysis of the longitude (φ) and latitude (θ) flow direction angles and their variations on different time scales (30 s and 3600 s) in solar wind large-scale streams of different types, using WIND spacecraft data. We also examine the relationships of the value and standard deviations SD of the flow direction angles with various solar wind parameters, regardless of the solar wind type. We have established that maximum values of longitude and latitude angle modulus, as well as their variations, are observed for Sheath, CIR, and Rare, with the probability of large deviations from the radial direction (>5°) increasing. The dependence on the solar wind type is shown to decrease with scale. We have also found that the probability of large values of SD(θ) and SD(φ) increases with increasing proton temperature (Tp) in the range 5–10 eV and with increasing proton velocity (Vp) in the range 400–500 km/s.

Estimated influence of stratospheric activity on the ionosphere according to measurements with ISTP SB RAS tools

We present the results of a comprehensive study of the manifestation of wave activity with periods of internal gravity waves (IGW) in various regions of the atmosphere: in the stratosphere, upper mesosphere, and in the F2-region of the ionosphere. The study is based on radiophysical and spectrometric measurements made with tools of the Institute of Solar-Terrestrial Physics (ISTP) SB RAS and the Era-Interim reanalysis data. The correlation coefficient with time shift between ionospheric and stratospheric activity for the annual interval varies in the range from 0.45 to 0.54, and for the 27-day interval it reaches the levels 0.4–0.8 in seventy percent of the cases. Thirty percent of correlation coefficients less than 0.4 can be explained by the influence of neutral wind, geomagnetic activity, and non-stratospheric IGW sources. Comparison between stratospheric activity and variations in characteristics of traveling ionospheric disturbances (TID) has shown that a ~15 day shift in stratospheric activity results in a fairly high correlation between stratospheric activity and disturbance of IGW characteristics (~0.6). The delay of about 15 days can be attributed to the delay in the temperature variations at heights of the lower thermosphere relative to the temperature variations at the altitude pressure level of 1 hPa. Comparative analysis of variations in mesospheric and ionospheric activity has revealed time intervals when their behavior is consistent.

Cluster analysis of lightning discharges: based on Vereya-MR network data

Monitoring thunderstorm activity can help you solve many problems such as infrastructure facility protection, warning of hazardous phenomena associated with intense precipitation, study of conditions for the occurrence of thunderstorms and the degree of their influence on human activity, as well as the influence of thunderstorm activity on the formation of near-Earth space. We investigate the characteristics of thunderstorm cells by the method of cluster analysis. We take the Vereya-MR network data accumulated over a period from 2012 to 2018 as a basis. The Vereya-MR network considered in this paper is included in networks operating in the VLF-LF range (long and super-long radio waves). Reception points equipped with recording equipment, primary information processing systems, communication systems, precision time and positioning devices based on global satellite navigation systems are located throughout Russia. In the longitudinal-latitudinal thunderstorm distributions of interest, the dependence on the location of recording devices might be manifested. We compare the behavior of thunderstorms on the entire territory of the Russian Federation with those in the Baikal natural territory. We have established the power of thunderstorms over the Baikal region is lower. The daily variation in thunderstorm cells we obtained is consistent with the data from other works. There are no differences in other thunderstorm characteristics between the regions under study. This might be due to peculiarities of the analysis method. On the basis of the work performed, we propose sites for new points of our own lightning location network, as well as additional methods of cluster analysis.

Features of short-period variability of total electron content at high and middle latitudes

The study presents the results of comparative analysis of features of a short-period (with periods of internal gravity waves) variability of total electron content (TEC) in the ionosphere at middle (Novosibirsk) and high (Norilsk) latitudes over a long period of time (2003–2020). The period analyzed makes it possible to estimate not only diurnal and seasonal variations in the variability, but also its changes within the solar activity cycle. The level of TEC variability is shown to experience pronounced seasonal variations with maxima in winter months. The difference between the level of variability in winter and summer is about two times for Novosibirsk and up to seven times for Norilsk. The variability features a distinct diurnal variation; however, the diurnal dependence at the mid- and high-latitude stations differs significantly. At high latitudes, the level of variability in the winter period strictly depends on solar activity. For the mid-latitude station, there is no clear dependence of variability level on solar activity; in the years of solar maximum, on the contrary, a slight decrease in the variability is observed. In summer, the level of variability at both middle and high latitudes remains practically unchanged and does not depend on solar activity. The main features in the dynamics of variability are shown to be similar at stations located at other longitudes, except for the East American sector. The result obtained suggests that the short-period TEC variability at high latitudes is primarily related to changes in solar activity, but regular variations in the variability at midlatitudes are probably not associated with heliophysical activity. The observed increase in the level of short-period variability in the winter mid-latitude ionosphere is assumed to be related to an increase in wave activity in the stratosphere.

Studying auroral activity using the SME index at the magnetic storm main phase during CIR and ICME events

. In this paper, we examine the relationship of the SME index with magnetic storm characteristics and interplanetary medium parameters during the main phase of magnetic storms caused by CIR and ICME events. Over the period 1990–2017, 107 magnetic storms driven by (64) CIR and (43) ICME events have been selected. In contrast to AE and Kp, a stronger correlation is shown to exist between the average SME index (SMEaver) and interplanetary medium parameters during the magnetic storm main phase. Close correlation coefficients between SMEaver and the SW electric field (southward IMF Bz) have been obtained for CIR and ICME events. SMEaver has been found to increase with the rate of magnetic storm development and |Dstmin|. For CIR and ICME events, no difference has been revealed between SMEaver and |Dstmin| in linear regression equations.

Interaction between long-period ULF waves and charged particle in the magnetosphere: theory and observations (overview)

The paper reviews the current state of the problem of interaction between long-period ultra-low-frequency (ULF) waves and high-energy particles. We consider elements of the theory of energy exchange between waves and particles, particle transport across magnetic shells under the influence of the electromagnetic field of a wave, the acceleration of radiation belt particles by both resonant and non-resonant mechanisms. We examine the mechanisms of generation of azimuthally-small-scale ULF waves due to instabilities arising from the wave–particle resonance. The cases of Alfvén, drift-compressional, and drift-mirror waves are analyzed. It is noted that due to the lack of a detailed theory of drift-mirror modes, the possibility of their existence in the magnetosphere cannot be taken as a proven fact. We summarize experimental data on the poloidal and compression ULF waves generated by unstable populations of high-energy particles. We investigate the mechanisms of modulation of energetic particle fluxes by ULF waves and possible observational manifestations of such modulation. Methods of studying the structure of waves across magnetic shells by recording fluxes of resonant particles with a finite Larmor radius are discussed.