Showing papers on "Very low frequency published in 2022"

Space-to-space very low frequency radio transmission in the magnetosphere using the DSX and Arase satellites

Abstract: Abstract Very low frequency (VLF) waves (about 3–30 kHz) in the Earth’s magnetosphere interact strongly with energetic electrons and are a key element in controlling dynamics of the Van Allen radiation belts. Bistatic very low frequency (VLF) transmission experiments have recently been conducted in the magnetosphere using the high-power VLF transmitter on the Air Force Research Laboratory’s Demonstration and Science Experiments (DSX) spacecraft and an electric field receiver onboard the Japan Aerospace Exploration Agency’s Arase (ERG) spacecraft. On 4 September 2019, the spacecraft came within 410 km of each other and were in geomagnetic alignment. During this time, VLF signals were successfully transmitted from DSX to Arase, marking the first successful reception of a space-to-space VLF signal. Arase measurements were consistent with field-aligned propagation as expected from linear cold plasma theory. Details of the transmission event and comparison to VLF propagation model predictions are presented. The capability to directly inject VLF waves into near-Earth space provides a new way to study the dynamics of the radiation belts, ushering in a new era of space experimentation. Graphical Abstract

Discovery and insights from DSX mission’s high-power VLF wave transmission experiments in the radiation belts

Abstract: Space weather phenomena can threaten space technologies. A hazard among these is the population of relativistic electrons in the Van Allen radiation belts. To reduce the threat, artificial processes can be introduced by transmitting very-low-frequency (VLF) waves into the belts. The resulting wave-particle interactions may deplete these harmful electrons. However, when transmitting VLF waves in space plasma, the antenna, plasma, and waves interact in a manner that is not well-understood. We conducted a series of VLF transmission experiments in the radiation belts and measured the power and radiation impedance under various frequencies and conditions. The results demonstrate the critical role played by the plasma-antenna-wave interaction around high-voltage space antennae and open the possibility to transmit high power in space. The physical insight obtained in this study can provide guidance to future high-power space-borne VLF transmitter developments, laboratory whistler-mode wave injection experiments, and the interpretation of various astrophysical and optical phenomena.

First Results of the Wave Measurements by the WHU VLF Wave Detection System at the Chinese Great Wall Station in Antarctica

Abstract: A Very Low Frequency (VLF) wave detection system has been designed at Wuhan University (WHU) and recently deployed by the Polar Research Institute of China at the Chinese Great Wall station (GWS, 62.22°S, 58.96°W) in Antarctica. With a dynamic range of ∼110 dB and timing accuracy of ∼100 ns, this detection system can provide observational data with a resolution that can facilitate space physics and space weather studies. This paper presents the first results of the wave measurements by the WHU VLF wave detection system at GWS to verify the performance of the system. With the routine operation for 3 months, the system can acquire the dynamic changes of the wave amplitudes and phases of various ground-based VLF transmitter signals emitted in both North America and Europe. A preliminary analysis indicates that the properties of the VLF transmitter signals observed at GWS during the X-class solar flare events are consistent with previous studies. As the HWU-GWS path crosses the South Atlantic Anomaly region, the observations also imply a good connection in space and time between the VLF wave disturbances and the lower ionosphere variation potentially caused by magnetospheric electron precipitation during the geomagnetic storm period. It is therefore well expected that the acquisition of VLF wave data at GWS, in combination with datasets from other instruments, can be beneficial for space weather studies related to the radiation belt dynamics, terrestrial lightning discharge, whistler wave propagation, and the lower ionosphere disturbance, etc., in the polar region.

Active VLF Transmission Experiments Between the DSX and VPM Spacecraft

Abstract: This study presents results from magnetic field line conjunctions between the medium-Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low-Earth orbiting (LEO) very low frequencies (VLF) Propagation Mapper (VPM) satellite. DSX transmitted at VLF toward VPM, which was equipped with a single-axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include (a) the predicted power at LEO from DSX transmissions was too low for VPM to observe; (b) VPM's trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or (c) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray-tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low-frequency (below 4 kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid- and high-frequency transmissions (∼8 and 28 kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5 μV/m and 1 μV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

Discovery and insights from DSX mission’s high-power VLF wave transmission experiments in the radiation belts

Abstract: Space weather phenomena can threaten space technologies. A hazard among these is the population of relativistic electrons in the Van Allen radiation belts. To reduce the threat, artificial processes can be introduced by transmitting very-low-frequency (VLF) waves into the belts. The resulting wave-particle interactions may deplete these harmful electrons. However, when transmitting VLF waves in space plasma, the antenna, plasma, and waves interact in a manner that is not well-understood. We conducted a series of VLF transmission experiments in the radiation belts and measured the power and radiation impedance under various frequencies and conditions. The results demonstrate the critical role played by the plasma-antenna-wave interaction around high-voltage space antennae and open the possibility to transmit high power in space. The physical insight obtained in this study can provide guidance to future high-power space-borne VLF transmitter developments, laboratory whistler-mode wave injection experiments, and the interpretation of various astrophysical and optical phenomena.

Electron Scattering by Very‐Low‐Frequency and Low‐Frequency Waves From Ground Transmitters in the Earth's Inner Radiation Belt and Slot Region

Abstract: The very-low frequency (VLF) and low frequency (LF) waves from ground transmitters propagate in the ionospheric waveguide, and a portion of their power leaks to the Earth’s inner radiation belt and slot region where it can cause electron precipitation loss. Using Van Allen Probes observations, we perform a survey of the VLF and LF transmitter waves at frequencies from 14 kHz to 200 kHz. The statistical electric and magnetic wave amplitudes and frequency spectra are obtained at 1

Unexpected VLF Bursty-Patches Above 5 kHz: A Review of Long-Duration VLF Series Observed at Kannuslehto, Northern Finland

Abstract: Abstract Unexpected short patches of natural VLF emissions at f > 5 kHz have been observed at the ground station of Kannuslehto (KAN, L ~ 5.5) in Northern Finland. In contrast with usual VLF emissions (e.g., chorus, hiss, and quasiperiodic emissions) these high-frequency bursty-patches are observed at frequencies higher than half of the equatorial electron gyro-frequency of the L shell of KAN. Moreover, most of these waves reached frequencies above the local equatorial electron gyrofrequency at L = 5.5. Thus, they cannot be attributed to the classical theory of electron-cyclotron interaction. We present a review of VLF bursty-patches at KAN during winters 2011–2021. These emissions have rarely been observed as they are usually hidden by sferics originating from lightning discharges. Therefore, a special numeric filtering technique was used to reduce noise from sferics. VLF bursty-patches typically occur as sequences of short right-hand polarized bursts separated by a few minutes and lasting several hours. Here, we discuss the spectral structure of long-lasting bursty-patches (6 + hours) and the properties of individual patches. We established two categories: (1) “ triggered-like” hiss-like bursts at f ~ 4–7 kHz with a very abrupt onset and detected under quiet geomagnetic conditions, and (2) “ dash-like” emissions at f > 6 kHz that resemble narrowband hiss and observed under moderate activity. Even though VLF bursty-patches in winters 2011–2021 were observed under weak or slightly disturbed magnetic activity, their annual cyclical occurrence was similar to variations in solar activity. The nature of these VLF patches has not been established yet, but they appear to be generated at L shells lower than that of KAN. Their exact generation region and propagation behavior remain unknown, with further theoretical and experimental research being required.

Impact of partial discharges on MV cable dissipation factor varying supply frequency and waveshape

Abstract: It is shown that under sinusoidal regime, at 50 Hz, measurements highlighted the inception of PD above PDIV level, while that was not the case for lowest test frequency (0.1 Hz), mostly due to the gradual reduction of repetition rate. Measurements under a cosine rectangular waveform were not able to highlight the inception of PD activity, which might indicate that this kind of waveform is unsuitable for $\tan(\delta)$ measurements. On the whole, $\Delta\tan\delta$ can be a quantity that is affected by the inception of PD mostly at industrial frequency, while one could expect significant changes of the values of this quantity under VLF only on the presence of large PD phenomena. In other words, VLF tests detect an increase of $\Delta \tan\delta$ with applied voltage, this may be likely addressable to lossy polarization mechanisms or conduction rather than PD inception.

An Amateur-Radio-Based Open-Source (HW/SW) VLF/LF Receiver for Lower Ionosphere Monitoring, Examples of Identified Perturbations

Abstract: The ground-based monitoring of the lower ionosphere by studying the perturbations of the subionospheric propagation of very-low-frequency/low-frequency (VLF/LF) signals is important in the research of a wide variety of geophysical and Sun/space extreme phenomena. Such perturbations are identified as anomalies in the signal received from the VLF/LF transmitters operating worldwide for military purposes, time code broadcasting, etc. Especially for the study of local ionosphere-influencing phenomena, such as earthquakes, volcanoes, typhoons, etc., the monitoring of several subionospheric propagation paths is necessary. However, it is very difficult to find in the market (or reproduce) hardware (HW) for wide-band VLF/LF receivers that could receive many different transmitters, while the involved software (SW) is mainly proprietary. Aiming to provide a low-cost and easy-to-build alternative for the scientists involved in this research field, we suggest a VLF/LF receiver setup based on amateur radio open-source HW and SW. Its key components are the so-called “mini-whip” active antenna and the freeware “SpectrumLab” and “GPS2Time”. The full HW schematics and all settings of the employed SW configuration for the proposed VLF/LF receiver setup are provided in the article. To check the reliability of the proposed receiver setup, two almost identical VLF/LF radio receivers were installed in the prefecture of Attica in Greece, in June and September of 2021, respectively. Examples of ionospheric perturbations due to different phenomena (solar flares, earthquakes, and a magnetic storm) are provided to show the ability of the proposed receiver setup to provide reliable data for ionosphere-related research.

Ionospheric response to the passage of typhoons observed by subionospheric VLF radio signals

Abstract: The response of the lower ionosphere to the passage of several dozen typhoons has been studied using a regional network of VLF stations in the Russian Far East. The experimental data presented in all cases clearly demonstrates wavelike disturbances of the subionospheric VLF signal amplitude and phase during the active stage of typhoons crossing radio paths. With the exception of magnetoactive and seismoactive days, this means that the disturbances generated by a typhoon, when propagating into the upper ionosphere, pass through the lower ionosphere, causing corresponding disturbances in the amplitude and phase of the VLF signal. Spectral analysis shows that the range of the wave disturbances detected corresponds to the periods of atmospheric internal gravity waves (IGW). A mechanism of the action of IGWs on the lower ionosphere is proposed which allows us to interpret the VLF signal phase variations observed. According to this mechanism, the action of IGW on the lower ionosphere is caused by polarization fields arising during the wave motion of plasma in the lower part of the F layer. These fields projected along geomagnetic field lines into the lower ionosphere cause the upper wall of the Earth—ionosphere waveguide to rise or fall.

On potential use of natural electromagnetic emissions in ELF, VLF and HF radio bands at active landslide areas: Preliminary results from Vinohrady nad Váhom site (Slovakia)

Abstract: The use of natural electromagnetic (EM) emissions in extreme low frequency (ELF) and very low frequency (VLF) radio bands for monitoring and predicting landslide activity is examined. The approach is based on measuring EM emissions using radio receivers and subsequent correlation of their intensity with the amount and character of deformation in the landslide. There are several sources of EM emissions in the rock environment, but it is mostly the piezoelectric impulse of quartz grains during the deformation or sliding movements within rock masses. Deformation was observed in the studied landslide body. Deformations were measured using a geodetic band and a network of stakes, which were installed before and re-measured after recording a natural EM signal attributed to the geodynamic activity. A method of direct monitoring of the entire VLF spectrum using sound cards such as AD converters was applied. So far, several candidate signals were captured and two permanent VLF monitoring stations have been built.

Compact quantum VLF/ELF sources for submarine to air communication

Abstract: For decades communicating between underwater and the air has remained an unsolved problem. Only very low frequency electromagnetic waves have sufficient penetration through seawater to enable an air-underwater RF link. Generation of such waves requires large antennas which cannot be installed on a submarine. In this paper, we demonstrate the feasibility of a compact very low radio frequency source which can be used on a submarine. Theoretical calculations show that by using highly excited Rydberg states of hydrogen atoms which are further split and brought closer together by the linear Stark effect, one can achieve sufficient VLF/ELF radiation power for a submarine at operational depths of hundreds of meters to communicate with a surface/air platform tens of kilometers away.

Synchronous globally observable ultrashort-period pulses

Abstract: We have studied the properties of impulsive geomagnetic disturbances, which are observed synchronously at the network of induction magnetometers of the Institute of Solar-Terrestrial Physics (ISTP SB RAS) and Canadian stations of the CARISMA project [Mann, et al., 2008]. A feature of the pulses we detected is that their frequency range (f~5–30 Hz) lies at the junction of the ranges of two known classes of electromagnetic oscillations: ultra-low-frequency (ULF) oscillations (f<5–10 Hz), or geomagnetic pulsations, and extra-low frequency (ELF) oscillations (f~30–300 Hz); therefore, the 5–30 Hz range is poorly studied. The work is of undoubted interest for physics of processes in the magnetosphere–ionosphere–atmosphere system. Morphological analysis of the pulses detected has been carried out using data from ISTP stations. As a result, we obtained statistical characteristics of the pulses, plotted their dynamic spectra, and determined a number of unusual properties that distinguish them, on the one hand, from geomagnetic pulsations of the pulsed type (irregular pulsations of the Pi1B type), and, on the other hand, from higher frequency ELF and VLF signals (atmospherics, whistlers, etc.). On the basis of the results, we have made an assumption that a source of the pulses under study can be electrical sprites caused by powerful thunderstorms at middle and low latitudes. Using the results obtained by Wang, et al. in 2019 on spatial and temporal fixation of sprites in North China, we have confirmed that ultra-short-period pulses occur following the emergence of sprites. Thunderstorm activity, both local and global, is considered to be one of the main sources of excitation of the ionospheric Alfvén resonator (IAR), which plays an important role in coupling the ionosphere and the magnetosphere. The pulsed oscillations of interest may be one of the agents through which the energy of thunderstorms is transferred to IAR, thereby including the atmosphere in the system considered.

Synchronous globally observable ultrashort-period pulses

Abstract: We have studied the properties of impulsive geomagnetic disturbances, which are observed synchronously at the network of induction magnetometers of the Institute of Solar-Terrestrial Physics (ISTP SB RAS) and Canadian stations of the CARISMA project [Mann, et al., 2008]. A feature of the pulses we detected is that their frequency range (f~5–30 Hz) lies at the junction of the ranges of two known classes of electromagnetic oscillations: ultra-low-frequency (ULF) oscillations (f<5–10 Hz), or geomagnetic pulsations, and extra-low frequency (ELF) oscillations (f~30–300 Hz); therefore, the 5–30 Hz range is poorly studied. The work is of undoubted interest for physics of processes in the magnetosphere–ionosphere–atmosphere system. Morphological analysis of the pulses detected has been carried out using data from ISTP stations. As a result, we obtained statistical characteristics of the pulses, plotted their dynamic spectra, and determined a number of unusual properties that distinguish them, on the one hand, from geomagnetic pulsations of the pulsed type (irregular pulsations of the Pi1B type), and, on the other hand, from higher frequency ELF and VLF signals (atmospherics, whistlers, etc.). On the basis of the results, we have made an assumption that a source of the pulses under study can be electrical sprites caused by powerful thunderstorms at middle and low latitudes. Using the results obtained by Wang, et al. in 2019 on spatial and temporal fixation of sprites in North China, we have confirmed that ultra-short-period pulses occur following the emergence of sprites. Thunderstorm activity, both local and global, is considered to be one of the main sources of excitation of the ionospheric Alfvén resonator (IAR), which plays an important role in coupling the ionosphere and the magnetosphere. The pulsed oscillations of interest may be one of the agents through which the energy of thunderstorms is transferred to IAR, thereby including the atmosphere in the system considered.

Measuring effects of height on the autonomic nervous system in middle-aged adolescents using the very low frequency band of heart rate variability

Abstract: Abstract Previous studies have revealed the association between falling accidents and stress, measured via heart rate variability (HRV). However, none have studied this association using the very low frequency (VLF) band of HRV in adolescent populations. This study aimed to fill this gap by recruiting 90 adolescents to perform a light physical task at varying heights. Heart rates were used to calculate short-term HRV. The results showed a positive correlation between VLF bands and parasympathetic indices and a negative correlation with sympathetic indices, demonstrating the balancing effects of the autonomic modulation associated with height. The lowest VLF bands were obtained as 79.25 ms2 at 10 m (p < 0.001) and 62.87 ms2 at 9 m (p < 0.001) for the experienced and non-experienced male groups, respectively, and 28.09 ms2 at 6 m (p = 0.001) for the female group. The results also suggested the need for a relatively lower height restriction for female adolescents than for males. Practitioner summary: Increased working heights can cause stress, which leads to falling accidents. The very low frequency band was shown to be associated with parasympathetic and sympathetic indices. Furthermore, the results suggested that the height limit necessary for providing a safe working environment may be lower for female adolescents than for males. Abbreviations: HRV: heart rate variability; VLF: very low frequency; ms2: absolute power; ANS: autonomic nervous system; PNS: parasympathetic nervous system; SNS: sympathetic nervous system; RR: intervals between two successive peaks of R waves; RMSSD: root mean square of successive RR interval differences; SD1: Poincaré plot index of standard deviation 1; SD2: Poincaré plot index of standard deviation 2; HF: high frequency; LF: low frequency; BMI: body mass index; ECG: electrocardiography; HR: heart rate; FFT: fast Fourier transformation; IQR: interquartile range; r: non-parametric partial correlation coefficient; η2: eta-squared; EM: experienced males; NM: non-experienced males; NF: non-experienced females; EEG: electroencephalogram

Spectrum Based Prediction for Seismic Activity

Abstract: Based on the retrospective study of seismic activity we have analyzed prospective seismic activity based on results from Continuous Wavelet Transform (CWT) and from studies on The logged data of Very Low Frequency (VLF) transmitted sub-ionospheric signals at 16.4 kHz from Novik, Norway (Lat: 66.97° S; Long: 13.9° E), 19.8 kHz from North West Cape, Australia (Lat: 21.82° S; Long: 114.16o E) and 25 kHz from Petropavlovsk-Kamchatsky, Russia (Lat: 53.15° N; Long:158.92° E,) at Kolkata (Lat: 22.56° N, Long: 88.5° E) are studied throughout the period of April 3, 2013–April 24, 2013, when there happened 18 large earthquakes with M ≥ 5. Here the introduction of other signals which are generated due to the seismic activity (considered as noise) is captured from the spectrum analysis using the method of CWT. In this method, we can watch a yellow region in the spectrum of blue color. Blue color spectrum is for VLF signals without any noise in fair-weather conditions and in this yellow color indicates the introduced noise which may also start to observe few hours (12 h) prior to the event of the earthquake. The identified event may have been the result of a combination of changes in seismicity patterns and the yellow color gives the forecasting of the main event as a signature of the prediction.

Lightning interaction with the ionosphere

Abstract: Lightning discharges, including cloud-to-ground (CG) and intracloud (IC) lightning, are known to emit electromagnetic pulses (EMPs) in a wide frequency band ranging from few Hz up to hundreds MHz [1]. During the breakdown and ionization processes (mostly from leader processes and streamers), there are strong emissions in the HF (3-30 MHz) and VHF (30-300 MHz) bands. When high currents occur in previously ionized channels (mostly from return strokes and the active stage of cloud flashes), the most powerful emissions concentrate in the very low frequency (3-30 kHz, VLF) and low frequency (30-300 kHz, LF) bands [2]. Among them, the VLF/LF waves of lightning discharges can propagate long distances with low attenuation by reflection between the ground surface and the lower D-region ionosphere (60-90 km), namely the so-called earth-ionosphere waveguide (EIWG).In order to investigate the lightning EMPs interaction with the ionosphere, a number of models and methods have been developed in the literature, such as the wave-hop (ray theory) method [3-6], the waveguide mode theory [7-9], or numerical methods such as the finite-difference-time-domain (FDTD) method [10-17] and the full-wave finite element method (FEM) [18,19]. Previous studies indicate that the amplitude and phase perturbation for lightning VLF/LF signals have a complicated relationship with the ionospheric D region parameters. The propagation of lightning EMPs between the earth ground surface and the lower D region ionosphere can be affected by many factors, such as the propagation distances [10,14,20], the ground conductivity [14,20], the electron and neutral particle densities [13,21,22], the Earth curvature [23,24], the presence of the Earth's magnetic field [22,25-27], and the presence of mountainous terrain [24].In this chapter, we will first introduce the propagation theory of lightning EMPs interaction with the ionosphere on the basis of the full-wave FDTD method. We will then investigate the propagation effect of lightning radiated electromagnetic (EM) fields in the EIWG by considering the effect of the Earth curvature, the effect of the ground conductivity, and the effect of different ionospheric profiles. Finally, we will present applications, including (1) propagation of narrow bipolar events (NBEs) at different distances, (2) lightning electromagnetic fields propagation over mountainous terrain, and (3) the optical emissions of lightning-induced transient luminous events in the nonlinear D-region ionosphere.

The Interaction of Specific Frequency Bands in the Geomagnetic Field Diurnal Spectrum, With Specific Frequency Bands in the Human Heart Rate Variability Diurnal Spectrum.

Abstract: Abstract BACKGROUND. For over 50 years, experiments have been demonstrating an interaction between the dynamics of the human heart and the dynamics of the earth’s magnetic field. In these experiments, specific frequency components in the variability of the geomagnetic field (GMF), involving micropulsations, are shown to significantly interact with specific frequency components in human heart rate variability (HRV). A significant difference was found in the HRV ratio of VLF/HF, on days of low geomagnetic variability (L gmf) compared with days of high geomagnetic field variability (H gmf) (P = 0.0001, n = 24, effect size 1.014). METHODS. Experiments were conducted prospectively, over a period of 3 years, on one 70 year old, healthy female subject, using a 24 hour holter electrocardiogram (ECG) machine, and the results were analysed for the HRV frequency spectral components. The local geomagnetic field was simultaneously monitored over 24 hour periods, using a 3-axis fluxgate magnetometer, and the frequency spectra were analysed for the various spectral power components. Statistical analysis was performed using XLSTAT software. RESULTS. It was shown that there was a correspondence in the power spectra of the high (HF) and very low (VLF) frequency components in HRV and GMF variability. It was shown that there was a significant difference in the ratio of the VLF and HF frequency band power of the HRV between the ECGs recorded during days of lower geomagnetic variability, and days of higher geomagnetic variability. CONCLUSION. These findings support the hypothesis of a homeostatic response of the human heart to the earth’s magnetic field. This may have implications for prognosis and management of heart disease, during times of significant environmental change.

Geoelectric section of the Selenga river water area based on the results of VLF-LF radio impedance soundings

Abstract: The surface impedance and geoelectric section of the Selenga River in the area of the Ulan-Ude water intake station were determined based on measurements by radio impedance sounding in the very-long-wave - long-wave (VLF-LF) radio wave bands. Radio impedance sounding by IPI-300 equipment on a 360 m long profile made it possible to determine the electrical properties and structure of the layered medium "ice-water-bottom soil" by changing the impedance and geoelectric section. The thickness of the ice on the ice track varied from 0.6 m to 1.2 m. The electrical resistivity of water from wells was 71 Ohm·m at a temperature of 1°C. An electromagnetic model of the Selenga River water area based on the local electrical characteristics of the underlying medium has been created.

Spectral Analysis and Information Entropy Approaches to Data of VLF Disturbances in the Waveguide Earth-Ionosphere

Abstract: Very low frequency (VLF) signals are considered as an important tool to study ionosphere disturbances. We have studied variations in signal amplitude of the Japanese JJI transmitter received by a network of eight Japan stations. The distinctions between characteristics of daytime and nighttime disturbances are considered. Signal processing based on spectral analysis is used to evaluate typical periodicities in the VLF signals in the time range from minutes to hours. In particular, we have retrieved quasi-wave oscillations of the received signal with periods of 4–10 and 20–25 min, which can be associated with atmospheric gravity waves excited by the solar terminator, earthquakes or other reasons. In addition, oscillations at periods of 3–4 h are observed, probably, caused by long-period gravity waves. We also calculate the information entropy to identify main details in daily VLF variations and influence of solar flares. It is shown that the information entropy increases near sunrise and sunset with seasonal variation, and that solar flares also lead to the growth in information entropy. A theoretical interpretation is given to the typical features of ultra-low frequency modulation of VLF electronagnetic wave spectra in Waveguide Earth-Ionosphere, found by processing the experimental data.

Using VLF Transmitter Signals at LEO for Plasmasphere Model Validation

Abstract: This study presents analysis of very low frequency (VLF) transmitter signal measurements on the Very-Low-Frequency Propagation Mapper (VPM) CubeSat in low-Earth orbit. Six months of satellite operation provided good data coverage, used to build global statistical maps of VLF power distribution. The power distribution above four powerful transmitters is used as input for ray tracing to study signal propagation to the conjugate hemisphere in two plasmaspheric density models. The ray tracing results are further compared with VPM measurements to determine which model provides better agreement with observations. As ray propagation largely depends on the background plasma density distribution, this indirect method can be used for plasmaspheric density model validation as an alternative to multipoint in situ plasma measurements that may not be readily obtainable. In addition, it can be used to investigate Landau damping and ducted versus non-ducted propagation of VLF signals.

Parametric Interaction of VLF and ELF Waves in the Ionosphere

Abstract: In this Chapter we analyze a non-linear parametric interaction between Very Low Frequency (VLF) and Extremely Low Frequency (ELF) waves in the ionosphere. We demonstrate that nonlinear parametric coupling between quasi-electrostatic Lower Oblique Resonance (LOR) and ELF waves significantly contributes to the VLF electromagnetic whistler wave spectrum. Analytical and numerical results are compared with experimental data obtained during active space experiments and satellite data. These data clearly show that presence of VLF waves in the region of plasmasphere boundary layer, where there are no injected due to substorm/storm activity energetic electrons with energies of tens keV can strongly affect the radiation belt boundary.

Age-related changes in heart rate variability and spectral parameters of blood pressure variability in the range of 20-90 years

Abstract: Целью нашей работы стало описание половозрастных особенностей показателей вариабельности сердечного ритма (СР), систолического и диастолического артериального давления (АДс и АДд), а также чувствительности барорефлекса (ЧБР). Методы: спироартериокардиоритмография, позволяющая регистрировать все перечисленные показатели у каждого человека одновременно. В исследовании приняли участие 1559 человек (1119 женщин и 440 мужчин), в возрасте 20-90 лет. Весь возрастной интервал был разбит на 10 диапазонов по 7 лет. Результаты. Показано, что большинство изученных параметров изменяются с возрастом. Для женщин было характерно линейное снижение частоты пульса при возрастании АДс, у мужчин такие закономерности не обнаружены. Для показателей вариабельности СР у обоих полов описана U-образная динамика: снижение с последующим возрастанием показателей SDNN (отражающего сбалансированность автономной регуляции) и общей мощности спектра, с минимумом в 55-76 лет; возрастание в самых старших возрастных группах (после 62 лет) связанных с вагусной активностью RMSSD, pNN50, и мощности диапазона HF; возрастание с последующим снижением стресс-индекса и вклада в спектр вариабельности СР диапазона VLF. Показано, что в возрастном диапазоне 34-55 лет у мужчин в спектре вариабельности СР был выше относительный вклад LF, у женщин - наоборот, HF, с соответствующими величинами отношения LF/HF. Выявлена зависимость от возраста, сходно у мужчин и женщин, спектральных показателей вариабельности АД: при отсутствии возраст-зависимой динамики общей мощности спектров вариабельности как АДс, так и АДд, обнаружено перераспределение относительной мощности разных диапазонов - линейное возрастание вклада диапазона VLF, и снижение после 55 лет вклада диапазона LF в спектрах вариабельности АДс и АДд. По амплитудным показателям барорефлекса минимальные величины зафиксированы у обоих полов в возрастном диапазоне 62-69 лет. Заключение. Полученные данные свидетельствуют о снижении с возрастом (после 55 лет) относительного вклада в регуляцию сердечно-сосудистой системы симпатических влияний, при возрастании представленности нейрогенных вагусных и гуморальных компонентов. Величины изученных показателей в различных половозрастных выборках могут быть взяты за основу при их нормировании в прикладных целях. The aim of this study was to describe the age and sex characteristics of heart rate variability (HR), systolic and diastolic blood pressure (sBP and dBP), as well as baroreflex sensitivity (BRS). Methods: Spiroarteriocardiorhythmography recored all reported indicators for each person. The study included 1559 people (1119 women and 440 men) aged 20-90 yrs. The age interval was divided into 10 ranges of 7 yrs each. Results. Most of the measured variables changed with age. Women had a linear decrease in HR with an increase in BP, but this pattern was not found in men. In both sexes, HR variability had a U-shaped dynamics, i.e., a decrease followed by an increase in SDNN. This reflected the balance of autonomic regulation and total spectrum power, with a minimum at 55-76 yrs. After 62 yrs, an increase in SDNN was observed in the oldest age groups that was associated with vagal activity RMSSD, pNN50, and HF band power. This increase was followed by a decrease in the stress index and the contribution to the HR spectrum of the VLF range. For men aged 34-55 years, the relative contribution of LF to the HR variability spectrum was greater, but in women the contribution of HF was greater. There were corresponding gender effects on values of the LF/HF ratio. The dependence on age of spectral indicators of BP variability was equal in men and women. In the absence of age-dependent changes of the total power of the variability spectra of both sBP and dBP, a redistribution of the relative power of different ranges was found. There was a linear increase in the contribution of the VLF range, and a decrease after 55 yrs in the contribution of the LF range to the variability spectra of sBP and dBP. According to the amplitude indicators of baroreflex, the minimum values were recorded for both sexes in the range of 62-69 yrs. Conclusion. The results indicate a decrease with age after 55 years in the relative contribution of sympathetic influences to regulation of the cardiovascular system that was associated with an increase in the representation of neurogenic vagal and humoral components. The values of the measured indicators in different sexes and ages can be applied in practice as a basis for their normalization.

ELF/VLF Wave Radiation Experiment by Modulated Ionospheric Heating Based on Multi-Source Observations at EISCAT

Abstract: Ground-based high-frequency modulated waves can periodically heat the ionosphere and create “virtual antennas”, which can radiate extremely low frequency (ELF, 0.3–3 kHz) or very low frequency (VLF, 3–30 kHz) waves for long-distance communication. Ionospheric X-mode and O-mode heating experiments using amplitude and beat-wave (BW) modulations were conducted on 21 November 2019. Experimental results were analyzed from multiple perspectives based on data from Dynasonde, a magnetometer, stimulated electromagnetic emissions, an ELF/VLF signal receiver, and ultra-high-frequency radar. The strongest excited ELF/VLF signals in previous BW modulation heating experiments were around 8–12 kHz; however, in this experiment, no signal excited in this frequency range was observed, and the signal with the highest signal/noise ratio was at the frequency of 3517 Hz, which will aid in understanding the best communication frequency under different ionospheric backgrounds. It is well-accepted that the electron temperature changes periodically with the modulation frequency. However, we noted that the electron temperature had insufficient cooling during the O-mode modulated heating process and then increased again, resulting in a continuous electron temperature increase. We found that this was related to the change in ion composition after analyzing ion-line spectra, which will be helpful in studying the effect of modulation heating on the ionosphere background.

Correlational Analysis of the ELF – VLF Nighttime Atmospherics Parameters

Abstract: Tweek-atmospherics (tweeks), along with radio transmission by VLF radio stations, are used to study the lower ionosphere. Electromagnetic pulse radiation, which has been excited by the lightning discharges, has a maximum spectral density at extra low frequencies range (ELF, 300...3000 Hz) and very low frequencies (VLF, 3...30 kHz). The Earth-ionosphere cavity serves as a waveguide for electromagnetic waves in these frequency ranges. On the spectrogram of the tweek, the initial part is a linearly polarized broadband signal, and then a number of individual harmonics are observed. Their instantaneous frequencies decrease, asymptotically approaching approximately multiples of the cutoff frequencies of the waveguide. The single position method for lightning location and estimation of the ELF wave’s reflection heights in the lower ionosphere by tweeks has been implemented into the computational algorithm. The clusters with approximately the same azimuths and distances to sources which have been obtained during the same night have been identified upon the ensemble of tweek-atmospheric records. The data were accumulated at the Ukrainian Antarctic Station "Akademik Vernadsky" in 2019. The location of the receiving complex in the near-polar region makes it possible to register tweek sources in two world thunderstorm centers with geographic azimuths from –60° to 130°. The results of processing these data have been used by studying the correlation matrix and partial correlation coefficients to identify causal relationships between the three main parameters of the tweek, such as (1) the average azimuth of the arrival of tweeks in regard to the magnetic meridian, (2) the average distance to the center of the cluster of tweek sources (lightning discharges), and (3) the average number of tweek harmonics. The same correlation analysis was applied to two groups with distances to sources of 2.2...7.5 Mm and 7.6...9.5 Mm used for study in detail. It is shown that the partial correlation coefficients between the number of tweek harmonics and the difference of the magnetic azimuth from the magnetic east are 0.624 (for the entire range of distances), 0.696 (for far tweek sources) and 0.595 (for main middle range), so, they always exceed the values of 0.1% significance level. The correlation of tweek spectrum with the distance to the tweek source in the range of 2.2…7.5 Mm has been shown to be comparable in magnitude or to exceed the correlation of tweek spectrum with the magnetic azimuth. The elimination of this masking effect by calculating the partial correlation coefficients made it possible to reveal the magnetic azimuth dependences of the tweek spectra if tweek propagates in a region outside the geomagnetic equator. Thus, the effect of non-reciprocity of propagation of ELF – VLF waves in regard to the magnetic meridian in the east – west and west – east directions is found in the spectra of tweek-atmospherics. It results in an increased probability of detecting tweeks with higher harmonics if their directions of arrival are close to the geomagnetic east. It is also shown that this effect, as a result of increased attenuation during the propagation of ELF – VLF radiation from the west and weakened attenuation during propagation from the east, leads to a highly significant correlation (with probability level more than 99.9%) between the magnetic azimuths of tweeks and the lengths of their paths to the receiving station.

The Relationship Between “Large” Early/Fast VLF Events and Modal Propagation Nulls

Abstract: Early/fast very low frequency (VLF) events evidence the direct impulsive coupling of lightning energy to the overlying mesosphere and lower ionosphere, and they are readily detected as rapid (<20 msec) changes to the amplitude and phase of VLF transmitter signals propagating within the Earth-ionosphere waveguide. Typical early/fast VLF events exhibit 0.1-1.0 dB amplitude changes, but can be smaller (<0.1 dB) or significantly larger (>10 dB). Large (>6 dB) early/fast events have not yet been satisfactorily simulated, and past work has suggested that large amplitude early/fast VLF events may only be detected when the receiver is located in a VLF propagation “null.” VLF propagation “nulls” result from the destructive interference of propagating waveguide modes, and they would result in deceptively large relative changes in field amplitude by reducing the magnitude of the ambient field, rather than by increasing the magnitude of the scattered field. In this paper, we present a new method to quantify the degree of destructive modal interference at the location of the VLF receiver, and we apply this method to analyze the impact of modal interference on the detected amplitude changes associated with 235 early/fast VLF events occurring over the Midwestern United States on 21 September 2016. No meaningful dependence on destructive modal interference is detected. We conclude that contemporary models of lightning-ionosphere interactions and/or VLF scattering require modification in order to accurately reproduce observations of large early/fast VLF events.