Ionosphere Cavity Research Articles

Terminator effect in the model Schumann resonance signals

We model Schumann resonance (SR) amplitude variations driven by the diurnal and seasonal motion of the day–night interface in the Earth – ionosphere cavity that is regarded as the terminator effect. The current moment of the equatorial source is a constant. The observer occupies the middle, the high latitudes, or the South Pole. Vertical electric field is calculated at the frequencies 8, 14, and 20 Hz. The following results were obtained. Amplitude modulations relevant to the day-night interface acquire ubiquitous patterns in the local time of the observer. The date–time changes have a convex lens-like structure at the mid-latitude observatory. The simultaneous displacement of the source and observer along the longitude causes an appropriate shift of the date–time pattern plotted in the universal time (UT). Relocation of the observer from the Northern to the Southern Hemisphere shifts the pattern by 6 months. These changes might totally alter the outline of the observed “12 months - 24 h” pattern. The shape of date–time variation alters from the convex lens-like to the cross-like when the observer is located in the high latitudes owing to the polar day and the polar night periods. When the observatory occupies a geographic pole, the season-diurnal variations resemble a ‘chessboard’. The Wentzel – Kramers – Brillouin (WKB) approximation allows obtaining the general outline of the date–time variations. The data obtained might be interpreted as a combination of two effects. The first one forms the general outline of temporal changes; and it is linked to the height of the Earth-ionosphere cavity above the source and the observer. The second one is related to "reflections" of ELF radio waves from the day–night interface, and it drives oscillations with the period equivalent to the wavelength at the particular frequency. All effects are related to the position of the day–night interface and occur in the observer's local time. The range of amplitude modulations in a non-uniform cavity is of ± (10–15)% relative to the average amplitude.

Physical effects from the powerful Tonga volcanic eruption of January 15, 2022, in the earth–atmosphere–ionosphere–magnetosphere system

The powerful Tonga volcanic explosion and eruption has been reliably determined to cause numerous processes on a global scale; nevertheless, a comprehensive model of these processes is absent in the literature, which remains an urgent scientific objective. The purpose of this paper is to thoroughly analyze and model the main physical processes that accompanied the volcanic explosion within the Earth (lithosphere, tectonosphere, ocean) – atmosphere – ionosphere – magnetosphere (EAIM) system on January 15, 2022. The study attempts, for the first time, to comprehensively model or estimate the magnitude of the main effects arising in the solid Earth, the troposphere, the ionosphere, and in the magnetosphere. The effects include a rich assortment of waves: Lamb, sound, seismic, tsunami, infrasonic, and atmospheric gravity waves, which propagated on a global scale, with the blast wave launching secondary tsunamis and seismic waves. The powerful waves nonlinearly distorted their profiles and suffered nonlinear attenuation, while the electrical processes in the troposphere caused perturbations in the global electric circuit enhancing the strength of the ionospheric electric fields by one to two orders of magnitude that led to the emergence of secondary processes in the magnetosphere and radiation belts. The volcanic explosion excited all resonators in the EAIM system, perturbing their parameters. Resonances occurred in the Earth's solid shell (Rayleigh wave), in the Earth–ionosphere cavity (Schumann resonances), in the atmosphere (acoustic resonance), in the ionosphere (Alfvén resonances), and in the magnetosphere (magnetospheric resonance). The magnetic effect has been estimated of the submarine volcanic explosion and eruption (∼100–1000 nT), the tsunami (∼0.1 nT), of the volcanic plume (∼1–10 nT), and in the ionosphere due to the ionospheric hole (ΔB ∼ 0.1–10 nT) and an external current in the atmospheric wave field (ΔB ∼ 0.1–1 nT). The magnetospheric effects were caused by electromagnetic emissions in the ∼10–100 kHz band generated by lightning in the volcanic plume, which triggered the precipitation of energetic radiation belt particles into the ionosphere. This is the first time that feedback and main coupling processes, positive and negative, have been observed to emerge among the EAIM system components.

A New SLF/ELF Algorithm of Fields Excited by a Radiator in a Soil Foundation in the Earth–Ionosphere Cavity

A New SLF/ELF Algorithm of Fields Excited by a Radiator in a Soil Foundation in the Earth–Ionosphere Cavity

Integrated Schumann Resonance Intensity as an Indicator of the Global Thunderstorm Activity

This paper addresses the accuracy of estimates for the contemporary level of global thunderstorm activity found from the synchronous records of integrated Schumann resonance (SR) intensity at two high-latitude observatories in the Northern and Southern hemispheres. The results are based on numerical simulations of electromagnetic fields in the frequency band of the global (Schumann) resonance in the Earth–ionosphere cavity characterized by a realistic conductivity profile in the middle atmosphere. The credible distribution is used for global thunderstorm activity in space and time. The paired observatory locations are considered either at the geographic poles or at Svalbard and the Antarctic Peninsula. The seasonal variations in the spatial distribution of global thunderstorms are adopted from the OTD satellite observations. The diurnal variations imply the spatial and temporal distribution of lightning strokes measured by the WWLLN network for an arbitrarily chosen date of 18 January 2022. The results obtained suggest that simultaneous records of the integrated SR intensity at Svalbard and in Antarctica provide errors below 3% in the diurnal variations of global thunderstorm activity with a temporal resolution of 10 min. The seasonal changes in global thunderstorm intensity are estimated with an error of ~10%. Since the level of global thunderstorm activity varies by a factor of two on the both time scales, the estimates confirm the appropriate accuracy of the estimate of thunderstorm activity from the concurrent measurements at the high-latitude SR observatories in the Arctic and Antarctic.

Power flux in the Schumann resonance band linked to the eruption of Tonga volcano on Jan. 15, 2022. (Two point measurements of Umov-Poynting vector)

Observations and model computations are compared of the Umov – Poynting vector (UPV) in the Schumann resonance (SR) band during the eruption of Tonga volcano on Jan. 15, 2022. The volcanogenic SR signals were recorded at the dual site: at the Cumiana observatory (44.96° N and 7.42° E) in the vertical electric field component and at the Sos Enattosobservatory (40.47° N, 9.48° E) in the two orthogonal ELF components of horizontal magnetic field. Initially, we present computations in the update model of the Earth – ionosphere cavity with the global lightning stroke distribution found by the Worldwide Lightning Location Network (WWLLN), which demonstrates the workability of our approach. The hodographs of UPV were obtained in the frequency (formally rigorous) and the time domain during the major phase of eruption. The positions of both of them suitably agree with each other and indicate on the wave arrival from the compact area around volcano during the major phase of Tonga eruption. The UPV magnitude indicates that the contribution from the Tonga volcano has elevated the observed level of ELF power flux by the factor of six relative to the quiet condition.

The Effect of an Extremely Low-Frequency Electromagnetic Field on the Drought Sensitivity of Wheat Plants.

Extremely low-frequency magnetic fields are thought to be capable of modulating the resistance of plants to adverse factors, particularly drought. Magnetic fields in this frequency range occur in nature in connection with so-called Schumann resonances, excited by lightning discharges in the Earth-ionosphere cavity. The aim of this work was to identify the influence of a magnetic field with a frequency of 14.3 Hz (which corresponds to the second Schumann harmonic) on the transpiration and photosynthesis of wheat plants under the influence of drought. The activity of photosynthesis processes, the crop water stress index, relative water content and leaf area were determined during drought intensification. At the end of the experiment, on the 12th day of drought, the length, and fresh and dry weight of wheat shoots were measured. The results obtained indicate a protective effect of the magnetic field on plants in unfavorable drought conditions; the magnetic field delayed the development of harmful changes in the transpiration and photosynthesis processes for several days. At the same time, in the absence of the stressor (drought), the effect of the electromagnetic field was not detected, except for a decrease in relative transpiration. In favorable conditions, there were only minimal modifications of the photosynthetic processes and transpiration by the magnetic field.

Using 3D Ray Tracing Technology to Study the Disturbance Effect of Rocket Plume on Ionosphere

In this paper, the initial neutral atmospheric parameters, background ionospheric parameters and geomagnetic field parameters of the ionosphere are obtained by NRLMSISE-00 model, IRI-2016 model and IGRF-13 model, respectively. Considering the neutral gas diffusion process, ion chemical reaction and plasma diffusion process, a three-dimensional dynamic model of chemical substances released by rocket plume disturbing the ionosphere is constructed. The influence of the disturbance on the echo path of high frequency radio waves with different incident frequencies is simulated by using three-dimensional digital ray-tracing technology. Using this model, the process of ionospheric disturbance caused by the main chemical substances H2 and H2O in the rocket plume under three different release conditions: fixed-point release at 300 km, vertical path at 250–350 km and parabolic path at 250–350 km, and the influence of the ionospheric cavity on the radio wave propagation of high frequency radio waves at different frequencies are simulated. The main purpose of the article is to focus on the effect of the cavity generated by the rocket exhaust on the propagation of radio waves. It mainly studies the perturbation effect on the ionosphere under different release conditions, considers the neutral gas diffusion process, ion chemical reaction and plasma diffusion process, and establishes the three-dimensional dynamics of the ionospheric electron density and the spatiotemporal distribution of the plume plasma learning model. Finally, the three-dimensional ray-tracing algorithm is used to simulate the propagation path of the radio wave through the disturbance area. We considered three different release conditions, including fixed-point release, vertical path and parabolic path. The ionospheric disturbances produced by these different releases are compared and analyzed, and their effects on the propagation path of radio waves are studied.

Finite-difference time-domain analysis of ELF radio wave propagation in the spherical Earth–ionosphere waveguide and its validation based on analytical solutions

Abstract. The finite-difference time-domain (FDTD) model of electromagnetic wave propagation in the Earth–ionosphere cavity was developed under assumption of an axisymmetric system, solving the reduced Maxwell equations in a 2D spherical coordinate system. The model was validated on different conductivity profiles for the electric and magnetic field components for various locations on Earth along the meridian. The characteristic electric and magnetic altitudes, phase velocity, and attenuation rate were calculated. We compared the results of numerical and analytical calculations and found good agreement between them. The undertaken FDTD modeling enables us to analyze the Schumann resonances and the propagation of individual lightning discharges occurring at various distances from the receiver. The developed model is particularly useful when analyzing ELF measurements.

Solar Cycle-Modulated Deformation of the Earth–Ionosphere Cavity

The Earth–ionosphere cavity resonator is occupied primarily by the electromagnetic radiation of lightning below 100 Hz. The phenomenon is known as Schumann resonances (SR). SR intensity is an excellent indicator of lightning activity and its distribution on global scales. However, long-term measurements from high latitude SR stations revealed a pronounced in-phase solar cycle modulation of SR intensity seemingly contradicting optical observations of lightning from satellite, which do not show any significant solar cycle variation in the intensity and spatial distribution of lightning activity on the global scale. The solar cycle-modulated local deformation of the Earth–ionosphere cavity by the ionization of energetic electron precipitation (EEP) has been suggested as a possible phenomenon that may account for the observed long-term modulation of SR intensity. Precipitating electrons in the energy range of 1–300 keV can affect the Earth–ionosphere cavity resonator in the altitude range of about 70–110 km and modify the SR intensities. However, until now there was no direct evidence documented in the literature supporting this suggestion. In this paper we present long-term SR intensity records from eight stations, each equipped with a pair of induction coil magnetometers: five high latitude (|lat| > 60°), two mid-high latitude (50° < |lat| < 60°) and one low latitude (|lat| < 30°). These long-term, ground-based SR intensity records are compared on the annual and interannual timescales with the fluxes of precipitating 30–300 keV medium energy electrons provided by the POES NOAA-15 satellite and on the daily timescale with electron precipitation events identified using a SuperDARN radar in Antarctica. The long-term variation of the Earth–ionosphere waveguide’s effective height, as inferred from its cutoff frequency, is independently analyzed based on spectra recorded by the DEMETER satellite. It is shown that to account for all our observations one needs to consider both the effect of solar X-rays and EEP which modify the quality factor of the cavity and deform it dominantly over low- and high latitudes, respectively. Our results suggest that SR measurements should be considered as an alternative tool for collecting information about and thus monitoring changes in the ionization state of the lower ionosphere associated with EEP.

Model sub-ionospheric ELF – VLF pulses

The propagation of natural wide-band pulsed radio signals in the spherical Earth – ionosphere cavity is numerically simulated. The frequency band considered is 1–10,000 Hz. We address the TM waves originated from a vertical lightning stroke of the Bruce and Golde, Williams, or Jones model. Major data are based on the Williams’ lightning model. The upper boundary of the cavity is horizontally stratified and the conductivity of each stratum depends on its altitude. We apply several models of vertical conductivity profile, and all of them adequately describe the global electromagnetic (Schumann) resonance. The electromagnetic fields are expanded into a series of normal waves (modes). The frequency dependence of the complex propagation constant of each mode is found by using the full wave solution in the form of Riccati equation. The roots of this equation are found by iterations. Various source–observer distances are used in the waveguides with different height conductivity profiles. The Fourier transform of complex spectra provides the time domain realizations of the wide-band pulsed signals. The results of different model profiles are compared and their correspondence to the known observational data is demonstrated. Computations show that the model pulses similar to either tweek or to slow tail atmospherics. They acquire the form of slow tail atmospherics when the conventional night or day conductivity profiles are used. In this case, a train of VLF oscillations precedes a typical Q-burst. The delay of Q-burst from the VLF precursor grows in proportion to the source – observer distance. The tweek atmospheric emerges combined with the Q-burst when the mesosphere conductivity profile has a section of rapid conductivity increase by 4–5 orders of magnitude in the altitude interval of a few kilometers around 90 km.

Global Lightning Quanta

AbstractThe World Meteorological Organization recently declared lightning an essential climate variable which makes the global lightning flash rate density a key quantity, currently assessed by geostationary satellites and ground‐based lightning location networks. Yet, no theory has been put forward to explain the physical relationships between the thermodynamic temperature of the Earth’s atmosphere T, the global lightning flash occurrence frequency fg, and its radiant energy E of resonant electromagnetic waves within the Earth ionosphere cavity. These three parameters are combined here by adapting the rigorous framework of quantum physics. The minimum amount of radiant energy produced by the lightning flash occurrence frequency is the global lightning quantum E = hfg, h being Planck’s constant. The superposition of numerous global lightning quanta distributes its radiant energy around the world as Earth ionosphere cavity resonances. The novel theory is in agreement with measurements using a radiometer at Arrival Heights, Antarctica, as part of the Stanford ELF/VLF Radio Noise Survey. It is found that the measurements agree with the theory within ∼30%. The operation of the theory is illustrated with an interpretation of the measurements for an exemplary thermodynamic energy. In this case, the measurements correspond to a radiant temperature ∼−30°C akin to the mixed phase region of thunderclouds where lightning discharges are initiated. The theory can help to assess the mutual impact of climate change and global lightning on each other as proposed by the World Meteorological Organization.

Automatic detection of Ionospheric Alfvén Resonances in magnetic spectrograms using U-net

Ionospheric Alfvén Resonances (IARs) are weak discrete non-stationary Alfvén waves along magnetic field lines, at periods of ∼0.5–20 Hz, that occur during local night-time, particularly during low geomagnetic activity. They are detectable through time–frequency analysis (spectrograms) of measurements made by sensitive search coil magnetometers. The IARs are generated by the interaction of electromagnetic energy partially trapped in the Earth–ionosphere cavity with the main geomagnetic field and their behaviour provides proxy information about atmospheric ion density between 100–1000 km altitude. Limited methods exist to automatically detect and analyse their properties and behaviour as they are difficult to extract using standard image and signal processing techniques. We present a new method for the detection of IARs based on the fully convolutional neural network U-net. U-net was chosen as it is able to perform accurate image segmentation and it can be trained in a supervised fashion on a relatively small labelled dataset utilizing data augmentation. We show that the resulting predictive model generated by training the U-net is able to detect IAR signals while mislabelling considerably less noise than other data analysis methods. We achieved our best results by using a training set of 178 hand-digitized examples from high-quality spectrograms measured at the Eskdalemuir Geophysical Observatory (UK). We find that the network converges in ten iterations with a final intersection over union (IoU) metric of 0.9 and a training loss of below 0.2. We use the trained network to extract IARs from over 2300 images, covering six years of search coil magnetometer data measured at the Eskdalemuir Observatory. U-net can also automatically handle missing data or days without IARs, giving a null result as expected. This constitutes the first use of a neural network for pattern recognition of unstructured image data such as spectrograms containing IAR signals, though the method is applicable to other types of resonances or geophysical features in the time–frequency domain.

Modifications of Schumann resonance spectra as an estimate of causative earthquake magnitude: The model treatment

Using a numerical model of the ELF radio wave scattering by seismogenic localized non-uniformity, we evaluate the earthquake (EQ) magnitude by using modifications in the power spectra of global electromagnetic (Schumann) resonance. The ionosphere non-uniformity is introduced as a disturbance in the vertical profile of atmosphere conductivity above the earthquake. Three types are considered of disturbance: the WHOLE, BOTTOM and TOP models, each depending on the EQ magnitude. The disturbance is axially symmetric; it varies along the radius according to the Gauss law with the scale corresponding to the EQ magnitude. The electromagnetic problem is solved by using the complex characteristic electric and magnetic heights relevant to conductivity profiles. These heights are found by solving the Riccati equation and are substituded into the two-dimensional telegraph equation (2DTE) for computing the vertical electric and two orthogonal magnetic fields in the Earth–ionosphere cavity. To avoid an impact of the field source coordinates, we apply the source model of independent lightning strokes uniformly distributed over the globe. The ionosphere non-uniformity is located above the North Pole, and the observer is positioned at the point of 63° N and 0° E, i.e., at 3 Mm distance from the disturbance. Comparison of power spectra in the regular and non-uniform cavities allowed us to single out their modifications at particular frequencies. Finally, equations are obtained connecting the EQ magnitude and the spectral modifications averaged over the frequency band covering four Schumann resonance modes. Computations indicate that reflections of radio waves from the localized seismogenic non-uniformity provide the noticeable impact when the EQ magnitude exceeds the M = 7 threshold. The BOTTOM model provides the highest impact on the Schumann resonance spectra, while the influence of the TOP model is the weakest one. The highest seismogenic effect exceeding the 20 dB level is observed in the meridional component HWE (the disturbance is found above the North Pole). Model data agree with available observations, and a further progress might be achieved in the case studies of particular EQs.

Scattering of ELF radio waves by a localized non-uniformity in the lower ionosphere

We numerically model the ELF radio wave scattering by a localized non-uniformity in the mesosphere conductivity characterized by a 25–30 km reduction of conventional profile from 70 to 80 km altitude. The disturbance is axially symmetric; it depends on the radius according to the Gaussian law of 1 Mm scale. The complex characteristic electric and magnetic heights are found by solving the Riccati equation. These heights are used in the 2D (two dimensional) telegraph equations (2DTE) when computing the ELF fields in the Earth–ionosphere cavity. The field source is located at the point (10° S; 0° E), the non-uniformity is positioned above the North Pole, and the observer moves across the entire surface of the Earth. Spatial distributions were computed of the vertical electric field amplitude in both regular and non-uniform cavity. Comparison of these data allowed us to single out the reflections from the localized non-uniformity. It is shown that interference takes place at an arbitrary observation point between direct wave and the wave reflected from the non-uniformity. Computational data might be interpreted by secondary currents induced in the non-uniformity by the incident radio wave. There are two types of secondary sources. The first one is a “monopole” radiating symmetrically in all directions. Spatial distribution of the field from this source coincides with the distribution of corresponding Schumann resonance mode at a particular frequency. The other secondary source is of “dipole” type, which has a cosine radiation pattern with the maximum oriented along the source – non-uniformity line. Its spatial distribution coincides with the derivative of the corresponding Schumann resonance mode eigen-function on the distance. Model data obtained will facilitate interpretation of experimentally observed seismogenic Schumann resonance signals.

Impact of the Ionospheric Day–Night Non-Uniformity on the ELF Radio-Wave Propagation

The real structure of the lower ionosphere should be taken into account when propagation of extremely low frequency (ELF) radio waves is modeled and the global electromagnetic (Schumann) resonance in the Earth–ionosphere cavity is studied. In the present work, we use a 2D telegraph equation (2DTE) to estimate the effect of the ionospheric day–night non-uniformity on the electromagnetic field amplitude at the Schumann-resonance and higher frequencies. The properties of the cavity are accounted for by using the full wave solution technique through conductivity profiles in the daytime and night-time conditions. Electromagnetic fields in the non-uniform cavity are found by using a 2DTE. Both the sharp terminator model and the model of a smooth day–night transition were considered. The main attention was focused on effects arising on 5000-km paths that are perpendicular or parallel to the solar terminator line. The data were computed for a series of frequencies. A comparison of the calculated data with observation results is also performed and an interpretation of the observed effects is given.

Возмущения пеленга источника в резонаторе земля–ионосфера с неоднородностью день–ночь

Возмущения пеленга источника в резонаторе земля–ионосфера с неоднородностью день–ночь

Amplitude variations of ELF radio waves in the Earth–ionosphere cavity with the day–night non-uniformity

The real structure of lower ionosphere should be taken into account when modeling the sub–ionospheric radio propagation in the extremely low frequency (ELF) band and studying the global electromagnetic (Schumann) resonance of the Earth–ionosphere cavity. In the present work we use the 2D (two dimensional) telegraph equations (2DTE) for evaluating the effect of the ionosphere day-night non-uniformity on the electromagnetic field amplitude at the Schumann resonance and higher frequencies. Properties of the cavity upper boundary were taken into account by the full wave solution technique for realistic vertical profiles of atmosphere conductivity in the ambient day and ambient night conditions. We solved the electromagnetic problem in a cavity with the day–night non-uniformity by using the 2DTE technique. Initially, the testing of the 2DTE solution was performed in the model of the sharp day–night interface. The further computations were carried out in the model of the smooth day–night transition. The major attention was directed to the effects at propagation paths "perpendicular" or "parallel" to the solar terminator line. Data were computed for a series of frequencies, the comparison of the results was made and interpretation was given to the observed effects.

Early time effects produced by neutral gas ionospheric chemical release

The artificial release of electron adsorbing material can cause electron density to be depleted in the ionosphere, forming the ionospheric holes rapidly. At the same time, the shell structure of the electron density enhancement around the hole is produced, owing to the extrusion of background plasma caused by rapid expansion of the release. The coexistence of ionospheric hole and enhancement structure is the significant characteristics of the early time effects. In this paper, the early time effects of neutral chemicals released into ionosphere are studied, and a physical model of spatiotemporal evolution about early time electron density is set up. At t=1 s, the maximum electron density in the enhanced region is 2.46106 cm-3, approximately 2.8 times as great as background electron density, then the electron density at the boundary gradually decreases. At t=30 s, the maximum electron density is 1.58106 cm-3, which is about 1.7 times the background electron density. At t=120 s the maximum electron density in the enhanced region is 1.12106 cm-3, which is 1.2 times the background electron density. Within 120 s after release, the size of the ionospheric cavity increases gradually; at t=5 s the distribution range of the released chemical material is of a sphere of about 10 km in diameter; at t=120 s the distribution diameter of the released chemical material is more than 70 km, and at the same time, the depletion depth of the ionospheric hole decreases slowly. At t=1 s, the depletion depth of the ionospheric hole is about 100%, and at t=120 s the depletion depth of the ionospheric cavity decreases to 95%. The effects of different-frequency radio waves propagating through ionospheric disturbance at t=10 s and t=120 s are simulated by the ray tracing. At t=10 s, the effect of electron density enhancement is remarkable, and the thickness of the enhancement is about 10 km, and the electronic density enhancement area can reflect the radio wave signal at a frequency as high as 14 MHz. At t=120 s, the phenomenon of electron density enhancement becomes weak, the thickness of the enhanced area continues to increase, and the radio wave signal that the electronic density enhancement area could reflect decreases to 11 MHz. The radio waves at a frequency range between 9 MHz and 12 MHz each have a complex diffraction, focusing and dispersing effect in the disturbed area. Furthermore, according to the working principle of ionospheric vertical measurement instrument and ray tracing theory, the vertical ionization detection figures are obtained through inversion. The results are consistent with previous experimental results of rocket exhaust, which testifies the correctness of proposed model.

СОПОСТАВЛЕНИЕ ВАРИАЦИЙ ТЕМПЕРАТУРЫ ВОЗДУХА НА АФРИКАНСКОМ КОНТИНЕНТЕ И ИНТЕНСИВНОСТИ ШУМАНОВСКОГО РЕЗОНАНСА ПО ДОЛГОВРЕМЕННЫМ НАБЛЮДЕНИЯМ В AНТАРКТИКЕ

СОПОСТАВЛЕНИЕ ВАРИАЦИЙ ТЕМПЕРАТУРЫ ВОЗДУХА НА АФРИКАНСКОМ КОНТИНЕНТЕ И ИНТЕНСИВНОСТИ ШУМАНОВСКОГО РЕЗОНАНСА ПО ДОЛГОВРЕМЕННЫМ НАБЛЮДЕНИЯМ В AНТАРКТИКЕ

Ionospheric Alfvén resonator observed at low‐latitude ground station, Muroto

AbstractThe ionospheric Alfvén resonator (IAR) is found in a dynamic power spectrum of the geomagnetic field variations as spectral resonance structures in the frequency range of 0.1–10 Hz. To date, observations of IAR at low latitude have only been made by a few studies employing a single ground observatory and are not sufficient enough to reveal its general characteristics. We analyze data measured with an induction magnetometer installed at Muroto, Japan (24.40° geomagnetic latitude) for the period from 28 December 2013 to 13 August 2016. A statistical analysis reveals that the occurrence probability of the low‐latitude IAR is (1) dominant during nighttime with a gradual increase from dusk to midnight and a broad maximum at 00–05 LT followed by a sudden decrease at dawn, (2) slightly higher during May through September, and (3) independent of the Kp index for Kp ≤ 5. We also find that (4) the frequency separation between the harmonics (Δf) of the low‐latitude IAR is 0.1–0.5 Hz with a peak at 0.2–0.275 Hz. These observational results are compared with the model calculation results along with previous studies about possible energy sources. It is found that the global thunderstorm activity is a likely source for IAR at Muroto; that is, it produces electromagnetic disturbances that are converted into the Alfvén waves and that are then trapped in the ionospheric cavity bounded by the conductive E layer and a steep gradient of Alfvén velocity above the F2 layer to form the resonant Alfvén waves.