VLF Emissions Research Articles

Relativistic electron flux growth during storm and non-storm periods as observed by ARASE and GOES satellites

Variations of relativistic electron fluxes (E ≥ 1 MeV) and wave activity in the Earth magnetosphere are studied to determine the contribution of different acceleration mechanisms of the outer radiation belt electrons: ULF mechanism, VLF mechanism, and adiabatic acceleration. The electron fluxes were measured by Arase satellite and geostationary GOES satellites. The ULF power index is used to characterize the magnetospheric wave activity in the Pc5 range. To characterize the VLF wave activity in the magnetosphere, we use data from PWE instrument of Arase satellite. We consider some of the most powerful magnetic storms during the Arase era: May 27–29, 2017; September 7–10, 2017; and August 25–28, 2018. Also, non-storm intervals with a high solar wind speed before and after these storms for comparison are analyzed. Magnitudes of relativistic electron fluxes during these magnetic storms are found to be greater than that during non-storm intervals with high solar wind streams. During magnetic storms, the flux intensity maximum shifts to lower L-shells compared to intervals without magnetic storms. For the considered events, the substorm activity, as characterized by AE index, is found to be a necessary condition for the increase of relativistic electron fluxes, whereas a high solar wind speed alone is not sufficient for the relativistic electron growth. The enhancement of relativistic electron fluxes by 1.5–2 orders of magnitude is observed 1–3 days after the growth of the ULF index and VLF emission power. The growth of VLF and ULF wave powers coincides with the growth of substorm activity and occurs approximately at the same time. Both mechanisms operate at the first phase of electron acceleration. At the second phase of electron acceleration, the mechanism associated with the injection of electrons into the region of the magnetic field weakened by the ring current and their subsequent betatron acceleration during the magnetic field restoration can work effectively.Graphical

Quasi-periodic ELF/VLF emissions with atypical time structure inside the plasmasphere

Some results of observations on board the Van Allen probe A spacecraft of atypical quasi-periodic VLF electromagnetic emissions inside the plasmasphere are discussed. We study in detail two events (December 6, 2015 and November 24, 2015) of the generation of deeply modulated quasi-periodic VLF emissions (QP) in the frequency band below 2kHz, when separate relatively short (on the order of 20 and 60 s) bursts of hiss without any notable frequency drift repeated with a period of 90 and 240s. The emissions selected for analysis were not accompanied by a hydromagnetic compression wave, which indicates the impossibility of their interpretation in terms of the concept of QP 1 modulated hiss. For the considered events, no clear periodicity of spectral bursts typical for QP 2 emissions take place. The dynamic spectra analysis makes it possible to conclude that the cyclotron generation of electromagnetic emissions occurs in the regions of the electron radiation belts with relatively dense cold plasma. The time course of these emissions can be explained under the assumption of a periodic modification of the conditions for the reflection of whistler wave incident on the ionosphere from above. Based on the available data and calculations, it was concluded that it is possible to interpret the analysed atypical quasi-periodic VLF emissions as a result of the resonant parametric action of ionospheric infrasonic waves with periods close to the repetition period of spectral bursts in quasi-periodic emissions.

The Response of a Plasma Magnetospheric Maser to Atmospheric Perturbations

The effect of weak external influences associated with infrasonic waves in the ionosphere on the operation of a plasma magnetospheric maser (PMM) is considered. It is shown that if the frequency of infrasonic waves is close to the eigen frequency oscillations of the PMM, then quasi-periodic (QP) electromagnetic VLF emissions with repetition periods of spectral forms of 10–300 s can be excited in radiation belts. It has been found that one of the possible reasons for this phenomenon may be the Q-switching of the magnetospheric resonator due to a change in the coefficient of reflection of whistler waves from the ionosphere from above by atmospheric infrasonic waves. For natural atmospheric sources of infrasonic disturbances with horizontal scales of approximately 100 km, model calculations of the depth of modulation of the energy density of electromagnetic waves in a magnetic field tube were carried out. It has been found that in the morning and daytime subauroral magnetosphere, even weak external influences lead to the appearance of signals with a sufficiently large modulation depth (tens of percent).

Effect of the Relativistic Electron Beam on Propagating Whistler-Mode Wave for Ring Distribution in the Saturn Magnetosphere

Cassini and many investigators reported whistler chorus near Saturn equatorial plane moving outwards. Whistler can propagate when going to high latitude and can alter its characteristics while interacting resonantly with available energetic electrons. Here investigating wave for a relativistic beam of the electron. It is observed and reported by Cassini Magnetospheric Imaging Instrument (MIMI) that inward radial injection of highly energetic particles is most dominant in Saturn intrinsic magnetosphere. Within this paradigm, an empirical energy dispersion relation for propagated whistler-mode oscillations in quasi Saturn magnetospheric plasma from such a non-monotonous ringed distribution function has been established. The kinetic approach and method of characteristics methodologies were used in the computations, which have been shown to be the best for building perturbed plasma states. The perturbed distribution function was estimated using the unperturbed particle routes. The ring distribution function was used to construct an unexpected growth rate expression for relativistic plasma in the inner magnetosphere. The results from the Saturn magnetosphere have been calculated and interpreted using a range of parameters. Temperature heterogeneity was shown to be a significant source of free energy that aided the propagation of a whistler-mode wave. By raising the peak value, the bulk injection of energetic hot electron injection impacts the growth rate. Growth was also demonstrated to be accelerated when the propagation angle increased. The research contributes to a better understanding of the relationship between wave and particle emissions and VLF emissions on a large scale.

WHISTLER-TRIGGERED VLF EMISSIONS RECORDED AT JAMMU DURING DAY TIME

Whistler-triggered VLF emissions recorded at low latitude station Jammu (Geomagnetic latitude = 220 26/ N; L = 1.17) during day time period on 19th February 1999 at 14:35 hrs. IST. The recorded data have been analyzed. Based on whistler-triggered VLF emissions spectrum, the VLF waves propagate along the path with L – values lying between L = 4.4 and 4.38. During the observation period, magnetic activity was very high. Mostly these types of emissions recorded at mid latitudes. These whistler-triggered emission waves propagate along the geomagnetic field lines either in a ducted mode or in a pro-longitudinal mode. Relative amplitude of whistlers waves is almost equal to relative amplitude of triggered emissions. The proposed generation mechanism explains through the dynamic spectra of the whistler-triggered emissions.

Impact of hot injected beam on whistler mode with alternating electric field (AC) in the magnetosphere of Saturn

Whistlers are believed to be generated by its own and responsible to evolve dynamical properties of magnetized planetary environment. Growing whistler instability can cause other uncertainties in the magnetosphere and evident to be generated by mean of injection events and temperature variance in plasma environment. In this paper the empirical dispersion relation has developed for parallel propagating whistler mode instability in an infinite saturnian magneto plasma in the presence of perpendicular electric field for ring distribution function having non-monotonous nature. Method of characteristics solutions alongside kinetic approach found to be most suitable in order to achieve perturbed plasma states. The perturbed and unperturbed particle trajectories have taken into consideration to determine perturbed distribution function. A remarkable growth rate expression with added hot plasma injection has been calculated in inner magnetosphere near 6.18 Rs. The results obtained using demonstrative value of the parameters suited to the Saturnian magnetosphere have been computed and discussed. Pressure (Temperature) anisotropy is found to be a peculiar source of free energy for whistler mode instability. The AC frequency irrespective of its magnitude, affects the growth rate significantly. The bulk of energetic hot electrons injection influences the growth rate by increasing its peak value. The result obtained provide the important view of wave particle interaction and useful to analyze the VLF emissions observed over a wide frequency range.

Short Periodic VLF Emissions Observed Simultaneously by Van Allen Probes and on the Ground

AbstractWe present simultaneous observations of very low‐frequency emissions with periodic bursts by Van Allen Probe near geomagnetic equator and Kannuslehto and Lovozero ground‐based sites. The repetition period and ground–spacecraft delay are consistent with guided whistler wave propagation between conjugate ionospheres. In contrast to lightning whistlers, the group velocity dispersion is not accumulated from one burst to another, thus implying a nonlinear mechanism of its compensation. Two regimes are observed. In one regime, Poynting flux direction alternates in the magnetosphere, and the burst period (2 s) is half of that detected on the ground (4 s), corresponding to single‐wave packet bouncing along the field line. This regime is switched to the other one, with burst period unchanged in the magnetosphere but halved on the ground. In this second regime, no alternating Poynting flux direction is observed. The second regime corresponds to two symmetrically propagating wave packets synchronously meeting at the equator.

On the propagation of VLF electric field emissions associated with earthquakes in the middle layer of the earth crust

An ensemble of elementary radiators is generated on the basement rock because of the applied stresses in the preparation zone of the earthquakes in the earth crust. Considering such an ‘ensemble’ as the source of electromagnetic signals, the strength of the electric field is estimated at different distances and frequencies lying in range (3 – 27 kHz) at three different conductivities of the crustal layers (10-8, 10-9, 10-10 S/m). The results of the computation are presented in this paper. Moreover, propagation distances for the seismogenic VLF emissions have also been calculated in the frequency band (3 – 27 kHz) at the conductivities lying in the range (10-8 - 10-10 S/m) within the limit of detectability of measuring instruments (10-7 V/m). It is observed that these distances increase with the decrease of conductivity of the middle layer of crust. Furthermore, theoretical results of comptations are verified from the experimental observations of the seismic event that occurred at the distance of 698 km from the observing station at Chaumuhan Mathura (Geographic lat. 27.490N, long. 77.670E). In addition to this, generation and propagation mechanisms of seismo-electromagnetic radiations have also been discussed briefly.

Daytime VLF Emissions during the Magnetic Storm Recovery Phase: the Event of January 5, 2015

This paper discusses the spectral and temporal features of the 7-h daytime VLF noise emission burst in the frequency band of 1‒10 kHz observed on the Earth’s surface during the late recovery phase of a moderate magnetic storm (Kр = 5 and Dst ~ –80 nT) under nearly quiet space weather. These VLF emissions were recorded simultaneously at two auroral stations located at a geomagnetic latitude of ~64° MLAT and at a longitude distance of ~400 km from each other: Finnish Kannuslehto Station (KAN) and Russian Lovozero Observatory (LOZ). The dynamic VLF wave spectrum is considered at different time scales. During the development of the discussed daytime VLF burst, the noise emissions at below 3.5 kHz gradually turn into periodic (QP) emissions with a quasi-repetition of ~3 s. Having compared the directions of short isolated high-frequency (above 5 kHz) VLF signals (so-called “birds”) arriving at KAN and LOZ, we draw conclusions on the possible location and spatial dynamics of the exit area of emission from the ionosphere.

First Results from VLF Observations during the Transarctica 2019 Polar Expedition

The first results are presented for observations of the VLF emissions (1–15 kHz) conducted at polar latitudes during the expedition Transarctica 2019, which was organized by the Arctic and Antarctic Research Institute. The observations were conducted onboard the Akademik Treshnikov vessel with highly sensitive equipment developed at the Polar Geophysical Institute. Bursts of the VLF emission of the auroral hiss type were registered on 13 of the 26 observational days. These bursts were simultaneously observed at Barentsburg Observatory, which is ~600 km to the west and ~160 km to the south. However, these bursts were not registered at lower auroral latitudes at the same meridian at Lovozero Observatory or at a Finnish station, Kannuslehto. One of the most typical cases, the auroral VLF hiss of April 11, 2019, is considered in detail. It is concluded that the region of the exit of the studied auroral hiss from the ionosphere to the Earth’s surface was local and limited in latitude.

A two year survey for VLF emission from fireballs

Here we report on a two year continuous survey to examine possible VLF signals associated with meteors brighter than magnitude −5. Our survey allowed both calibrated temporal and spatial correlations between VLF signals and fireball light curves. We used continuous observations from the Atmospheric Weather Electromagnetic System for Observation Modeling and Education (AWESOME) VLF receiver system (Cohen et al., 2010) deployed at the Elginfield Observatory near London, Ontario, Canada (43N, 81W) to monitor VLF radio signals and correlate with all-sky video recordings of fireballs. This survey from May 2017 to March 2019 was cued using fireballs detected by the Southern Ontario Meteor Network (Brown et al., 2010; Weryk et al., 2008). The GPS conditioned timing of the AWESOME system was continuously synchronized with video recordings directly in the video stream to ensure sub-frame VLF-optical time calibration. The AWESOME system has two orthogonal VLF antennas which permits directional calculation of incoming VLF signals, which were compared to the apparent optically measured locations of simultaneously detected fireballs. VLF events potentially linked to fireballs were checked against the National Lightning Detection Network (NLDN) database to remove false positive association with lightning. During the two year survey interval, over 80 bright meteors (apparent magnitude brighter than −5, brightest recorded event −7.8) were detected and compared to VLF signals detected by the AWESOME system. No definitive evidence was found for VLF emission from meteors up to a limiting magnitude of −7.8.

Whistler Growth Rates in the Magnetosphere According to Measurements of Energetic Electron Fluxes on the Van Allen Probe A Satellite

Measurements of the differential fluxes of energetic electrons in the equatorial region of the magnetosphere in the energy range from units of keV to hundreds of keV carried out on Van Allen Probes make it possible to obtain the distribution function of electrons resonantly interacting with whistlers in the VLF range and to calculate the corresponding local growth rates (or increments) of the waves. The article presents the results of the reconstruction of the distribution function of energetic electrons, as well as the direct calculation of the wave increments without any model assumptions about the form of the distribution function. The data from the Van Allen Probe A satellite on electron fluxes in the energy range of 33–143 keV are used, and the dependence of the increment on frequency along the satellite path is calculated. The obtained increment values are compared with the spatial and temporal variations in VLF emissions detected by the satellite. An important feature of the calculated increments is their significant variation during the time between successive measurements of the fluxes, i.e., at times on the order of 11 s.

Effect of geomagnetic storms on VLF waves at low latitudes based on the analysis of whistlers and VLF emissions observed at Indian ground stations: A Review

Effect of geomagnetic storms on VLF waves at low latitudes based on the analysis of whistlers and VLF emissions observed at Indian ground stations: A Review

VLF Emissions With Banded Structure in the 16‐ to 39‐kHz Frequency Range Measured by a High‐Latitude Ground‐Based Receiver

AbstractVery low frequency (VLF) emissions of natural origin were identified for the first time by analyzing 1‐hr ground‐based magnetic field spectrograms in the 0.2‐ to 39‐kHz frequency range. Data were used from the Kannuslehto radio receiver (L‐shell ~5.5), recorded during different campaigns between 2006 and 2019. The spectrograms exhibit banded structures, which consist of several strip elements that vary in time and frequency over the event duration. Statistical analysis of 95 events shows that they are observed in the frequency range that extends from 2 to ~37 kHz, and mainly appearing above 16 kHz. The events span from 4 to 110 min and occur in the evening sector (~17–01 magnetic local time), mostly during quiet geomagnetic conditions. Furthermore, they are primarily left‐handed polarized and are associated with bursts of lightning‐related radio emissions such as sferics and tweeks.

Bursts of Auroral-Hiss VLF Emissions on the Earth’s Surface at L ~ 5.5 and Geomagnetic Disturbances

We have studied the geomagnetic conditions during the occurrence of auroral-hiss VLF emissions on the Earth’s surface in the form of noise bursts at frequencies above 5–6 kHz. The study is based on an analysis of auroral-hiss observations at auroral latitudes in northern Finland at Kannuslehto station (KAN, L ~ 5.5) during the winter campaigns of 2013–2018. It is shown that auroral-hiss bursts are most often observed in the interval of 20–01 MLT under low geomagnetic activity (Kp < 3). It has been found that the bursts are typical for the growth phase of a magnetospheric substorm; the bursts cease abruptly at the onset of the substorm (the break-up of auroras), which is apparently due to a drastic increase in the absorption of VLF waves in the ionosphere. The bursts are often accompanied by the generation of geomagnetic Pi2 pulsations. VLF observations have shown that auroral-hiss bursts are not observed on the Earth’s surface in the main phase of magnetic storms; however, they are typical of the recovery phase. It has been found from model data that, during the occurrence of auroral hiss, KAN station is usually projected to the near-equatorial region of the auroral oval or to the zone of diffuse precipitation of more energetic electrons, i.e., to lower latitudes than the typical position of visible auroras during that time.

Features of the Spectrum of Natural VLF Emissions in the Near-Equatorial Region of the Upper Ionosphere from DEMETER Satellite Observations

The generation of the U-shaped spectrum, an unusual wave phenomenon observed in the equatorial part of the DEMETER satellite orbit, is studied. This wave phenomenon is explained for the first time based on the assumption that this emission is formed by the waves generated by lightning discharges, while the shape of the spectrum is determined by the features of wave propagation and damping in the near-equatorial region of the upper ionosphere.

262. Assessment of occupational exposure to electromagnetic field around 1.5 T and 3.0 T MRI scanners

Purpose MRI uses a combination of strong Static Magnetic Field ( B 0 , 0 Hz), which is constantly present inside and around the MRI facility, switching gradient fields oscillating in the Very Low Frequency range (VLF, 0.3–3 KHz) and radiofrequency pulses in the Very High Frequency range (VHF, 30–300 MHz). Staff entering an MRI scanner room may experience instantaneous SMF exposure, as well as time-varying magnetic field exposure (dB/dt) at extremely low frequency, when they move through the non-uniform static magnetic field nearby the scanner. In some situations MRI staff are present inside the scanner room during image acquisition, implying the exposure to the different Electromagnetic Field (EMF). In this study we investigated the compliance of occupational exposure to MRI-related EMF with the 2013/35/EU Directive. Methods The personnel exposure to both dB/dt and B0 during work activity were evaluated on 1.5 T and 3.0 T clinical whole body MRI scanner by using a portable electronic dosimeter for static magnetic field (Talete® Tecnorad, Italy). The dosimeters was capable of measuring B0 at 18 Hz sampling rate. The estimated values were compared with the Exposure Limit Values (ELVs) established from 2013/35/EU Directive. The electric field and magnetic field associated with VLF and VHF were measured at different positions inside the scanner room for three different acquisition technique. The measured values were compared with the Action Levels (ALs) given in 2013/35/EU Directive. For f Results The ALs and the ELVs set by the 2013/35/EU Directive were not exceeded. Conclusion The assessment of occupational exposure to electromagnetic field around 1.5 T and 3.0 T MRI scanners is difficult because of many different EMF. Furthermore many signals and modulations are present in to VHF and VLF emission spectra, besides the use into clinical practice of different equipment and acquisition techniques. In the routine daily work activities, the respect of ALs and ELVs given by 2013/35/EU Directive is demonstrated.

Localization of the sources of narrow-band noise VLF emissions in the range 4–10 kHz from simultaneous ground-based and Van Allen Probes satellite observations

This paper presents the results of simultaneous observations of narrow-band noise VLF emissions in the frequency range 4–10 kHz at Kannuslehto ground station in Northern Finland and by Van Allen Probes (previously RBSP) in the equatorial part of the magnetosphere. The event of December 25, 2015, is considered. During the event, narrow-band noise VLF emissions were detected on the Earth in two frequency ranges, f = 3.5–6 kHz and f = 8–10 kHz, between 1100 and 1300 UT. Narrow-band VLF emissions in the equatorial zone were also observed during that time by the RBSP-B satellite; their frequency was close to the electron equatorial half-gyrofrequency and gradually increased from 3 to 11 kHz during the satellite motion from L = 5.0 to L = 3.0. Analysis of the fine structure of the emissions on the ground showed that their spectral and temporal characteristics corresponded to emissions by the satellites in localized zones at different L-shells. The ground-based observations at lower frequencies correlated with the satellite observations at larger L-shells. In order to localize the regions of the generation of the VLF emissions observed at Kannuslehto auroral station at different frequencies, we calculated the ray trajectories of waves from the equator for the plasma density distributions detected by Van Allen Probes. The calculations of the trajectories showed that the VLF waves detected at Kannuslehto station could travel to the ground only if they propagated in the large-scale density ducts (700–900 km) observed by Van Allen Probes.

Plasma perturbations in the coexisting environment of VLF transmitter emission, lightning strokes and seismic activity

We analyzed plasma perturbations occurring in the coexisting environment of powerful VLF transmitter emission, intense lightning strokes and strong seismic activity during pregnant period. The results suggest that anomalous electron bursts with energy dispersion in the range of ~100–350 keV, forming the “wisp” signature, are due to cyclotron resonance of electrons with monochromatic waves from the powerful NWC VLF transmitters during nighttime. The intense broad band VLF emissions (up-going 0+ whistlers) are observed while the DEMETER satellite goes through the region of intense thunderstorm activities at mid-latitudes. However, the effects of intense lightning activity and pregnant earthquake have little impact on this kind of stable energy-dispersed electron structures, despite the fact that they are presumably two primary reasons for the particle precipitation in the ionosphere. The case studied here provides us a valuable opportunity to address the various sources triggering the anomalous plasma perturbations in the ionosphere.

Study of Oblique Propagating Whistler Mode Waves in Presence of Parallel DC Electric Field in Magnetosphere of Saturn

In this paper whistler mode waves have been investigated in magnetosphere of Saturn. The derivation for perturbed distribution function, dispersion relation and growth rate have been determined by using the method of characteristic and kinetic approach. Analytical expressions for growth rate and real frequency of whistlers propagating oblique to magnetic field direction are attained. Calculations have been performed at 6 radial distances in plasma sheet region of Saturn’s magnetosphere as per data provided by Cassini. Work has been extended for bi-Maxwellian as well as Loss-cone distribution function. Parametric analysis show that temperature anisotropy, increase in number density, energy density and angle of propagation increases the growth rate of whistler waves along with significant shift in wave number. In case of Loss-cone distribution, increase in growth rate of whistlers is significantly more than for bi-Maxwellian distribution function. Generation of second harmonics can also be seen in the graphs plotted. It is concluded that parallel DC field stabilizes the wave and temperature anisotropy, angle of propagation, number density and energy density of electrons enhances the growth rate. Thus the results are of importance in analyzing observed VLF emissions over wide spectrum of frequency range in Saturnian magnetosphere. The analytical model developed can also be used to study various types of instabilities in planetary magnetospheres.