Showing papers on "Very low frequency published in 2018"

Diagnosing the Role of Alfvén Waves in Magnetosphere-Ionosphere Coupling: Swarm Observations of Large Amplitude Nonstationary Magnetic Perturbations During an Interval of Northward IMF

Abstract: High-resolution multispacecraft Swarm data are used to examine magnetosphere-ionosphere coupling during a period of northward interplanetary magnetic field (IMF) on 31 May 2014. The observations reveal a prevalence of unexpectedly large amplitude (>100 nT) and time-varying magnetic perturbations during the polar passes, with especially large amplitude magnetic perturbations being associated with large-scale downward field-aligned currents. Differences between the magnetic field measurements sampled at 50 Hz from Swarm A and C, approximately 10 s apart along track, and the correspondence between the observed electric and magnetic fields at 16 samples per second, provide significant evidence for an important role for Alfven waves in magnetosphere-ionosphere coupling even during northward IMF conditions. Spectral comparison between the wave E- and B-fields reveals a frequency-dependent phase difference and amplitude ratio consistent with interference between incident and reflected Alfven waves. At low frequencies, the E/B ratio is in phase with an amplitude determined by the Pedersen conductance. At higher frequencies, the amplitude and phase change as a function of frequency in good agreement with an ionospheric Alfven resonator model including Pedersen conductance effects. Indeed, within this Alfven wave incidence, reflection, and interference paradigm, even quasi-static field-aligned currents might be reasonably interpreted as very low frequency (ω → 0) Alfven waves. Overall, our results not only indicate the importance of Alfven waves for magnetosphere-ionosphere coupling but also demonstrate a method for using Swarm data for the innovative experimental diagnosis of Pedersen conductance from low-Earth orbit satellite measurements.

China Seismo-Electromagnetic Satellite search coil magnetometer data and initial results

Abstract: Four levels of the data from the search coil magnetometer (SCM) onboard the China Seismo-Electromagnetic Satellite (CSES) are defined and described. The data in different levels all contain three components of the waveform and/or spectrum of the induced magnetic field around the orbit in the frequency range of 10 Hz to 20 kHz; these are divided into an ultra-low-frequency band (ULF, 10–200 Hz), an extremely low frequency band (ELF, 200–2200 Hz), and a very low frequency band (VLF, 1.8–20 kHz). Examples of data products for Level-2, Level-3, and Level-4 are presented. The initial results obtained in the commission test phase demonstrated that the SCM was in a normal operational status and that the data are of high enough quality to reliably capture most space weather events related to low-frequency geomagnetic disturbances.

Void discharge behaviours as a function of cavity size and voltage waveform under very low-frequency excitation

Abstract: Measurement of partial discharge (PD) is a vital method to assess the health of the electrical insulation in high-voltage power equipment. As diagnostic testing at very low frequency (VLF) is increasingly being used, it is important to investigate PD behaviours under such a low-frequency voltage excitation. This study presents the PD characteristics at VLF excitation under different applied voltage waveforms, including sinusoidal and trapezoid-based waves. Also, the effects of cavity size on PD behaviours at VLF are investigated. Experiments were performed to measure PDs in a cylindrical void bounded by solid insulation. The results show that discharge activities at VLF, in general, exhibit lower magnitude and repetition rate when compared with power frequency. Also revealed is the strong dependency of discharge parameters on the rate of voltage rise. Physical mechanisms to explain discharge behaviours are given.

Cable commissioning and diagnostic tests: The effect of voltage supply frequency on partial discharge behavior

Abstract: CIGRE documents, IEEE and IEC standards permit alternate power supply frequencies to reduce the size of the power supply for offline tests, such as those using resonant systems, Very Low Frequency (VLF) and Damped Alternating Current (DAC). To provide insight into the risks of adopting these alternatives, partial discharge (PD) measurements are reported herein for medium voltage (MV) cables with artificial defects, in a range of supply frequencies from 0.01 Hz to 60 Hz and at different temperatures. The results show that varying temperature and frequency changes the PD repetition rate, amplitude and energy, raising questions about the interpretation of diagnostic measurements made at VLF and any other alternate method allowed by the standard.

Whistler Influence on the Overall Very Low Frequency Wave Intensity in the Upper Ionosphere

Abstract: We investigate the influence of lightning‐generated whistlers on the overall intensity of electromagnetic waves measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft (2004–2010, quasi Sun‐synchronous polar orbit with an altitude of about 700 km) at frequencies below 18 kHz. Whistler occurrence rate evaluated using an onboard neural network designed for automated whistler detection is used to distinguish periods of high and low whistler occurrence rates. It is shown that especially during the night and particularly in the frequency‐geomagnetic latitude intervals with a low average wave intensity, contribution of lightning‐generated whistlers to the overall wave intensity is significant. At frequencies below 1 kHz, where all six electromagnetic wave components were measured during specific intervals, the study is accompanied by analysis of wave propagation directions. When we limit the analysis only to fractional‐hop whistlers, which propagate away from the Earth, we find a reasonable agreement with results obtained from the whole data set. This also confirms the validity of the whistler occurrence rate analysis at higher frequencies.

VLF signal anomalies during cyclone activity in the Atlantic Ocean

Abstract: In this paper we present ionospheric disturbances during the simultaneous presence of two to three Large Meteorological Systems, classified as hurricanes and tropical storms, in the Atlantic Ocean from August to November 2016. The ionospheric disturbances were detected by very low frequency (3–30 kHz) signals from two North American transmitters observed in Algiers (36.75°N, 03.47°E). The results show clear anomalies in the amplitude both at nighttime and at daytime. At nighttime, the anomalies were observed in association with all Large Meteorological Systems even at low stage of storm intensity (tropical depression). The anomalies showed periodicities between 2 and 3 hr with a strong decrease in the signal amplitude. The wave‐like features were confirmed by the mother wavelet analysis of the normalized signal amplitude. These signal anomalies may result from traveling ionospheric disturbances generated by tropical storms and hurricanes associated gravity waves.

Spectral Characteristics of VLF Sferics Associated With RHESSI TGFs

Abstract: We compared the modeled energy spectral density of very low frequency (VLF) radio emissions from terrestrial gamma ray flashes (TGFs) with the energy spectral density of VLF radio sferics recorded by Duke VLF receiver simultaneously with those TGFs. In total, six events with world wide lightning location network (WWLLN) defined locations were analyzed to exhibit a good fit between the modeled and observed energy spectral densities. In VLF range the energy spectral density of the TGF source current moment is found to be dominated by the contribution of secondary low-energy electrons and independent of the relativistic electrons which play their role in low-frequency (LF) range. Additional spectral modulation by the multiplicity of TGF peaks was found and demonstrated a good fit for two TGFs whose VLF sferics consist of two overlapping pulses each. The number of seeding pulses in TGF defines the spectral shape in VLF range, which allows to retrieve this number from VLF sferics, assuming they were radiated by TGFs. For two events it was found that the number of seeding pulses is small, of the order of 10. For the rest of the events the lower boundary of the number of seeding pulses was found to be between 10 to 103.

The Concept of an Airborne VLF Transmitter with Vertical Electric Dipole Antenna

Abstract: This paper presents the idea of a Very Low Frequency facility based on an airborne, mobile transmitter equipped with a symmetric-dipole antenna. The antenna, to be flown under an aerostat to high altitudes (between 12 and 60 km), is to present, in comparison to ground facilities, less financial issues and modified field intensity. The antenna concept was proven during three experiments flown in 2014 and 2015, with the longest aerial having the length of 200 m. The prototype transmitter is planned to be launched in 2019.

Mechanical Antennas: Emerging Solution for Very-Low Frequency (VLF) Communication

Abstract: This paper presents the concept of mechanical antennas for long range and under water communication systems and addresses its challenges and the corresponding solutions. In this approach, instead of using conventional antennas driven by a power source or oscillator, the radiation is mainly due to charge acceleration under mechanical forces provided in an engineered fashion. As illustrating examples, the radiation associated with rotating dipole charges and rotating magnets are theoretically derived and radiated power dependency on the frequency of rotation, dipole moment, required mechanical force and radius of rotation are investigated. The concept of magneto-dielectric resonance is finally utilized to enhance the radiated electric field from the mechanical antenna system by around 100 dB.

Parametric excitation of very low frequency (VLF) electromagnetic whistler waves and interaction with energetic electrons in radiation belt

Abstract: The concept of a parametric antenna in ionospheric plasma is analyzed. Such antennas are capable of exciting electromagnetic radiation fields, specifically the creation of whistler waves generated at the very low frequency (VLF) range, which are also capable of propagating large distances away from the source region. The mechanism of whistler wave generation is considered a parametric interaction of quasi-electrostatic whistler waves (also known as low oblique resonance (LOR) oscillations) excited by a conventional loop antenna. The interaction of LOR waves with quasi-neutral density perturbations in the near field of an antenna gives rise to electromagnetic whistler waves on combination frequencies. It is shown in this work that the amplitude of these waves can considerably exceed the amplitude of whistler waves directly excited by a loop. Additionally, particle-in-cell simulations, which demonstrate the excitation and spatial structure of VLF waves excited by a loop antenna, are presented. Possible applications including the wave-particle interactions to mitigate performance anomalies of low Earth orbit satellites, active space experiments, communication via VLF waves, and modification experiments in the ionosphere will be discussed.

On the Tempo-Spatial Evolution of the Lower Ionospheric Perturbation for the 2016 Kumamoto Earthquakes from Comparisons of VLF Propagation Data Observed at Multiple Stations with Wave-Hop Theoretical Computations

Abstract: There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evolution of VLF propagation anomalies with special attention to one particular propagation path. The most important advantage of this paper is the simultaneous use of several propagation paths. A succession of earthquakes (EQs) happened in the Kumamoto area in Kyusyu Island; two strong foreshocks with magnitude of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15 April (UT). Because the EQ epicenters are not far from the VLF transmitter (with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously 8 observing stations of our network all over Japan. Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs. It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock. When the perturbation is most disturbed, the maximum change in vertical direction is depletion in the VLF effective ionospheric height of the order of 10 km, and its horizontal scale (or its radius) is about 1000 km. These spatio-temporal changes of the seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe with the same magnitude and of the same fault-type, and a brief discussion on the generation mechanism of seismo-ionospheric perturbation is finally made.

Coordinated Satellite Observations of the Very Low Frequency Transmission Through the Ionospheric D Layer at Low Latitudes, Using Broadband Radio Emissions From Lightning

Abstract: Both ray theory and full-wave models of Very Low Frequency transmission through the ionospheric D-layer predict that the transmission is greatly suppressed near the geomagnetic equator. We use data from the low-inclination Communication/Navigation Outage Forecast System satellite to test this semi-quantitatively, for broadband Very Low Frequency emissions from lightning. Approximate ground-truthing of the incident wavefields in the Earth Ionosphere Waveguide is provided by the World Wide Lightning Location Network. Observations of the wavefields at the satellite are provided by the Vector Electric Field Instrument aboard the satellite. The data set comprises whistler observations with the satellite at magnetic latitudes < 26 deg. Thus our conclusions, too, must be limited to the near-equatorial region, and are not necessarily predictive of mid-latitude whistler properties. We find that in most broadband recordings of radio waves at the satellite, very few of the lightning strokes result in a detectable radio pulse at the satellite. However, in a minority of the recordings, there is enhanced transmission of Very Low Frequency lightning emissions through the D-layer, at a level exceeding model predictions by at least an order-of-magnitude. We show that kilometric-scale D-layer irregularities may be implicated in the enhanced transmission. This observation of sporadic enhancements at low magnetic latitude, made with broadband lightning emissions, is consistent with an earlier review of D-layer transmission for transmission from powerful man-made radio beacons.

The polarization characteristics of ELF/VLF waves generated via HF heating experiments of the ionosphere by EISCAT

Abstract: This paper presents the experimental results of modulation and heating of the ionosphere using the European Incoherent Scatter facility in Troms ϕ, Norway, heating facility. The frequency characteristics of the excited extremely low frequency (ELF)/very low frequency (VLF) radiation sources in the amplitude modulated (AM) and dual-beam beat-wave (BW) modes are compared, and the polarization of the horizontal ELF/VLF magnetic field received at 15 km from the heating facility is studied. The results show that in the AM mode, the amplitude of the ELF/VLF signal decreases as the radiation frequency increases when the amplitude of the ELF/VLF signal reaches the maximum at 2017 Hz. In the BW mode, the maximum appears at 2017 Hz, but the overall amplitude of the ELF/VLF signal increases as the radiation frequency increases. The AM and BW modes have different frequency characteristics on ELF/VLF radiation sources formed by modulation and heating of the ionosphere. When the frequency is low, the amplitude of the ELF/VLF signal excited by the AM mode is greater than that excited by the BW mode, but as the frequency increases, the BW mode performs better than the AM mode. Under the same background conditions, the polarization characteristics of the ELF/VLF radiation signals excited by the AM and BW modes are different. The former tends to produce circularly polarized waves, while the latter produces linearly polarized waves.

An excitation mechanism for discrete chorus elements in the magnetosphere

Abstract: . A beam pulsed amplifier mechanism responsible for effective amplification of short very low frequency (VLF) electromagnetic pulses is proposed. Effective amplification near the magnetic equator outside the plasmasphere is considered. A conditional growth rate of short electromagnetic pulses is calculated. Obtained results can explain some important features of the oblique electromagnetic chorus emissions without hiss-like radiation background.

Radiated field of rotating charged parallel plates and its frequency spectrum

Abstract: Antennas in the ultra low frequency (ULF, 300 Hz to 3 kHz) and very low frequency (VLF, 3∼30 kHz) bands are usually very large. In this study, we developed a very small antenna, which can radiate electromagnetic waves in these frequency bands. The antenna consisted of two rotating charged plates (RCPs), which were charged by the voltage source. Numerical results of the radiated far field and its frequency spectrum are presented for three cases: (a) the antenna is charged with DC voltage, (b) the antenna is charged with AC voltage, and (c) the antenna rotates with variable speed and is fed with DC voltage. The predominant frequency component was equal to the rotating frequency of the RCPs fed with DC voltage. If the RCPs were fed with AC voltage two dominant frequencies were generated, which were determined by the difference in frequency between the RCP rotation frequency and the AC source frequency. When the RCPs rotated with variable speed and were fed with DC voltage, many frequency components were gene...

Detailed Properties of Equatorial Noise With Quasiperiodic Modulation

Abstract: Equatorial noise (EN) emissions are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. Although they are typically continuous in time, they sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity, with modulation periods on the order of minutes. We perform a detailed analysis of 118 EN events with the QP modulation. The events are observed preferentially outside the plasmasphere. We determine the times and frequencies of individual QP elements forming the events. Apart from the event modulation period, this allows us to characterize the intensity and the frequency drift of individual QP wave elements. It is shown that the element intensity peaks at the magnetic equator. The modulation period within a single event is usually quite stable, with variations lower than 25% of the median value in the vast majority of cases. The events with shorter modulation periods are typically more intense, and they tend to have larger frequency drifts. These relations resemble the relations formerly revealed for extra low frequency/very low frequency quasiperiodic emissions, suggesting that the origin of the QP modulation of the wave intensity of EN and ELF/VLF emissions might be similar.

Stratospheric VLF vertical electric mono-and dipole antenna tests in 2014–2015

Abstract: This paper presents the assumptions, executions and results of the three tests of vertical electric mono-and dipole antennas in the Earth's stratosphere, flown using helium balloons to the altitudes between 23 and 30 km in 2014 and 2015. The antennas varied in lengths and presented different technical aspects subjected to the tests (the corona protection, flight mechanics of extremely long flying objects, end weight for stabilizing the flight during the descent phase etc.). All aerials were safely retrieved after the landings. The data gathered during these missions helped to evaluate the basic concept of an airborne vertical electric aerial for Very Low Frequency range; it showed that very long aerials may induce electric charge flow when passing through the cloud layer, need additional weight at their ends to assure safe descent and, when composed of a tape-like structure, automatically move in a helical way, which, with a sufficient antenna length, may in the future enable the design of an elliptically-polarized vertical VLF antenna. The outcomes of the experiments shall be used in the design of a stratospheric VLF transmitter.

Simulation on the Electromagnetic Environment in the Carrier of Airborne VLF Wire Antennas

Abstract: The airborne VLF (Very low frequency) transmitting system is commonly used in underwater communication with a very high power, which produces a complement electromagnetic field. It may threaten to the equipment and personnel on the aircraft. In this paper, the electromagnetic environment inside the aircraft with airborne DTWA (Dual Trailing Wire Antenna) is studied based on the numerical simulation software FEKO. The aircraft model and the VLF transmitting system model are established. By calculating the radiation near field of the DTWA, the electromagnetic field distribution and the induced current of the cables inside the aircraft cabin are analyzed. The conclusions are helpful for the further research on the application of airborne VLF communication system.

Comparison of Offline VLF PD Measurements and Online PD Measurements on a 50-Year-Old Hydrogenerator Stator in Norway

Abstract: Stator winding insulation is the part of the hydrogenerator experiencing the highest number and the most damaging failures. Partial discharge (PD) measurement, both offline and online, are commonly used for condition assessment and monitoring of electrical machines. The main concern of using very low frequency (VLF) methods is the changed electrical field distributions compared to that at power frequency. Hence, PD measurements performed at VLF should be carefully assessed and compared to PD measurements at power frequencies. In this work, offline PD measurement at VLF are presented and compared to power frequency online measurements of a 50-year-old hydrogenerator in Norway, using statistical analysis of phase resolved PD recordings. It is found that both offline VLF and online assessment can identify unnormal PD activity in a specific phase, although the phase resolved PD patterns are not similar for VLF offline and online assessment.

Ionospheric response to an intense solar flare in equatorial and low latitude region

Abstract: Multi-instrument data recorded at multi-stations are used to study the equatorial and low-latitude ionospheric response to an intense solar flare of class X7 (2B) in the current solar cycle 24 with the peak at 08:05 UT on 09 August 2011. Rapid changes in ionospheric total electron contents (TEC) measured by global positioning system (GPS) showed an enhancement of 2–3 TECU. The very low frequency (VLF) data recorded at a low latitude station Varanasi showed an enhancement of VLF signal amplitude during the solar flare period which is attributed to the sudden enhancement of D-region ionization. Ground based GPS measurements are also validated by analyzing the electron density profiles measured from COSMIC satellite mission. COSMIC-derived electron density profile shows a decrease below F2 peak altitude and increase above F2 peak. The D-region ionospheric perturbation observed during the solar flare could be caused by flare time enhanced level of photo-ionization due to X-ray flux enhancement, whereas for E and F-region ionosphere, enhanced EUV flux causes photo-ionization and hence perturbed the TEC.

The Application Research of Very Low Frequency Diagnostic Methods in Acceptance Tests on New Cable Systems of Medium Voltage

Abstract: Tangent delta (TD) measurements using very low frequency (VLF) sinusoidal waveform in aged cable system have been proved sensitive to water tree degradation and insulation aging. Whereas, the application of VLF in acceptance tests on new cable systems of medium voltage is still on research and development. This paper presents an experimental study of the capability of the main VLF testing methods for newly installed 10kV XLPE cable systems in acceptance tests. Compared with oscillation wave excitation, the different waveforms including sinusoidal (Sinus) and cosine-rectangular (CR) of VLF voltage were employed to stress the nine samples of artificial defects of different types. Various characteristics of TD measurement and partial discharge (PD) measurement were analyzed. The results showed that VLF-Sinus voltage has a distinctive ability of detection for a wet or lossy accessory through TD or PD measurements. It is recommended to employ a combination of the monitored VLF-Sinus withstand with TD measurement and the PD measurement under damped alternative current (DAC) or VLF-CR as field diagnostic tools in acceptance test for newly installed cables.

Excitation of electromagnetic whistler waves due to a parametric interaction between magnetosonic and lower oblique resonance modes in a cold, magnetized plasma

Abstract: We have studied the parametric interaction between a fast magnetosonic (FM) mode and a lower oblique resonance (LOR) mode in a cold, magnetized plasma using a kinetic, three dimensional Particle-in-Cell simulation code called the Large Scale Plasma. The FM mode is excited with a loop antenna driven at a frequency below the lower hybrid frequency (ωLH), while the LOR is excited at a frequency above ωLH. For historical purposes explained in the Introduction, we call the antennas which drive the FM mode and LOR mode Extremely Low Frequency (ELF) and Very Low Frequency (VLF) antennas, respectively. The antennas are modeled as magnetic dipoles (ρant = 0) and are assigned a time varying current density within a finite sized current loop. The VLF and ELF antennas are driven at 10 A and 3 A, respectively. The parametric interaction is excited with a combined ELF/VLF antenna (which we call a parametric antenna) and includes both antennas driven simultaneously in the same simulation domain. We show that the parametric antenna non-linearly excites electromagnetic (EM) whistler waves to a greater extent than the VLF antenna alone. We also show that the parametric excitation of EM whistler waves leads to greater emitted EM power (measured in Watts) compared with a VLF antenna alone.We have studied the parametric interaction between a fast magnetosonic (FM) mode and a lower oblique resonance (LOR) mode in a cold, magnetized plasma using a kinetic, three dimensional Particle-in-Cell simulation code called the Large Scale Plasma. The FM mode is excited with a loop antenna driven at a frequency below the lower hybrid frequency (ωLH), while the LOR is excited at a frequency above ωLH. For historical purposes explained in the Introduction, we call the antennas which drive the FM mode and LOR mode Extremely Low Frequency (ELF) and Very Low Frequency (VLF) antennas, respectively. The antennas are modeled as magnetic dipoles (ρant = 0) and are assigned a time varying current density within a finite sized current loop. The VLF and ELF antennas are driven at 10 A and 3 A, respectively. The parametric interaction is excited with a combined ELF/VLF antenna (which we call a parametric antenna) and includes both antennas driven simultaneously in the same simulation domain. We show that the paramet...

Robust cancellation of minimum shift keying-encoded radio interference in data from the transient electromagnetic method

Abstract: Interference from radio signals in the very low frequency (VLF, 3–30 kHz) band constitutes a significant part of the noise in data, measured using the transient electromagnetic method (TEM) Most radio signals in this frequency band convey streams of binary data, encoded using minimum shift keying or variants thereof We have developed a framework for cancellation of these signals by modeling and subtracting the interfering signals from the recorded data The framework uses several techniques to ensure robust and accurate modeling of the radio signals in a typical airborne TEM environment These include combination of signals from multiple receiver coils, tracking of time-varying parameters for each receiver coil, use of a priori knowledge pertaining to the TEM primary field, and use of a high-performance decoding algorithm Examples using recorded noise data from the field combined with synthetic TEM signals indicate a significant reduction in noise power This results in a reduction of the stand

Multiple-Mode Interference and Phase Standard Deviation of VLF Wave Propagation in an Anisotropic Earth–Ionosphere Waveguide

Abstract: In this paper, the multiple-mode interference (MMI) and phase standard deviation of very low frequency (VLF) wave propagation in an anisotropic earth–ionosphere waveguide are treated analytically. The phase stability of a VLF wave is dependent on the characteristics of each propagable mode, such as the excitation factor, the attenuation rate, the phase velocity, as well as their derivatives with respect to the effective ionosphere height. Computations show that the phase stability in the multiple-mode zone is much poorer than that in the single-mode zone, and the phase standard deviations evaluated considering the ionospheric anisotropy are more consistent with the measured data. It is also found that there exist pronounced directional differences in the interference distributions, and the effect of MMI is weakened as the geomagnetic inclination angle increases. By comparison of the phase standard deviation and the field strength, we find that the maxima of the phase standard deviation coincide with the minima of the field strength. Moreover, the MMI zone is much broader in nighttime than in daytime, and in an anisotropic waveguide, the interference will present evident aggregation at certain heights.

New high-frequency (7–12 kHz) quasi-periodic VLF emissions observed on the ground at L ∼ 5.5

Abstract: . We reveal previously unknown quasi-periodic (QP) very low frequency (VLF) emissions at the unusually high-frequency band of ∼ 7–12 kHz by applying the digital filtering of strong atmospherics to the ground-based VLF data recorded at Kannuslehto station (KAN). It is located in northern Finland at L ∼ 5.5 . The frequencies of QP emissions are much higher than the equatorial electron gyrofrequency at L ∼ 5.5 . Thus, these emissions must have been generated at much lower L shells than KAN. Two high-frequency QP emission events have been studied in detail. The emissions were right-hand polarized waves indicating an overhead location of the exit area of waves in the ionosphere. In one event, the spectral–temporal forms of the emissions looked like a series of giant “bullets” due to the very abrupt cessation. Unfortunately, we could not explain such a strange dynamic spectral shape of the waves. In the second event, the modulation period was about 3 min under the absence of simultaneous geomagnetic pulsations. The studied emissions lasted about 4 h and were observed under the very quiet geomagnetic activity. The adequate mechanisms of the generation and propagation of the revealed high-frequency QP emissions have not yet been established. We speculate that studied QP emissions can be attributed to the auto-oscillations of the cyclotron instability in the magnetospheric plasma maser.

Electrical Tree Growth Under Very Low Frequency (VLF) Voltage Excitation

Abstract: Electrical trees are a mechanism of failure of high voltage polymeric insulation. Their growth is associated to partial discharges (PD). Conventionally, PD are analyzed using phase-resolved PD plots and statistical parameters. An alternative is to exploit the tools of nonlinear time series analysis to analyze PD as a nonlinear dynamical process. Standards for field testing of power cable include the measurement of PD under very low frequency (VLF) voltage excitation. Here we calculated the correlation dimension of the reconstructed dynamic object obtained from the time series of PD during tree propagation. Also, conventionally PD analysis was carried out to compare with the alternative approach. Electrical trees grown at 0.1 Hz 14, 16 and 18 kV have been analyzed, along a tree grown at 50 Hz 14 kV for comparison. Different PD dynamics were found, depending on the applied voltage and frequency, and the stage of tree-growth. The results show that the approach using nonlinear time series analysis can be an alternative method of analyzing tree growth and PD under VLF voltage excitation.