Extremely Low Frequency Waves Research Articles

Raytracing of extreamely low frequency waves radiated from ionospheric artificial modulation at low latitude

Powerful high-frequency radio waves modulated at extremely low frequency (ELF) and very low frequency (VLF) can efficiently modify the lower ionospheric current which can act as an equivalent ionospheri'c antenna for the generation of ELF/VLF wave and these signals can propagate into the magnetosphere and the investigation of the propagation properties is of significance for understanding of radiation belt energetic electrons precipitation. In this paper, based on the raytracing theory, the ray paths of ELF waves in the magnetosphere are obtained using numerical modeling and the properties are analyzed. The results shows that the ELF waves generated from the low latitude reflect between the southern and northern hemisphere and propagate to the farther place gradually. For the signals with different frequencies, the higher their frequencies, the shorter their propagation distances are, the lower their frequencies, the longer their propagtion distances are. In the process, ELF waves tend to reflect near a fixed region. The normal angel increases to 90° gradually and ray direction tends to propagate along the direction of geomagnetic field.

A survey of ELF and VLF research on lightning‐ionosphere interactions and causative discharges

Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.

Penetration of ELF currents and electromagnetic fields into the Earth's equatorial ionosphere

The penetration of extremely low frequency (ELF) transient electromagnetic fields and associated currents in the Earth's equatorial E‐region plasma is studied theoretically and numerically. In the low‐frequency regime, the plasma dynamics of the E‐region is characterized by helicon waves since the ions are viscously coupled to neutrals while the electrons remain mobile. For typical equatorial E‐region parameters, the plasma is magnetically insulated from penetration of very long timescale magnetic fields by a thin diffusive sheath. Wave penetration driven by a vertically incident pulse localized in space and time leads to both vertical penetration and the triggering of ELF helicon/whistler waves that carry currents obliquely to the magnetic field lines. The study presented here may have relevance for ELF wave generation by lightning discharges and seismic activity and can lead to new concepts in ELF/ULF injection in the earth‐ionosphere waveguide.

Propagation velocity of VLF EM waves from lightning discharges producing Q-bursts observed in the range 10–15 Mm

The electric fields in ELF (extremely low frequency) to VLF (very low frequency) range were observed in Kochi, Japan (latitude 33.3° north, longitude 133.4° east) on 27 December 2007. The SODs (source-to-observer distances) for 72 Q-bursts of positive polarity obtained in 2 h after 0401.00 UT were estimated by analyzing the waveforms. The VLF component from lightning discharges producing the Q-bursts was detected preceding ELF at the SODs less than 16 Mm (mega-meters). Using time separation between arrivals of VLF and ELF waves in the SOD range 10–15 Mm, the propagation velocity of VLF waves in the Earth-ionosphere spherical guide is estimated as 295 ± 3 Mm/s in comparison with the velocity of ELF waves (Schumann resonance frequency range) of 265 ± 1 Mm/s (Ogawa and Komatsu, 2007).

New insights on Titan's plasma-driven Schumann resonance inferred from Huygens and Cassini data

After a preliminary analysis of the low-frequency data collected with the electric antenna of the Permittivity, Wave and Altimetry (PWA) experiment onboard the Huygens Probe that landed on Titan on 14 January, 2005, it was anticipated in a previous article [Béghin et al., 2007. A Schumann-like resonance on Titan driven by Saturn's magnetosphere possibly revealed by the Huygens Probe. Icarus, 191, 251–266] that the Extremely Low-Frequency (ELF) signal at around 36 Hz observed throughout the descent, might have been generated in the upper ionosphere of Titan, driven by a plasma instability mechanism associated with the co-rotating Kronian plasma flow. The involved process was proposed as the most likely source of a Schumann resonance in the moon's atmospheric cavity, the second eigenmode of which is actually found by models to occur at around 36 Hz. In this paper, we present a thorough analysis of this signal based upon the Huygens Probe attitude data deduced from the Descent Imager Spectral Radiometer (DISR), and relevant measurements obtained from the Radio Plasma Wave Science (RPWS) experiment and from the magnetometer (MAG) onboard Cassini orbiter during flybys of Titan. We have derived several coherent characteristics of the signal which confirm the validity of the mechanism initially proposed and provide new and significant insights about such a unique type of Schumann resonance in the solar system. Indeed, the 36 Hz signal contains all the characteristics of a polarized wave, with the measured electric field horizontal component modulated by the antenna rotation, and an altitude profile in agreement with a Longitudinal Section Electric (LSE) eigenmode of the atmospheric cavity. In contrast to Earth's conditions where the conventional Transverse Magnetic mode is considered, the LSE mode appears to be the only one complying with the observations and the unexpected peculiar conditions on Titan. These conditions are essentially the lack of any lightning activity that can be ascertained from Cassini observations, the presence of an ionized layer centered around 62 km altitude that was discovered by the PWA instrumentation, and the existence of a subsurface conducting boundary which is mandatory for trapping ELF waves. A simple theoretical model derived from our analysis places tentatively consequential constraints on the conductivity profile in the lower ionosphere. It is also consistent with the presence of a conductive water ocean below an icy crust some tens of kilometers thick.

Electromagnetic wave propagation in the surface‐ionosphere cavity of Venus

The propagation of extremely low frequency (ELF) waves in the Earth surface‐ionosphere cavity and the properties of the related Schumann resonances have been extensively studied in order to explain their relation with atmospheric electric phenomena. A similar approach can be used to understand the electric environment of Venus and, more importantly, search for the evidence of possible atmospheric lightning activity, which remains a controversial issue. We revisit the available models for ELF propagation in the cavity of Venus, recapitulate the similarities and differences with other planets, and present a full wave propagation finite element model with improved parameterization. The new model introduces corrections for refraction phenomena in the atmosphere; it takes into account the day‐night asymmetry of the cavity and calculates the resulting eigenfrequency line splitting. The analytical and numerical approaches are validated against the very low frequency electric field data collected by Venera 11 and 12 during their descents through the atmosphere of Venus. Instrumentation suitable for the measurement of ELF waves in planetary atmospheres is briefly addressed.

A new numerical model for the simulation of ELF wave propagation and the computation of eigenmodes in the atmosphere of Titan: Did Huygens observe any Schumann resonance?

The propagation of extremely low frequency (ELF) electromagnetic waves in the Earth's ionospheric cavity and the associated resonance phenomena have been extensively studied, in relation with lightning activity. We perform a similar investigation for Titan, the largest moon of Saturn. There are important differences between Earth and Titan, as far as the cavity geometry, the atmospheric electron density profile, and the surface conductivity are concerned. We present an improved 3D finite element model that provides an estimate of the lowest eigenfrequencies, associated quality factors ( Q-factors), and ELF electric field spectra. The data collected by the electric antenna of the Permittivity, Waves, and Altimetry (PWA) instrument reveals the existence of a narrow-band signal at about 36 Hz during the entire descent of Huygens upon Titan. We assess the significance of these measurements against the model predictions, with due consideration to the experimental uncertainties.

Determination of the height of the modulated electrojet region generating ELF radiation

A virtual ionospheric antenna to generate ELF (extremely low frequency) waves is achievable by using amplitude modulated HF (high frequency) wave to modulate the electrojet current. The results of the numerical analysis show that the radiation intensities of ELF waves produced by modulated electrojet vary with the modulation frequency. Moreover, a ratio of two radiation intensities at two frequencies is height dependent. Thus, with the aid of the numerical results, the experimental measurements on the ratios of the radiation intensities of the ELF waves can provide unambiguous information on the height of the electrojet contributing to the ELF radiation. An experiment using the HAARP heating facility has been performed in 2005. The experimental results of three independent intensity ratios (25 Hz/205 Hz, 25 Hz/325 Hz, and 25 Hz/625 Hz) are used to demonstrate this new diagnostic technique. The height of the source region, where the modulated electrojet radiates most effectively, is determined to be between 94 Km and 97 Km.

Narrow‐band extremely low frequency (ELF) wave phenomena observed at South Pole Station

Extremely low frequency (ELF) magnetic pulsations which show a whistler‐like spectral structure have been measured at South Pole Station. An initial survey of the first 70 days of data revealed narrow‐band ELF pulsations with slowly varying frequency over time. The narrow‐band (a few Hz) signatures maintain their bandwidth as their frequencies decrease from 110 to 50 Hz (and their second harmonics) over the course of 40–90 s. Although the narrow‐band ELF waves have a signature somewhat similar to very low frequency (VLF) whistlers, their duration and frequency range are much different from those of whistlers; they also show an unusual characteristic where their frequency often increases before it decreases. These events show a peak in their magnetic local time (MLT) occurrence distribution near the pre‐midnight region, a result that contrasts sharply with the only three previously published studies of these waves. Two possibilities of generation mechanism are discussed.

Influence of solar terminator passages on Schumann resonance parameters

The Terminator Effect (TE) on extremely low frequency (ELF) electromagnetic waves or Schumann Resonances (SR) was investigated on the basis of observations during a number of years at different mid-latitude stations [Hollister (36.8° N, 121.5° W, SAO, USA), Mitzpe Ramon (30.5° N, 34.4° E, MR, Israel) and Nagycenk (47.6 N, 16.7 E, NC, Hungary)]. Definite shifts are observed in the SR magnetic and electric field characteristics during local sunrise and sunset. The terminator passage influences all magnetic field components at SAO, with more significant effects in the east-west component. At MR and NC stations notable effects were observed only in the vertical electric field. The TE is more pronounced in higher SR modes (14, 20 Hz ) compared with the 8 Hz mode. Furthermore, the TE is more pronounced in the morning than evening, and more pronounced in winter than in summer. All these observations indicate that, in addition to diurnal-seasonal thunderstorm source motions, lateral ionospheric gradients across the terminator region have significant influences on ELF wave propagation.

FDTD Analysis of ELF Wave Propagation for Realistic Subionospheric Waveguide Models

The space formed by the ground and ionosphere is known to act as a resonator for extremely low frequency (ELF) waves. Lightning discharges trigger this global resonance, which is known as Schumann resonance. Even though the inhomogeneity (like day-night asymmetry, local perturbation etc.) is important for such a subionospheric ELF propagation, the previous analyses have been always made by some approximations because the problem is too complicated to be analyzed by any exact full-wave analysis. This paper presents the application of the conventional numerical FDTD (finite difference time domain) method to such a subionospheric ELF wave propagation, in which any kinds of inhomogeneities can be included in this analysis. The present paper is intended to demonstrate the workability of this method for a realistic waveguide (with day-night asymmetry), by comparing the results from this method with those by the corresponding analytical method.

FDTD analysis of ELF wave propagation and Schumann resonances for a subionospheric waveguide model

The space formed by the ground and ionosphere is known to act as a resonator for extremely low frequency (ELF) waves. Lightning discharges trigger this global resonance, which is known as Schumann resonance. Even though the inhomogeneity (like day‐night asymmetry, local perturbation, etc.) is important for such a subionospheric ELF propagation, the previous analyses have been always made by some approximations because the problem is too complicated to be analyzed by any exact full‐wave analysis. This paper presents the first application of the conventional numerical FDTD method to such a subionospheric ELF wave propagation, in which any kind of inhomogeneity can be included in this analysis. However, the present paper is intended to demonstrate the workability of this method only for a uniform waveguide (without day‐night asymmetry), by comparing the results from this method with those by the corresponding analytical method.

Nonlinear Excitation of ULF Atmosphere–Ionosphere Waves and Magnetic Perturbations Caused by ELF Seismic Acoustic Bursts

Nonlinear excitation of ultra-low frequency (ULF) atmosphere-ionosphere waves by extremely low frequency (ELF) seismic acoustic bursts is discussed. An analysis of the nonlinear transformation of ELF acoustic waves (f ~ 100 Hz) into ULF (f ≤ 1Hz) waves is presented. The ELF wave is excited as the burst-like envelope of a finite transverse scale by the seismic motion of the Earth’s surface. Then, it propagates upwards and is subject to both nonlinearity and dissipation. Nonlinearity leads to the generation of higher harmonics and, thus, to a saw-tooth-like wave structure, and also to an increase in the ULF part of the wave spectrum. This process takes place at the altitudes of z = 0–100 km. For altitudes z > 100 km, the ELF wave dissipates fully, and only ULF acoustic wave can reach the ionosphere dynamo layer (120–200 km), where this wave excites fluctuating currents. Those currents are the sources of Alfvén and fast magnetosonic plasma waves that penetrate into the higher ionosphere and magnetosphere. The simulations of the nonlinear transformation of the acoustic waves are based on the slowly varying profile equation derived from the hydrodynamic equations of air motion. A high efficiency of the nonlinear transformation of ELF acoustic waves into ULF ones is shown. Like a possible linear transformation, this nonlinear transformation causes ULF magnetic field perturbations in the ionosphere of 10-4–101 nT at altitudes z ~ 200 km, possibly accompanied by temperature variations.

Evolution of electron beam generated waves resulting in transverse ion heating and filamentation of the plasma

We present here a systematic simulational study on electron beam driven waves and their consequences in terms of plasma electrodynamics. The study is performed by using three‐dimensional particle‐in‐cell code, parallelized to simulate a large volume of plasma. Our simulation shows that an initial electron beam of finite radius with beam velocity along the ambient magnetic field triggers a series of events in the evolution of the waves and the plasma. In the initial stage (t ≤ 200 ωpo−1, ωpo being the electron plasma frequency with the total electron density n0), high frequency waves near ω ∼ ωpo are driven. These waves progressively disappear giving way to the dominance of lower hybrid (LH) waves. The phase of the lower hybrid waves lasts over the time interval . In this time period the LH waves stochastically accelerate ions transverse to B0, and the beam electrons are scattered outside the initial beam volume to occupy the entire volume of the simulation plasma. The ion acceleration leads to the formation of elongated tail in the perpendicular velocity distribution. The large‐amplitude LH waves are seen to undergo a parametric decay into resonance cone waves at frequencies ω < Ωi, the ion cyclotron frequency. Such extremely low‐frequency (ELF) waves are the electrostatic version of the inertial Alfven waves. The phase of the strong LH waves is followed by a stage in which HF waves with frequencies ω ∼ ωpo appear again, but in this stage they are strongly modulated by the already present ELF waves and other low‐frequency waves in the frequency range near the ion cyclotron frequency Ωi and its harmonics. Beginning with the LH wave stage and continuing into the late stages of ELF waves, the plasma density is highly filamented and the filaments oscillate with the ELF frequencies. The relevance of our results to the observations on plasma waves from satellites is brought out.

The impact of the August 27, 1998, γ-ray burst on the Schumann resonances

On 27 August 1998, at 10:22 UT, an extremely intense gamma ray flare passed through the solar system. The γ-ray burst lasted only 5 min and resulted in the ionization of a large portion of the Earth's night-side upper atmosphere to levels found normally only during the daytime. During this period we were continuously monitoring extremely low frequency (ELF) electromagnetic waves in the range 1– 50 Hz , at a new monitoring site established in the Negev Desert, Israel. We have analysed our ELF data, including the Schumann resonances, during this period, and no noticeable changes in the ELF signals are observed as a result of this event. This is very different to observations in the VLF range that showed extreme changes in amplitude and phase resulting from this γ-ray burst. Theoretical modeling points to three factors that can explain the differences between the VLF and ELF signal responses: (i) different properties of the ELF and VLF signals within the time scale of the γ-ray burst; (ii) different paths of these signals during the event; and (iii) the different influences of the day/night asymmetry on the propagation of ELF and VLF waves.

Ionospheric E region remote sensing with ELF radio atmospherics

Radio remote sensing of the ionospheric E region can be difficult, particularly in the nighttime E region valley which lies between two regions of higher electron density. We show that extremely low frequency (ELF) electromagnetic waves launched from below penetrate this region because of their low attenuation and can be reflected from both the D region and the top of the E region valley. This double reflection, also caused by sporadic E layers, produces a measurable effect on the subionospheric propagation of ELF waves, which we demonstrate with numerical Earth‐ionosphere waveguide simulations. This sensitivity opens the possibility of remotely sensing the E region and detecting sporadic E layers with broadband ELF propagation measurements using lightning discharges as the source. From nighttime observations of ELF lightning radiation over the same propagation path on different days, we extract E region electron density profiles that show significant variability not only from day to day but over the course of a single night. This technique is inherently a path‐integrated measurement, enabling tomographic large‐scale ionospheric remote sensing with multiple sources and receivers.

Electrostatic broadband ELF wave emission by Alfvén wave breaking

The linear and nonlinear kinetic properties of electrostatic oblique waves below the lower hybrid frequency are investigated. For propagation angles the waves are damped by either parallel electron Landau or ion cyclotron damping. For Ti/Te ≫ 1 the waves are only weakly damped and can propagate. These waves are called slow ion cyclotron (SIC) and slow ion acoustic (SIA) Waves. A fluid‐kinetic model, comprised of hot linear kinetic ions and cold nonlinear fluid electrons, is proposed to describe a nonlinear wave breaking process of small‐scale Alfvén waves resulting in broadband extremely low‐frequency (ELF) wave emission. Numerical solutions of the fluid‐kinetic model are compared to the electric and magnetic fields of solitary kinetic Alfvén waves and broadband ELF waves observed by the Freja satellite within a hot ion environment. The agreement in waveform morphology and amplitude between the fluid‐kinetic simulations and the observed waves provides support for the theory that observed SIA waves are the result of a nonlinear emission process from SIC waves.

The generation of ELF and VLF radio waves in the ionosphere using powerful HF transmitters

The generation of Extremely Low Frequency (ELF) and Very Low Frequency (VLF) waves by ionospheric heating using powerful High Frequency (HF) radio waves is reviewed. Most attention is given to natural current modulation using modulated HF heating and results are presented from all the major heating facilities in chronological order. Both ground based and satellite measurements are described, as are techniques for enhancing ELF/VLF wave generation efficiency. Techniques for ELF/VLF wave generation which do not require the presence of natural ionospheric currents are also reviewed.

Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines

Extremely low‐frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted‐V‐type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy (≈100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction.

Electron pitch-angle scattering by low frequency waves at the geomagnetic Equator

Recent satellite observations1 have revealed details on the strength of the pulsing extremely low frequency (ELF) hiss in the equatorial region which maps down to the auroral zone. It is shown here to be insufficient to produce pitch–angle scattering of energetic electrons into the loss cone at a sufficiently high rate to explain the characteristics of the measured pitch-angle distributions in pulsating auroras, within the framework of the theory proposed by Coroniti and Kennel2. It is suggested that wave–particle interactions involving both broadband incoherent ELF waves and narrow band coherent very low frequency (VLF) waves may be able to produce a sufficiently high rate of pitch-angle scattering.