Showing papers on "Very low frequency published in 1988"

Subionospheric VLF signatures of nighttime D region perturbations in the vicinity of lightning discharges

Abstract: A 12-hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud-to-ground lightning may be important in producing the perturbations. High-resolution analysis of event signatures indicated the presence of two different classes of events. For one set of events, observed during the most active central 6 hours of the observation period, a ∼0.6-s delay between the causative lightning and VLF event onset and a ∼1-s onset duration was observed, consistent with previously suggested models of the gyroresonant whistler-particle interaction that leads to particle precipitation and perturbation of the Earth-ionosphere waveguide. However, another set of events, observed during the first 2 hours of the observation period, exhibited a very different temporal signature, characterized by a much smaller (<50 ms) delay and sometimes also very short (<50 ms) rise times. Such events are possibly related to previously reported cases of similarly early/fast events and may involve a more direct coupling between the lightning discharge and the lower ionosphere.

VLF wave stimulation experiments in the magnetosphere from Siple Station, Antarctica

Abstract: The Earth's magnetosphere is host to remarkable very low frequency (VLF) electromagnetic signals of natural origin. One of these, called a whistler, originates in lightning. Others, such as hiss and chorus, originate within the plasma itself. They are important for at least three reasons. First, they reveal the properties of the plasma through which they travel and thus can be used as remote sensing tools. Second, their high intensity and narrow bandwidths indicate the presence of a previously unknown kind of wave particle interaction that converts the kinetic energy of charged particles to coherent electromagnetic radiation. This process is called the coherent wave instability (CWI). Third, energetic charged particles are precipitated into the ionosphere through resonant scattering by these same waves, causing enhanced thermal ionization, X rays, light, and heat. To better understand and use the CWI, controlled VLF signals have been injected into the magnetosphere from Siple Station, Antarctica and received on satellites and near the conjugate point in Quebec, Canada. In addition to reproducing many puzzling natural phenomena, these experiments have provided critical new data on the CWI, laying a foundation for various theories and computer simulations. Key findings are as follows: 1) Coherent VLF signals often exhibit exponential temporal growth (∼30 dB) and saturation at levels estimated to be of order 5 pT. 2) Temporal growth requires that the input signal exceed a threshold that varies widely with time. The probable cause of the growth threshold is in situ background noise that reduces the efficiency of phase bunching by a coherent input signal whose intensity is comparable to the noise level within the frequency band of the interaction (∼100 hz). 3) Narrowband triggered emissions can be entrained by Siple frequency ramps of different slope but of much lower (−20 dB) amplitude. The mechanism of entrainment is not yet understood. 4) For two equal amplitude input waves spaced 20 Hz apart, the temporal growth of each component is almost totally suppressed. For larger spacings, 40–100 Hz, the lower frequency is more suppressed than the upper. For 10 < Δƒ < 100 Hz, unsymmetrical sidebands at integer multiples (up to seventh order) of Δƒ are created, along with subharmonics. The integer sidebands are attributed to emission growth triggered by one beat and suppressed by the next. Taken together, the spectrum of the stimulated sidebands and sub-harmonics is thus more noise-like than the transmitted spectrum. 5) Simulated hiss shows coalescence of selected noise wavelets into longer and stronger chorus-like emissions, suggesting that chorus and hiss originate in the same mechanism. Future objectives of a VLF wave injection facility include (1) new experiments on the physics of wave growth and wave-induced particle scattering and precipitation, (2) testing of the predictions of theories of VLF wave-particle interaction, (3) development of new techniques for remote sensing and control of space plasmas using VLF techniques, and (4) improvements in the design and operation of VLF communication and navigation systems.

Phase and amplitude perturbations on subionospheric signals explained in terms of echoes from lightning-induced electron precipitation ionization patches

Abstract: Phase and amplitude perturbations (trimpis) on VLF signals propagating in the earth-ionosphere waveguide are usually explained in terms of precipitation-induced modification along the great circle path as though it were a one-dimensional (1-D) waveguide. The alternative 2-D view presented here is that precipitation-induced ionization patches, laterally displaced from the great circle path, produce echoes at the receiver slightly delayed relative to the direct signal. Interference between the direct and echo signals can produce both phase and amplitude trimpis of both signs. Single-mode propagation is assumed here, since it is likely that the second mode amplitude exceeded that of the first by ∼12 dB. We apply this theory to a sequence of trimpis observed on the North West Cape (NWC)-Dunedin path on July 10, 1987. This indicates a north–south drift of the precipitation region consistent with drift speeds of whistler ducts. The theory is also applied to events published in the literature.

Lightning‐associated precipitation of MeV electrons from the inner radiation belt

Abstract: Transient perturbations of subionospheric very low frequency (VLF) radiowave signals provide new evidence for lightning-induced electron precipitation (LEP) events involving short ( 1 MeV electrons from the earth's inner radiation belt at L ≤1.8. The signal amplitude changes are attributed to increased absorption in the earth-ionosphere waveguide and/or alterations of the waveguide mode structure due to localized secondary ionization enhancements produced in the nighttime lower ionosphere and the mesosphere by the precipitating electrons. The otherwise stably trapped electrons are believed to be scattered in pitch angle during cyclotron resonant interactions in the magnetosphere with the lightning-generated whistler waves. That some precipitation bursts consist partly of MeV electrons is suggested by (i) confinement of the perturbed subionospheric signal path to low magnetic latitudes (L ≤1.8), for which corresponding electron energies for gyroresonance with typical whistler-wave frequencies in the magnetosphere are >1 MeV, and (ii) the temporal signatures of the perturbation events, which often exhibit an unusually rapid initial recovery (time constant of τ 1 MeV electrons.

Spectral broadening of VLF radio signals traversing the ionosphere

Abstract: Two different source mechanisms that can cause the spectral broadening of VLF radio signals traversing the ionosphere, a phenomenon first observed by Bell et al. (1983), are proposed. The first is a nonlinear scattering of the whistler-mode VLF signals by preexisting ionospheric density fluctuations that render a mode conversion to lower hybrid waves. In the absence of ionospheric irregularities, a second mechanism, that involves a parametric instability, can excite the lower hybrid waves, as was proposed by Lee and Kuo (1984). Since both types of spectra were recorded in experiments, it is concluded that the two suggested source mechanisms contribute additively to the observed spectral broadening of injected VLF waves.

Remote sensing of the magnetospheric plasma by means of whistler mode signals

Abstract: Early in the past decade of U.S. Antarctic research, the whistler method of measuring equatorial electron density was found to agree with in situ satellite electron density measurements by a radio technique. Furthermore, the whistler method of measuring the east-west component of the convection electric field in the outer plasmasphere was found to agree, under conditions of mapping in a dipole magnetic field, with simultaneous results from incoherent scatter radar. A global model of the east-west convection electric field in the outer plasmasphere during substorms was developed. The detection of whistlers and their use for magnetospheric diagnostics have been important elements in recent studies of burst precipitation into the ionosphere induced by whistlers and by other transient whistler mode waves propagating in the magnetosphere. Whistlers have also been used to obtain data on the L values and equatorial electron densities associated with the propagation paths of signals from the Siple VLF transmitter. The process of untrapping of downcoming wave energy from ducts in the upper ionosphere and the upward repropagation of portions of the energy following reflection in the lower ionosphere lead to the excitation of adjacent ducts as well as to upward propagation in the nonducted mode. Efficient interduct coupling has been found to occur over north-south ionospheric distances of >1000 km. Studies of the outer limits of observed ducting revealed dayside path radii in the range 6–8 RE and nightside limits of ∼5.5 RE. Ducted propagation beyond the plasmapause was found to occur regularly in the 0000–1800 MLT time range, but with variable rates and at various locations with respect to the plasmapause position. The special features of this propagation are believed to be related to conditions of lightning excitation, ionospheric penetration, and wave-particle interactions that are special to the region beyond the plasmasphere. New aspects of Siple wave injection experiments were demonstrated by the application of a new phase measurement method to Siple signals that did not exhibit fast temporal growth during passage through the magnetosphere. This method, a refinement of techniques developed previously by New Zealand workers, is capable of detecting fluctuations in phase path with period of ∼10 s and greater and thus can be used to study magnetospheric convection and coupling fluxes along field lines of propagation as well as pulsations associated with ultralow-frequency perturbations of the geomagnetic field. Additional topics discussed include results from direction-finding experiments and evidence of the dependence of whistlers upon magnetospheric wave amplification.

Ionospheric heater beam scanning: a realistic model of this mobile source of ELF/VLF radiation

Abstract: The generation of ELF/VLF radio waves by heating the auroral ionosphere with a powerful HF transmitter, modulated at the ELF/VLF frequency, is now well documented. Recently, results were presented of the phase of the ELF/VLF signals received at a distance of 500 km from such a source as the HF beam was deflected toward and away from the receiver by up to 37° from the vertical. The phase data were modeled assuming the source to be a simple point dipole located in the ionosphere at the point of maximum HF radiation. This simple model provided good agreement with experimental phase data only when the HF beam was deflected toward the ELF/VLF receiver. The model of the ELF/VLF generation mechanism presented here includes contributions from the majority of the HF radiation pattern illuminating the ionosphere and assumes it to be a continuous source of ELF/VLF excitation. Computations based on this new model provide excellent agreement with the experimental phase data at all beam deflection angles. The use of this model to improve the power efficiency of the generation of ELF/VLF radio waves by HF heating facilities by more than a factor of 3 is discussed.

Satellite observations of a new type of discrete VLF emission at L < 4

Abstract: A new type of discrete whistler mode emission has been observed in the magnetosphere at L below 4. The emission elements are confined to a bandwidth of 1-5 kHz, with the lower cutoff frequency of the band varying with L shell, being equal to about 0.2-0.5 f(Heq), where f(Heq) is the equatorial electron gyrofrequency. The discrete and burstlike nature of the emissions is similar to that of chorus emissions typically observed at higher L; however, dispersion of individual elements is often different from typical chorus, and the emissions are observed inside as well as outside the plasmapause. The phenomenon seems to occur mainly in the early morning local time sector and is well correlated with geomagnetic activity, occurring mostly at high gasmagnetic activity. The analysis of data from the low-altitude ISIS-2 and the high-altitude De-1 satellites indictes that the emissions may be generated near the equatorial plane at frequencies of about 0.2 f(Heq) inside and about 0.35 f(Heq) outside the plasmapause. The parallel energy of electrons for gyroresonance with the observed waves is found to be about 20-50 keV in both cases.

VLF wave stimulation by pulsed electron beams injected from the space shuttle

Abstract: Among the investigations conducted on the Space Shuttle flight STS-3 of March 1982 was an experiment in which a 1-keV, 100-mA electron gun was pulsed at 3.25 and 4.87 kHz. The resultant waves were measured with a broadband plasma wave receiver. At the time of flight the experimental setup was unique in that the electron beam was square wave modulated and that the Shuttle offered relatively long times for in situ measurements of the ionospheric plasma response to the VLF pulsing sequences. In addition to electromagnetic response at the pulsing frequencies the wave exhibited various spectral harmonics as well as the unexpected occurrence of 'satellite lines' around those harmonics. Both phenomena occurred with a variety of different characteristics for different pulsing sequences.

A new component of terrestrial radio emission observed from ISEE-3 and ISEE-1 in the solar wind

Abstract: At a time when the AE index was 1300, the radio receivers on board ISEE-3 (216 RE sunward of Earth) and ISEE-1 recorded a new type of terrestrial radio emission between 20 and 50 kHz. The radiation had a smooth time profile, with a negative frequency drift; its angular diameter at 47 kHz was at least 170°. This smooth component followed the onset of a TKR burst whose angular diameter was 50° at the same frequency 47 kHz and varied as f−2, i.e., was controlled by interplanetary scattering. Because of its time association with the TKR burst, the source of the smooth radiation must have been in the Earth environment. We suggest that it was produced or leaked into the interplanetary medium very far from Earth in order that the scattering path to ISEE-3 be long enough (2000 RE) to account for the observed angular diameter. More than 50 such events, clearly distinct from the Earth non thermal continuum, were observed between 1978 and 1983.

Ray tracing of electrostatic waves in a hot plasma and its application to the generation of terrestrial myriametric radiation

Abstract: The first results from the program HOTRAY for ray tracing electrostatic and electromagnetic waves in a hot plasma are presented. The calculations are applied to the linear mode-conversion window theory for the generation of Terrestrial Myriametric Radiation corresponding to a specific event observed by the DE-1 satellite. Results show that backward propagating electrostatic waves can refract in a density gradient normal to the magnetic field to become Z-mode waves and can have access to the radio window. During propagation the waves remain minimally damped. Variations of approximately 0.01° in the initial wave normal angle ψ can result in a large difference in the transmitted wave intensities. However, if such variations do occur, waves from different locations can still access the radio window so that energy transfer to the conversion point remains efficient. Strong refraction of electrostatic waves at latitudes > 1° show that the magnetic equator is the preferred location for mode conversion to take place.

A very low frequency radio astronomy observatory on the Moon

Abstract: Because of terrestrial ionospheric absorption, very little is known of the radio sky beyond 10 m wavelength. An extremely simple, low cost very low frequency radio telescope is proposed, consisting of a large array of short wires laid on the lunar surface, each wire equipped with an amplifier and a digitizer, and connected to a common computer. The telescope could do simultaneous multifrequency observations of much of the visible sky with high resolution in the 10 to 100 m wavelength range, and with lower resolution in the 100 to 1000 m range. It would explore structure and spectra of galactic and extragalactic point sources, objects, and clouds, and would produce detailed quasi-three-dimensional mapping of interstellar matter within several thousand parsecs of the Sun.

Near‐surface ocean ambient noise measurements at very low frequencies

Abstract: Ambient noise in the frequency range 1–20 Hz was measured for 1‐day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface‐suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow‐band spectral analysis was performed at 1‐h intervals to form time series of noise spectral level for each site. Statistical analyses of the power‐spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind‐speed dependence of mean spectral level. Comparisons with data from bottom‐mounted hydrophones and studies of inter‐ and intra‐array coherence indicate that the data were, with few exceptions, uncontaminated by self‐noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5‐ to 3‐Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was ...

Outer zone electron precipitation produced by a VLF transmitter

Abstract: : Using high resolution pitch angle measurements made by a magnetic focusing electron spectrometer on the S3-3 satellite while in the drift loss cone region of the magnetosphere, characteristics of fluxes of 108 keV to 654 keV electrons precipitated in the inner zone, in the slot region, and in the outer zone of the magnetosphere are all shown to be consistent with the precipitation having been produced by the same ground-based VLF transmitter, UMS. Pitch angle measurements are used to locate the longitude of precipitation. The temporal pattern of transmitter operation obtained from synoptic data from a ground-based VLF receiver is used along with drift-rate calculations to predict the electron energies as a function of L-shell which should be observable by the S3-3 instrument. The predicted energy response is then compared with the in-situ observations, getting complete agreement. Finally, wave-particle resonance calculations are made for each of the three regions. The study indicates that ground-based VLF transmitters, which have previously been shown to produce precipitation in the inner zone and slot regions, are almost certainly instrumental in precipitating electrons in the outer zone also.

Design of an elf/vlf satellite for under the ice submarine communications

Abstract: : This thesis proposes the design of a space based tethered antenna satellite system for ELF/VLF communications with submarines in the far northern latitudes, and under the polar ice. By using the dynamo effect of a moving wire in a (geo)magnetic field, the tether cable can produce sens of kilowatts of its own radiation power. The transmitted signal of IKHz-3KHz will be whistler mode propagation to couple to the earth's field lines and follow them down to the surface. The signal can penetrate 100m of seawater, and ice of unlimited thickness. A constellation of 12 satellites will provide 75% duty cycle coverage for each submarine operating area of over four million square kilometers. Issues examined are: tether electrodynamics and mechanics, debris survivability, ionospheric radio and plasma physics, plasma contactors, satellite and constellation design concepts, cost analysis, and military mission needs analysis.

Non-stationary effects in wave-particle interactions during storm sudden commencements (SSC)

Abstract: Simultaneous observations of energetic electrons (16-300 keV) and ELF/VLF waves (200 Hz-3 kHZ) are analysed, that are obtained onboard GEOS 2 in the dayside magnetosphere during Storm Sudden Commencements (SSC's). The electron distribution exhibits a large temperature anisotropy, leading to the amplification of whistler waves. Wave particle interactions (WPI's) are studied by calculating the temperature anisotropy and the wave growth rate from one minute averages of the particle data. Following the SSC, an increase in the electron distribution anisotropy results in an increase of the wave spectral intensity Bf2. Oscillations with a period of 2-4 min are observed in the maximum intensity Bfmax2 that are in antiphase both with the corresponding growth rate and the anisotropy. This last effect is interpreted in terms of the quasilinear pitch-angle diffusion of the resonant electrons.

Exit Areas of VLF Waves Inferred from the Multistation Measurements at Roberval, Canada

Abstract: The characteristics of the wave exit area on the lower boundary of the ionosphere is inferred from the distribution of the magnetic intensities of the VLF waves simultaneously received at six observation stations near the Siple conjugate point at Roberval in Canada on July 23, 1979. In order to estimate the characteristics of the wave exit area, we have to make some assumptions about the conditions of the input wave injected to it and the propagation mechanism in free space. Applying the principle of Huygence for electromagnetic wave in free space to the wave radiated from the exit area, we compute the distributions of the wave magnetic intensity on the ground. By a least square technique, we infer the location and shape of the exit area making the wave intensity distribution observed on the ground. The obtained results are as follows. It was found that three wave exit areas at least existed over the observation station network. It was confirmed that the signals radiated from the Siple transmitter (Siple waves) were propagated along the ducts in the magnetosphere. The wave exit areas were generally elongated in the east-west direction, suggesting that the shapes of the ducts are also elongated in the east-west direction.

Ultralow- and very-low-frequency seismic and acoustic noise in the Pacific

Abstract: A discussion of the sources of seafloor noise is most conveniently broken into four frequency bands since the noise is dominated by different physics in each of these bands. The first, and most familiar, band is from 3–50 Hz. This band is known as the very‐low‐frequency (VLF) or infrasonic band but is termed high‐frequency noise for these puposes. The best documented mechanisms for the generation of ambient noise in this band is shipping. The next lower band, from 80 mHz to 3 Hz, is commonly called the microseism band after the high‐level microseismic noise that is clearly recorded at all sites on the Earth's surface and results from nonlinear wave‐wave interactions. The third band, the noise notch (20–80 mHz), has a variable bandwidth and is observed on both the continents and in the ocean. Noise levels within this notch appear to be controlled largely by currents and turbulence in the seafloor boundary layer. The final ultralow‐frequency (ULF) band extends from dc to 20 mHz and the levels can be attribu...

Comparison of Swallow Float, Ocean Bottom Seismometer, Sonobuoy Data in the VLF (Very Low Frequency) Band

Abstract: : VLF data collected concurrently by three sets of instruments--the MPL's freely drifting Swallow floats, an array of OBSs deployed by SIO, and sonobuoys from the NADC are compared. The data were collected in April and May, 1987, near the site of DSDP hole 469, 32.5 deg N, 120.5 W deg. To make the data comparisons quantitative, the OBS and sonobuoy data were resampled to a sampling frequency of 50 Hz and the time bases for the 3 sets of instruments were aligned using coherent arrivals. Spectra, spectral differences, and coherence squared estimates were calculated and plotted. Results indicate that the Swallow float geophone data are consistent i.e., the spectra from different instruments of the same type do not differ within the confidence limits of the spectral estimates as are most of the OBS vertical-axis geophone data. The inter-Swallow-float coherences are significantly different from zero at frequencies thought to be broadcast by the research vessels in the experiment. The inter-vertical -axis- OBS coherences are also significantly different from zero at frequencies below 6 Hz as well as at the ship frequencies. The high coherences below 6 Hz are probably due to the close spacing of the OBSs in the experiment. On the other hand, the OBS horizontal-axis geophone data from different instruments can vary over several tens of decibels in spectral level. The OBS hydrophone time series contain 0.5-second spikes of various amplitudes, and the sonobuoy data appear to suffer from contamination below 10 Hz.

Practical VLF/LF radio test system

Abstract: A practical VLF/LF radio test system designated as the AN/URM-212 Test-Set Radio is presented. The purpose of the AN/URM-212 is to verify the operational readiness of VLF/LF receive communication systems installed in submarines, US Navy aircraft, and tenders from antenna to printer. The AN/URM-212 tests processor-controlled VLF/LF receive components (e.g. VERDIN) in a quantitative manner and tests non-processor-controlled VLF/LF receive components (e.g. receivers, antennas and associated couplers) in a qualitative manner. It tests the receive components by measuring small performance degradations not discovered by automated performance monitoring, self-test or other preventive maintenance procedures. >

Generation of VLF and ELF Waves for Active Probing

Abstract: Generation of electrical power at any frequency in this range is quite simple. Significant generation of waves at frequencies below, say, 5kHz requires antennas some kilometres in length and height above the ground plane. Antenna methods used with some success to probe the magnetosphere and ionosphere are reviewed: balloon lofted vertical monopoles (Alaska and N. Z.), a dipole over ice (Siple), borrowed power lines (arctic Norway and N. Z.), electrojet modulation (U. S. S. R. and Norway).