Low-frequency Emission Research Articles

Infrasonic monitoring of snow-avalanche activity: What do we know and where do we go from here?

The first recordings of acoustic signals associated with avalanches go back to the 1970s when premonitory low-frequency acoustic emission was observed in the snow cover just before the setting off of natural avalanches (Sommerfeld, 1977; Sommerfeld and Gubler, 1983). Recently, it has been demonstrated that avalanches also produce strong infrasonic vibrations in air during their movement. These infrasonic vibrations propagate great distances and can follow the natural relief. It was then a question of automatically detecting this emission to monitor avalanche activity, with a view to improving forecasting and assisting in the study of certain aspects of avalanche dynamics.With the double aim of pursuing investigations on the mechanics of the acoustic emission from avalanches and designing an automatic detection system, our laboratory has developed specific instrumentation, the essential link of which is an array of infrasonic microphones associated with a goniometer. Systematic measurements of the infrasonic emissions are being carried out, not only of the natural or released avalanches themselves but of all events with the aim of characterizing the former in relation to “infrasonic noise”.

Infrasonic monitoring of snow-avalanche activity: What do we know and where do we go from here?

The first recordings of acoustic signals associated with avalanches go back to the 1970s when premonitory low-frequency acoustic emission was observed in the snow cover just before the setting off of natural avalanches (Sommerfeld, 1977; Sommerfeld and Gubler, 1983). Recently, it has been demonstrated that avalanches also produce strong infrasonic vibrations in air during their movement. These infrasonic vibrations propagate great distances and can follow the natural relief. It was then a question of automatically detecting this emission to monitor avalanche activity, with a view to improving forecasting and assisting in the study of certain aspects of avalanche dynamics.With the double aim of pursuing investigations on the mechanics of the acoustic emission from avalanches and designing an automatic detection system, our laboratory has developed specific instrumentation, the essential link of which is an array of infrasonic microphones associated with a goniometer. Systematic measurements of the infrasonic emissions are being carried out, not only of the natural or released avalanches themselves but of all events with the aim of characterizing the former in relation to “infrasonic noise”.

Detection on the Intercosmos-24 satellite of VLF and elf waves stimulated in the topside ionosphere by the heating facility Sura

Results are presented of a satellite experiment on the detection of VLF and ELF waves excited by irradiation of the night ionosphere F region by the field of a nonmodulated powerful radio wave. The excited VLF and ELF waves have been detected at topside ionosphere altitudes h = 500–1000 km in the frequency bands 7.5 kHz < f < 15 kHz and 8 Hz < f < 1.5 kHz at distances less than 50–100 km from the centre of the magnetic force tube passing through the heated spot of the ionosphere. The region of wave occurrence has well-defined boundaries. This points to propagation (guiding) of the observed low frequency waves along geomagnetic field lines. Possible generation mechanisms for the low frequency emission observed are discussed. It is shown that longitudinal ELF waves may be excited at altitudes of the topside ionosphere by beams of suprathermal electrons resulting from the action of a high-power radio wave on the ionosphere. These waves have a small group velocity in the direction orthogonal to the magnetic field. Occurrence of ELF oscillations detected onboard of satellite may be caused also by satellite motion in spatially inhomogeneous electrostatic field, connected with artificial irregularities of plasma density and accelerated electron beams. Whistler waves in the VLF band may result from conversion of lower-hybrid oscillations excited in the reflection region of powerful radio wave due to scattering by artificial small scale field-aligned irregularities. The whistlers generated can propagate to the satellite along artificial ducts formed within the disturbed magnetic force tube.

The charged‐particle fluxes at auroral and polar latitudes and related low‐frequency auroral kilometric radiation‐type and high‐frequency wideband emission

The results of simultaneous observations of charged particle fluxes within the energy range 50 eV to 20 keV (plasma energy‐angle spectrometer, PEAS experiment) and plasma waves within the frequency range 0.1 to 10 MHz (plasma radio spectrometer, PRS 3 experiment) on board the APEX spacecraft are presented. The data were obtained at polar and auroral latitudes in the dawn‐dusk and noon‐midnight time sectors. The low‐frequency (LF) sporadic emission mainly on frequencies lower than the local gyrofrequency was commonly observed within a comparatively narrow latitude interval (∼5°– 6°) of the auroral oval. The maximum of spectral wave intensity was revealed at a frequency of ∼ 200 kHz. The sporadic character of the observed emissions, the spectra, are very similar to those reported for auroral kilometric radiation (AKR), and the temporal and spatial occurrences imply its relation to the source of AKR emissions measured at the higher altitudes, with regards to our wave measurements as LF AKR‐type emissions. Comparative analysis of spatial distributions of charged particle energy spectra and variations of wave spectra at auroral oval latitudes showed that several conditions should be fulfilled for LF AKR generation. These conditions are related both to the intensity and energy of precipitating ion and electron fluxes, and to albedo electrons and the ionospheric plasma parameters. The wideband HF emission was observed together with the LF AKR‐type emission during the morning auroral oval crossing. At spacecraft apogee altitudes, in the polar cap zone intense electron precipitations and an increase of plasma noise intensity at frequencies equal or below ƒHe were observed at the period when the IMF was northward These precipitations and emissions were accompanied by the horizontal currents crossing the polar cap, the so‐called theta structure.

Spectral Diagnostics of Blazar Central Engines. III. Submillimeter Flares in Blazars

This article investigates the difference in submillimeter flares in BL Lac objects and flat-spectrum radio quasars. It is shown that a strong distinction can be found in the low-frequency emission at centimeter wavelengths. This is ascribed to a mildly varying centimeter-peaked background spectral component that is far more powerful in quasar radio cores than in BL Lac objects. There is also a highly variable millimeter peaked component, the flaring component, which is similar in quasars and BL Lac objects. This analysis is supportive of the two-component spectral model discussed in Papers I and II of this series.

Temperature dependence of spontaneous otoacoustic emissions in the edible frog ( Rana esculenta)

The change in frequency of individual emissions in the European edible frog Rana esculenta) when the temperature of the frog is modified, is part of a complex pattern of interaction between spontaneous otoacoustic emissions. At high temperatures (above 24°C) two emissions are always detected (e.g., one near 800 Hz and one near 1200 Hz). The higher-frequency emission is lower in level and has a wider bandwidth than the lower-frequency emission. It is also often asymmetric and sometimes breaks into two emissions when an external suppressor tone is applied. When the temperature is decreased, these emissions are reduced in frequency at a rate of 0.04 octave/°C. The higher-frequency emission becomes narrower and taller, and the lower-frequency emissions becomes broader and less intense. At approximately 18°C the lowest of these emissions (now between 600 and 700 Hz) disappears and is replaced by a new emission approximately 100 Hz lower in frequency. When the temperature is carefully controlled the two emissions can exist simultaneously. The lowest-frequency emission changes 0.015°C/octave suggesting that the mechanisms controlling the frequency of this emission may be different than those determining the frequencies of the other emissions. All but the lowest-frequency emissions are maximal in level and have minimal bandwidth when the frequency is close to 700 Hz, which is interpreted as evidence that these emissions are filtered by a temperature-independent process.

Analysis of shielded-room error for screened rooms and its application to low-frequency emission measurements from electrically small sources

Screened rooms are an alternative to open area test sites for radiative emission measurements. The cost of screened anechoic rooms by comparison is considerable, especially for low-frequency operation. The paper considers the use of screened rooms operated below resonance in obtaining low-frequency emission measurements from electrically-small sources. The screened room is modelled as a lossless rectangular cavity using a rapidly convergent Green's function formulation. This is used to establish the shielded-room error for electric and magnetic current sources and examine the main coupling mechanisms between the source and observation point in a screened room. Emission measurements are presented for a self-contained electrically-small test source. Comparison with open-air measurements and emission measurements from a TEM cell show agreement to within ± 5 dB.

A Cocoon Transparency and the 3C 345 Low Frequency Variability

The structure of the quasar 3C 345 is studied at λ = 49cm. The core has a low-frequency cut off spectrum and α ≃ 3. The brightest knot is in the nearest part of the jet. The cocoon wall absorbs low-frequency emission and changes the polarization orientation, RM = 3500 rad m–2, B|| ≃ 100 μG at the core region.

Interplanetary type III radio bursts observed simultaneously by ICE

We analyze two solar type III radio bursts that were observed simultaneously by the ICE and Ulysses spacecraft. Both bursts originated behind the solar limb as viewed from either spacecraft. At the time of these events, ICE was in the ecliptic plane at ∼1 AU and Ulysses was ∼ 35° south of the ecliptic plane at ∼4 AU. For one event on 931117, the ratios of the peak flux densities measured at each spacecraft, at each observing frequency, were consistent with the most probable source locations relative to ICE and Ulysses. The second event on 931004 was a complex burst consisting of two distinct components at high frequencies. At low frequencies, the intensity of the first component decreased rapidly at each spacecraft. The second component, however, dominated the low frequency emission observed at Ulysses but not at ICE. These differences in the observed radiation must be related to the different viewing geometries of the two spacecraft. The measured onset times as a function of observing frequency were consistent with a constant exciter speed through the interplanetary medium and suggest that there are significant propagation delays, especially for the radiation propagating within the ecliptic plane.

Appearance of VLF and ELF-noises in topside ionosphere under the action of high power radio wave from data of satellite intercosmos-24

Results of a satellite experiment are presented on detection of VLF and ELF-waves excited by irradiation of the night ionosphere F-region by the field of a nonmodulated high-power radio wave. The excited VLF and ELF-waves have been detected at the topside ionosphere heights h=500–1000 km in the frequency bands 8 kHz<f<1.5 kHz and 10 Hz<f<1.5 kHz at distances less than 50–100 km from the centre of the magnetic force tube passing through the heated spot in the ionosphere. The region of wave appearance has well-defined boundaries. This points to propagation (guiding) of the observed low frequency waves along geomagnetic field lines. Possible mechanisms of observed low frequency emission generation are discussed. It is shown that longitudinal ELF-waves may be excited at the heights of topside ionosphere by beams of suprathermal electrons resulting from the action of high-power radio wave on the ionosphere. These waves have a small group velocity in the direction orthogonal to the magnetic field. Whistler waves in the VLF-band may result from conversion of lower-hybrid oscillations excited by the high-power wave into the whistler mode due to scattering by artificial small-scale field-aligned irregularities. The whistlers generated can propagate to the satellite along artificial ducts formed within the disturbed magnetic force tube.

Algorithms for the retrieval of rainfall from passive microwave measurements

The retrieval of rainfall intensity from radiances measured by spaceborne microwave radiometers can be understood in terms of well established physics. At frequencies below about 40 GHz over an ocean background the relationship between the rainfall and the observations is particularly well understood. In this part of the spectrum, the radiances are principally determined by the liquid hydrometeors with only a modest amount of ambiguity. In very intense convection, ice aloft may increase this ambiguity somewhat. At high frequencies, such as the 85.5 GHz channel of the SSM/I, scattering by the frozen hydrometeors becomes more significant and quantitative rainfall retrieval becomes more problematic. In spite of the ambiguities, the use of the higher frequencies is desirable on a number of counts including: applicability over land, spatial resolution and dynamic range. A total of 16 algorithms were submitted for the PIP‐1. These include algorithms that are based on high frequency (scattering) measurements and low frequency (emission) measurements with a few combinations and variations on these themes. The calibration of the algorithms varies from mostly empirical to essentially first principles with most falling somewhere in‐between. All of the algorithms retrieved rainfall and one also retrieved a profile of the liquid and frozen hydrometeors.

Low-frequency acoustic emissions in fresh and salt water

The impact of a jet of water onto a still water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Densely populated bubble plumes are generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. The detached bubble plume, which is roughly spherical in shape, then undergoes volume pulsation and radiates relatively large-amplitude, low-frequency sound. The results of a laboratory study of the noise produced by this process were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 94, 2809–2820 (1993)]. In this paper we report the results of a field study of noise produced by this process in both fresh water (Lake Washington, WA) and salt water (Puget Sound, WA). Studies of acoustic emissions from transient bubble plumes as a function of cylinder parameters are described, with specific attention devoted to a comparison of the results obtained in salt and fresh water. The measurements, which exhibit good agreement with laboratory study, indicate that there is a correlation between the acoustic intensity radiated from bubble plumes and the total potential energy of the water jet.

Miocrowave spectral imaging, H-alpha and hard X-ray observations of a solar limb flare

We compare the microwave, H-alpha, and hard X-ray observations for a west limb C7.3 flare that occurred at 17:10 UT, 1992 June 26. H-alpha movies were obtained at Big Bear Solar Observatory. Before the onset of the flare, overexposed H-alpha images show the complicated flux loop structure above the limb. Material was observed to descend along the loops toward the site where the flare occurred hours later. Using the five-antenna solar array at Owens Valley Radio Observatory, we obtain two-dimensional maps of flare emission from 1.4 to 14 GHz. In all three temporal peaks of the microwave bursts, the maps show the same characteristics. The peak low-frequency emission comes from the top of one bundle of the H-alpha loops and gradually shifts to the foot-point of the loops (the location of H-alpha flare) as the frequency increases. The location of the emission peak shifts 88 sec between 1 and 14 GHz. Seventy percent of the shift occurs between 1 and 5 GHz. The locus of the shift of the emission peak follows the shape of an H-alpha surge that occurred after the flare. For each point along the locus, we create the microwave brightness temperature spectrum and compare the radio-derived electron distribution with that derived from the high-resolution hard X-ray spectra measured with Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO). We find that the peak frequency changes from approximately 3 GHz at the loop top to approximately 7 GHz at the footprint, presumably due to the increase of the magnetic field from approximately 160 GHz at the loop top to approximately 300 G at the footpoint. The high-frequency slope of the microwave power-law spectrum decreases from approximately 10 at the loop top to approximately 5 at the footprint due to a change in the energy distribution of the dominant electrons. The microwave brightness temperature spectral index predicted by the BATSE power-law hard X-ray spectra agrees with the measured value only at the footpoint. At the loop top, the emission may be thermal gyrosynchrotron with a temperature of 3.5 x 10<SUP>7</SUP> K, which is likely to correspond to the superhot component seen in the hard X-ray emission.

High-resolution observations of a complete sample of radio sources variable at 151-MHz

High-resolution VLA and Ryle Telescope observations, at 0.33, 1.5, 8.4 and 15 GHz are presented of a complete sample of 20 radio sources that are variable at 151 MHz. In 18 of the sources, the emission is dominated by structure on a scale ≤0.1 arcsec at 8.4 GHz; the spectra of these compact components indicate that structure on this scale probably also dominates the low-frequency emission. Most of the sources belong to the compact steep-spectrum class. Four are identified with quasars with redshifts greater than 1. The remainder are unidentified or identified with a stellar object; it is argued that these are also likely to be at redshifts in excess of 1. The most extreme low-frequency variability is shown by the unresolved sources with steep high-frequency and flat low-frequency spectra, suggesting that these objects have the most compact components at low frequencies.

Experimental study of low-frequency emission from a mobile ionospheric source

The results of an investigation of VLF radiation generated by a mobile ionospheric source are presented. Radiation at frequency F appears due to the operation of two powerful HF transmitters at frequencies f and f±F fed to two space-separated antennas. The intensity of the signals received at the ground depends on the location of the source with respect to the receiving point and is determined by the sign and value of the frequency shift F. The proposed model of the phenomena satisfactorily explains the experimental results.

Induced photon decay and photon-beam-induced Langmuir turbulence

A pair of quasi-linear-like equations is derived to describe the effect of three-wave interactions between high-frequency photons and Langmuir waves in a differential approximation. Induced photon decay leads to terms analogous to those for spontaneous emission, but involving the square of the photon occupation number. The effect on the Langmuir waves is evaluated for axisymmetric photons. The effect on the photons is shown to be similar to but weaker than induced Compton scattering by thermal electrons. The absorption coefficient for the Langmuir waves is evaluated for an axisymmetric distribution of photons, and used to discuss a photon-beam-induced instability. Possible astrophysical applications to solar ‘spike’ bursts, the eclipse of a radio pulsar and the variable low-frequency emission from some active galactic nuclei are discussed briefly, and it is concluded that the process can account for the observed properties of the eclipse of PSR 1957 + 20.

Search for effects of comet S-L 9 fragment impacts on low radio frequency emission from Jupiter

Decametric radio observations of Jupiter were made before, during, and after the impacts of the fragments of the comet S-L 9 with the planet, from the University of Florida Radio Observatory, the Maipu Radio Astronomy Observatory of the University of Chile, and the Owens Valley Radio Observatory of the California Institute of Technology. The decametric radiation was monitored at frequencies from 16.7 to 32 MHz. The minimum detectable flux densities were on the order of 30 kJy, except for that of the large 26.3 MHz array in Florida, which was about 1 kJy. There was no significant enhancement or suppression of the decametric L-burst or S-burst emission with respect to normal activity patterns that might be attributed to the fragment entries. However, a burst of left-hand elliptically polarized radiation having a considerably longer duration than an L-burst was observed almost simultaneously with the impact of the large fragment Q2, and another with right-hand elliptical polarization was observed simultaneously with Q1. We consider the possibility that these two bursts were emitted just above the local electron cyclotron frequencies from the southern and northern ends, respectively, of magnetic flux tubes that had been excited in some way by the proximity of fragments Q2 and Q1. In addition to the monitoring of the decametric radiation, a search was conducted for possible comet-enhanced Jovian synchrotron radiation at 45 MHz using a large dipole antenna array at the observatory in Chile. This frequency is above the cutoff of the decametric radiation, but is considerably below the lowest frequency at which the synchrotron emission has previously been detected. The minimum detectable flux density with the 45 MHz antenna was about 5 Jy. No synchrotron emission at all was found before, during, or after the entry of the comet fragments.

The Effect of Middle Ear Pressure on Transient Evoked Otoacoustic Emissions

To determine the effect of middle ear pressure on transient evoked otoacoustic emissions (TEOAEs), emissions were recorded in ears with tympanometric peak pressures < or = -100 daPa and audiometric thresholds < or = 30 dB HL at 500 through 2000 Hz. TEOAEs were alternately recorded at ambient pressure and at the tympanometric peak pressure. As demonstrated for the 14 ears tested, equalization of the middle ear pressure increased TEOAE amplitude. Reproducibility was similarly improved in 12 of 14 ears. Unequalized middle ear pressure attenuated low frequency emissions more than high frequency emissions. These amplitude and spectrum differences were consistent with previously reported observations of the effects of ear canal pressure on otoacoustic emissions. Results suggest that unequalized middle ear pressure may increase the occurrence of false positive failures, if otoacoustic emission testing is used for hearing screenings without consideration of middle ear pressure.

Stimulated low-frequency emission from anisotropic molecules in microdroplets

Spectral broadening of the input-laser line and the stimulated Raman line have been observed in microdroplets of carbon disulfide, benzyl alcohol, toluene, chloroform and a binary mixture of carbon disulfide in ethanol. The broadening is asymmetrical and extends more than 500 cm −1 towards the low-frequency side of the input-laser line and the stimulated Raman lines. The broadening is proposed to arise from multiple-order stimulated low-frequency scattering associated with librational and dipole—induced-dipole translational motions.

The low-frequency interplanetary radiation

Observations of the low-frequency interplanetary radiation have continued, although no major activity has begun since mid-1991. Theoretical work has been completed which suggests that the long-speculated source at the termination shock is feasible and could at least qualitatively account for several aspects of the observations, although some difficulties exist. The Ulysses encounter with Jupiter occurred in February 1992 and has renewed discussion of a Jovian source for the emissions. We reconsider the arguments used previously to rule out Jupiter as a source and conclude that the existence of a relatively high-density boundary to the heliosphere renders most of these arguments ineffective in eliminating Jupiter as a source. We conclude, therefore, that Jupiter must be considered as likely a candidate source as the termination shock for the low-frequency emissions and, given that Jupiter is a known source of emissions in the few-kHz range, perhaps Jupiter is the more likely candidate. Even with a Jovian source, however, the low-frequency emissions continue to be useful sounders of the heliosphere which can provide information on the size of the heliospheric cavity. No conclusive evidence is available which identifies either Jupiter or the termination shock as the source.