Showing papers on "Very low frequency published in 1981"

Precipitation of inner zone electrons by whistler mode waves from the VLF transmitters UMS and NWC

Abstract: The precipitation of energetic electrons which are commonly observed in the drift loss cone east of 60/sup 0/ east longitude between Lapprox.1.6 and Lapprox.1.8 can be accounted for by a Doppler-shifted cyclotron resonance between the electrons and nonducted whistler mode waves from high-power, ground-based VLF transmitters. A ray-tracing analysis using a diffusive-equilibrium model shows that 17.1-kHz waves starting with vertical wave normals between 23/sup 0/ and 31/sup 0/ magnetic latitude cross the magnetic equator between Lapprox.1.6 and f Lapprox.1.8 with wave normals of approximately 63/sup 0/. A relativistic cyclotron-resonance analysis for the same model plasmasphere using the ray-tracing results gives an energy versus L shell dependence for the precipitated ray electron which is in excellent agreement with the observed dependence. The primary VLF transmitter is most probably the UMS transmitter located near Gorki, USSR. It transmits on 17.1 kHz. VLF records covering this frequency band were available for only three of the time periods when electrons were observed. In two cases UMS was transmitting at the time required to account for the observations. In the third case a higher frequency is required to fit the data. At the time, the NWC transmitter at North West Cape, Australia was operating atmore » 22.3 kHz. These data are consistent with a model in which weak pitch angle scattering by whistler mode waves from NWC does not completely fill the drift loss cone at the longitude of NWC.« less

Narrowband Electromagnetic Emissions from Jupiter's Magnetosphere.

Abstract: Recent studies of wideband plasma wave data from Voyagers 1 and 2 have revealed the existence of narrowband radio emissions escaping from Jupiter's magnetosphere in the frequency range 1-12 kHz. These narrowband emissions are very similar to narrowband emissions previously discovered near earth and Saturn, and are believed to be produced by mode conversion from locally generated upper hybrid resonance waves at odd half-integral harmonics of the electron cyclotron frequency. This mode conversion process is believed to be one of the basic mechanisms for generating planetary radio emissions.

The role of hiss in magnetospheric chorus emissions

Abstract: : Many researchers have reported that narrowband hiss emissions are simultaneously present with ELF chorus emissions outside of the plasmasphere. In data from the SCATHA satellite chorus emissions are often observed to start at frequencies that are within a hiss band. Hiss band spectra averaged for 6.4 s with a resolution of 5 Hz are very smooth. Relative maxima are typically less than 2 db above adjacent minima. Spectra obtained on a 200 ms sample show large variations in amplitude between adjacent bins with relative maxima 10 to 15 db above adjacent minima. Electrons in a narrow range of energies and pitch angles can be organized in phase by the doppler-shifted cyclotron resonance with the larger amplitude spectral components in the hiss band. The bandwidth of the cyclotron resonance is found to be sufficiently narrow so that the electrons are not dephased by waves in adjacent portions of the highly structured spectrum. The amplitude of the hiss is sufficient to significantly phase bunch the electrons in the calculated interaction time. The chorus emission is then generated as the phase bunched electrons moved adiabatically along the geomagnetic field line. No evidence for monochromatic input waves such as power line harmonic radiation are found in the SCATHA data within the hiss bands from which chorus is observed to be triggered. This mechanisms can also account for the chorus emissions detected at frequencies above a hiss band in the Jovian magnetosphere. (Author)

Very low‐frequency electromagnetic interpretation using tilt angle and ellipticity measurements

Abstract: During this study, a plane‐wave computer modeling approach was used to gain more insight into the response of vertical sheet conductors at very low frequency (VLF). The medium which surrounds the conductors is assumed to be a dissipative one. Magnitude and shape of the anomalous field caused by a vertical “thin” conductor at VLF is found to be dependent mainly on (1) conductivity‐thickness product of conductor, (2) resistivity of host medium, (3) depth to the top of conductor, and (4) overlying conductive overburden. Other variables being the same, a shift in frequency in the 17 to 25 kHz range does not produce an appreciable change in the overall response of modeled conductors. The lateral distance between the maximum and minimum on a tilt angle profile is related to conductivity and thickness of conductor, resistivity of the host rock, and depth to the top of the conductor in a complex manner. An interpretation scheme has been proposed to determine the conductivity‐thickness product and depth of vertica...

VLF emissions from a modulated electron beam in the auroral ionosphere

Abstract: A discrete VLF frequency of 3 kHz was successfully radiated by a modulated electron beam on a rocket launched into an active aurora. Instrumentation on this flight included a programable electron accelerator on the aft section with various particle and field detectors on the aft section as well as the ejected forward payload. The accelerator programmer included a current modulation period at fixed electron energy for 0.45-s duration approximately every 11 s throughout the flight. In each of these program steps, 4-kV electrons are current modulated at a 3-kHz rate between Imin=0 or 10 mA and Imax ≅80 mA. The forward payload, which was ejected at about 10 m/s, included a pair of spherical double probes separated by 2.75 m and connected to a VLF receiver operating between 30 Hz and 18 kHz. Both this broadband receiver output as well as various narrow band channel outputs were directly telemetered to ground. Post flight spectrum analysis of the broadband VLF data clearly indicates that signals during the 3-kHz accelerator modulation periods were propagated to the forward payload. A detailed analysis of these modulated pulses detected by the VLF receiver is presented. A time-delay analysis between the start of the modulation and detection at the forward payload indicates time delays up to 0.2 s. The electron beam is believed to have produced a beam-plasma discharge making a radiation efficiency calculation difficult. However, absolute received signal strength was about 1 mV/m at 1.4-km separation.

A ground‐satellite study of wave‐particle correlations

Abstract: Very low frequency (VLF) waves recorded at Siple, Antarctica (Lapprox.4; 84/sup 0/W, geographic), are compared with low-energy ( 5-keV electrons detected at the synchronous altitude, and its generation region is inferred to be outside the plasmapause. The chorus upper cutoff frequency increases with time in a characteristic manner, consistent with the expected adiabatic motion of injected electrons in cyclotron resonance with the waves. The second type of chorus, which we refer to as ''plasmaspheric chorus,'' occurs inside the plasmapause, has no apparent relationship to particle injection at the synchronous altitude and shows clear evidence of being triggered by whistlers, power line radiation, and other signals. The two different types of chorus are readily distinguishable in frequency-time spectrograms.

Correlations between very low frequency chorus bursts and impulsive magnetic variations at L ~ 4.5

Abstract: Coordinated observations of aurora, ULF, and VLF waves were made at 13 stations in Canada in January and February, 1980. The analysis of simultaneous ULF and VLF data obtained at Park Site (L = 4.4...

Computer-Program Model for Predicting Horizontally and Vertically Polarized VLF Atmospheric Radio Noise at Elevated Receivers

Abstract: : A computer program has been developed that can predict horizontally and vertically polarized atmospheric radio noise at any altitude or location in the earth-ionosphere waveguide in the very-low-frequency (VLF) range from 10 to 30 kHz. The new program, HORNS, uses the outputs of two previously written programs, COMPWR and NOISLAN, which predict the vertical electric noise field at the ground. The HORNS program computes all the field components at any altitude using the vertical electric field at the earth's surface as a basis. Predicted values from several versions of the new model have been compared with presently available data. The results are encouraging, but more data are needed to test the model.

VLF Wave Observation by Satellite JIKIKEN (EXOS-B)

Abstract: Instrumentation, data processing and preliminary results of EXOS-B VLF wave receiver are presented The VLF receiver is connected, by command, either to a long dipole antenna or to a loop antenna, and observes both electromagnetic and electrostatic waves in a frequency range between 150Hz and 95kHz Up to the present time after the launch, various VLF phenomena including whistler echo trains, whistler triggered emissions, hiss, chorus, and possible power line harmonic radiation have been observed In addition to these natural phenomena, artificial VLF waves excited by electron beam injection have also been observed

Calculation of Ionospheric Conductivity Profiles by Inverting VLF/LF Reflection Data: I. Isotropic Propagation.

Abstract: : This report presents an iterative method for inverting VLF/LF ionosounding data to obtain ionospheric conductivity profiles. The relationship between the detail that can be realized in the calculated profile and the quality and quantity of the reflection data is given explicitly. Only altitudes below about 70 km where the propagation can be assumed isotropic are considered, although the method can be extended to anisotropic propagation. The method is demonstrated by applying it to two qualitatively different ionospheric models--a strong SPE disturbance and a C-layer exhibiting a well-defined conductivity peak. Ground-level reflection data contain information only about those heights from which significant energy is returned. The calculated profiles agree well with the true profiles over the height range where the strongest reflections occur, and break down outside of that range. The solutions converge toward the true profiles despite the use of initial estimates that contain no information about the true ionospheric structure. The height resolution actually achieved for the two examples is much better than the theoretically predicted limit. (Author)

Transequatorial propagation of very low frequency radiowaves

Abstract: Abstract : This study reports on observations of dawn/dusk transition fading of very low frequency radiowaves on a long, transequatorial northeasterly propagation path. A mode conversion model is developed and numerical modeling results are presented for transition fading on long transequatorial paths, both northeasterly and southwesterly. From these results the following general conclusions are drawn: (a) Anomalous transequatorial transition fading is not observed for the northeasterly propagation path. The transition fading data support completely the mode conversion model of transition fading developed by Crombie. (b) The transequatorial sunrise transition reported in the literature as anomalous is completely explained by correctly allowing for variations of the geomagnetic field along the path. The geomagnetic field influences the nighttime modal parameters to such an extent for the southwesterly path considered that the approximate WKB method of mode summation cannot be employed. (c) Careful application of model techniques appears to be capable of describing all LF and VLF propagation in the earth-ionosphere waveguide, provided the effect of the earth's magnetic field is properly included at night. However, an improved model of the ionosphere at altitudes below 100 km is desired.

VLF/LF/MF Whispering Gallery Propagation Studies.

Abstract: : The propagation of very low frequency (VLF) and low frequency (LF) radio waves in the earth-ionosphere waveguide is affected profoundly by the earth's curvature. In particular, for frequencies greater than about 20 kHz, some modes are possible for which the energy is concentrated near the base of the ionosphere and the field strength near the ground is small. It is useful to think of such modes as being composed of waves repeatedly reflected at the inside surface of the ionosphere. Using analogy with sound waves, we can call these modes whispering-gallery modes. Since they depend only very slightly on ground conductivity, these modes are also referred to as earth-detached modes. The Navy currently is pursuing efforts leading to a feasibility demonstration of a balloon gateway communications system that would employ a balloon-to-balloon cross link based on the whispering-gallery propagation modes. This report discusses some recent whispering-gallery theoretical advances and simplifications. (Author)

Transverse Electric Waves for VLF/LF Communication between Aircraft.

Abstract: : This report compares the excitation and propagation of VLF/LF transverse electric (TE) and transverse magnetic (TM) long waves in the earth-ionosphere waveguide. It calculates the dependence of TE and TM mode parameters on three factors: (1) ground conductivity, (2) state of the ionosphere, and (3) elevation of terminals above the ground. It also briefly addresses the effects of atmospheric noise and ground-based jammers on the performance of TE links. Results are given for frequencies between 20 and 50 kHz. Only TM signals are efficiently radiated by ground-based transmitters. However, TE signals are strongly excited by nearly horizontal trailing-wire antennas at elevations exceeding 20 kft and, thus, can be important for air-to-air links. Poorly conducting ground heavily degrades TM propagation, but it will not affect TE propagation if the terminals are elevated at least 5000 ft. TE signals are, therefore, better suited than TM signals for air-to-air links that traverse Greenland and much of Canada. For paths over highly conducting ground, TM signals suffer less degradation than TE signals during intense disturbances; for ground conductivities less than about .0001 mhos/m, however, TM signals are more adversely affected. TE signals also provide protection against ground-based jammers under disturbed ionospheric conditions where geomagnetic conversion is slight. (Author)

Very low frequency electromagnetic profiles at Woodlawn

Abstract: The Woodlawn orebody produces large Very Low Frequency electromagnetic (VLF EM) anomalies. These anomalies demonstrate the need to explore with at least two VLF stations whose primary magnetic field directions are approximately orthogonal.- Large VLF anomalies of comparable magnitude and character to those over the orebody were detected both near the orebody and over the weakly mineralized Black Shale about 2.5km to the south. These anomalies suggest that it is unlikely that the VLF response of the Woodlawn orebody would be sufficiently diagnostic to distinguish it from other conductive geological formations in the region.