Whistler Mode Signals Research Articles

The path of NLK whistler-mode signals received at Wellington

VLF whistler-mode signals transmitted from NLK in Seattle and received at Wellington, New Zealand, were found to travel mainly along magnetic field lines near Wellington ( L ~ 2.3) rather than those near Seattle ( L ~ 3.0). This is found to be explicable in terms of the expected attenuation for the subionospheric part of the path together with the guiding to be expected from likely duct enhancements.

Electric fields from whistler-mode doppler shifts

Westward electric fields of 0–0.4 mV/m well inside the plasmasphere ( L ~ 2.3) are found from the measured doppler shifts and group delay times of whistler-mode signals during quiet times.

Quiet-time characteristics of middle-latitude Whistler-mode signals during an 8-yr period

Results of eight years of measurements on Whistler-mode signals from station NLK (18.6 kHz) propagating in the vicinity of L = 2.5 are presented. The statistics of occurrence indicate a considerable extent (at least 6000 km) for the longitudinal window within which it is assumed ducts must be if a signal is to be received at a particular station. A significant increase in signal amplitude 2–3 days after a magnetic storm is found. A consistent pattern is found in the nightly variations of doppler shift, but there is a marked difference between the June and December solstice data. The magnitudes of the shifts are comparable with those expected from Sq electric fields, but there is little agreement between the predicted and observed variations of doppler shift. This may be related to ionosphere—magnetosphere coupling fluxes.

Frequency-time behavior of artificially stimulated VLF emissions

Artificially stimulated VLF emissions (ASE's) are emissions triggered in the magnetosphere by the whistler mode signals from VLF transmitters. These emissions may be separated into two classes, rising and falling, depending on whether the final value of df/dt is positive or negative. Several hundred ASE's triggered by three transmitters (NAA, Maine, 14.7 kHz, 1 MW; Omega, New York, 10.2 kHz, 100 W; Siple station, Antarctica, 5.5 kHz, 400 W) have been analyzed using the fast Fourier transform with a filter spacing of 25 Hz and an effective filter width of about 45 Hz. The study was limited to the initial frequency-time behavior of ASE's. Averages taken over many events reveal that both rising and falling tones show the same initial behavior. The emissions begin at the frequency of the triggering signal rather than at an offset frequency, as has previously been reported. Both tones initially rise in frequency, falling tones reversing slope at a point 25–300 Hz above the triggering signal. The slope of rising tones, particularly those triggered by NAA, often abruptly levels off in this same frequency range; as a result, a short (∼40 ms) plateau is formed that precedes the final rising phase. The initial frequency offset commonly observed in individual events appears to result from the frequent coincidence with this plateau of a peak in amplitude. Emissions stimulated by all three transmitters show essentially the same features; this finding indicates that their frequency behavior does not depend strongly on transmitter power. The process appears to be asymmetric in frequency; no evidence of initial growth below the triggering frequency has been found.

Nonlinear frequency shift and self-modulation of the quasi- monochromatic whistlers in the inhomogeneous plasma (magnetosphere)

The nonlinear frequency shift arising from the interaction of the quasimono- chromatic whistler-mode wave with resonant particles in an inhomogeneous plasma is derived. The modulational instability caused by this shift is investigated. The results are applied to the propagation of long-duration VLF whistler-mode signals along the magnetic field in the magnetosphere. It is shown that the modulational instability of these waves in the equatorial region leads to pulsations very similar to those observed experimentally

Alaska to New Zealand whistler-mode transmission at 6.8 kHz

OBSERVATIONS of whistler-mode signals from v.l.f. communications transmitters have been used1,2 to study the magnetosphere in a way similar to the use of naturally occurring whistlers from lightning. Existing naval communications transmitters provide high radiated power (up to 1 MW) but are restricted to transmission at allocated frequencies mostly above about 15 kHz, are not well located geographically for this work and are not often available to transmit special programmes.

Propagation velocity of whistler-mode signals in an unstable and abnormally dispersive plasma-beam system

A propagation velocity of a whistler-mode signal in an unstable and abnormally dispersive plasma-beam system was investigated. Responses of the plasma in real time and space were computed by the inverse Fourier-Laplace transformation. The computed results of the propagation velocity are compared with various velocities analytically determined from the dispersion equation.

Measurements of wave normal direction of whistler mode signals in the ionosphere by means of the Rocket-Doppler technique

Wave normal directions of VLF signals propagating through the ionosphere can be determined by measuring Doppler frequency shift of the signals by means of rocket borne receivers. Two rockets were launched to detect the NWC signal of 22.3 kHz which was transmitted from Australia and propagated on two completely different paths, one being propagated through the Earth-ionosphere waveguide and up to the rocket, the other propagated down to the rocket by the whistler mode directly from the source in the opposite hemisphere. The wave normal directions of the latter mode were almost vertically downward in the ionosphere in the northern hemisphere, although substantial error was involved in the determination of the wave normal direction for a part of the upgoing flight of the rockets, due to the relative geometry of the directions of the rocket flight and the geomagnetic field. The effect of the horizontal gradients of the ionosphere on the above results were found to be not significant. From the experimental results it is concluded that field aligned ducts stretching down to the rocket altitudes did not exist, at least, during the rocket flights.

Ring current effect on magnetospheric electron density profiles derived from plasmapause whistlers.

THE longitudinal drift of van Allen radiation belt charged particles, with opposite charges drifting in opposite directions in the geomagnetic field, constitutes a westward directed ring current at a distance of several Earth radii1. Although at magnetically quiet times this current scarcely neither reduces the geomagnetic field nor affects the parameters that determine the group velocity of a whistler mode signal propagating along a particular field line, these can be significantly changed at magnetically active times, for example during the main and recovery phases of a magnetic storm, when the ring current is stronger2.

Satellite observations of whistler-mode signals in the conjugate region of a 200-kilohertz station

Study of the signals recorded by a narrow-band (about 200 Hz) receiver at a broadcast station operating at 200 kHz and in the conjugate region of Ashkhabad. The latitude of the station is nearly low enough for propagation of a 200-kHz signal in the ducted whistler mode to the conjugate hemisphere along field lines terminating at the station. In the dawn-dusk orbital plane signals are indeed relatively often observed in the conjugate region, but the source of the signals and their path of propagation is not completely clear. The pattern of observations is consistent with propagation over the long magnetospheric path in field-aligned ducts spread in longitude near 22 deg invariant latitude, but an interpretation involving nonducted propagation is preferred, in which the occasionally high electric-field intensities encountered (greater than 10 microvolts/m) result from focusing effects or from propagation near the resonance angle.

Phenomenological study of LHR hiss

The 1963 and 1964 Alouette 1 records from the Ottawa station were studied to determine the statistical characteristics of VLF hiss exhibiting a sharp lower-cutoff frequency associated with the lower-hybrid resonance (LHR). A particular class of midlatitude hiss events were emphasized and designated ‘S-type hiss.’ These events have lower-cutoff frequencies that vary smoothly with time, have limited bandwidths, and do not appear to be associated with other VLF phenomena. Their diurnal time of peak occurrence provides evidence that their sources may be connected with the type of magnetospheric activity usually associated with auroral-zone phenomena; yet, they are definitely midlatitude events. A likely means for transporting the electromagnetic energy is a horizontal duct that occurs as a result of the presence of more than one ion species in the topside ionosphere. Nonducted whistler-mode signals may be trapped in the duct, and satellites within the region observe signals with characteristics determined by this guiding structure. Other forms of LHR hiss, such as whistler-triggered events, may also be associated with these ducts. The characteristics of noise at altitudes above such ducts, or in the absence of ducts, may be different from that of S-type hiss. Some of the observed Alouette 1 events may be such nontrapped noises.

Pulsation phenomena observed in long-duration VLF whistler-mode signals

Whistler-mode signals from station NAA (14.7 and 17.8 kHz), Cutler, Maine, show periodic fluctuations (?pulsations') in amplitude and bandwidth. The data were recorded at Eights station, Antarctica, during unmodulated (?key-down') transmissions from NAA lasting up to 2 min. In three of four instances, the pulsations consist of a series of moderate enhancements of the amplitude and bandwidth of the signal, each pulsation lasting about 50 msec. The fourth instance, however, was unusual in that the key-down signal exhibited remarkably regular and intense amplitude variations. In all four occurrences, the period of the pulsation was in the range from 0.3 to 0.6 sec. In three occurrences, this period was roughly the same as the one-hop whistler-mode delay along the field-line path; however, no demonstrable mechanism to explain this association could be found. An explanation of pulsations in terms of multipath fading effects could not be supported by the data. More likely explanations include intrinsic oscillation in the emission generation mechanism, natural oscillation in the energetic-particle population, or modulation of the VLF growth rate by Pc 1 micropulsations in the region of wave growth.

VLF radio observations of magnetic impulses in the plasmasphere

Observations of VLF radio waves traveling via the whistler mode have indicated a common origin for the abrupt frequency shifts of these signals and the simultaneous sudden impulses and sudden commencements of magnetic storms, as registered by ground-based magnetometers. A study of this relationship suggests that, during the disturbances, observations of whistler mode signals from VLF transmitters can yield useful information on the behavior of the plasmasphere.

Frequency Shifts on Whistler Mode Signals from a Stabilized VLF Transmitter

VLF signals received in New Zealand from NPG, Seattle, via the whistler mode are usually shifted in frequency during transit. They traverse a region of the magnetosphere which is bounded approximately by the magnetic longitudes of transmitter and receiver and which may vary in latitude during a single night over a range of several degrees.In general, signals are detected only when entry and exit points of the appropriate field lines are in darkness. They are subject to a form of geomagnetic control. The roles of ionosphere and magnetosphere as frequency shifting devices are examined, and it is concluded that the chief agency is an equatorward movement of preferred paths during the night

The reception of whistler mode signals at a point remote from the transmitter's magnetic conjugate point

An attempt is made to describe the propagation characteristics of a whistler mode radio signal over the complete one-hop path from a transmitter to a receiver that is up to several thousand kilometers away from the transmitter's magnetic conjugate point. It is shown that there are likely to be two preferred propagation paths with less total transmission loss than for other possible paths: (1) under the ionosphere from the transmitter to the receiver's conjugate point, then along a magnetospheric path to the receiver; (2) along a magnetospheric path from the transmitter to its conjugate point, then under the ionosphere to the receiver. A long series of observations of whistler mode signals from station NPG, near Seattle, on 18.6 kc/s, received near Wellington, New Zealand, 2570 km from the Vestine conjugate of NPG, using three different methods, gave results that are in accord with the idea of two preferred paths. More recent observations of 24.0-kc/s transmissions from the same station also gave general support to this idea.

Frequency Spectrum of Doppler-shifted Whistler-mode Signals

To further our knowledge of very-low-frequency signals received via the whistler mode of propagation we have constructed a 25-channel spectrum analyser. Other work1,2 has shown the existence of apparent Doppler shifts on these signals of up to two or three parts in 105 of the carrier frequency; but the equipment used did not allow the variations of doppler shift with time to be resolved to better than 30 min. It sometimes appeared that there could be large positive and negative shifts occurring within a single 30-min period.

Fundamentals of very low frequency emission generation mechanisms

The transfer of energy between whistler-mode signals and energetic charged particles is examined. Resonance conditions are derived, leading to a classification of the mechanisms previously suggested for the generation of VLF emissions. The relationship between change in energy and change in pitch angle of the particles is derived for the transverse resonance interaction with longitudinal whistler-mode waves. Features of the transverse resonance plasma instabilities and the anomalous Doppler effect are clarified.

Frequency variations of whistler mode signals from NPG Seattle as received in New Zealand

Study of whistler mode signals from the station NPG at Seattle operating on 18.6 kc/s reveals that the received frequency of these signals can depart from the nominal frequency by as much as ±1 part in 105. The average frequency shift varies systematically during the course of a night, and it is possible that two modes, one more common than the other, are present, with frequency shifts of opposite signs. Signals have been recorded within 15 minutes having large frequency shifts of opposite sign. They are likely to have occurred simultaneously.

Artificially stimulated very-low-frequency radiation from the ionosphere

In the course of experimental studies of whistlers and related ionospheric effects conducted by Stanford University, a new phenomenon has been discovered. It is the triggering of VLF emissions in the ionosphere by whistler-mode signals transmitted from VLF stations operated by the U.S. Navy. Although much remains to be learned about this remarkable phenomenon, certain important features are already sufficiently clear to warrant reporting at this time. Evidence of triggered emissions was first found on the IGY-IGC synoptic tape recordings made at Wellington, New Zealand, on November 28, 1959, at 0835 UT [Helliwell, 1962]. A spectrum analysis, from 15 to 35 kc/s, of a short section of that record is shown in Figure 1. The triggering signals, Morse code transmissions from station NPG (Jim Creek, Washington), can be seen as a broken line at 18.60 kc/s on the spectrogram. The emissions consist mainly of rising tones (called ‘risers’) lasting about 0.1 or 0.2 sec. Analysis of the starting time of each of these risers with respect to the transmitted Morse code characters showed that the risers associated with 96 per cent of the dashes and only 4 per cent of the dots.

Variations in phase path of man-made one-hop whistler mode signals at 18.6 kilocycles per second

The phase stability of one-hop whistler mode signals on 18.6 kc/s from the VLF transmitter NPG near Seattle, Washington, has been observed in New Zealand. These signals are characterized by a change in frequency, presumably introduced by variations in the phase path length. The observations indicate that the phase path length can alter at a rate of about 1 km/sec, and total phase path changes of the order of 200 km can occur in a period of 3 min. Possible methods of determining where and how the phase variations occur are suggested.