Natural Whistlers Research Articles

The study of bulk plasma motions and associated electric fields in the plasmasphere by means of whistler-mode signals

Whistler-mode waves propagating to ground stations along geomagnetic-field-aligned paths provide powerful tools for investigating bulk motions of the magnetospheric plasma and thus the corresponding convection electric fields. Natural whistlers from lightning as well as signals from very low frequency (VLF) transmitters have been employed. The whistler method emphasizes measurement of temporal variations in the frequency versus time, or dispersion, properties of whistlers, while the transmitter method focuses upon measurement of the phase and group paths of fixed-frequency signal propagation. The methods depend upon wave properties that are sensitive to inhomogeneities in the geomagnetic field, and thus provide information on what are essentially cross- L plasma motions in a frame of reference rotating with the Earth. In addition, whistler data on the duskside plasmasphere bulge have been used to estimate values of the radial convection electric field (GSE Y direction) near dusk. In this topical review we discuss the development, beginning in the 1960s, of the whistler and transmitter methods, as well as a few of their geophysical applications. Whistlers have provided substantial new information on the spatially and temporally structured manner in which convection electric fields penetrate the plasmasphere, one example being the still unexplained reversal from inward to outward of the post-midnight radial flow direction following temporally isolated substorms. Whistlers have also been useful in identifying the plasmaspheric drifts associated with quiet-day electric fields of ionospheric dynamo origin and in showing that the E y (duskward), component of the convection electric field in the outer plasmasphere is substantially larger near dusk than it is near mid-night. Whistler-mode signals from transmitters have been found to be a powerful means of tracking cross- L motions in the plasmasphere near L=2.5 while separately identifying the effects of interchange fluxes with the ionosphere on the signal phase and group paths.

Plasmaspheric storm time erosion

Unusually low whistler mode group delay times are observed by VLF Doppler receivers at both Faraday, Antarctica, and Dunedin, New Zealand, following magnetic storms. These are typically caused by plasmaspheric electron concentration depletions near L=2.4 and not by changes in the VLF wave propagation path. Using a data set that is almost continuous since 1986, we find that depletions during storms in the solar minimum of 1995 are significantly deeper than in the minimum of 1986. Event studies at Faraday show that the electron concentration depletions caused by storms were about a factor of 2 in 1986 and a factor of 3–4 in 1995, independent of the time of year. However, the depletions observed by both sites are significantly deeper than those observed in 1958 and 1961 using natural whistlers (i.e., factors of 2–4 compared to 1.3). The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to investigate possible causes of the plasmaspheric electron concentration depletions observed in the whistler mode data. Thermospheric parameters, including a reduction in the concentration of neutral hydrogen and oxygen at all altitudes, were perturbed by a factor of 10 from their normal levels. However, the plasmaspheric depletions produced were only of the order of 10% after 27 hours. It is unlikely therefore that thermospheric modifications alone are responsible for the depletions observed in the data. Additionally, a tube of plasma was moved to higher L shell under the influence of an equatorial meridional E × B drift velocity of 1000 m s−1 and showed levels of depletion of about a factor of 2. Although it is possible to generate plasmaspheric concentration depletions using the drifting tube model, the depletions are smaller than those observed and the outward E × B drift velocity needed is a factor of 2 greater than those reported previously at L=2.4. It is therefore unlikely that the tube drifting mechanism is the principal cause of the observed plasmaspheric electron concentration depletions at L=2.4. Although no mechanism is clearly identified in this study, the ground‐based results presented in this paper indicate erosion levels of similar structure and magnitude to electron concentration profiles from the ISEE 1 satellite in the aftermath of magnetic disturbances during 1983, thus providing a long‐term record of plasmaspheric erosion.

The propagation of low-latitude whistlers: A review

The purpose of this paper is to review the results of investigation on low- and very low-latitude whistlers carried out since the previous review paper by Hayakawa and Tanaka (1978, Rev. Geophys. Space Phys. 16, 111–123). The propagation (ducted or non-ducted) of whistlers at low latitudes has been discussed based on the ground and satellite observations of both natural whistlers and VLF transmitter signals. By using the whistler propagation characteristics, the behavior of the electron density of the inner plasmasphere and also the electric field have been discussed. In the discussion of each subject the unsolved problems to be pursued in the future have been indicated.

Excitation of whistler mode signals via injection of polarized VLF waves with the Siple transmitter

Whistler mode waves of various polarizations were transmitted by the Siple Station, Antarctica, VLF transmitter and received near the geomagnetic conjugate point at Lake Mistissini, Quebec. Crossed 21‐km horizontal dipole antennas on top of the 2‐km‐thick ice sheet were used to transmit 2‐ to 4‐kHz waves alternately with right‐hand circular, left‐hand circular, and linear polarizations. Excitation of a multiplicity of magnetospheric propagation paths and the received signal strength were observed to depend on the transmitter antenna polarization. Where whistler mode growth and emission triggering occurred, saturated peak values of received signals were independent of antenna polarization and initial injected power levels, in agreement with previous findings. Propagation paths of ducted Siple signals observed at Lake Mistissini were identified with propagation paths deduced from natural whistlers, from which the L shell values and equatorial number densities for the paths were calculated. A combination of L shell data and models of antenna coupling into the whistler mode may aid in the location of ducts. Dynamics Explorer 1 satellite recordings of unducted Siple signals showed trends similar to the ground data on ducted signals. The observations are discussed in the context of a simplified model of the coupling from the Siple antenna into the ionosphere, which provides reasonable agreement with observations.

On whistlers with very low dispersion

The purpose of this note is to show that the non-ducted, low latitude VLF path proposed earlier to explain certain whistler mode observations of signals from a VLF transmitter (Andrews, 1978), may also provide an explanation for the observation of natural whistlers with dispersions of ~10s 1 2 at very low latitudes. The existence of the path requires the O + to H + transition level in the topside ionosphere to be above about 700 km altitude. If this condition is fulfilled, one or two refracting “channels”, ~0.5°wide, are produced which enable all frequencies within a whistler to arrive with approximately constant dispersion and with wavenormals vertical, and so be transmitted out of the ionosphere.

Initial results from a tracking receiver direction finder for whistler mode signals

A new apparatus combining a tracking receiver and a direction‐finding signal processor for observation of VLF whistler mode signals is described. The system is expected to achieve greater bearing accuracy than can be obtained from a conventional goniometer and is usable on a wider variety of signals than can be processed by most other techniques. Convenience in data output has been emphasized. In July–August 1975 the new tracker/direction finder (TR/DF) was successfully field tested at Roberval, Canada (L ≃ 4). Observations were made on natural whistlers and emissions and on signals from the Siple, Antarctica, VLF transmitter. As a test of accuracy on arrival bearing of whistler mode signals, the TR/DF output was compared with results from a goniometer operating from the same set of crossed loop and vertical antennas. The two systems agreed within ≃10° on several‐minute averages of bearings of both Siple transmitter pulses and natural signals. Effects of multipath propagation and polarization error have been identified. On the basis of the initial data sets it is found that whistler mode signal paths in the outer plasmasphere have ionospheric exit points that can frequently be resolved to within a region 30–40 km on a side. The apparent whistler mode signal path endpoints are relatively close to the ionospheric projections of the field lines along which the wave energy propagated, within 50–100 km (in the north‐south direction). Bearings of Siple transmitter signals show an apparent concentration to the southwest of the receiving station.

Linear and nonlinear amplification in the magnetosphere during a 6.6‐kHz transmission

Reception at Dunedin of the magnetospheric signal at 6.6 kHz transmitted from Anchorage, Alaska, showed both linear and nonlinear amplification during an event lasting some 20 min near local midnight. Linear amplification of the transmitter signal was ∼20 dB. Natural whistlers were also amplified but often at frequencies sharply limited to those from the transmitter frequency upward. Nonlinear amplification (NLA) produced a signal positively offset from the transmitter frequency by 20–150 Hz at amplitudes over 40 dB above the unamplified transmitter signal. This signal appeared as a largely self‐sustaining embryo emission (EE) under the control of the transmitter signal. The phase of this EE signal in each NLA event was tracked with respect to a recorded phase reference. These phase studies showed that the accumulating phase of the offset EE signal is frequently interrupted by negative phase steps (‘N events’) which tend to reduce the offset frequency. Five of the NLA events during key‐down transmission were quenched by whistlers which themselves triggered free emissions at ½ƒBO. The theory of nonlinear wave‐wave interaction between the transmitter or input wave (IW) and the embryo emission is developed to explain these features. It is shown that coupling depends on the offset frequency δƒ and the ‘control frequency’ Fc: for δƒ > Fc the emission is effectively free; for δƒ < Fc, EE is controlled by IW. Curiously, Fc is determined by the EE amplitude (Bw) as Fc ∞ BW1/2, and is almost independent of IW amplitude. This control applies whether the emission was originally generated by IW or captured by it. Fc is determined from Bw measurement to be 60–120 Hz, which fits the observed behavior quite well. For δƒ < Fc a fraction of the phase‐bunched electrons are trapped by IW as they are detrapped by EE in the growth region. This superimposes a strong component oscillating in phase which can produce N events, effectively phase locking the low‐amplitude end of EE to IW. Amplitude fluctuations, δƒ, and Fc are interrelated in a complicated way which gives rise to short‐term instabilities and somewhat longer‐term stabilizing influences.

Causes of the frequency shift of whistler-mode signals

Whistler-mode signals received at Wellington, New Zealand, from NLK in Seattle are typically doppler shifted ~0.1 Hz from the transmitter frequency, implying a changing phase path. At least during quiet times, this is shown to be primarily caused by (inward) drifts of the whistler duct rather than by changing electron density along the path. The mechanism is established by measuring the ratio of the changes in phase and group times for the path during the nights studied. On one of these nights, the drift is further confirmed by measurements on natural whistlers.

Amplitude Variations in Artificially Stimulated v.l.f. Emissions

AN understanding of the basic mechanisms involved in wave-particle interactions in a magneto-active plasma is currently of interest in contexts ranging from planetary and pulsar magnetospheres to galactic jets. In the Earth's magnetosphere, which is an excellent laboratory for the study of an infinite collisionless plasma with an inhomogeneous magnetic field, ground and satellite observations of discrete very low frequency (v.l.f.) emissions are providing information on the interaction of waves in the whistler mode with asymmetric velocity distributions of energetic electrons in the radiation belts. The emissions observed consist of rising, falling, hook-shaped, or approximately steady tones, triggered in the equatorial region of the magnetosphere by natural whistlers, by unknown natural sources, by short (150 ms) pulses from high power (106 W) ground transmitters1, or by long (∼ 1 s) pulses from low power (∼ 100 W) ground transmitters of the Omega network2. The basic properties3 of the emissions to be accounted for on a microscopic theory are the narrow bandwidth, the frequency variations, the amplitudes observed (sometimes an order of magnitude greater than the amplitude of the triggering pulse), the triggering delay of order 100 ms and the fact that for ground observations triggering is generally seen at frequencies ω such that ω≈0.5 Ω0, where Ω0 is the equatorial value of the electron gyrofrequency on the appropriate field line. I shall report here the observation of an additional property: large regular and irregular classes of amplitude fluctuation within artificially stimulated v.l.f. emissions triggered by low power Omega pulses of frequency 10.2 kHz received on the ground in the magnetically conjugate zone of the transmitter after following a magnetospheric trajectory.

A dispersion anomaly in whistlers received on Alouette 1

Natural whistlers observed in the ionosphere, but not by nearby ground receivers, may exhibit a frequency versus time curve that differs appreciably from the typical curve of whistlers observed at ground level. For wave frequencies well below the minimum value of electron gyro-frequency along the path, the frequency-time curve of most ground-observed whistlers can be approximated within a few per cent by the relation ((1))where t represents travel time at frequency ƒ. However, a number of recent Alouette whistler observations at 1000 km show relatively large departures from this relation. The purpose of this note is to describe a particular departure in which the quantity D(ƒ) = tƒ1/2 exhibits an anomalous increase with increasing frequency.

Whistlers excited by nuclear explosions

The dynamic spectra of whistlers excited by five nuclear explosions are found to be identical to those of natural whistlers observed within a few minutes of shot time. Two shots fired at the Nevada proving ground produce normal two- hop whistlers at Stanford, 600 km to the west. Three shots fired over Johnston Island (about 14 deg N geomagnetic latitude) produce one-hop whistlers, some of which are received at northern hemisphere stations as far away as Stanford, 5100 km to the northeast. No whistlers are observed on Kauai, only 1200 km northeast of Johnston Island. Analysis shows that the magnetospheric paths of natural and explosion-excited whistlers are essentially the same, and that these paths are fixed in the ionosphere and are not necessarily close to the impulse source or the receiver. From analysis of impulse-whistler delays of shot Teak, it is deduced that the one-hop whistler received at Stanford is excited by an impulse that first crosses the equator in the earth-ionosphere wave guide before entering the whistler duct in the southern hemisphere, as suggested in the concept of the hybrid whistler. The high-altitude shot Starfish Prime (July 9, 1962) is detonated at 400-km altitude, but the dispersion of themore » source impulse is similar to that of ordinary lightning impulses. This result is interpreted to mean that the impulse is produced below the ionosphere, thus providing support for the Compton-electron mechanism of impulse generation. The similarity between the natural and shot-excited whistlers leads to the conclusion that shotexcited whistlers cannot be used for the purpose of detecting nuclear explosions. (auth)« less