Siple Station Research Articles

A nonlinear theory of sideband stability in ducted whistler mode waves

This paper is concerned with large amplitude ducted VLF waves (> 1.5pT) in the Earth's magnetosphere. The waves are excited by nonlinear cyclotron resonance with a continuous distribution of energetic electrons. The nonlinear sideband stability of such VLF waves is investigated. In addition the paper looks at wave particle interaction effects in waveflelds consisting of multiple sidebands, discrete waves plus hiss bands, or intense band limited hiss. The treatment is non-selfconsistent. The wavefield is specified a priori and the nonlinear resonant particle current is directly computed. The wavefield and current are spatially DFT'd, and the nonlinear growth rate is calculated as a function of frequency. Sideband stability behaviour is controlled by the magnitude and sign of the inhomogeneity. The magnetic field strength provides a parabolic inhomogeneity, and it was found that under these circumstances the upper sideband was unstable and the lower sideband damped. In band limited signals the uppermost frequencies get the most power, and intermediate frequencies are usually relatively damped. The computations lead directly to a nonlinear theory of quiet band, and also suggest a mechanism for PLHR line drift. Results with intense band limited hiss suggest that nonlinear wave particle interactions may cause a broadband spectrum to become spectrally structured. Assuming strong nonlinearity, constant inhomogeneity and weak sidebands, an analytic theory of sideband stability is developed. This is found to agree well with both computations and the experimental results from the Siple station.

Generation of band‐limited VLF noise using the Siple transmitter: A model for magnetospheric hiss

Band‐limited VLF noise generated by the experimental transmitter at Siple Station, Antarctica, is used to simulate the interaction of natural magnetospheric hiss with energetic radiation belt particles. While the observed spectrum of the generated noise at times closely resembles that of natural VLF hiss, at other times discrete emissions are found to be generated from the incoherent noise signal. Results imply that magnetospheric chorus can be triggered by hiss signals, indicating the similarity of generation mechanisms for coherent and incoherent VLF emissions. A model based on second‐order cyclotron resonance can explain the conversion of the relatively short duration wavelets of a hiss spectrum into the longer, semicoherent discrete emissions that are typical of chorus.

Beat excitation of whistler mode sidebands using the Siple VLF transmitter

The process of whistler mode sideband generation in the magnetosphere was studied by transmitting two monochromatic signals closely spaced in frequency (Δf = 5–45 Hz) from the experimental VLF transmitter at Siple Station, Antarctica. The signals were observed following ducted magnetospheric propagation to the conjugate station at Roberval, Quebec. Sidebands up to seventh order were generated extending to ±100 Hz with respect to the average frequency of the carriers. New frequencies were observed both under conditions of little or no growth of the input signals and when one of the input signals did grow significantly. At times the sideband amplitudes exceeded the intensity of either input signal. A small signal mechanism is proposed in which emissions are triggered by each beat between the input waves but are then suppressed by the following beat. The energy from the phase bunched particles is believed to feed preferentially into the sidebands rather than causing growth of the input waves themselves. In this model, phase trapping of resonant electrons by the wave is not required. The observed process of sideband generation provides a mechanism to break down the coherence of relatively narrowband waves in the magnetosphere and may thus account for existing evidence of the transformation of relatively coherent wave packets into broader more variable bands of noiselike signals.

A new type of VLF emission triggered at low altitude in the subauroral region by Siple Station VLF transmitter signals

VLF wave data from the ISIS 2 satellite has revealed the existence of a new phenomenon in which coherent VLF signals from the Siple Station, Antarctica, VLF transmitter are observed to trigger a new type of VLF emission as these signals propagate upward to the satellite at 1400 km altitude through the ionosphere and low‐altitude magnetosphere. The emissions have the form of band‐limited impulses of approximately 20–30 ms duration. The bandwidth of the emissions is as much as 1 kHz and their amplitude is as much as 20 dB above that of the triggering signal. The emissions are thought to be the result of a rapidly evolving quasi electrostatic plasma instability triggered by the transmitter signals. The effect occurs generally when the transmitter signals lie just below a band of impulsive VLF hiss which is commonly observed to occur in the subauroral region poleward of the plasmapause. The impulsive VLF hiss band has a lower cutoff frequency in the range 3–4 kHz and appears to be the subauroral extension of the well‐known auroral VLF impulsive hiss Band. This phenomenon is possibly the transient analog of the recently reported spectral broadening effect [Bell et al., 1983b] but the connection is not clear. In the present paper we show examples of this new phenomenon and delineate the conditions under which it occurs. The role of precipitating energetic particles in producing the phenomenon is discussed.

X ray microbursts and VLF chorus

On January 4, 1978, at 1140 UT, a SuperArcas sounding rocket was launched from Siple Station, Antarctica (L = 4.2, 76°S, 84°W), during a geomagnetically disturbed period (Kp = 6−) with intense X ray and VLF chorus activity. The parachuted payload observed an intense microburst precipitation event of 10‐minute duration. These data have been correlated with measurements of VLF chorus by receivers on the ground at both Siple and its magnetic conjugate point, Roberval, Quebec. Detailed one‐to‐one correspondence between the microbursts and the chorus was not a consistent feature of the data. Time series analysis of the data did indicate a significant correlation between the Siple X ray precipitation and the Roberval VLF waves with an arrival time delay of 0.1±0.3 seconds.

Increase of Atmospheric Methane Recorded in Antarctic Ice Core

Air entrapped in bubbles of cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation. Measurements of the methane concentration in air extracted by two different methods from ice samples from Siple Station in western Antarcitica allow the reconstruction of the history of the increase of the atmospheric methane during the past 200 years.

DE 1 observations of siple transmitter signals and associated sidebands

VLF signals from the Siple Station, Antarctica, transmitter received on the DE 1 spacecraft provide new information on whistler mode signal propagation paths in the magnetosphere. In two case studies, the measured group delay in conjunction with in situ density measurements and ray tracing analysis are used to distinguish between direct nonducted propagation and a hybrid mode consisting of one‐hop propagation in a duct followed after ionospheric reflection by nonducted propagation. The extent of the observations both in space and time indicates that such a hybrid propagation mode may be an important means by which whistler mode signals generated or amplified in ducts can populate the magnetosphere. Computed group time delays based on a diffusive equilibrium model for the density distribution along the field lines provide good agreement with measurements over a wide range of magnetic latitudes (50°S–25°N). In one case, observed Siple signals are associated with sidebands of ±30 Hz spacing, and emissions are occasionally triggered. Sidebands are associated with signal components that are believed to have propagated on direct nonducted paths as well as in a hybrid (ducted/nonducted) mode with the sideband spacing being equal in both cases. The sideband spacing is found to be independent of the carrier frequency and amplitude and the satellite location with respect to the magnetic equator.

Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries

Precise and continuous measurements of atmospheric CO2 concentration were first begun in 1958 and show a clear increase from 315 parts per million by volume (p.p.m.v.)1 then to 345 p.p.m.v. now. A detailed knowledge of the CO2 increase since preindustrial time is a prerequisite for understanding several aspects of the role of CO2, such as the contribution of biomass burning to the CO2 increase and the sensitivity of climate to the CO2 concentration in the atmosphere. Estimates of the preindustrial CO2 concentration are in the range 250–290 p.p.m.v. (ref. 2), but the precise level then and the time dependence of the increase to the present levels remain obscure. The most reliable assessment of the ancient atmospheric CO2 concentration is derived from measurements of air occluded in ice cores. An ice core from Siple Station (West Antartica) that allows determination of the enclosed gas concentration with very good time resolution has recently become available. We report here measurements of this core which now allow us to trace the development of the atmospheric CO2 from a period overlapping the Mauna Loa record back over the past two centuries.

A new VLF method for studying burst precipitation near the plasmapause

VLF signals in the 2–4 kHz range transmitted from Siple Station, Antarctica (L ∼ 4.3), and received at various Antarctic locations have been used to detect the occurrence of burst precipitation of electrons (E ≥ 50 keV) into the nighttime lower ionosphere. The receiving stations, each ∼1400 km from Siple, were located to the north at Palmer (L ∼ 2.3), to the east at Halley (L ∼ 4.3), and to the south at South Pole (Λ ∼ 74°). Rapid changes in the received phase and amplitude of the Siple signal (“Trimpi events”) were observed in conjunction with the reception of one‐hop whistlers. In one case involving propagation paths lying poleward of the plasmapause, amplitude decreases by ∼20% in ∼2 s, decaying in ∼7 s, were recorded simultaneously at Halley and South Pole. Post facto analysis of the South Pole records using a new digital processing scheme showed corresponding fast phase advances of ∼15 µs. In a case of phase perturbations at Palmer, the Siple signal path crossed the plasmapause projection, and the associated whistlers propagated in the outer plasmasphere. Phase measurements appear to be a particularly sensitive means of detecting burst precipitation activity under experimental conditions of the kind described. The length and spatial distribution of the signal paths provide a basis for studying the occurrence and approximate location of burst precipitation in regions outside the observing range of most instruments used for detection of precipitation. Direction finding on correlated whistler events as well as dispersion analysis may be used to increase the spatial resolution of the method.

Variable frequency VLF signals in the magnetosphere: Associated phenomena and plasma diagnostics

Coherent variable frequency signals (ramps) extending from 1 to 8 kHz, injected into the magnetosphere from Siple Station, Antarctica (L=4.3) exhibit upper and lower cutoffs when received at the conjugate station, Roberval, Quebec. Ramp group delay measurements and ionospheric sounding data are used for the first time to determine the cold plasma density and L shell of the propagation path. Relationships among f, df / dt, and the “phase equator” for gyroresonance are calculated using second‐order resonance equations generalized to relativistic electrons. Observed upper cutoff characteristics are interpreted in terms of off‐equatorial gyroresonant interaction regions and ducted propagation limited to frequencies below half the local gyrofrequency. The observed lower cutoff frequencies varied systematically with transmitted ramp slope, suggesting a threshold in the resonant electron number density above which rapid temporal wave growth and saturation can occur. This concept is used to develop a hot plasma diagnostic technique which, for an assumed g(α)υ−n electron distribution, provides an estimate of the energy dependence n. A test of this technique is given using the data and a simplified wave‐particle interaction simulation. Additional aspects of the magnetospheric response to ramp injection, including emission triggering, are discussed.

Diurnal modulation of the quiet time penetrating electron flux

Three sounding rocket measurements of energetic electron precipitation in the 65 to 80 km altitude range above Siple Station, Antarctica suggest a possible diurnal modulation of the precipitation. Flights made on January 10 and 11, 1981, to investigate this possibility showed that no electrons were present after local noon, while a significant flux of penetrating electrons was observed the following day near local dawn, even though the level of magnetic activity was higher during the postnoon flight. A previous flight from Siple Station also observed a large flux of penetrating electrons near local dawn. Calculations of electron drift trajectories in a model magnetic field indicate that moderate values of the convection electric field could cause significant local time variations in the penetrating electron flux at the western edge of the South Atlantic anomaly. These calculations indicate that Siple Station is shadowed by eastern Siberia when it is at local noon, but not when it is at local dawn.

The inflection point method of determining riometer quiet day curves

A computer technique is described for determining the quiet day curve (QDC) from riometer measurements of cosmic radio noise. In this technique, the QDC value for a given sidereal time interval is taken to be the signal level corresponding to the inflection point on the high‐signal side of the peak of the distribution of the measured cosmic radio noise power for that interval. A comparison is made with earlier methods of calculating the QDC and the superiority of the present method is demonstrated. This method has been applied to 30 MHz riometer data from Siple station, Antarctica (75°36'S, 83°36'W) for 1975 and 1980. Despite the presence of high levels of propagated interference during some months in 1980, the technique is able to generate useable QDC's for each month. The uncertainty in the determination of the QDC is estimated to be about 0.1 dB when interference is present, but much lower at other times. Changes in the QDC levels from month to month have been found which are qualitatively consistent with expected seasonal (i.e., solar zenith angle) effects. The technique described here provides an objective operational definition of the QDC which is relatively easy to adapt to computer calculation, and it appears to be a promising tool for the detection and monitoring of long‐term variations in background radio‐wave absorption by the ionosphere.

On the suppression of wave growth and triggering in VLF multi-wave experiments

In experiments employing two monochromatic whistler-mode wave trains, simultaneously injected from Siple Station, Antarctica, two kinds of suppression phenomena were identified and recently reported by Helliwell (1983). The first occurs for small frequency separations (∼ 20–30 Hz) and involves the mutual suppression of wave growth and triggering of both signals; the second type of suppression is unsymmetrical: it occurs only in the lower frequency wave, for frequency separations larger than those for which mutual suppression is detected. Here, we discuss the mechanisms underlying these effects based on a wave-particle interaction description, in the presence of two waves. Mutual suppression of wave growth and triggering is understood as a consequence of the mutual inhibition of the waves to phase trap resonant electrons: this is expected to perturb the development of transverse resonant currents supported by the electrons, thereby causing suppression of growth and triggering of both waves. In what concerns the origin of the assymmetric-type suppression occurring in the lower frequency ( ƒ 1 ) wave, a different two-wave interaction mechanism is contemplated: we assume the suppression to be related to the perturbing effects caused by the higher frequency ( ƒ 2 ) wave on the electron population that would have resonated with ƒ 1 in the absence of ƒ 2 . These effects shall depend on the capability of ƒ 2 to trap these electrons, thus avoiding their resonant interaction with ƒ 1 and consequently affecting its growth. We analysed, theoretically and by computer simulation, the influence of the perturbing second wave on the energetic electrons that would have resonated with the first wave. A model was developed to quantify the critical frequency spacing associated with unsymmetrical interaction effects and a comparison was made with the critical frequency spacing associated with mutual interaction effects. For the considered range of frequencies, corresponding to whistler-mode ducted propagation in the geomagnetosphere near L = 4, it was shown that the unsymmetrical effect may dominate over the mutual effect, above critical amplitude levels within the moderate range of wave amplitudes (∼ 1–10 pT). A good agreement was obtained between predicted results for critical frequency separations and those reported in the examples corresponding to experiments in the range 3–4 kHz (Heliwell, 1983), where detected unsymmetrical effects seem to extend to frequency separations larger than 200 Hz.

Age difference between polar ice and the air trapped in its bubbles

Air entrapped in bubbles formed in cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation. The analysis of dated ice samples therefore enables the history of atmospheric composition to be investigated.1–3 The age of the entrapped air is, however, not the same as that of the surrounding ice because air bubbles only become isolated from the atmosphere during the transition from firn to ice. Typically the age of the ice at this transition is between 100 and 3,000 yr, depending mainly on firn temperature and snow accumulation rate. The mean age difference between ice and enclosed air, as well as the age distribution width for a given sample, are especially important for the investigation of the anthropogenic increase of CO2 and trace gases in the atmosphere over the last centuries, and for the comparison of climatic parameters recorded in the ice with parameters recorded in the bubbles. For Siple Station (Antarctica), this age difference and age distribution width were deduced from the bubble volume measured as a function of depth. The values are 95 yr and 22 yr respectively.

VLF two-wave-electron interactions in the magnetosphere

The mutual influence between two whistler mode waves, through cyclotron resonant interaction of each wave with the same set of energetic electrons, is analysed both theoretically and by computer simulations ; this two-wave interaction mechanism seems to be an important process in understanding recently observed phenomena in Siple Station VLF multi-wave injection experiments. A criterion is established to estimate the threshold for the critical frequency spacing (for given wave amplitudes) for a significant mutual interaction between two monochromatic waves to occur. This criterion is based on the overlap of coherence bandwidths associated with the trapping domains of each wave and it takes into account the geomagnetospheric medium inhomogeneity. The effects of a perturbing second wave on electrons trapped by a first wave is discussed, considering the general situation of varying-frequency waves, and a simulation model is used to track the motion of test-electrons in the two-waves field. Conditions leading to detrapping and subsequent trapping by the second wave of previously first-wave trapped electrons are analysed and suggest the possibility of this phenomenon to play an important role in frequency entrainment and energy exchange between two waves.

Temporal variations in the Siple Station conjugate area

The Siple Station conjugate locations in the northern hemisphere are examined for the epoch 1975–1990 according to the predictions of three internal source geomagnetic field models (IGRF 1980, MAGSAT 1980, Barraclough 1975) and one external field model (Mead/Fairfield) under quiet and disturbed conditions. The conjugate location systematically changes in the interval studied, making it imperative that changes be made in the locations of the northern hemisphere geophysical stations during the reactivation of Siple Station in 1985–1987.

Direct multiple path magnetospheric propagation: A fundamental property of nonducted VLF waves

ISEE 1 satellite observations of nonducted whistler mode signals from the Siple Station VLF transmitter show that all well‐defined pulses (S/N ≈ 20 dB) are elongated by 20 ms to 200 ms. This elongation is attributed to closely spaced multiple paths of propagation between the ground and the satellite. The presence of multiple paths is further confirmed by the observed amplitude fading pattern where fading occurs not only at half the spin period (1.52 s) of the rotating antenna but also at time scales of the order of 1 s. Electric field measurements show a 2 to 10‐dB amplitude variation, which is again consistent with direct multiple path propagation. We illustrate the results with two cases: one on October 29, 1977, inside the plasmapause and the other on May 7, 1979, outside the plasmapause. Our results establish that, in general, at any point in the magnetosphere the direct signals transmitted from the ground arrive almost simultaneously along two or more closely spaced direct ray paths. It is shown that multiple paths can be explained by assuming field‐aligned irregularities of 1 to 10‐km horizontal scale in the ionosphere with a few percent enhancement or depletion in the plasma density. We discuss the implications of our results for nonducted wave‐particle interaction in the earth's magnetosphere. We show that for reasonable parameters of nonducted multiple path propagation, a cyclotron resonant electron will experience a wave doppler broadening of a few tens of hertz to a few hundreds of hertz.

Phase measurements of whistler mode signals from the Siple VLF transmitter

A digital signal processing program has been developed to measure the phases of coherent VLF signals from analog tape recordings made in the field. The program uses a constant frequency pilot tone recorded with the VLF data to correct tape speed errors and reconstruct the signal phases. We analyze several examples of whistler mode signals from the VLF transmitter at Siple Station, Antarctica, as received at Roberval, Quebec. Pulses with temporal growth show a relative phase advance with time, and thus a positive frequency offset from the transmitted signal, often from the beginning of the pulse. Amplitude beating is often seen toward the end of a pulse, sometimes with phase cycle‐skipping as the emission becomes unlocked from the input signal. Current theories of wave‐particle interaction are reviewed and found to explain some of the observed signal features, though no theory predicts the initial frequency offset of a growing pulse.

Whistler induced suppression of VLF noise

New evidence has been found connecting whistlers with transient reductions in the amplitude of magnetospheric VLF noise bands. Until now, the only reported examples of such effects were observed during a several hour period at Siple Station, Antarctica, when bursts of particle precipitation were correlated with whistlers that suppressed a background hiss band. Data acquired at South Pole Station during 1981 showed similar suppression events on 20% of all winter days. A review of data from Siple, Byrd, Eights, and Palmer Stations in Antarctica and Roberval in Quebec, Canada, has since identified many more events. The ground signature of an event is characterized by the attenuation of an existing VLF noise band following the arrival of a whistler. The attenuation typically reaches a maximum of 3–5 dB in 5–10 s, and the noise band recovers to the pre‐event amplitude 15–30 s after the whistler. The noise bands are generally mid‐latitude hiss, but suppression associated with polar chorus has been seen. Regularly observed features of suppression events include multi‐hop echoing of the driving whistler; confinement of the whistler echoes to the frequency band occupied by the suppressed noise; suppression during the first pass of the whistler; continuing suppression by the whistler echoes as the suppressed noise amplitude reaches a minimum and then recovers; and recovery of the noise band to the pre‐event level. Event duration was found to increase with the duration of the whistler echo train. Some data suggest that this phenomenon may be a natural analog of the previously reported quiet band effect in which signals from the VLF transmitter at Siple Station were observed to attenuate portions of noise bands.

Controlled stimulation of VLF emissions from Siple Station, Antarctica

Coherent VLF (1.8–7 kHz) signals injected into the magnetosphere from Siple Station (L = 4.2), Antarctica, are amplified (30 dB) and trigger intense, narrowband whistler mode emissions. The mechanism of growth is believed to be wave‐induced phase bunching of cyclotron resonant electrons in an interaction region near the equatorial region. A key parameter in these experiments is the spectral purity of the transmitted signal. If the width of the signal spectrum exceeds ∼ 10 Hz, there is significant loss of gain. In one experiment (f ≈ 3 kHz), two equal‐amplitude signals are transmitted with a frequency spectrum of Δf. When Δf = 20 Hz, the gain is reduced as much as 20 dB below its single‐frequency value. For f ≥ 100 Hz, the two signals tend to behave independently of one another, indicating that they interact with different electron populations. Other effects of interest include entrainment of strong free‐running emissions by a relatively weak Siple signal and the frequent generation of sidebands (2–100 Hz) on Siple signals that have reached saturation. Future work at Siple Station will emphasize frequencies below 4 kHz in order to raise the energies (to > 10 keV) of the precipitating electrons and hence make them more readily detectable by ground‐based techniques. To achieve this goal, the dipole length will be doubled (to 43 km), which is expected to increase the maximum radiated power at 2.5 kHz by 7 dB. Later, it is planned to install a circularly polarized crossed dipole that will double the effective radiated power and permit new experiments on the polarization properties of the ionosphere and magnetosphere at VLF.