Siple Station Research Articles

Review of Controlled Excitation of Non-linear Wave-Particle Interactions in the Magnetosphere

Controlled experiments involving injection of 0.5 Hz – 8 kHz electromagnetic waves into the Earth’s magnetosphere have played an important role in discovering and elucidating wave-particle interactions in near-Earth space. Due to the significant engineering challenges of efficiently radiating in the ELF/VLF: 300 Hz – 30 kHz band, few experiments have been able to provide sustained transmissions of sufficient power to excite observable effects for scientific studies. Two noteworthy facilities that were successful in generating a large database of pioneering and repeatable observations were the Siple Station Transmitter in Antarctica and the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Both facilities were able to excite Doppler shifted cyclotron resonance interactions leading to linear and nonlinear wave amplification, triggering of free running emissions, and pitch angle scattering of energetic electrons. Amplified and triggered waves were primarily observed on the ground in the geomagnetic conjugate region after traversal of the magnetosphere along geomagnetic field aligned propagation paths or in the vicinity of the transmitter following two traversals of the magnetosphere. In several cases, spacecraft observations of the amplified and triggered signals were also made. The observations show the amplifying wave particle interaction to be dynamically sensitive to specific frequency and also specific frequency-time format of the transmitted wave. Transmission of multiple coherent waves closely spaced in frequency showed that the wave particle interaction requires a minimum level of coherency to enter the nonlinear regime. Theory and numerical simulations point to cyclotron resonance with counter streaming particles in the 10-100 keV range as the dominant process. A key feature of the nonlinear interaction is the phase-trapping of resonant particles by the wave that is believed to drive non-linear wave amplification and the triggering of free-running emissions. Observations and modeling of controlled wave injections have important implications for naturally occurring whistler mode emissions of hiss and chorus and the broader phenomena of radiation belt dynamics. A review of observational, theoretical, and numerical results is presented and suggestions for future studies are made.

Parallel optimization of signal detection in active magnetospheric signal injection experiments

Signal detection and extraction requires substantial manual parameter tuning at different stages in the processing pipeline. Time-series data depends on domain-specific signal properties, necessitating unique parameter selection for a given problem. The large potential search space makes this parameter selection process time-consuming and subject to variability. We introduce a technique to search and prune such parameter search spaces in parallel and select parameters for time series filters using breadth- and depth-first search strategies to increase the likelihood of detecting signals of interest in the field of magnetospheric physics. We focus on studying geomagnetic activity in the extremely and very low frequency ranges (ELF/VLF) using ELF/VLF transmissions from Siple Station, Antarctica, received at Québec, Canada. Our technique successfully detects amplified transmissions and achieves substantial speedup performance gains as compared to an exhaustive parameter search. We present examples where our algorithmic approach reduces the search from hundreds of seconds down to less than 1 s, with a ranked signal detection in the top 99th percentile, thus making it valuable for real-time monitoring. We also present empirical performance models quantifying the trade-off between the quality of signal recovered and the algorithm response time required for signal extraction. In the future, improved signal extraction in scenarios like the Siple experiment will enable better real-time diagnostics of conditions of the Earth's magnetosphere for monitoring space weather activity.

Modulation Analysis of Whistler Mode Sidebands in VLF‐Triggered Emissions and Implications for Conditions of Nonlinear Growth

AbstractExperimental observations of very low frequency (VLF) triggered emissions are an important resource in investigation of nonlinear wave‐particle interactions between whistler mode waves and energetic electrons in the Earth's radiation belts. Magnetospherically generated whistler mode sidebands observed during the Siple Station wave injection experiment are analyzed using a mixed modulation model and the MINUIT minimization package. The observed sidebands are found to exhibit features of both amplitude and frequency modulation of the input carrier wave with frequency modulation becoming more prominent as the observed amplitudes of the carrier and sidebands increase. A nonlinear whistler mode wave growth formulation based on phase bunching of counterstreaming electrons within a well‐defined phase trap is shown to reproduce the salient features of the sideband observations. Whistler mode sideband amplitude is shown to be affected by the shape and uniformity of the trap.

Predicting conditions for the reception of one‐hop signals from the Siple transmitter experiment using the Kp index

AbstractWave injection experiments provide an opportunity to explore and quantify aspects of nonlinear wave‐particle phenomena in a controlled manner. Waves are injected into space from ground‐based ELF/VLF transmitters, and the modified waves are measured by radio receivers on the ground in the conjugate hemisphere. These experiments are expensive and challenging projects to build and to operate, and the transmitted waves are not always detected in the conjugate region. Even the powerful transmitter located at Siple Station, Antarctica in 1986, estimated to radiate over 1 kW, only reported a reception rate of ∼40%, indicating that a significant number of transmissions served no observable scientific purpose and reflecting the difficulty in determining suitable conditions for transmission and reception. Leveraging modern machine‐learning classification techniques, we apply two statistical techniques, a Bayes and a support vector machine classifier, to predict the occurrence of detectable one‐hop transmissions from Siple data with accuracies on the order of 80%–90%. Applying these classifiers to our 1986 Siple data set, we detect 406 receptions of Siple transmissions which we analyze to generate more robust statistics on nonlinear growth rates, 3 dB/s–270 dB/s, and nonlinear total amplification, 3 dB–41 dB.

An empirical profile of VLF triggered emissions

AbstractThe Siple Transmitter Experiment operated from 1973 to 1988 and generated a wealth of observations of nonlinear wave‐particle interactions including extensive recordings of triggered emissions generated by VLF signals injected into the magnetosphere from the transmitter at Siple Station, Antarctica. Due to their complex appearance and immensely varied behavior, triggered emissions remain poorly described and understood. This work provides a comprehensive statistical description of observed triggered emissions and establishes statistical bounds on triggered emission type (fallers, risers, and positive and negative hooks) and behavior (frequency changes between 1 kHz and 2.5 kHz with initial sweep rates between −2.5 kHz/s and 2.5 kHz/s, with risers undergoing a median frequency change of 556 Hz and fallers a median frequency change of −198 Hz). The statistical study also reveals an apparent dependence of the triggered emission behavior on the transmitted signal itself. Long tones and rising ramps generate more risers and positive hooks, while short tones and falling ramps produce more fallers and negative hooks. Triggered emissions also appear to favorably initiate with sweep rates similar to that of the triggering element, with the 1 kHz/s rising ramps triggering initial risers with a median sweep rate of 1.03 kHz/s and −1 kHz/s triggering initial fallers with a median sweep rate of −0.73 kHz/s. These results improve observations of wave modification resulting from wave‐particle interactions in the radiation belts and can be used to validate numerical simulations of triggered emissions.

Preferential amplification of rising versus falling frequency whistler mode signals

AbstractAnalysis of ground‐based ELF/VLF observations of injected whistler mode waves from the 1986 Siple Station experiment demonstrates the preferential magnetospheric amplification of rising over descending frequency‐time ramps. From examining conjugate region receptions of ±1 kHz/s frequency‐time ramps, we find that rising ramps generate an average total power 1.9 times higher than that of falling frequency ramps when both are observed during a transmission. And in 17% of receptions, only rising ramps are observed above the noise floor. Furthermore, the amplification ratio inversely correlates with the noise and total signal power. Using a narrowband Vlasov‐Maxwell numerical simulation, we explore the preferential amplification due to differences in linear growth rate as a function of frequency, relative to the frequency which maximizes the linear growth rate for a given anisotropy, and in nonlinear phase trapping. These results contribute to the understanding of magnetospheric wave amplification and the preference for structured rising elements in chorus.

Finite difference modeling of coherent wave amplification in the Earth's radiation belts

AbstractModeling of gyroresonant wave‐particle interactions in the radiation belts requires solving the Vlasov‐Maxwell system of equations in an inhomogenous background geomagnetic field. Previous works have employed particle‐in‐cell methods or Eulerian solvers (such as the Vlasov Hybrid Simulation code) to provide numerical solutions to the problem. In this report, we provide an alternative numerical approach by utilizing a first order finite difference upwind scheme. When coupled to the narrowband Maxwell's equations, the model reproduces linear as well as nonlinear wave growth of coherent signals. Wave growth is nonlinear growth when the wave amplitude exceeds the minimum value for phase trapping of counterstreaming particles and is linear otherwise. The model also demonstrates free‐running frequency variation for a case with a high linear growth rate. In addition, the model confirms the theoretical prediction of a stable “phase‐space hole” during the nonlinear growth process. The plasma parameters and L shell used in this study are typical of those associated with the Siple Station wave injection experiment.

Theoretical and numerical analysis of radiation belt electron precipitation by coherent whistler mode waves

AbstractThe interaction between coherent whistler mode waves and energetic radiation belt electrons can result in pitch angle scattering of electrons into the bounce loss cone and subsequent precipitation. In studying the effects of VLF transmitter signals on particle precipitation, past modeling efforts have focused on the computation of diffusion coefficients for a Fokker‐Planck model. In contrast, to capture the nonlinear effects of large‐amplitude coherent waves, we evaluate particle precipitation using a Vlasov‐Liouville (VL) model which computes the phase space particle distribution function directly using a characteristic‐based solution of the Vlasov equation. Previous work has shown that in the case of large‐amplitude coherent waves, phase trapping can significantly perturb resonant particles from their adiabatic paths. We evaluate the importance of phase trapping over a range of wave amplitudes (up to 200 pT); the percentage of particles that precipitate after being phase trapped is computed over a phase space grid in the loss cone. The results demonstrate that phase trapping contributes significantly to precipitation when a large‐amplitude wave (>100 pT) is present. Additionally, linear theory can be valid over a broad range of amplitudes and the relative accuracy of linear theory in calculating the precipitated flux depends strongly on the initial particle distribution. Additionally, we demonstrate the ability of the VL model to calculate the time evolution of the precipitated flux due to short‐duration whistler mode pulses. The physical parameters used in this study are typical of those associated with the Siple Station wave injection experiment.

Analysis of magnetospheric ELF/VLF wave amplification from the Siple Transmitter experiment

Controlled experiments with dedicated ground‐based ELF/VLF (0.3–30 kHz) transmitters are invaluable in investigating nonlinear whistler mode wave‐particle interactions in the Earth's magnetosphere. The most productive such experiment operated between 1973 and 1988 near L=4 at Siple Station, Antarctica. A major effort has been undertaken to digitize and preserve a significant portion of the historical data set from the original magnetic tapes, and we describe here the data set and the processing techniques used to remove artifacts introduced during recording and playback. We analyze a commonly transmitted diagnostic format from 1986 and present statistics on the occurrence and properties of amplified ELF/VLF waves received by a ground‐based receiver at the geomagnetic conjugate location to Siple at Lake Mistissini, Quebec. For the interval examined, only 11% of Siple transmissions are successfully received in the conjugate hemisphere with quiet geomagnetic conditions being significantly more conducive to successful reception. The total growth for the events examined is estimated to be 5–40 dB, and nonlinear growth rates are in the range of 20–350 dB/s. The observations show that as the nonlinear growth rate increases, the duration of nonlinear growth decreases. Significant linear correlation is found between the noise floor and the saturation level, with higher noise floors resulting from increases in natural magnetospheric emissions. Finally, we find a lack of correlation between the nonlinear growth rate and the noise, threshold, and saturation levels.

Magnetospheric amplification and emission triggering by ELF/VLF waves injected by the 3.6 MW HAARP ionospheric heater

The HF dipole array of the High Frequency Active Auroral Research Program (HAARP) in Gakona, Alaska, was recently upgraded to 180 elements, facilitating operations at a total radiated power level of 3.6 MW and an effective radiated power of ∼575 MW. In the first experiments at the new power level, the HAARP array is used for magnetospheric wave injection. Modulated heating of auroral electrojet currents in the ionosphere yields radiation in the ELF/VLF frequency range. The HAARP‐generated signals are injected into the magnetosphere, where they propagate in the whistler mode in field‐aligned “ducts,” allowing them to be observed at the conjugate point on a ship‐borne receiver and on autonomous buoy platforms. The observation of the 1‐hop signals is accompanied by the observation of associated 2‐hop components in the northern hemisphere, which have reflected from the ionospheric boundary in the southern hemisphere. The observed signals are accompanied by triggered emissions and exhibit temporal amplification of 15–25 dB/s and bandwidth broadening to ∼50 Hz. Amplification occurs at injected signal frequencies selected in near real time on the basis of observations of natural emission activity, and only certain components of the frequency‐time formats transmitted are amplified. Observations at multiple sites and dispersion analysis show that the signals are injected into the magnetosphere directly above the HF heater. The duration of echo observation and the prevalence of 1‐hop observations are consistent with statistics from 1986 Siple Station experiments. The particle‐trapping wave amplitude near the magnetic equator is estimated in the range 0.1–0.4 pT and gyroresonance with 10 keV–100 keV electrons.

Characterization of terminal impedance and radiation properties of a horizontal VLF antenna over Antarctic ice

Measurements of the input impedance of two very low frequency (VLF) transmitters in Antarctica are used to validate existing dielectric models for the ice substrate at these frequencies. Using a finite difference time domain approach, Maxwell's equations are solved in the presence of dispersive media, representing the layers of ice which form the Antarctic ice sheet as a single‐pole Debye material. This model provides simulated input impedance values in good agreement with measured data for both the South Pole beacon and the former Siple Station VLF transmitters. Using the validated simulation tool, the radiation characteristics of the South Pole beacon VLF transmitter are characterized. Results for the radiation resistance, efficiency, and far‐field pattern are provided for the current South Pole transmitter. The power pattern for a two‐element array is also determined.

A parametric study of the numerical simulations of triggered VLF emissions

Abstract. This work is concerned with the numerical modelling of VLF emissions triggered in the equatorial region of the Earth's magnetosphere, using a well established 1-D Vlasov Hybrid Simulation (VHS) code. Although this code reproduces observed ground based emissions well there is some uncertainty regarding the magnitude of simulation parameters such as saturation wave amplitude, cold plasma density, linear growth rate and simulation bandwidth. Concentrating on emissions triggered by pulses of VLF radio waves from the transmitter at Siple Station, Antarctica (L=4.2), these parameters, as well as triggering pulse length and amplitude, are systematically varied. This parametric study leads to an understanding of the physics of the triggering process and also of how the properties of these emissions, particularly their frequency time profile, depend upon these parameters. The main results are that weak power input tends to generate fallers, intermediate power input gives stable risers and strong growth rates give fallers, hooks or oscillating tones. The main factor determining the frequency sweep rate - of either sign - turns out to be the cold plasma density, lower densities giving larger sweep rates.

A 15‐year West Antarctic climatology from six automatic weather station temperature and pressure records

Apart from a small number of mostly coastal stations with human observers, automatic weather stations (AWS) are the dominant source of direct measurements of near‐surface climate parameters on the West Antarctic ice sheet. To help alleviate the shortage of surface meteorological data in Antarctica and to better exploit this invaluable data resource, we have used artificial neural network (ANN)‐based techniques to extend and fill data gaps in selected AWS records. Climatological analyses of the complete 15‐year temperature and pressure records (1979–1993) are reported here for six West Antarctic AWS sites (Siple Station, Byrd, Lettau, Marilyn, Elaine, and Ferrell) spanning ∼90° of longitude. Three sites (Siple, Lettau, and Marilyn) show significant warming trends during the austral summer season (December–February). Warm anomalies of 2°–5°C appeared at all sites during 1980 (winter) and 1989 (winter‐spring) with a cold anomaly of up to 3°C during fall 1982. Average intersite seasonal correlation comparisons are highest during fall and winter and lowest in summer; Byrd correlates with no other sites in summer. All sites have short, sharp temperature transitions during the fall and spring seasons and a relatively stable winter season with occasional early to midwinter warmings. Typical winter season conditions are present for up to 5–6 months of the year. An exception occurs at Byrd during 1988 and 1989, when climatologically normal winter conditions did not appear to become fully established. Mean monthly temperatures during this unusual period were 5°–10°C above normal.

Christmas Card from Siple Station, Antarctica

Christmas Card from Siple Station, Antarctica

Triggering of whistler mode emissions by the band‐limited impulse (BLI) Associated with amplified Vlf signals from Siple Station, Antarctica

At termination of an amplified and saturated (Bsat) Vlf pulse from Siple Station, Antarctica, a band‐limited (50–500 Hz range) impulse (BLI) is generated which in turn triggers a narrowband emission, of about the same amplitude, that starts at about 50–150 Hz above the frequency of Bsat, called the positive frequency offset (PFO). The BLI creates the PFO by reducing the parallel velocities of existing phase‐bunched cyclotron resonant electrons that form the source currents (J⟂) of the stimulated radiation. The PFO is proportional to the magnetic field intensity BBLI of the BLI. Since BBLI ≈ Bsat the PFO then becomes a measure of the saturated field Bsat. An example for L = 4 and a pitch angle of 60°, gives Bsat ≅ 5pT, in good agreement with previous estimates of Bsat.

Probing properties of the magnetospheric hot plasma distribution by whistler mode wave injection at multiple frequencies: Evidence of spatial as well as temporal wave growth

This is the second of two papers on the use of whistler mode wave injection to investigate properties of the magnetospheric hot plasma. Paper 1, [Sonwalkar et al., this issue] emphasized the use of signals at a single frequency to identify longitudinal structures ranging from 100 to 25,000 km in extent in ∼1–10 keV electrons drifting azimuthally through whistler ducts. This short paper discusses and illustrates the use of wave injection at multiple discrete frequencies to study temporal changes in magnetospheric hot electrons with parallel (gyroresonant) velocities in various nonoverlapping ranges. As in paper 1, the data studied were acquired during a special 9‐hour period of 1.9 – 2.9 kHz VLF transmissions from Siple Station, Antarctica, to Lake Mistissini, Canada, on January 23–24, 1988. The amplitudes of the leading edges of constant frequency pulses at 1900, 2150, and 2400 Hz were found to vary independently with time. This is interpreted as evidence of a spatial amplification process that accompanied the well known and more readily identifiable phenomena of exponential temporal growth to a saturation level. Evidence of wave‐hot plasma interactions showed a dependence on df/dt of the input signal frequency versus time format; in general, the slow frequency ramps showed the highest amplitudes and the fast ramps and parabolas the lowest, in agreement with past work.

Properties of the magnetospheric hot plasma distribution deduced from whistler mode wave injection at 2400 Hz: Ground‐based detection of azimuthal structure in magnetospheric hot plasmas

Siple station VLF wave injection experiments aimed at finding the properties of the magnetospheric hot plasma were conducted for a 9‐hour period between 1705 and 0210 UT on January 23–24, 1988. A special frequency versus time format, lasting l min and transmitted every 5 min, consisted of a sequence of pulses, frequency ramps, and parabolas, all in a 1‐kHz range centered on 2400 Hz. The transmitted signals, after propagating along a geomagnetic field‐aligned duct, were recorded at Lake Mistissini, Canada. At various times during the 9‐hour interval the Siple signals showed features characteristic of wave‐particle interactions, including wave growth, sidebands, and triggered emissions. Our observations, primarily at 2400 Hz, show that (1) there were no correlations between the initial levels, the growth rates, and the saturation levels of constant‐frequency pulses; (2) in general, the values of growth rate and saturation level of two pulses injected within 30 s were nearly the same; (3) the initial level, growth rate, and saturation level showed temporal variations over 5–15 min and 1–2 hour timescales; (4) the leading edges of constant‐frequency signals underwent spatial amplification; and (5) under conditions of saturation the received signal bandwidth (∼ 20 Hz) remained constant over a 1‐hour period, although the saturation level and growth rate varied during the same period. On the assumption that gyroresonant interactions were responsible for the observed wave growth and saturation, the timescales over which those phenomena varied provide constraints on the possible energetic electron population within the duct. In the reference frame of the duct (L ∼ 5.1, Ne ∼ 280 cm−3) the particle fluxes showed no variation over a 30‐s timescale but varied over 5–15 min and 1–2 hour timescales. The 5–15 min timescale variations indicate longitudinal structures ranging from ∼ 0.2° or ∼100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 5° or ∼2800 km for electrons with energy E = 11 keV and pitch angle α = 80°. The hour‐long time variations indicate longitudinal structures ranging from ∼ 2° or ∼1100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 45° or ∼25,000 km for electrons with energy E = 11 keV and pitch angle α = 80°. We conclude that ground‐based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.

Ray-tracing analysis of multicomponent whistlers propagating in the asymmetric interhemispheric plasma

A ray-tracing model in which an interhemispheric plasma asymmetry exists, has been used to investigate the propagating characteristics of multicomponent whistlers recorded at Siple Station (L = 4.3), during local dawn on 4 Jul 1982, under very quiet geomagnetic conditions. The 3 kHz rays were started in the summer hemisphere at 300 km altitude, with vertical wave normals at 0.001 0 latitude intervals and over initial latitude ranges from which rays became trapped in eight ducts (L = 2.9 – 4.6). Final latitudes and final wave normal distributions at 300 km altitude in the winter hemisphere were calculated. The best results for trapping rays in each duct were obtained for a duct modeled ( δ = 0.15 and σ d = 50 km) to terminate at 300 km altitude and to reach full enhancement at 2100 km in both hemispheres, disregarding the seasonal variations. The oxygen ion, electron density ratio and ratio of temperature asymmetry, between summer and winter hemispheres were selected for each duct in such a way that electron density and oxygen ion concentrations were similar to data from the ISIS-2 satellite. Ray-tracing calculations were performed for 3 kHz ray that propagated in the magnetosphere in ducted mode or guided by gradient trapping.

Intensities and polarization rates of whistler mode VLF signals observed from a ground network near L = 4

VLF signals transmitted from Siple station in Antarctica that propagated through the ionosphere and magnetosphere to the northern hemisphere and triggered emissions generated by the Siple signals in the magnetosphere are radiated from wave exit areas on the ionospheric lower boundary. The locations and extent of the wave exit areas are deduced from the wave intensity distributions measured on the ground. In this method we use measurements of wave magnetic intensity made simultaneously at six stations. The validity of this method is assessed by comparing the results with values simply estimated from a mathematical evaluation and with results of full wave calculations carried out by Nagano et al. (1986, 1987). We find close agreement among results obtained from the three methods and conclude that the wave intensity distribution analysis is valid. The observed distribution of polarization rate also agrees well with the results of the full wave calculations carried out by Nagano et al. (1986, 1987) and with those obtained by the mathematical evaluation. As a result of analyzing a triggered emission in detail, the extent of the wave exit area of the downgoing triggered emission is estimated to be 25‐50 km or so at an altitude of 80 km. We believe that this value relates to the horizontal extent of the duct through which the triggered emission propagated in the magnetosphere and ionosphere and that essentially only direct waves transmitted from the wave source arrive within a range of 200 km from the wave intensity peak on the ground.

An appreciation of Paul A. Siple

A major inspiration for writing this long overdue appreciation of Paul A. Siple was a wind chill temperature in Washington D.C. of −39°F.Other inspirations have come over the years from colleagues who either know about Paul Siple or Siple Station but not both. In 19571 had the unique fortune to spend a year with Paul Siple as a scientific member of the first wintering‐over party at the U.S. Amundsen‐Scott South Pole Station at 90°S. He was the leader of the station during the first winter of the International Geophysical Year (IGY) 1956–1957.