Whistler Mode Signals Research Articles

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.

Whistler mode bursts in the Venus ionosphere due to lightning: Statistical properties using Venus Express magnetometer observations

The Venus Express mission has completed over four years in polar orbit about Venus with periapsis altitudes as low as 180 km. On each orbit around periapsis the fluxgate magnetometer samples the magnetic field at 128 Hz. The data reveal short‐lived bursts with peak‐to‐peak amplitudes up to 1.5 nT in the frequency range 42 to 60 Hz. These signals are whistler mode waves with burst durations of about 100 ms and Poynting vectors similar to terrestrial whistler mode signals generated by atmospheric lightning when detected in the ionosphere. We have examined the occurrence of these bursts as a function of background magnetic field strength, altitude, latitude and local time. The burst rates are highest for magnetic fields of 15–30 nT, at altitudes near 215 km, and at local times near the terminators. The characteristics of these signals are consistent with generation in the dynamic Venus atmosphere, entry into the ionosphere, propagation along the ionospheric magnetic field, and ultimately damping in the ionospheric plasma.

Nonlinear VLF effects in the topside ionosphere

The Demeter satellite observed intense broadband lower‐ and upper‐hybrid electrostatic waves and plasma perturbations associated with high‐power whistler‐mode signals from the very‐low frequency (VLF) transmitter NWC. This paper shows that the Demeter observations can be explained by nonlinear interactions driven by VLF pump waves, thereby suggesting that nonlinear effects are responsible for energy losses of high‐power VLF signals in the ionosphere.

Cross modulation of whistler mode and HF waves above the HAARP ionospheric heater

[1] The HF facility of the High Frequency Active Auroral Research Program (HAARP) is employed to generate ELF/VLF radiation by modulation of overhead auroral electrojet currents for magnetospheric wave injection and propagation in the Earth-ionosphere waveguide. HAARP induced ducted magnetospherically amplified whistler mode signals are observed to return to the vicinity of the HF facility and experience cross modulation with simultaneous HF transmissions. The cross modulation effect results from modification of the attenuation properties of the ionospheric medium by HAARP HF waves in the daytime ionosphere. The cross modulation concept is subsequently investigated as a new method for ELF/VLF wave generation with the HAARP heating facility using two co-located HF beams. The new generation method is observed to generate radiation from 630 Hz–37 kHz. Observed amplitudes are an order of magnitude lower than fundamental frequencies generated using conventional AM, but of the same order as AM second harmonics.

Ducted whistlers propagating in higher-order guided mode and recorded on board of Compass-2 satellite by the advanced Signal Analyzer and Sampler 2

[1] The advanced electromagnetic wave detector and analyzer, Signal Analyzer and Sampler 2, successfully operated on board of Compass-2 satellite (launched May 2006). One of the peculiarities of this experiment was that the efficient sensitivities of both electric and magnetic channels were very close to being identical. Between the interesting events detected we found the evidence of whistler mode signals propagating in higher- (third-) order guided mode, most probable between two layers (i.e., “onionskin” structure was in the plasmasphere at this time). We present in the paper the real full wave ultrawideband interpretation of these propagating signals using the exact solution of Maxwell's equations in this boundary problem.

Space plasma disturbances caused by NAU-launched whistler waves

Radio signals from Naval (NAU) transmitter in Puerto Rico can interact effectively with naturally occurring or HF heater wave-induced large-scale ionospheric irregularities, allowing them to propagate as whistler-modes in the ionosphere and to the inner radiation belts. NAU-generated whistler-modes have intensities sufficient to parametrically excite lower hybrid waves and ten-meter and meter-scale ionospheric irregularities over Arecibo. Subsequent heating of electrons and ions by the lower hybrid waves yield a sequence of ionospheric plasma effects such as airglow, short-scale density depletion and plasma line enhancements in a range of altitudes which far exceed that caused by the HF heater. Furthermore, they can interact with trapped energetic electrons in inner radiation belts at L=1.35 and trigger precipitation of electrons into the lower ionosphere. We suggest that disturbances in the ionosphere above NAU caused by whistler-mode signals can significantly affect heater-induced perturbations and partially explain unique results obtained at other heater sites.

Saturation effects in the VLF‐triggered emission process

An in‐depth study is performed of the saturation characteristics of the VLF‐triggered emission process, a plasma instability associated with the amplification of whistler mode signals in the magnetosphere. A survey of data from the 1986 operating year of the Siple VLF wave injection experiment reveals that long‐period oscillations (characterized by a pattern of growth to saturation then subsequent suppression of the wave), short‐period oscillations (previously identified as sidebands), and generation of incoherent wave energy around the frequency of the triggering signal are all characteristics of the instability at saturation. A model is developed to study these phenomena. The model is in some respects similar to the Vlasov Hybrid Simulation but modified so that saturation is a natural consequence of the modeled growth process. Results from the model indicate that the growth and eventual saturation are caused by wave amplitude gradients, most notably gradients that indicate a transition from an amplitude where the wave cannot trap electrons in its potential in the presence of the inhomogeneous magnetospheric magnetic field to an amplitude where such trapping is possible. That is, resonant currents are enhanced in the region where the phase space electron hole created by the trapping condition is allowed to mix with the ambient energetic electrons. This growth process is found to naturally lead to saturation of the instability.

Detection of ionospheric perturbations associated with Japanese earthquakes on the basis of reception of LF transmitter signals on the satellite DEMETER

Abstract. There have been recently reported a lot of electromagnetic phenomena associated with earthquakes (EQs). Among these, the ground-based reception of subionospheric waves from VLF/LF transmitters, is recognized as a promising tool to investigate the ionospheric perturbations associated with EQs. This paper deals with the corresponding whistler-mode signals in the upper ionosphere from those VLF/LF transmitters, which is the counterpart of subionospheric signals. The whistler-mode VLF/LF transmitter signals are detected on board the French satellite, DEMETER launched on 29 June 2004. We have chosen several large Japanese EQs including the Miyagi-oki EQ (16 August 2005; M=7.2, depth=36 km), and the target transmitter is a Japanese LF transmitter (JJY) whose transmitter frequency is 40 kHz. Due to large longitudinal separation of each satellite orbit (2500 km), we have to adopt a statistical analysis over a rather long period (such as 3 weeks or one month) to have reliable data set. By analyzing the spatial distribution of JJY signal intensity (in the form of signal to noise ratio SNR) during a period of 4 months including the Miyagi-oki EQ, we have found significant changes in the intensity; generally the SNR is significantly depleted before the EQ, which is considered to be a precursory ionospheric signature of the EQ. This abnormal effect is reasonably explained in terms of either (1) enhanced absorption of whistler-mode LF signals in the lower ionosphere due to the lowering of the lower ionosphere, or (2) nonlinear wave-wave scattering. Finally, this analysis suggests an important role of satellite observation in the study of lithosphere-atmosphere-ionosphere coupling.

Lightning on Venus inferred from whistler-mode waves in the ionosphere

The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. Although much evidence has been reported for lightning on Venus, some searches have been negative and the existence of lightning has remained controversial. A definitive detection would be the confirmation of electromagnetic, whistler-mode waves propagating from the atmosphere to the ionosphere. Here we report observations of Venus' ionosphere that reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s, and have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds.

LF/MF Whistler mode dispersive signals observed with rocket‐borne instruments in the auroral downward current region

The SIERRA sounding rocket launched 14 January 2002, northward from Poker Flat, Alaska, to an altitude of 735 km in active aurora. On the downleg, in the altitude range 500–700 km, the onboard high‐frequency wave experiment detected structured whistler mode signals in the frequency range 100–1000 kHz, lasting nearly 200 s during which the rocket moved approximately 200 km downrange. The most prominent feature consisted of narrowband signals with frequency descending from 500 kHz to 250 kHz in about 0.5 s, with bandwidths ∼10 kHz, with frequency spacings ∼25 kHz, and with amplitudes in the range 10–400 μV/m. The structured whistler mode signals coincide with a region of Alfvénically accelerated electrons of the type often associated with the downward current region, poleward of the inverted‐V electron structures usually associated with the upward current region. However, on short timescales there was no clear correlation between the whistler mode signals and the locally detected electron flux or density. Ray‐tracing calculations indicate that concentrated sources of waves originating on the whistler mode resonance cone and moving upward through a model ionosphere, with speeds of 1000–5000 km/s, to rocket altitudes, exhibit the magnitude of dispersion of the observed structured whistler mode features. We evaluate the strengths and weaknesses of the hypothesis that these concentrated sources could be associated with phase space electron holes shedding electrostatic whistlers as they move upward along the field line at altitudes much higher than the rocket in the downward current region.

Lightning detection on the Venus Express mission

Radio waves and optical flashes consistent with the lightning generation have been reported frequently at Venus. These observations point to the presence of electrical discharges in the sulfuric acid clouds of Venus. A particularly strong whistler-mode signal has been found propagating parallel to the magnetic field in the night ionosphere near 100 Hz by the Pioneer Venus spacecraft. At high (radio) frequencies, intermittent signals are also seen reminiscent of terrestrial lightning. However, these signals appear to be weaker than their terrestrial counterparts. On Venus Express, the magnetometer bandwidth is sufficient to record the lightning signals propagating in the whistler mode and will be used to map the occurrence of lightning across the nightside of the planet.

Satellite observations of lightning-induced hard X-ray flux enhancements in the conjugate region

Abstract. Preliminary examination of October-December 2002 SONG (SOlar Neutron and Gamma rays) data aboard the Russian CORONAS-F (Complex Orbital Near-Earth Observations of the Activity of the Sun) low-altitude satellite has revealed many X-ray enhanced emissions (30–500 keV) in the slot region (L ~ 2–3) between the Earth's radiation belts. In one case, CORONAS-F data were analyzed when the intense hard X-ray emissions were seen westward of the South Atlantic Anomaly in a rather wide L shell range from 1.7 to 2.6. Enhanced fluxes observed on day 316 (12 November) were most likely associated with a Major Severe Weather Outbreak in Eastern USA, producing extensive lightning flashes, as was documented by simultaneous optical observations from space. We propose that whistler mode signals from these lightning discharges cause precipitation of energetic electrons from terrestrial trapped radiation belts, which, in turn, produce atmospheric X-rays in the Southern Hemisphere.

Simulation of nose whistlers: An application to low latitude whistlers

Simulation technique for whistler mode signal propagating through inhomogeneous plasma using WKB approximation has been developed (Singh, K., Singh, R.P., Ferencz, O.E., 2004. Simulation of whistler mode propagation for low latitude stations. Earth Planet Space 56, 979–987). In the present paper, we have used it for the analysis of recorded signals at low latitudes and estimated the nose frequency, which is not observed on the dynamic spectra. At low latitudes nose frequency is ∼100 kHz or more and therefore it is absent in the dynamic spectra due to attenuation of the signal at higher frequencies. The importance of nose frequency is in determining the exact path of propagation, which is required in probing of ambient medium. It is shown that the method permits to study the nose frequency variation, it can be used to deduce physical parameters as the global electric field. A case study permits to get a reasonable value of the electric field, which up to now could not be done at very low latitude.

CLUSTER observations of lower hybrid waves excited at high altitudes by electromagnetic whistler mode signals from the HAARP facility

[1] We report new observations from the CLUSTER spacecraft of strong excitation of lower hybrid (LH) waves by electromagnetic (EM) whistler mode waves at altitudes ≥20,000 km outside the plasmasphere. Previous observations of this phenomenon occurred at altitudes ≤7000 km. The excitation mechanism appears to be linear mode coupling in the presence of small scale plasma density irregularities. These observations provide strong evidence that EM whistler mode waves are continuously transformed into LH waves as the whistler mode waves propagate at high altitudes beyond L ∼ 4. This may explain the lack of lightning generated whistlers observed in this same region of space.

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.

A quantitative estimate of the ducted whistler power within the outer plasmasphere

From a statistical study of ducted whistlers observed at Halley, Antarctica, in 1996, which had propagated on paths in the range L=2.5–4.5, we report mean occurrence rates, numbers of components per whistler, intensities, etc. for night and day conditions and in different seasons at solar minimum. We found an average whistler rate of 5 min −1 and 3 components per whistler. Received whistler amplitudes were measured as a function of frequency and were in the range 2–40 fT, typically 10 fT at 4 kHz. Combining these results with a propagation model, we estimate mean whistler duct output powers to be around 1–10 mW, (≃0.1–1 mJ per whistler in the 3–5 kHz band). Inferred typical equatorial wave fields for ducted whistlers of 0.3 pT led to estimated radiation belt lifetimes for 1–100 keV electrons due to gyroresonance with ducted whistlers of 2×10 6 days. This compares with published lifetimes due to plasmaspheric hiss of order 10 5 days or less, and we conclude that, on average, lightning which enters and propagates in magnetospheric ducts, although known to cause pitch angle scattering and precipitation of trapped electrons, does not significantly affect the radiation belt fluxes in a statistical sense. We have compared our results with those from a similar study by Burgess and Inan (J. Geophys. Res. 98 (1993) 15,643–15,665). In a separate investigation of multi-component whistlers received in winter at quiet times, using the same methodology, we have found that the duct output power generally decreases with increasing L. This is consistent with previous theoretical work and parallels a similar experimental conclusion with respect to higher-frequency whistler-mode signals from VLF transmitters.

High‐frequency and time resolution rocket observations of structured low‐ and medium‐frequency whistler mode emissions in the auroral ionosphere

High bandwidth electric field waveform measurements on a recent auroral sounding rocket reveal structured whistler mode signals at 400–800 kHz. These are observed intermittently between 300 and 500 km with spectral densities 0–10 dB above the detection threshold of 1.5 × 10−11 V2/m2 Hz. The lack of correlation with local particle measurements suggests a remote source. The signals are composed of discrete structures, in one case having bandwidths of about 10 kHz and exhibiting rapid frequency variations of the order of 200 kHz per 100 ms. In one case, emissions near the harmonic of the whistler mode signals are detected simultaneously. Current theories of auroral zone whistler mode emissions have not been applied to explain quantitatively the fine structure of these signals, which resemble auroral kilometric radiation (AKR) rather than auroral hiss.

Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

Abstract. In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S,171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, and using only ducts with exit regions that were >500km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with Kp=2–8 and this result is discussed in terms of changes in wave-particle interactions and duct size.

A satellite study of low latitude electron and proton whistlers

Whistler dispersion, measured on the ISIS 2 satellite, at an altitude of 1400km, showed a variation with latitude from about 4.5 s12 at a magnetic latitude (λ) of 30 ° to about 12 ± 2 s12 at the equator. Frequently, fractional hop whistlers were observed from both hemispheres with dispersions which converged to the same value at the equator. Ray tracing calculations revealed that whistler mode signals of different frequencies take different paths from a source on the ground to the satellite and yet their dispersion is indistinguishable from the Eckersley form. The extent of the entry region to the ionosphere depends on the satellite location and may be more than 300km for different frequencies. The fractional hop whistlers frequently excite proton whistlers and their crossover frequencies have been used to determine fractional hydrogen ion concentrations. The measurements show that the average fractional H+ concentrations at 1400 km increase from about 0.80 at λ = 25 ° to 0.88 at the equator.

Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S,171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, and using only ducts with exit regions that were >500km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with Kp=2–8 and this result is discussed in terms of changes in wave-particle interactions and duct size.