Ducted Whistlers Research Articles

Prevalence and Propagation of Lightning‐Generated Whistlers in Van Allen Probes EFW Burst Data

AbstractWe assess the prevalence of ducted and non‐ducted whistler propagation using burst mode data from the Van Allen Probes Electric Field and Waves instrument (EFW). We have identified burst periods containing lightning‐generated whistlers (LGWs), resulting in a data set available for future use. The entire burst data set is filtered through an analysis of the search coil magnetometer (SCM) noise, identifying signals in frequency space that exceed an adaptive noise threshold. We implement DBSCAN (Density‐Based Spatial Clustering of Applications with Noise) to identify individual whistlers and clusters of whistlers. With magnetic spectral analysis, we calculate the mean wave normal angle (WNA) for each LGW group. We use ray tracing to estimate the expected WNA distributions for non‐ducted LGWs to compare to the data and determine methods for identifying potentially ducted LGWs. The ray‐tracing results provide a clear threshold in WNA for ducted whistlers. Using this threshold, we estimate that at least of LGWs in this data set are ducted. We find that the majority of potentially ducted whistlers are below and nearly half of LGWs below and within 9–18 MLT may be ducted.

Role of Ducting in Relativistic Electron Loss by Whistler‐Mode Wave Scattering

AbstractResonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating only keV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.

Ground and Space Signatures of VLF Noise Suppression by Whistlers

AbstractVLF (VeryLow Frequency) spectrograms registered at Kannuslehto ground station, after cleaning them from strong sferics, reveal VLF noise suppression by whistlers and whistler echo trains, which consists in significant reduction in the noise spectral power after a strong whistler event. We have found similar effect in the VLF data from Van Allen Probe B taken in the equatorial region on ‐shell 3. Detailed analysis of the data shows that the whistler echo train and the VLF noise have small wave normal angles. Based on this observation, we limit our analysis to parallel (ducted) whistler wave propagation. The persistence of whistler echo train, as well as the VLF noise, suggests that in the events under discussion, plasma is unstable in the frequency range corresponding to the observed VLF noise band. In an attempt to explain the effect of VLF noise suppression, we follow up the long‐standing idea that relates this effect to the reduction of free energy in the unstable plasma distribution by whistler echo train. To develop this idea into qualitative model, we have studied the motion of energetic electrons, responsible for the noise generation, in the field of ducted whistler echo train. We show that energetic electrons that make the main contribution to the growth rate of VLF noise, during their bounce oscillations in the magnetosphere, are subject to multiple resonant impacts from the whistler echo train. These lead to energetic electron diffusion in the phase space and the corresponding reduction in free energy of the unstable distribution.

Investigation of VLF Transmitter Signals in the Ionosphere by ZH‐1 Observations and Full‐Wave Simulation

AbstractIn this study, we investigate the very low frequency (VLF) signals in the ionosphere propagating from ground‐based transmitters by using the data from the newly launched China Seismo‐Electromagnetic Satellite (CSES), which is also called ZH‐1. The simultaneous ionospheric perturbations and electron precipitations induced by the NWC transmitter are also analyzed, and the possible mechanisms are discussed. The ZH‐1 and DEMETER observations of VLF transmitters are compared morphologically, which are generally consistent with the full‐wave simulations. The results show that the electromagnetic waves excited by the VLF transmitters penetrate into the ionosphere and propagate in the magnetosphere to its conjugate hemisphere as a ducted or nonducted whistler. When the VLF transmitter is located at L < 1.5, the wave propagates more easily as a nonducted whistler. When the wave propagates as a ducted whistler, the point of maximum observed transmitter‐induced electric field in the conjugate hemisphere will move poleward in the northern hemisphere and equatorward in the southern hemisphere. The results also show that the data stability of electromagnetic field recorded by ZH‐1 satellite is almost the same as DEMETER satellite, and there is a good correlation between the ZH‐1 observation and simulated results. These conclusions demonstrate that the nightside VLF band electromagnetic observation of ZH‐1 satellite is stable and reliable.

Excitation of whistler waves below the lower hybrid frequency by a loop antenna located in an enhanced density duct

Whistler wave radiation from a loop antenna located in a cylindrical duct with enhanced plasma density is considered in the case where the wave frequency is less than the lower hybrid frequency. Using the full-wave formulation, the total radiation resistance and the partial radiation resistances corresponding to guided eigenmodes of such a duct and unguided waves radiating to the background magnetoplasma are calculated and analyzed as functions of the plasma and source parameters. The emphasis is placed on the radiation characteristics of the considered source in the presence of an artificial near-antenna duct that can be created during active experiments in the ionosphere. Conditions are revealed under which the total radiation resistance is predominantly determined by the excitation of the eigenmodes of the duct. It is shown that the presence of an enhanced density duct can lead to a notable increase in the radiation resistance of a loop antenna in the discussed frequency range even if the duct is rather narrow and capable of guiding only a single low-order eigenmode. The results obtained can be helpful in understanding the basic features of excitation of the ducted whistlers and planning the related ionospheric and laboratory experiments.

Whistler wave propagation in the antenna near and far fields in the Naval Research Laboratory Space Physics Simulation Chamber

In previous papers, early whistler propagation measurements were presented [W. E. Amatucci et al., IEEE Trans. Plasma Sci. 33, 637 (2005)] as well as antenna impedance measurements [D. D. Blackwell et al., Phys. Plasmas 14, 092106 (2007)] performed in the Naval Research Laboratory Space Physics Simulation Chamber (SPSC). Since that time there have been major upgrades in the experimental capabilities of the laboratory in the form of improvement of both the plasma source and antennas. This has allowed access to plasma parameter space that was previously unattainable, and has resulted in measurements that provide a significantly clearer picture of whistler propagation in the laboratory environment. This paper presents some of the first whistler experimental results from the upgraded SPSC. Whereas previously measurements were limited to measuring the cyclotron resonance cutoff and elliptical polarization indicative of the whistler mode, now it is possible to experimentally plot the dispersion relation itself. The waves are driven and detected using balanced dipole and loop antennas connected to a network analyzer, which measures the amplitude and phase of the wave in two dimensions (r and z). In addition the frequency of the signals is also swept over a range of several hundreds of megahertz, providing a comprehensive picture of the near and far field antenna radiation patterns over a variety of plasma conditions. The magnetic field is varied from a few gauss to 200 G, with the density variable over at least 3 decades from 107 to 1010 cm−3. The waves are shown to lie on the dispersion surface for whistler waves, with observation of resonance cones in agreement with theoretical predictions. The waves are also observed to propagate without loss of amplitude at higher power, a result in agreement with previous experiments and the notion of ducted whistlers.

Some unusual high dispersion whistler triplets recorded at a low L-value (L=1.17)

A detailed analysis of the VLF data collected during January to December, 2003 at the low latitude station Jammu (geomag. lat., 22° 26′ N; L = 1.17) has yielded some unusual VLF events like whistler triplets of very high dispersion D ∼ 70 − 90 sec1/2. From the dispersion analysis of the whistler triplets, it is found that the individual whistlers of the triplets are one-hop high dispersion ducted whistlers having propagation path along higher and closely spaced L-values (L∼4.0) than the L-value of the recording station. The dispersion and occurrence characteristics of these high dispersion whistlers of the triplets are compared with those of the majority of low dispersion whistlers (D ∼ 30 − 40 sec1/2) recorded during the same period. Generation and propagation mechanisms are briefly discussed.

Observations of unusual whistlers during daytime at Jammu

In this paper, we report observations of unusual whistlers recorded at Jammu (geomag. lat. = 22°26′N; L = 1.17), India on March 8, 1999 during the daytime. They are interpreted as one-hop ducted whistlers having propagated along higher L-values in closely spaced narrow ducts from the opposite hemispheres. After leakage from the duct, the waves might have propagated in the earth-ionosphere waveguide towards the equator in surface mode. Tentative explanation of the dynamic spectra of these events is briefly presented.

Magnetospheric effects from man‐made ELF/VLF modulation of the auroral electrojet

A model for the magnetospheric effects of ELF/VLF modulation of the auroral electrojet using high‐power ground‐based high‐frequency radio waves is developed. The effects of ducted topside whistler wave power flux, as generated from ELF/VLF auroral electrojet modulation, on the magnetospheric equatorial pitch angle diffusion coefficient are computed for a range of ELF/VLF modulation frequencies. We find that the precipitation lifetime in the presence of this whistler wave flux is smaller than for purely collisional processes. We find that aspects of the model are in agreement with recent experimental observations.

Dispersion properties of ducted whistlers, generated by lightning discharge

Abstract. Whistler-mode wave propagation in magnetospheric ducts of enhanced cold plasma density is studied. The case of the arbitrary ratio of the duct radius to the whistler wavelength is considered, where the ray-tracing method is not applicable. The set of duct eigenmodes and their spatial structure are analysed and dependencies of eigenmode propagation properties on the duct characteristics are studied. Special attention is paid to the analysis of the group delay time of one-hop propagation of the whistler wave packet along the duct. We found that, in contrast to the case of a wide duct, the group delay time in a rather narrow duct decreases as the eigenmode number increases. The results obtained are suggested for an explanation of some types of multi-component whistler signals.

LHR effects in nonducted whistler propagation – new observations and numerical modelling

Abstract. VLF-ELF broadband measurements onboard the MAGION 4 and 5 satellites at heights above 1 Re in plasmasphere provide new data on various known phenomena related to ducted and nonducted whistler wave propagation. Two examples are discussed: magnetospherically reflected (MR) whistlers and lower hybrid resonance (LHR) noise band. We present examples of rather complicated MR whistler spectrograms not reported previously and argue the conditions for their generation. Analytical consideration, together with numerical modelling, yield understanding of the main features of those spectrograms. LHR noise band, as well as MR whistlers, is a phenomenon whose source is the energy propagating in the nonducted way. At the plasmaspheric heights, where hydrogen (H+) is the prevailing ion, and electron plasma frequency is much larger than gyrofrequency, the LHR frequency is close to its maximumvalue in a given magnetic field. This frequency is well followed by the observed noise bands. The lower cutoff frequency of this band is somewhat below that maximum value. The reason for this, as well as the possibility of using the LHR noise bands for locating the plasma through position, are discussed.Key words. Magnetospheric physics (plasmasphere; wave propagation)

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.

Satellite observations of lightning‐induced electron precipitation

Lightning‐induced electron precipitation (LEP) from the Earth's radiation belt has been observed on numerous occasions with detectors on the low‐altitude S81‐1/SEEP satellite. A sequence of seven LEP events on September 9, 1982, and eight events on October 20, 1982, are correlated on a one‐to‐one basis with one‐hop whistlers at Palmer, Antarctica. The temporal profile within a LEP burst has a remarkable fine structure. It is shown to be associated with bunches of magnetically guided and focused 100‐to‐200 keV electrons that are repeatedly scattered by the atmosphere and bounce between the northern and southern hemispheres. The delay time between the lightning sferic and the arrival of the first electron bunch increases with increasing L as predicted by the first‐order gyroresonance theory. The global distribution of strong LEP events observed with the SEEP payload correlates with lightning activity and shows a preferred distribution at 2 < L < 3. This L shell range corresponds to the slot region in the electron radiation belt. A single LEP burst (10−3 erg s−1 cm−2) in the slot region is estimated to deplete ∼0.001% of the particles in the region covered by the burst magnetic field lines. The evidence supports the production of structured LEP by ducted rather than nonducted whistlers. It is found that ducted whistlers can be an important pitch angle diffusion mechanism for 100–250 keV electrons in the 2 < L < 3 range although a number of uncertainties in the various parameters remain to be resolved; It is suggested that observations of LEP can be a new tool to measure the presence and transverse dimensions of plasmaspheric whistler mode ducts.

A new parametric reflection mechanism for ducted whistlers and an explanation of precursors

Abstract A new reflection mechanism of ducted whistler-mode waves in the magnetosphere, which can be defined as a parametric reflection, is discussed. Unlike the well-known process of backscattering, parametric reflection occurs when the scale length of electron density inhomogeneities is much less than the whistler wave-length; the reflected whistler wave appears simultaneously with the excitation of short-scale lower hybrid resonance (LHR) plasma waves. The efficiency of such a parametric reflection is higher than that of traditional backscattering. Two cases are analysed. The first is when a reflected whistler wave arises via the process of the parametric decay instability of a sufficiently intense whistler wave into two quasielectrostatic short-scale waves, that is a LHR wave and an ionic (ion cyclotron or sound) wave. The second case is when the initial whistler interacts with naturally occurring ionic waves. Quantitative estimates show that both cases can exist in the magnetosphere. In the case of ducted whistler, generated by a lightning discharge, reflected signals at a given frequency are formed near two points along the magnetic field line where this frequency coincides with the local LHR frequency. This leads to appearance on a spectrogram of signals with a hook-like shape, which precede the whistler. It is shown that these signals can be associated with different types of whistler precursors.

Middle and High Latitude Whistlers Observed Simultaneously in a Low Latitude Station at Agra.

The characteristics of very high dispersion (D ∼ 54-70 sec1/2) whistlers recorded simultaneously in a low latitude ground station at Agra (geomag. lat. 17°1'N, L =1.15) during the night-time observation on 2 April, 1979 are studied and their propagation paths are determined using the curve fitting technique. The results show that some of these whistlers are high latitude whistlers which propagated along the magnetic field line of L = 3.6 while the others are one hop and three hop mid-latitude whistlers which propagated along the field lines between L = 1.9 to 2.5. Some of these whistlers are recorded simultaneously in our ground station. The dispersion and occurrence characteristics of these high dispersion whistlers are compared with those of the majority of relatively low dispersion ducted whistlers (D ∼ 30-35 sec1/2) which were recorded in our station during the same period of observation and then we discuss the propagation characteristics of high dispersion whistlers in terms of nonducted Pro-Longitudinal mode (PL mode) of propagation.

Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler‐induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.

The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons

New experimental evidence suggests that every ducted whistler component may precipitate bursts of radiation belt electrons into geomagnetically conjugate ionospheric regions. Strong spatial and temporal associations are seen between transient ionospheric disturbances observed in conjugate regions and ducted whistlers monitored at Palmer Station, Antarctica. The ionospheric disturbances were detected by their characteristic perturbing effects (“Trimpi events”) on subionospheric VLF, LF, and MF signals recorded at Palmer Station and at northern hemisphere sites. Of 74 such events examined on four different days, all were time‐associated with ducted whistlers. In no case was the arrival azimuth or dispersion of the associated whistlers inconsistent with the locations of the conjugate ionospheric disturbances, which were inferred from the configuration of perturbed signal paths. Other whistlers occurring independently of detected disturbances were found to be either weak or to have arrived from regions where a disturbance would not have been detected for lack of monitored signal paths. Signal perturbation onset behavior was consistent with multiple regions of precipitation induced by components of multipath whistlers and with theoretical predictions for ducted whistler‐induced precipitation. The results not only support the hypothesis that ducted whistlers are responsible for burst precipitation of energetic electrons but imply that such bursts may be induced by every ducted whistler component. Since radio energy from a lightning discharge can excite whistler ducts located 2500 km or more away from the flash, every ducted whistler observed at Palmer Station may indicate the presence of precipitation bursts associated one‐to‐one with excited whistler ducts distributed over a 5000‐km‐wide portion of the Earth's surface, and over its geomagnetic conjugate as well. The estimated effect of this precipitation on 70‐ to 200‐keV radiation belt electron populations for 2 < L < 3 is comparable to that predicted as a result of plasmaspheric hiss, indicating that ducted whistlers may contribute as significantly as hiss to radiation belt equilibrium at those electron energies.

The Lorentz polarization term for magnetospheric whistler propagation

If the particles of a dielectric are arranged in a cubic lattice, the effective field acting on a particle is E + lP/εo, where l = 1/3. The second term is the Lorentz polarization term. Appleton believed that this term should be omitted from magneto‐ionic theory, whereas Hartree included it. Various arguments have been presented to prove Appleton's contention that this term is zero for a plasma. Some proofs have considered the perturbation of the individual orbits of the electrons, but careful consideration of these treatments shows that they would predict a nonzero Lorentz term for some velocity distributions. It has also been argued that l ≠ 0 would violate causality but it can be shown that this only applies in the zero frequency limit for positive l. Other proofs that have been presented also possess significant limitations, in particular being limited to rather simpler plasma configurations than exist in the magnetosphere and weak field strengths. The theoretical problem for more general conditions is very difficult and thus an appeal to experiment results is considered relevant. It is shown that a very small nonzero Lorentz term would be significant for whistler‐mode propagation. The observed lower cutoff frequencies of whistlers put a very low limit (l ∼ 10−5) on the maximum value of positive l but put no limit on negative l. It is shown, however, that l < 0 would increase the cutoff frequency for ducted propagation. Thus the upper cutoff frequency is determined for values of l in the range −5×10−6 to −2×l0−3 by ray tracing in a model Gaussian cross‐section duct in a realistic model of the magnetospheric plasma corresponding to winter night conditions. Cutoffs (normalized to the equatorial electron gyrofrequency determined by curve fitting to the ray‐traced whistler spectra) are found to be in the range 0.49 to 0.55 for a 15% enhancement duct. The range of cutoffs for l ∼ −10−4 is in much better agreement with experimentally observed whistler cutoffs than the assumption that l = 0 (no Lorentz term). This provides experimental evidence for a nonzero (negative) Lorentz term for ducted whistler propagation.

Electric fields at L = 2.5 during geomagnetically disturbed conditions

We have used observations of the group time delay and Doppler shift of ducted VLF whistler mode signals propagating near L = 2.5 to deduce the azimuthal component of the plasmaspheric electric field during geomagnetically disturbed periods in June and July 1986. The whistler mode signals originated in the US Navy transmitters NAA and NSS and were recorded at Faraday, Antarctica. The average E w versus LT curve for periods when K p > 2 + has been compiled; when this is compared with the E w versus LT curve for quiet times, it is found that the electric field is more eastwards from 18–22 LT and more westwards from 00–01 LT in disturbed times. This difference is consistent with published calculations of the penetration of the dawn-dusk electric field to L = 2.5. A variety of behaviour is evident when the data are examined on a case to case basis. Sometimes the dawn-dusk electric field becomes apparent during isolated intense substorms; this is attributed to increased penetration due to an increase in the auroral zone Pedersen conductivity. On one night the drifts seemed to be partly due to the ionospheric disturbance dynamo.

Fine structure of whistlers recorded digitally at Halley, Antarctica

Two whistlers recorded digitally at Halley, Antarctica ( L = 4.3) were analyzed by matched filtering with 10 Hz frequency resolution. For the construction of the matched filter, a more realistic description of the whistler waveform than has been used for lower latitude whistlers was applied. The ( ƒ, t) pairs obtained, together with the corresponding magnitudes, gave a high resolution dynamic spectrum which revealed the fine structure of the whistlers. Whistlers which appeared as well defined, discrete, single traces on a conventional spectrogram, turned out to be composed of several components covering various parts of the spectrum. Further analysis of the strongest and longest component of a whistler resulted in high resolution travel time residual curves similar to those obtained by the ray tracing method or by averaging of numerous occurrences of whistlers in the same duct. The matched filter method of analysis can provide new insights into the details of ducted whistler propagation. It also yields the amplitude variation vs time (or vs frequency) along the whistler trace, thus potentially giving information about the spectrum of the source spheric, and/or the frequency dependence of magnetospheric amplification due to wave-particle interactions.