Lightning-generated Whistlers Research Articles

Lightning-induced relativistic electron precipitation from the inner radiation belt.

The Earth's radiation belts are maintained by a number of acceleration, loss and transport mechanisms, and the electron fluxes at any given time are highly variable. Microbursts, which are rapid (sub-second) bursts of energetic electrons entering the atmosphere from the magnetosphere, are one of the key loss mechanisms controlling radiation belt fluxes. Such rapid bursts are typically observed from the outer radiation belt and driven by interactions with whistler mode chorus waves, but they can also occur in the inner belt and slot region, driven by lightning-generated whistlers. This lightning-induced electron precipitation is typically observed at 10s-100s keV, but here we present direct observations of this phenomenon at MeV energies. This unveils a coupling between near-Earth processes, such as lightning, and radiation belt processes, such as relativistic electron microbursts, bridging the gap between Earth weather and space weather.

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.

Specularly reflected whistler: A low-latitude channel to couple lightning energy to the magnetosphere.

Lightning-generated whistlers profoundly affect the energetic particle population in Earth's radiation belts, influencing space weather and endangering astronauts and satellites. We report the discovery of specularly reflected (SR) whistler in which the lightning energy injected into the ionosphere at low latitudes reaches the magnetosphere after undergoing a specular reflection in the conjugate ionosphere, contradicting previous claims that lightning energy injected at low latitudes cannot escape the ionosphere. SR whistlers provide a low-latitude channel to transport lightning energy to the magnetosphere. We calculate the relative contributions of SR, magnetospherically reflected, subprotonospheric, and ducted whistlers to the lightning energy reaching the magnetosphere. When SR whistlers are considered, the global lightning energy contribution to the magnetosphere doubles, implying that the previous estimates of the impact of lightning energy on radiation belts may need substantial revisions. Whistler dispersion and intensity analyses quantitatively confirm our results and suggest new remote-sensing methods of the magnetosphere, ionosphere, Earth-ionosphere waveguide, and lightning flashes.

On the Wave‐Normal Distribution of Lightning‐Generated Whistlers and Their Propagation Modes

AbstractObservations from Van Allen Probes are analyzed to obtain the statistical wave normal distribution of lightning‐generated whistlers (LGWs). An automatic algorithm is developed to identify burst mode waveform data with LGW signals and analyze the wave polarization information for these LGW signals. The spatial distribution of the LGW occurrence and the probabilities of different propagation types demonstrate that most LGWs can be observed in the low L‐shell region (L < 3), where the dominant propagation type is oblique outward. Parallel propagation dominates in the high L‐shell region, but the LGW occurrence is very small. Additionally, a small group of oblique but inwardly propagating LGWs are observed at low altitudes (<0.2RE). A ray tracing simulation is performed not only to confirm predominant oblique and outward wave vectors in the vast region of the plasmasphere but also to verify the existence of these inward propagating LGW signals at low altitudes near the topside ionosphere.

Lightning-generated Whistlers recognition for accurate disaster monitoring in China and its surrounding areas based on a homologous dual-feature information enhancement framework

Natural disasters, such as earthquakes and volcanic eruptions, pose a significant threat to Earth’s biodiversity and ecological environment. The ability of Lightning-generated Whistlers (LWs) to foresee these events is invaluable. However, the accurate recognition of LWs is hindered due to the spatial environmental interference and a lack of comprehensive information. This study proposes a novel framework called Dual-features Information Enhancement Framework (DIEF) with three versions: the cost-effective DIEF-B, the highly accurate DIEF-M, and the lightweight yet efficient DIEF-T. This framework aims to integrate homologous dual-feature and mitigate space environment effects for LWs recognition. Specifically, the Dual-feature Information Enhancement (DIE) module, which is based on Transformers, merges the waveform signal with the time-frequency spectrum of LWs to enhance the information representation within the feature space. In addition, Multi-scale Feature Integration (MFI) is designed to address the challenge of recognizing faint LWs in waveform signal. To correct errors in time-frequency spectrum recognition caused by space environmental interference, we adopt Mel-scale Frequency Cepstrum Coefficients (MFCCs) to enhance waveform signal features. Afterwards, the long-distance dependences between signals are improved through the Bi-directional Long Short-Term Memory (BiLSTM) network. Finally, an efficient Lightning-generated Whistlers Classifier (LWC) is developed. Numerous tests demonstrate the excellent performance and robustness of the DIEF series, which achieve 99.30% recognition accuracy on the 10,200 segments of LWs dataset acquired by Zhangheng-1 (ZH-1) satellite. The DIEF series achieves accuracy of 95.27% in audio recognition on the UrbanSound8k dataset, which is better than most current ones. Our framework can quickly and accurately recognize valuable LWs events in an interference environment, thereby benefiting for global natural disaster monitoring. Source Code is available at https://github.com/KotlinWang/DIEF.

An assessment of whistlers generated from tree-like gigantic jets

The characteristics of whistlers generated from an observed gigantic jet (GJ) are assessed and possible locations to detect these waves are deduced. Modeling is based on disturbances in the electric field, as measured by NCKU ELF/VLF station, associated with a tree-like GJ event over typhoon Lionrock. The power spectrum of GJ differs from that of common cloud-to-ground lightning; therefore, this study also investigates differences between GJ-generated signals and common lightning-generated whistlers. Detectability is evaluated by considering the absorption of amplitudes resulted from collisional damping associated with the propagation of generated waves. Our results show that in the ionosphere the waves are subject to greater attenuation as the frequency increases; however, a reversal occurs at lower frequencies of a few hundred Hz. The calculated waveforms show that the whistlers generated by the tree-like GJs are preceded by small fluctuations at high frequencies generated by the initiating lightning. Overall, the amplitudes increase with the passage of time; however, they are more randomly-distributed over time for whistlers generated from common lightning. The amplitudes decrease again when lower-frequency components below a few hundred Hz arrive. The amplitudes drop to the order of 1 mV/m as the waves propagate in the ionosphere, which puts them within a range detectable by the instruments on most satellites. Based on the locations of tree-like GJ events observed by ISUAL (Imager of Sprites and Upper Atmospheric Lightning), regions of the western and southeastern Pacific Ocean, as well as northern Africa region are the most likely locations to detect these whistlers.

Corrigendum: Propagation and dispersion of lightning-generated whistlers measured from the Van Allen Probes

Corrigendum on: Ripoll J-F, Farges T, Malaspina DM, Cunningham GS, Hospodarsky GB, Kletzing CA and Wygant J R (2021) Propagation and Dispersion of Lightning-Generated Whistlers Measured From the Van Allen Probes. Front. Phys. 9:722355. doi: 10.3389/fphy.2021.722355.In the published article, there was an error in Figure 8 as published. Figure 8 shows incorrectly the mean refractive index (µ "), which is shown correctly afterward in Figure 10, instead of showing the mean wave normal angle ( ̅ ). Therefore, Figure 10

Complementary and Catalytic Roles of Man‐Made VLF Waves and Natural Plasma Waves in the Loss of Radiation Belt Electrons

AbstractBy simulating the energy and pitch angle diffusions of radiation belt electrons caused by ground‐based VLF transmitter waves, plasmaspheric hiss and lightning‐generated whistlers (LGWs), we quantitatively estimated the electron change rates caused by individual and multiple waves. We found that man‐made VLF waves and naturally generated hiss or LGWs play complementary and catalytic roles in the loss of radiation belt electrons. Man‐made VLF waves mainly reduce the hundreds of keV electrons in inner radiation belt (L < 2), but they alone cannot effectively remove the MeV electrons in slot region (L ∼ 2–3) because of unefficient diffusions at small pitch angles (<60°). On the contrary, natural hiss and LGWs do not affect the hundreds of keV electrons in the inner belt, but they are able to effectively reduce the hundreds of keV and MeV electrons at small pitch angles in the slot region. The loss of the small pitch electrons caused by hiss or LGWs offer a necessary phase space density gradient for the diffusion of large pitch angle MeV electrons (>60°) driven by man‐made VLF waves toward the loss cone. The combined diffusions by three types of the waves ultimately catalyze the loss of the large pitch angle MeV electrons in the slot region, and thus cause more electron losses in the wider energy and pitch angle ranges.

Propagation and Dispersion of Lightning-Generated Whistlers Measured From the Van Allen Probes

We study the propagation and attenuation of lightning-generated whistler (LGW) waves in near-Earth space (L ≤ 3) through the statistical study of three specific quantities extracted from data recorded by NASA’s Van Allen Probes mission, from 2012 to 2019: the LGW electric and magnetic power attenuation with respect to distance from a given lightning stroke, the LGW wave normal angle in space, and the frequency-integrated LGW refractive index. We find that LGW electric field wave power decays with distance mostly quadratically in space, with a power varying between -1 and -2, while the magnetic field wave power decays mostly linearly in space, with a power varying between 0 and -1. At night only, the electric wave power decays as a quadratic law and the magnetic power as a linear law, which is consistent with electric and magnetic ground measurements. Complexity of the dependence of the various quantities is maximal at the lowest L-shells (L < 1.5) and around noon, for which LGW are the rarest in Van Allen Probes measurements. In-space near-equatorial LGW wave normal angle statistics are shown for the first time with respect to magnetic local time (MLT), L-shell (L), geographic longitude, and season. A distribution of predominantly electrostatic waves is peaked at large wave normal angle. Conversely, the distribution of electromagnetic waves with large magnetic component and small electric component is peaked at small wave normal angle. Outside these limits, we show that, as the LGW electric power increases, the LGW wave normal angle increases. But, as the LGW magnetic power increases, the LGW wave normal angle distribution becomes peaked at small wave normal angle with a secondary peak at large wave normal angle. The LGW mean wave-normal angle computed over the whole data set is 41.6° with a ∼24° standard deviation. There is a strong MLT-dependence, with the wave normal angle smaller for daytime (34.4° on average at day and 46.7° at night). There is an absence of strong seasonal and continental dependences of the wave-normal angle. The statistics of the LGW refractive index show a mean LGW refractive index is 32 with a standard deviation of ∼26. There is a strong MLT-dependence, with larger refractive index for daytime 36) than for nighttime (28). Smaller refractive index is found during Northern hemisphere summer for L-shells above 1.8, which is inconsistent with Chapman ionization theory and consistent with the so-called winter/seasonal anomaly. Local minima of the mean refractive index are observed over the three continents. Cross-correlation of these wave parameters in fixed (MLT, L) bins shows that the wave normal angle and refractive index are anti-correlated; large (small) wave normal angles correspond with small (large) refractive indexes. High power attenuation during LGW propagation from the lightning source to the spacecraft is correlated with large refractive index and anti-correlated with small wave normal angle. Correlation and anti-correlation show a smooth and continuous path from one regime (i.e. large wave normal angle, small refractive index, low attenuation) to its opposite (i.e. small wave normal angle, large refractive index, large attenuation), supporting consistency of the results.

Properties of Lightning Generated Whistlers Based on Van Allen Probes Observations and Their Global Effects on Radiation Belt Electron Loss

AbstractLightning generated whistlers (LGWs) play an important role in precipitating energetic electrons in the Earth's inner radiation belt and beyond. Wave burst data from the Van Allen Probes are used to unambiguously identify LGWs and analyze their properties at L < 4 by extending their frequencies down to ~100 Hz for the first time. The statistical results show that LGWs typically occur at frequencies from 100 Hz to 10 kHz with the major wave power below the equatorial lower hybrid resonance frequency, and their wave amplitudes are typically strong at L < 3 with an occurrence rate up to ~30% on the nightside. The lifetime calculation indicates that LGWs play an important role in scattering electrons from tens of keV to several MeV at L < ~2.5. Our newly constructed LGW models are critical for evaluating the global effects of LGWs on energetic electron loss at L < 4.

Coexistence of Lightning Generated Whistlers, Hiss and Lower Hybrid Noise Observed by e-POP (SWARM-E)–RRI

Whistler mode waves play a major role in regulating the lifetime of trapped electrons in the Earth’s radiation belts. Specifically, interaction with whistler mode hiss waves is one of the mechanisms that maintains the slot region between the inner and outer radiation belts. The generation mechanism of hiss is a topic still under debate with at least three prominent theories present in the literature. Lightning generated whistlers in their ducted or non-ducted modes are considered to be one of the possible sources of hiss. We present a study of new observations from the Radio Receiver Instrument (RRI) on the Enhanced Polar Outflow Probe (ePOP: also known as SWARM-E). RRI consists of two orthogonal dipole antennas, which enables polarization measurements, when the satellite boresight is parallel to the geomagnetic field. Here we present 105 ePOP - RRI events from 2014–2018, in which lightning whistlers(75) and hiss waves(39) were observed. In more than 50% of those whistler observations, hiss found to co-exist. Moreover, the whistler observations are correlated with observations of wave power at the lower-hybrid resonance. The observations and a whistler mode ray-tracing study suggest that multiple-hop lightning induced whistlers can be a source of hiss and plasma instabilities in the magnetosphere.

A statistical study over Europe of the relative locations of lightning and associated energetic burst of electrons from the radiation belt

Abstract. The DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) spacecraft detects short bursts of lightning-induced electron precipitation (LEP) simultaneously with newly injected upgoing whistlers. The LEP occurs within < 1 s of the causative lightning discharge. First in situ observations of the size and location of the region affected by the LEP precipitation are presented on the basis of a statistical study made over Europe using the DEMETER energetic particle detector, wave electric field experiment, and networks of lightning detection (Météorage, the UK Met Office Arrival Time Difference network (ATDnet), and the World Wide Lightning Location Network (WWLLN)). The LEP is shown to occur significantly north of the initial lightning and extends over some 1000 km on each side of the longitude of the lightning. In agreement with models of electron interaction with obliquely propagating lightning-generated whistlers, the distance from the LEP to the lightning decreases as lightning proceed to higher latitudes.

Three‐dimensional electron radiation belt simulations using the BAS Radiation Belt Model with new diffusion models for chorus, plasmaspheric hiss, and lightning‐generated whistlers

The flux of relativistic electrons in the Earth's radiation belts is highly variable and can change by orders of magnitude on timescales of a few hours. Understanding the drivers for these changes is important as energetic electrons can damage satellites. We present results from a new code, the British Antarctic Survey (BAS) Radiation Belt model, which solves a 3‐D Fokker‐Planck equation, following a similar approach to the Versatile Electron Radiation Belt (VERB) code, incorporating the effects of radial diffusion, wave‐particle interactions, and collisions. Whistler mode chorus waves, plasmaspheric hiss, and lightning‐generated whistlers (LGW) are modeled using new diffusion coefficients, calculated by the Pitch Angle and Energy Diffusion of Ions and Electrons (PADIE) code, with new wave models based on satellite data that have been parameterized by both the AE and Kp indices. The model for plasmaspheric hiss and LGW includes variation in the wave‐normal angle distribution of the waves with latitude. Simulations of 100 days from the CRRES mission demonstrate that the inclusion of chorus waves in the model is needed to reproduce the observed increase in MeV flux during disturbed conditions. The model reproduces the variation of the radiation belts best when AE, rather than Kp, is used to determine the diffusion rates. Losses due to plasmaspheric hiss depend critically on the the wave‐normal angle distribution; a model where the peak of the wave‐normal angle distribution depends on latitude best reproduces the observed decay rates. Higher frequency waves (∼1–2 kHz) only make a significant contribution to losses for L∗<3 and the highest frequencies (2–5 kHz), representing LGW, have a limited effect on MeV electrons for 2<L∗<5.5.

Understanding the dynamic evolution of the relativistic electron slot region including radial and pitch angle diffusion

[1] It has been suggested that the equilibrium structure of the slot region, which separates the inner and outer radiation belts, forms as the result of a balance between inward radial diffusion and pitch angle scattering of relativistic electrons by interactions with three types of whistler mode waves: plasmaspheric hiss, lightening-generated whistlers, and ground-based Very Low Frequency (VLF) transmitters. In this study, using the time-dependent 3D Versatile Electron Radiation Belt (VERB) code, we examine how effectively the slot can be formed by a combination of radial diffusion and pitch angle diffusion, together with Coulomb scattering, and compare the simulations with the CRRES MEA 1 MeV electron observations to examine the viability of the various scattering mechanisms. The results show that the overall time evolution of the observed two-zone structure is in a good agreement with our model simulations, which suggests a balance between inward radial diffusion due to Ultra Low Frequency (ULF) electromagnetic fluctuations and pitch angle scattering due to plasmaspheric hiss and lightning-generated whistlers. However, when inward radial diffusion due to the electrostatic fluctuations is included, agreement between the observed and simulated fluxes becomes weaker, suggesting that it is important to understand and quantify the radial diffusion rates in the slot region.

Lightning-driven inner radiation belt energy deposition into the atmosphere: implications for ionisation-levels and neutral chemistry

Abstract. Lightning-generated whistlers lead to coupling between the troposphere, the Van Allen radiation belts and the lower-ionosphere through Whistler-induced electron precipitation (WEP). Lightning produced whistlers interact with cyclotron resonant radiation belt electrons, leading to pitch-angle scattering into the bounce loss cone and precipitation into the atmosphere. Here we consider the relative significance of WEP to the lower ionosphere and atmosphere by contrasting WEP produced ionisation rate changes with those from Galactic Cosmic Radiation (GCR) and solar photoionisation. During the day, WEP is never a significant source of ionisation in the lower ionosphere for any location or altitude. At nighttime, GCR is more significant than WEP at altitudes <68 km for all locations, above which WEP starts to dominate in North America and Central Europe. Between 75 and 80 km altitude WEP becomes more significant than GCR for the majority of spatial locations at which WEP deposits energy. The size of the regions in which WEP is the most important nighttime ionisation source peaks at ~80 km, depending on the relative contributions of WEP and nighttime solar Lyman-α. We also used the Sodankylä Ion Chemistry (SIC) model to consider the atmospheric consequences of WEP, focusing on a case-study period. Previous studies have also shown that energetic particle precipitation can lead to large-scale changes in the chemical makeup of the neutral atmosphere by enhancing minor chemical species that play a key role in the ozone balance of the middle atmosphere. However, SIC modelling indicates that the neutral atmospheric changes driven by WEP are insignificant due to the short timescale of the WEP bursts. Overall we find that WEP is a significant energy input into some parts of the lower ionosphere, depending on the latitude/longitude and altitude, but does not play a significant role in the neutral chemistry of the mesosphere.

First results from the Cluster wideband plasma wave investigation

Abstract. In this report we present the first results from the Cluster wideband plasma wave investigation. The four Cluster spacecraft were successfully placed in closely spaced, high-inclination eccentric orbits around the Earth during two separate launches in July – August 2000. Each spacecraft includes a wideband plasma wave instrument designed to provide high-resolution electric and magnetic field wave-forms via both stored data and direct downlinks to the NASA Deep Space Network. Results are presented for three commonly occurring magnetospheric plasma wave phenomena: (1) whistlers, (2) chorus, and (3) auroral kilometric radiation. Lightning-generated whistlers are frequently observed when the spacecraft is inside the plasmasphere. Usually the same whistler can be detected by all spacecraft, indicating that the whistler wave packet extends over a spatial dimension at least as large as the separation distances transverse to the magnetic field, which during these observations were a few hundred km. This is what would be expected for nonducted whistler propagation. No case has been found in which a strong whistler was detected at one spacecraft, with no signal at the other spacecraft, which would indicate ducted propagation. Whistler-mode chorus emissions are also observed in the inner region of the magnetosphere. In contrast to lightning-generated whistlers, the individual chorus elements seldom show a one-to-one correspondence between the spacecraft, indicating that a typical chorus wave packet has dimensions transverse to the magnetic field of only a few hundred km or less. In one case where a good one-to-one correspondence existed, significant frequency variations were observed between the spacecraft, indicating that the frequency of the wave packet may be evolving as the wave propagates. Auroral kilometric radiation, which is an intense radio emission generated along the auroral field lines, is frequently observed over the polar regions. The frequency-time structure of this radiation usually shows a very good one-to-one correspondence between the various spacecraft. By using the microsecond timing available at the NASA Deep Space Net-work, very-long-baseline radio astronomy techniques have been used to determine the source of the auroral kilometric radiation. One event analyzed using this technique shows a very good correspondence between the inferred source location, which is assumed to be at the electron cyclotron frequency, and a bright spot in the aurora along the magnetic field line through the source.Key words. Ionosphere (wave-particle interactions; wave propagation) – Magnetospheric physics (plasma waves and instabilities; instruments and techniques)

Aspects of wave-particle interactions at mid-latitudes

Arguably the most significant source of particle precipitation into the ionosphere at mid-latitudes ( L ∼2–3), at least for electrons in the tens to hundreds of keV energy range, is that which arises from the interaction between trapped particles and whistler-mode waves in the magnetosphere. Quasisteady plasmaspheric hiss has long been postulated as a major class of waves contributing to the loss of radiation belt particles but recent studies have suggested that under some conditions, impulsive precipitation caused by interactions with lightning-generated whistlers may be equally important as a loss process. Such lightning-induced electron precipitation (LEP) and its ionospheric signature is the subject of this paper. Although LEP may be detected and studied by a variety of ground-based, balloon-borne and satellite-borne sensors, through optical emissions, X-ray production, enhanced conductivity, or the direct measurement of the precipitating particles themselves, a technique using ground-based narrow-band VLF receivers to measure the Trimpi effect (the transient perturbations in amplitude and/or phase of received narrow-band VLF transmissions) caused by LEP-associated ionisation enhancements has become increasingly popular due to its simple instrumentation and wide field of view. Most work has concentrated on the 2 < L < 3 region where typicalwhistler spectra, trapped electron energy distributions and magnetospheric plasma densities and magnetic field strengths are most favourable for the Trimpi effect. In order to use the technique to study in detail the characteristics and distribution of LEP (and its importance as a trapped-particle loss mechanism), using a network of intersecting transmitter-receiver great-circle paths (TRGCPs), a consensus on how to interpret the observational data is crucial, though this has recently been the subject of controversy. Whilst most studies suggest that only LEP within ∼200 km of the TRGCP gives rise to an observable Trimpi event, some have suggested that non-gaussian perturbations well off the TRGCP can be important. In this paper, the current state of LEP research is summarised and some recent results are presented, using data from a network of both widely and closely spaced observing sites in Antarctica.

Observation at Moshiri(L=1.6)of Whistler-Triggered VLF Emissions in the Electron Slot and Inner Radiation Belt Regions.

The morphological characteristics of whistler-triggered VLF emissions including diurnal, seasonal variations, Kp dependence, latitudinal distribution and spectral shape, were investigated based on the VLF data obtained at Moshiri (L=1.6) in Japan during a ten year span (1976 to 1985). The following results emerged; (1) There is no clear tendency for whistler-triggered emissions to occur at a particular local time, (2) an equinoctial maximum in occurrence probability is recognized, (3) the occurrence probability increases with increasing Kp in the range from 3 to 7, (4) the occurrence L shell is localized in two regions; one is L=2.1 to 3.4 (the electron slot region) and the other is just around L=1.6 (the inner radiation belt), and (5) a whistler-triggered emission is characterized by an initial component quasi-constant frequency and a subsequent large frequency drift with df/dt=10-20kHz/s. These characteristics are satisfactorily interpreted in terms of the gyroresonance interaction between lightning-generated whistlers and energetic electrons, with reference to previous results on lightning-induced particle precipitation.

Whistlers in Neptune's magnetosphere: Evidence of atmospheric lightning

During the Voyager 2 flyby of Neptune a series of 16 whistlerlike events were detected by the plasma wave instrument near closest approach. These events were observed at radial distances from 1.30 to 1.99 RN and magnetic latitudes from −7° to 33°. The frequencies ranged from 6.1 to 12.0 kHz, and the dispersions fit the Eckersley law for lightning‐generated whistlers. Lightning in the atmosphere of Neptune is the only known source of such signals. The frequency range of the whistlers (up to 12 kHz) indicates that the local electron densities are substantially higher (Ne &gt; 30 to 100 cm−3) than indicated by the in situ plasma measurements. The dispersion of the whistlers is very large, typically 26,000 s Hz1/2. On the basis of existing plasma density models and measurements, the dispersions are too large to be accounted for by a single direct path from the lightning source to the spacecraft. Therefore multiple bounces from one hemisphere to the other are required. The most likely propagation path probably involves a lighting source on the dayside of the planet, with repeated bounces through the dense dayside ionosphere at low L values.