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Nonlinear processes associated with the generation process of whistler-mode chorus emissions are summarized. The nonlinear dynamics of energetic electrons interacting with a coherent whistler-mode wave and the formation of electromagnetic electron holes or hills in the velocity phase space are described. The condition for resonant electrons to be free from the anomalous trapping at low pitch angles is obtained. In the presence of the inhomogeneity due to the frequency variation and the gradient of the magnetic field, the electron holes or hills result in resonant currents generating rising-tone emissions or falling-tone emissions, respectively. After formation of a coherent wave at a frequency of the maximum linear growth rate, triggering of the nonlinear wave growth takes place when the wave amplitude is above the threshold amplitude. The wave grows to a level close to the optimum wave amplitude as an absolute instability near the magnetic equator. The nonlinear growth rate at a position away from the equator is derived for a subtracted Maxwellian momentum distribution function with correction to the formulas in the past publications. The triggering process is repeated sequentially at progressively higher frequencies in the case of a rising-tone emission, generating subpackets forming a chorus element. With a higher plasma density as in the plasmasphere, the triggering of subpackets takes place concurrently over a wide range of frequency forming discrete hiss elements with varying frequencies. The mechanism of nonlinear wave damping due to quasi-parallel propagation from the equator is presented, which results in the formation of a gap at half the electron cyclotron frequency, separating a long rising-tone chorus emission into the upper-band and lower-band chorus emissions. The theoretical formulation of an oblique whistler mode wave and its interaction with energetic electrons at the n-th resonance is also presented along with derivation of the inhomogeneity factor.

We conduct electromagnetic particle simulations in a uniform magnetic environment to verify the nonlinear wave growth process of plasmaspheric hiss in the equatorial plasmasphere. The satisfaction of the separability criterion for coexisting multiple frequency waves in the initial stage of wavenumber-time evolution declares that wave packets are coherent and capable of growing nonlinearly. Spatial and temporal evolutions of two typical modes located in wavenumber-time evolution demonstrate the consistency among wave growths, frequency variations, and inhomogeneity factor S in coherent wave packets, showing that rising and falling tones occur at negative and positive S values, respectively, and an obvious wave growth happens in a reasonable range of S satisfying the second-order resonance condition. Wave packets extracted from wave fields in space and time by a frequency band-pass filter confirm a good agreement between the nonlinear theory and simulation results. The nonlinear growth rates of the extracted wave packets possess similar magnitudes to the growth rates of wave packets in the simulation, and they are much greater than the theoretical linear growth rate, indicating that the nonlinear process is essential in the generation of plasmaspheric hiss.

In the Outer Radiation Belt, the acceleration and loss of high‐energy electrons is largely controlled by wave‐particle interactions. Quasilinear diffusion coefficients are an efficient way to capture the small‐scale physics of wave‐particle interactions due to magnetospheric wave modes such as plasmaspheric hiss. The strength of quasilinear diffusion coefficients as a function of energy and pitch‐angle depends on both wave parameters and plasma parameters such as ambient magnetic field strength, plasma number density and composition. For plasmaspheric hiss in the magnetosphere, observations indicate large variations in the wave intensity and wavenormal angle, but less is known about the simultaneous variability of the magnetic field and number density. We use in‐situ measurements from the Van Allen Probe mission to demonstrate the variability of selected factors that control the size and shape of pitch‐angle diffusion coefficients: wave intensity, magnetic field strength and electron number density. We then compare with the variability of diffusion coefficients calculated individually from co‐located and simultaneous groups of measurements. We show that the distribution of the plasmaspheric hiss diffusion coefficients is highly non‐Gaussian with large variance, and that the distributions themselves vary strongly across the three phase‐space bins studied. In most bins studied, the plasmaspheric hiss diffusion coefficients tend to increase with geomagnetic activity, but our results indicate that new approaches that include natural variability may yield improved parameterizations. We suggest methods like stochastic parameterization of wave‐particle interactions could use variability information to improve modelling of the Outer Radiation Belt.

Whistler mode waves are a right-hand circularly polarized electromagnetic waves in the very low frequency (VLF) range, which can be excited and propagate in the region through the Earth's atmosphere to the magnetosphere. Typical examples of whistler mode waves include chorus, plasmaspheric hiss, lightning generated whistler (LGW), VLF waves ejected by ground transmitters. The whistler mode wave can interact with energetic electrons and plays an important role in both electron loss and acceleration in the magnetosphere.

The Earth's inner magnetosphere is populated by a host of high frequency plasma waves which, via wave-particle interactions, can shape the dynamics of the terrestrial radiation belts. One of these is an incoherent whistler-mode plasma wave commonly referred to as hiss. Hiss is nearly always present in the inner magnetosphere, acting to diffusively scatter electrons into the atmosphere. While hiss waves are often low amplitude, their persistence allows them to wear away at the radiation belts, carving out new slot regions over days to weeks.

The geomagnetic space environment is a natural laboratory for rich nonlinear plasma physics. The auroral zone ionosphere and magnetosphere of the Earth are replete with plasma waves and radiations. The energetic electrons precipitating to the auroral zone naturally excite various waves in the magnetosphere as well as in the ionosphere, which can be detected on the ground or by in situ spacecraft. Energetic electrons trapped in the dipolar field of the magnetosphere also excite naturally occurring plasma waves. One may use these radio emissions as an effective tool for diagnosing complicated plasma processes occurring in the natural environment. Among the most well-known electromagnetic radio emission processes in the tens to hundreds kHz frequency range is the terrestrial kilometric radiation (also known as the Auroral Kilometric Radiation or AKR for short) [1] , [2] . Terrestrial continuum radiation is also a well-known example [3] , [4] . Other types of naturally occurring auroral radio waves in the medium frequency range have also been identified. The hiss is a broadband emission below 1 MHz, medium frequency (MF) burst is a broadband emission at 1.5-4.5 MHz, and auroral roar, which is a narrowband emission near cyclotron harmonics [5] . Besides these nonlinear emission mechanisms may also be operating. For instance, recently it was proposed that a radiation emission mechanism similar to the plasma emission process, which is relevant for solar radio bursts, may be operative near the reconnection region [6] . The present paper will discuss kinetic theories of these radio emission processes that may be relevant to the naturally occurring radio emission phenomena in space plasma environment.

An amendment to this paper has been published and can be accessed via the original article.

Equatorial magnetosonic waves (EMS), together with chorus and plasmaspheric hiss, play key roles in the dynamics of energetic electron fluxes in the magnetosphere. Numerical models, developed following a first principles approach, that are used to study the evolution of high energy electron fluxes are mainly based on quasilinear diffusion. The application of such numerical codes requires statistical models for the distribution of key magnetospheric wave modes to estimate the appropriate diffusion coefficients. These waves are generally statistically modeled as a function of spatial location and geomagnetic indices (e.g., AE, Kp, or Dst). This study presents a novel dynamic spatiotemporal model for EMS wave amplitude, developed using the Nonlinear AutoRegressive Moving Average eXogenous machine learning approach. The EMS wave amplitude, measured by the Van Allen Probes, are modeled using the time lags of the solar wind and geomagnetic indices as inputs as well as the location at which the measurement is made. The resulting model performance is assessed on a separate Van Allen Probes data set, where the prediction efficiency was found to be 34.0% and the correlation coefficient was 56.9%. With more training and validation data the performance metrics could potentially be improved, however, it is also possible that the EMS wave distribution is affected by stochastic factors and the performance metrics obtained for this model are close to the potential maximum.

A numerical model of auroral hiss propagation from the region of its generation to the ground surface is developed for the interpretation of results from ground-based high-latitudinal VLF observations. The model includes modules describing the statistical properties of electrostatic whistler waves generated due to Cerenkov resonance at the heights of 6000–20 000 km, the propagation of these waves in the magnetosphere to the region of upper ionosphere (under 5000 km), which is filled with small-scale irregularities of electron concentration, the scattering of electrostatic waves from these irregularities into the transition cone, and further propagation of the waves through the lower ionosphere down to the ground surface. The modeling results agree with the observations.

We investigate the statistical distribution of energetic electron precipitation from the equatorial magnetosphere due to hiss waves in the plasmasphere and plumes. Using Van Allen Probes measurements, we calculate the pitch angle diffusion coefficients at the pitch angle of bounce loss cone, and evaluate the energy spectrum of precipitating electron flux using quasi-linear theory. Our ~6.5 years survey shows that, during disturbed times, the plasmaspheric hiss mostly causes the electron precipitation at L > 3 near the dayside in the plasmasphere, and hiss waves in plume cause the precipitation at L > 5 near dayside and L > 3.5 near the dusk side. The precipitating energy flux increases with increasing geomagnetic index, and is typically higher in the plasmaspheric plume than the plasmasphere. The characteristic energy of precipitation increases from ~20 keV at L = 6 to ~100 keV at L = 3, potentially causing the loss of electrons at several hundred keV. Although the total precipitating energy flux due to hiss waves is generally lower than the precipitation due to whistler mode chorus waves, the characteristic energy of precipitation due to hiss is higher, and the precipitation extends closer to the Earth.

The spatial scales of whistler-mode waves, determined by their generation process, propagation, and damping, are important for assessing the scaling and efficiency of wave-particle interactions affecting the dynamics of the radiation belts. We use multi-point wave measurements in 2013-2019 by two identically equipped Van Allen Probes spacecraft covering all MLTs at L=2-6 near the geomagnetic equator to investigate the spatial extent of active regions of chorus and hiss waves, their wave amplitude distribution in the source/generation region, and the scales of chorus wave packets, employing a time-domain correlation technique to the spacecraft approaches closer than 1000 km, which happened every 70 days in 2012-2018 and every 35 days in 2018-2019. The correlation of chorus wave power dynamics using two spacecraft measurements is found to remain significant up to inter-spacecraft separations of 400 km to 750 km transverse to the background magnetic field direction, consistent with previous estimates of the chorus wave packet extent, but indicating the likely presence of two different scales of about 400 km and 750 km. Our results further suggest that the chorus source region can be slightly asymmetrical, more elongated in either the azimuthal or radial direction, which could also explain the aforementioned two different scales. An analysis of average chorus and hiss wave amplitudes at separate locations similarly reveals different radial and azimuthal extents of the corresponding wave active regions, complementing previous results based on THEMIS spacecraft statistics mainly at larger L>6. Both the chorus source region scale and the chorus active region size appear smaller inside the outer radiation belt (at L< 6) than at higher L-shells.

Plasmaspheric hiss waves is important in the radiation belt. Previous papers have shown that considering the variability of wave parameters will improve the effectiveness of modeling wave-particle interactions in the Radiation Belt, but less is known about how rapidly (and by how much) wave characteristics vary. We use measurements from the Van Allen Probe mission to study the correlation and ratio of wave amplitudes over a range of frequencies covering the plasmaspheric hiss band as a function of separation and time delay between two satellites. A total of 1851 events with small separation (E) were found. The statistical results show that, as separation between spacecraft increase, the characteristics of hiss change in both correlation of the wavepacket and amplitude. Moreover, we find that the coherence between spacecraft is strong at low-L, and decreases strongly with increasing L. We investigate the coherence of plasmaspheric hiss on geomagnetic indices and solar wind driving. We discuss the ramifications of our results with relevance to understanding the global characteristics of plasmaspheric hiss waves.

The scattering of electrons via plasmaspheric hiss whistler-mode plasma waves has profound consequences for the dynamics of electrons in the inner terrestrial magnetosphere, including the radiation...

Very low frequency (VLF) auroral hiss at Kannuslehto (KAN), Finland, was analyzed with reference to the progress of 98 isolated magnetic substorms measured during the winter months of 2015–2018. Of these, 91 were accompanied by auroral hiss during the substorm growth phase. No auroral hiss was recorded during the expansion and recovery phases. We found that auroral hiss was observed under rising polar cap (PC) index, showing an increased solar wind energy input into the magnetosphere during the substorm growth phase. We also found that in 58 of the 65 events studied, KAN was located in the vicinity of enhanced field‐aligned currents (FACs) during auroral hiss occurrence. For the first time, it was established that auroral VLF hiss generation in the equatorial part of the auroral oval is a typical signature of a substorm growth phase. Plain Language Summary Auroral hiss is a well‐known type of nighttime natural VLF emission with a noise‐like structure generated by the Cherenkov instability of precipitating soft electrons above the ionosphere. Auroral hiss occurrence up to 39 kHz was studied in the equatorward region of the auroral oval at the Finnish station Kannuslehto (KAN, MLAT = 64.2°N) during 11 winter months in 2015–2018. During this time interval, 98 isolated and rather powerful magnetic substorms were recorded over Scandinavia. In 93% of the substorms studied, an auroral VLF hiss was recorded at the same time as enhancement of field‐aligned currents (FACs). FACs are caused by soft electron precipitation which could be a plausible source of the auroral VLF hiss generation. For the first time, it was found that auroral VLF hiss occurrence in the equatorward region of the auroral oval is a typical signature of the substorm growth phase.

Exohiss is a low-frequency structureless whistler-mode emission potentially contributing to the precipitation loss of radiation belt electrons outside the plasmasphere. Exohiss is usually considered the plasmaspheric hiss leaked out of the dayside plasmapause. However, the evolution of exohiss after the leakage has not been fully understood. Here we report the prompt enhancements of exohiss waves following substorm injections observed by Van Allen Probes. Within several minutes, the energetic electron fluxes around 100 keV were enhanced by up to 5 times, accompanied by an up to 10-time increase of the exohiss wave power. These substorm-injected electrons are shown to produce a new peak of linear growth rate in the exohiss band (< 0.1fce). The corresponding path-integrated growth rate of wave power within 10° latitude of the magnetic equatorial plane can reach 13.4, approximately explaining the observed enhancement of exohiss waves. These observations and simulations suggest that the substorm-injected energetic electrons could amplify the preexisting exohiss waves.

A review of the impact of the cold-ion and cold-electron populations in the Earth’s magnetosphere is presented. The cold populations are defined by total energy less than approximately 100 eV, i.e. in the energy range which is strongly affected by spacecraft charging and that often dominates the total plasma density. We also include the warm plasma cloak in the review, since it overlaps partially with the cold energy range and is a population that is still not well understood. The known impacts of cold ions and cold electrons that are discussed are: the source of hot magnetospheric plasma, solar-wind/magnetosphere coupling, magnetotail reconnection and substorms, Kelvin–Helmholtz instabilities on the magnetopause, chorus, hiss, electromagnetic-ion-cyclotron and ultra-low-frequency wave–particle interactions, aurora structuring and spacecraft charging. Other possible impacts are associated with refilling on open-drift trajectories, the remnant layer and plasmapause disruption. A discussion of the difficulty of cold-plasma measurements and the need for new measurement techniques that measure the full cold-ion and cold-electron distribution functions is also presented. There remain a lot of unknowns about the cold-ion and cold-electron populations, associated with their origin, properties, drivers and impacts. These populations will need to be fully understood before the magnetosphere–ionosphere system can be fully understood.

Plasmaspheric hiss is an important wave mode for the dynamics of inner terrestrial magnetosphere plasma populations. It acts to scatter high energy electrons out of trapped orbits about Earth and into the atmosphere, defining the inner edge of the radiation belts over a range of energies. A low-frequency component of hiss was recently identified and is important for its ability to interact with higher energy electrons compared to typically considered hiss frequencies. This study compares the statistical properties of low and high frequency plasmaspheric hiss in the terrestrial magnetosphere, demonstrating that they are statistically distinct wave populations. Low frequency hiss shows different behavior in frequency space, different spatial localization (in magnetic local time and radial distance), and different amplitude distributions compared to high frequency hiss. The observed statistical properties of low frequency hiss are found to be consistent with recently developed theories for low frequency hiss generation. The results presented here suggest that careful consideration of low frequency hiss properties can be important for accurate inclusion of this wave population in predictive models of inner magnetosphere plasma dynamics.

The Juno Waves instrument detected a new broadband plasma wave emission (~50 Hz to 40 kHz) on 27 August 2016 as the spacecraft passed over the low-altitude polar regions of Jupiter. We investigated the characteristics of this emission and found similarities to whistler mode auroral hiss observed at Earth, including a funnel-shaped frequency-time feature. The electron cyclotron frequency is much higher than both the emission frequency and local plasma frequency, which is assumed to be ~20–40 kHz. The E/cB ratio was about three near the start of the event and then decreased to one for the rest of the period. A correlation of the electric field spectral density with the flux of an upgoing 20 to 800 keV electron beam was found, with a correlation coefficient of 0.59. We conclude that the emission is propagating in the whistler mode and is driven by the energetic upgoing electron beam.

A study of dayside plasmaspheric hiss at frequencies from ~22 Hz to ~1.0 kHz was carried out using one year of Polar data. It is shown that intense, dayside plasmaspheric hiss is correlated with solar wind pressure with P > 2.5 nPa. The dayside effect is most prominent in the ~300 to ~650 Hz range. Intense dayside waves are also present during SYM-H

Hiss is the name given to the collection of incoherent whistler-mode waves that populate the near-Earth region in and around the plasmasphere, a cold plasma torus about Earth. Hiss waves scatter electrons in pitch angle over a broad energy range, often causing them to be lost from the inner magnetosphere and deposited into the upper atmosphere. The persistent electron scattering and loss driven by hiss waves is important to the dynamics of the inner magnetosphere and radiation belts. These waves determine the shape of the Earthward edge of the radiation belts and are largely responsible for the formation of the gap between electron radiation belts (the slot region).

In 1979, after plasma envelope exploration, Voyager 1 and 2 revealed that Jovian magnetosphere consists of an unusual mixture of ions like hydrogen, sulphur, oxygen etc., in similar proportions. The present study observed that the waves in Jovian magnetosphere propagate in whistler-mode, with some similarities to whistler-mode auroral hiss in the Earth’s magnetosphere. The dispersion relation has been deduced and calculated in detail for oblique propagating waves in presence of parallel AC electric field for bi-Maxwellian distribution function. Magnetic field model for different values of latitude at radial distance $17 R_{J}$ has been reported. By using the method of characteristic solution, relativistic growth rate has been calculated. Data provided by spacecrafts like Pioneer 10 and 11, Voyager 1 and 2, while exploring the magnetosphere of Jupiter, has been used to plot graphs of variation of growth rate for different values of various plasma parameters like temperature anisotropy, angle of wave propagation, AC frequency etc. The effect on growth rate by these plasma parameters is shown by graphs.

Saturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short-lived ∼1-hr quasiperiodic high-energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time-resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low-frequency waves and instead directly caused by recurrent small-scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent-back to a more dipolar configuration. This manifests as a sawtooth-shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.

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.

In this paper, using the Polar Orbiting Environment satellites (POES) in the year of 2011, we present global distributions of energetic electron precipitation (EEP) events that may driven by lower band chorus waves. Since the footprint of plasmapause in the ionospheric height can basically equal to mid-latitude trough minimum, it can be identified through the global total electron content (TEC) maps. Then we distinguish events perhaps driven by chorus waves outside the plasmapause or those driven by hiss waves inside the plasmapause. Based on the simultaneous observations of EEP in the E1 0o (>30 keV) and E2 0o (>100 keV) channels from POES satellites, a total of 4455 potentially chorus-driven events are identified. The potentially chorus-driven events are mainly distributed from midnight to noon which is similar to the distribution of lower band chorus waves. As the level of geomagnetic substorm activity increase, the occurrence rate is higher, which could be due to excitation of chorus waves associated with substorm electron injection. During higher level of substorm, a large number of events occur in lower L ~ shells. Besides, since the magnetosphere in the dayside is compressed and strong chorus waves are limited to the region where the ratio between the plasma frequency and electron gyro-frequency is less than 5, under the strong substorm, the events in the nightside are confined to lower L ~ shells due to smaller electron gyro-frequencies relative to those in the dayside. The occurrence rate of the events in the dayside also increase with enhancement of solar wind dynamic pressure, which suggests that the solar wind dynamic pressure can contribute to the excitation of events in the dayside. The statistics of potentially chorus-driven events are helpful to analyze the distribution of lower band chorus waves and their contributions to the loss of energetic electrons in the inner magnetosphere.

Previous studies have shown that low-energy ion heating occurs in the magnetosphere due to strong equatorial noise emission. Observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument recently determined that there was a depletion in the 1–10 eV ion population in the postmidnight sector of Earth during quiet times at L < 3. The diurnal variation of equatorially mirroring 1–10 eV H+ ions at 2 < L < 3 is connected with similar diurnal variation in the electric field component of plasma waves ranging between 150 and 600 Hz. Measurements from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) data set are used to analyze waves of this frequency in near-Earth space. However, when we examine the polarization of the waves in the 150 to 600 Hz range in the equatorial plane, the majority are right-hand polarized plasmaspheric hiss waves. The 1–10 eV H+ equatorially mirroring population does not interact with right-hand waves, despite a strong statistical relationship suggesting that the two are linked. We present evidence supporting the relationship, both in our own work and the literature, but we ultimately conclude that the 1–10 eV H+ heating is not related to the strong enhancement of 150 to 600 Hz waves.

In this work, Van Allen Probes data are used to derive terrestrial plasmaspheric hiss wave power distributions organized by (1) distance away from the plasmapause and (2) plasmapause distance from Earth. This approach is in contrast to the traditional organization of hiss wave power by L parameter and geomagnetic activity. Plasmapause-sorting reveals previously unreported and highly repeatable features of the hiss wave power distribution, including a regular spatial distribution of hiss power with respect to the plasmapause, a standoff distance between peak hiss power and the plasmapause, and frequency-dependent spatial localization of hiss. Identification and quantification of these features can provide insight into hiss generation and propagation and will facilitate improved parameterization of hiss wave power in predictive simulations of inner magnetosphere dynamics.

The in-situ exploration of the magnetospheres of Jupiter and Saturn has revealed different periodic processes. In particular, in the Saturnian magnetosphere, several studies have reported pulsations in the outer magnetosphere with a periodicity of about 1 h in the measurements of charged particle fluxes, plasma wave, magnetic field strength and auroral emissions brightness. The Low-Energy Magnetospheric Measurement System detector of the Magnetospheric Imaging Instrument (MIMI/LEMMS) on board Cassini regularly detects 1-hour quasi-periodic enhancements in the intensities of electrons with an energy range from a hundred keV to several MeV. We extend an earlier survey of these relativistic electron injections using 10 years of LEMMS observations in addition to context measurements by several other Cassini magnetospheric experiments. The one-year extension of the data and a different method of detection of the injections do not lead to a discrepancy with the results of the previous survey, indicating an absence of a long-term temporal evolution of this phenomenon. We identified 720 pulsed events in the outer magnetosphere over a wide range of latitudes and local times, revealing that this phenomenon is common and frequent in Saturn’s magnetosphere. However, the distribution of the injection events presents a strong local time asymmetry with ten times more events in the duskside than in the dawnside. In addition to the study of their topology, we present a first statistical analysis of the pulsed events properties. The morphology of the pulsations shows a weak local time dependence which could imply a high-latitude acceleration source. We provide some clues that the electron population associated with this pulsed phenomenon is distinct from the field-aligned electron beams previously observed in Saturn’s magnetosphere, but both populations can be mixed. We have also investigated the signatures of each electron injection event in the observations acquired by the Radio and Plasma Wave Science (RPWS) instrument and the magnetometer (MAG). Correlated pulsed signatures are observed in the plasma wave emissions, especially in the auroral hiss, for 12% of the electron injections identified in the LEMMS data. Additionally, in about 20% of the events, such coincident pulsed signatures have been also observed in the magnetic field measurements, some of them being indicative of field-aligned currents. This analysis combined with the multi-instrument approach sets constraints on the origin and significance of the pulsed events. Hence, our results suggest that the acceleration process providing the quasi-periodic relativistic electrons takes place at high-latitudes.

The radial and local diffusion processes induced by various plasma waves govern the highly energetic electron dynamics in the Earth's radiation belts, causing distinct characteristics in electron distributions at various energies. In this study, we present our simulation results of the energetic electron evolution during a geomagnetic storm using the University of California, Los Angeles 3-D diffusion code. Following the plasma sheet electron injections, the electrons at different energy bands detected by the Magnetic Electron Ion Spectrometer (MagEIS) and Relativistic Electron Proton Telescope (REPT) instruments on board the Van Allen Probes exhibit a rapid enhancement followed by a slow diffusive movement in differential energy fluxes, and the radial extent to which electrons can penetrate into depends on energy with closer penetration toward the Earth at lower energies than higher energies. We incorporate radial diffusion, local acceleration, and loss processes due to whistler mode wave observations to perform a 3-D diffusion simulation. Our simulation results demonstrate that chorus waves cause electron flux increase by more than 1 order of magnitude during the first 18 h, and the subsequent radial extents of the energetic electrons during the storm recovery phase are determined by the coupled radial diffusion and the pitch angle scattering by EMIC waves and plasmaspheric hiss. The radial diffusion caused by ULF waves and local plasma wave scattering are energy dependent, which lead to the observed electron flux variations with energy dependences. This study suggests that plasma wave distributions in the inner magnetosphere are crucial for the energy-dependent intrusions of several hundred keV to several MeV electrons.

The spatial–temporal variations in aurora and VLF emissions during an weak intensification in the auroral zone morning sector on December 30, 2011, have been analyzed. The event was accompanied by a negative bay (~70 nT) in the X component of the magnetic field at ground stations in northern Scandinavia. At the recovery phase of this bay, the precipitation zone moved and VLF emission frequency simultaneously increased over ten minutes, which may indicate that waves and precipitating electrons had a common source. VLF noise bursts in the 600–1000 Hz band with a characteristic modulation scale of ~10 s and the corresponding aurora intensifications localized in the ~100 km region were observed during the following ten minutes, which also confirms that recorded waves are related to electron precipitation. This correspondence of the pulsating aurora periods and VLF noise modulation has been revealed for the first time. The role of VLF wave generation processes during the cyclotron interaction with electrons in the magnetosphere and the propagation of these waves from the magnetosphere to the observation point are discussed.

Over the course of about 6 h on Day 129, 2008, the UV imaging spectrograph (UVIS) on the Cassini spacecraft observed a repeated intensification and broadening of the high latitude auroral oval into the polar cap. This feature repeated at least 5 times with about a 1 h period, as it rotated in the direction of corotation, somewhat below the planetary rotation rate, such that it moved from noon to post-dusk, and from roughly 77° to 82° northern latitudes during the observing interval. The recurring UV observation was accompanied by pronounced ∼1 h pulsations in auroral hiss power, magnetic perturbations consistent with small-scale field aligned currents, and energetic ion conics and electrons beaming upward parallel to the local magnetic field at the spacecraft location. The magnetic field and particle events are in phase with the auroral hiss pulsation. This event, taken in the context of the more thoroughly documented auroral hiss and particle signatures (seen on many high latitude Cassini orbits), sheds light on the possible driving mechanisms, the most likely of which are magnetopause reconnection and/or Kelvin Helmholtz waves.

Whistler mode waves play a major role in regulating the lifetime of trapped electrons in the Earth&rsquo, 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&ndash, 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.

The STAFF-DWP wave instrument on board the equatorial spacecraft (TC1) of the Double Star Project consists of a combination of 2 instruments which are a heritage of the Cluster mission: the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment and the Digital Wave-Processing experiment (DWP). On DSP-TC1 STAFF consists of a three-axis search coil magnetometer, used to measure magnetic fluctuations at frequencies up to 4 kHz and a waveform unit, up to 10 Hz, plus snapshots up to 180 Hz. DWP provides several onboard analysis tools: a complex FFT to fully characterise electromagnetic waves in the frequency range 10 Hz-4 kHz, a particle correlator linked to the PEACE electron experiment, and compression of the STAFF waveform data. The complementary Cluster and TC1 orbits, together with the similarity of the instruments, permits new multi-point studies. The first results show the capabilities of the experiment, with examples in the different regions of the magnetosphere-solar wind system that have been encountered by DSP-TC1 at the beginning of its operational phase. An overview of the different kinds of electromagnetic waves observed on the dayside from perigee to apogee is given, including the different whistler mode waves (hiss, chorus, lion roars) and broad-band ULF emissions. The polarisation and propagation characteristics of intense waves in the vicinity of a bow shock crossing are analysed using the dedicated PRASSADCO tool, giving results compatible with previous studies: the broad-band ULF waves consist of a superimposition of different wave modes, whereas the magnetosheath lion roars are right-handed and propagate close to the magnetic field. An example of a combined Cluster DSP-TC1 magnetopause crossing is given. This first case study shows that the ULF wave power intensity is higher at low latitude (DSP) than at high latitude (Cluster). On the nightside in the tail, a first wave event comparison - in a rather quiet time interval - is shown. It opens the doors to future studies, such as event timing during substorms, to possibly determine their onset location.

International audience; We present a global model of plasmaspheric hiss, using data from eight satellites, extending the coverage and improving the statistics of existing models. We use geomagnetic activity dependent templates to separate plasmaspheric hiss from chorus. In the region 22–14 magnetic local time (MLT) the boundary between plasmaspheric hiss and chorus moves to lower values with increasing geomagnetic activity. The average wave intensity of plasmaspheric hiss is largest on the dayside and increases with increasing geomagnetic activity from midnight through dawn to dusk. Plasmaspheric hiss is most intense and spatially extended in the 200 to 500 Hz frequency band during active conditions, 400

Whistler mode waves play an important role in space weather dynamics. Plasmaspheric hiss and chorus are types of whistler mode waves prevalent in the magnetosphere. The source location of whistler mode wave energy has been a topic of research for nearly half a century. We present results of warm plasma whistler mode raytracing considering three source locations at $L=2$, 3.8 and 5 in the magnetosphere. Comparison of simulation results with frequency normalized observations from the Van Allen Probes spacecraft suggest that sources within the plasmasphere (L∼2) provide a significant contribution to the observed whistler mode wave power distribution.

The Juno spacecraft successfully entered Jupiter orbit on 5 July 2016. One of Juno’s primary objectives is to explore Jupiter’s polar magnetosphere for the first time. An obvious major aspect of this exploration includes remote and in-situ observations of Jupiter’s auroras and the processes responsible for them. To this end, Juno carries a suite of particle, field, and remote sensing instruments. One of these instruments is a radio and plasma wave instrument called Waves, designed to detect one electric field component of waves in the frequency range of 50 Hz to 41 MHz and one magnetic field component of waves in the range of 50 Hz to 20 kHz. Juno’s first perijove pass with science observations occurred on 27 August 2016. This paper presents some of the first observations of the Juno Waves instrument made during that first perijove. Among radio emissions, kilometric, hectometric, and decametric emissions were observed. From a vantage point at high latitudes, many of Jupiter’s auroral radio emissions appear as V-shaped emissions in frequency–time space with vertices near the electron cyclotron frequency where the emissions intensify. In fact, we present observations suggesting Juno flew through or close to as many as five or six sources of auroral radio emissions during its first perijove. Waves made in-situ observations of plasma waves on auroral field lines such as whistler–mode hiss, a common feature of terrestrial auroral regions. We also discuss observations of dust at the equator and lightning whistlers observed over mid-latitudes.

This chapter presents a neural-network-based technique that allows for the reconstruction of the global, time-varying distribution of some physical quantity Q, that has been sparsely sampled at various locations within the magnetosphere, and at different times. We begin with a general introduction to the problem of prediction and specification, and why it is important and difficult to achieve with existing methods. We then provide a basic introduction to neural networks, and describe our technique using the specific example of reconstructing the electron plasma density in the Earth’s inner magnetosphere on the equatorial plane. We then show more advanced uses of the technique, including 3D reconstruction of the plasma density, specification of chorus and hiss waves, and energetic particle fluxes. We summarize and conclude with a general discussion of how machine learning techniques might be used to advance the state-of-the-art in space weather prediction, and insight discovery.

We present the first statistical analysis of ELF/VLF emissions observed on the ground at subauroral latitudes that includes their features, occurrences, and association with solar wind and geomagnetic variations. Using a 100 kHz sampling loop antenna located in Athabasca, Canada (54.60°N, 246.36°E, L = 4.3), we monitored these emissions, including chorus, quasiperiodic emissions, and hiss, from November 2012 to October 2013. We found a maximum occurrence rate in the morning sector (06–07 MLT, magnetic local time) and a minimum in the night sector (∼18 to 02 MLT), in agreement with previous satellite measurements in the inner magnetosphere. We also found correlation between the ongoing substorm and storm activity and the increase of occurrence rates. The observed waves usually had a central frequency ∼1–3 kHz lower than the half-gyrofrequency at the conjugate equatorial plane, indicating a wave source at higher latitudes. A superposed epoch analysis showed that the starting time of the ELF/VLF emissions is preceded by a rise in AE both on short (hours) and long (days) terms. Solar wind speed also started slowly rising ∼1.5 days before, while density and dynamic pressure decreased shortly afterward. This may signify that high-speed solar wind conditions also contribute to the generation of ELF/VLF emissions detected at subauroral latitudes.

We investigate a non-typical very low frequency (VLF) 1–4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency–time domain, each one lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, &varphi; = 67.74° N, λ = 26.27° E; L ∼ 5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2–3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8–1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth–ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron–cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1–10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF–VLF (ultra low frequency–very low frequency) wave interaction and the ionosphere waveguide propagation.

©2015. American Geophysical Union. All Rights Reserved. A novel technique capable of inferring wave amplitudes from low-altitude electron measurements from the Polar Operational Environmental Satellites (POES) spacecraft has been previously proposed to construct a global dynamic model of chorus and plasmaspheric hiss waves. In this paper we focus on plasmaspheric hiss, which is an incoherent broadband emission that plays a dominant role in the loss of energetic electrons from the inner magnetosphere. We analyze the sensitivity of the POES technique to different inputs used to infer the hiss wave amplitudes during three conjunction events with the Van Allen Probes. These amplitudes are calculated with different input models of the plasma density, wave frequency spectrum, and electron energy spectrum, and the results are compared to the wave observations from the twin Van Allen Probes. Only one parameter is varied at a time in order to isolate its effect on the output, while the two other inputs are set to the values observed by the Van Allen Probes. The results show that the predicted hiss amplitudes are most sensitive to the adopted frequency spectrum, followed by the plasma density, but they are not very sensitive to the electron energy spectrum. Moreover, the standard Gaussian representation of the wave frequency spectrum (centered at 550-Hz) peaks at frequencies that are much higher than those observed in individual cases as well as in statistical wave distributions, which produces large overestimates of the hiss wave amplitude. For this reason, a realistic statistical model of the wave frequency spectrum should be used in the POES technique to infer the plasmaspheric hiss wave intensity rather than a standard Gaussian distribution, since the former better reproduces the observed plasmaspheric hiss wave amplitudes. Key Points Inferred amplitudes are very sensitive to frequency spectrum and plasma density All estimates remain within a factor of 0.48 to 2.09 of the observed hiss waves Statistical frequency spectra should be used instead of standard Gaussian models

The features of the dynamics of solar wind and IMF parameters were studied during the initial phase of the weak magnetic storm of December 8, 2013, where a nontypical two hour lasting wide-band (∼4–10 kHz) hiss burst was recorded during VLF observations in auroral latitudes near Sodankyla observatory (L ∼ 5.5), which differed from classical auroral hiss. A similar VLF hiss burst was recorded at Russian Lovozero observatory, which is located ∼400 km to the east. In contrast to a typical auroral hiss, the VLF emissions at both points were left-polarized and arrived at the observation point from the southeast. Although the VLF hiss burst coincided in time with the development of a substorm and the appearance of zenithal bright auroras near the stations traveling north-south, the excitation of the VLF hiss apparently has no relation to the auroras. It is suggested that the VLF emissions were generated due to cyclotron instability far to the east of Scandinavia in the region of plasmapause at L ∼ 3.5, where the equatorial gyrofrequency (f He) is about 20 kHz. The generated VLF waves could be ducted in the plasmapause at frequencies lower than a half of f He, i.e., below ∼10 kHz, they arrive at the Earth’s surface near the projection of their source and propagate in the Earth-ionosphere waveguide to large distances, as left-polarized waves. A sharp increase in the solar wind dynamic pressure was noted during the hiss burst under study, which resulted in a significant contraction of the daytime magnetosphere, a shift of the plasmapause and radiation belt to lower L shells, and the development of a substorm and southward travel of auroral arcs. The VLF hiss may have been generated in the region where energetic particles of the radiation belt crossed the plasmapause. The fact that the hiss under study was generated not in the early morning sector (Scandinavian meridian), but much further to the east, could be indirectly confirmed by quasiperiodic modulation of the VLF noise intensity in the range of Pc5 geomagnetic pulsations, which were observed in this period at eastern stations but not at the Scandinavian meridian.