Rising-tone Elements Research Articles

Generation Mechanisms of Hiss‐Like Chorus in the Earth's Magnetosphere

AbstractWhistler mode chorus waves appear either as a series of discrete elements (rising or falling tones) or as banded hiss‐like emissions in the dynamic spectrogram. Although the generation of rising‐tone chorus waves has been extensively studied, few studies have focused on the generation of hiss‐like emissions. In this paper, using the one‐dimensional (1‐D) general curvilinear particle‐in‐cell (gcPIC) simulation model, we have thoroughly studied the generation mechanisms of hiss‐like emissions. Based on the spectrogram, we first classify hiss‐like emissions into four types (I, II, III, and IV). We find that the type I hiss‐like waves are linearly excited by the anisotropic electrons, while other types are excited through the nonlinear process. For the type I waves, they do not undergo a frequency chirping process and have a relatively broadband spectrum. The high proportion of hot electrons or the large temperature anisotropy of hot electrons will decrease the threshold for nonlinear growth and cause the wave amplitudes to exceed the threshold, leading to the nonlinear generation of type II hiss‐like waves. The fresh hot electrons continually injected into the system with a large drift velocity significantly reduces the time interval between the rising tone elements to form a continuous spectrum, which are the type III waves. The intense and long‐lasting hiss‐like emissions, that is, type IV, which are the most frequently observed, should be generated due to the combined effect of the rapid injection and the high proportion of hot electrons (or the large temperature anisotropy). Our findings provide a comprehensive understanding of the generation of hiss‐like chorus waves in the Earth's magnetosphere.

Short Chorus Wave Packets: Generation Within Chorus Elements, Statistics, and Consequences on Energetic Electron Precipitation.

Short and intense lower‐band chorus wave packets are ubiquitous in the Earth's outer radiation belt. In this article, we perform various Vlasov hybrid simulations, with one or two triggering waves, to study the generation of short chorus packets/subpackets inside long rising tone elements. We show that the length of the generated short wave packets is consistent with a criterion of resonance non‐overlap for two independent superposed waves, and that these chorus packets have similar characteristics as in Van Allen Probes observations. We find that short wave packets are mainly formed near the middle/end of long rising tones for moderate linear growth rates, and everywhere for stronger linear growth rates. Finally, we analyze an event characterized by Time History of Events and Macroscale Interactions during Substorms spacecraft measurements of chorus rising tones near the equator and simultaneous measurements by low altitude ELFIN CubeSats of precipitating and trapped electron fluxes in the same sector. The measured precipitating electron fluxes are well recovered by test particle simulations performed using measured plasma and wave properties. We show that short chorus wave packets of moderate amplitudes (160–250 pT) essentially lead to a more diffusive‐like transport of 50–200 keV electrons toward the loss cone than long packets. In contrast, long chorus packets are found to produce important nonlinear effects via anomalous trapping, which significantly reduces electron precipitation below 150 keV, especially for higher wave amplitudes.

Subpacket structure in strong VLF chorus rising tones: characteristics and consequences for relativistic electron acceleration

Van Allen Probes in situ observations are used to examine detailed subpacket structure observed in strong VLF (very low frequency) rising-tone chorus elements observed at the time of a rapid MeV electron energization in the inner magnetosphere. Analysis of the frequency gap between lower and upper chorus-band waves identifies fceEQ, the electron gyrofrequency in the equatorial wave generation region. Initial subpackets in these strong chorus rising-tone elements begin at a frequency near 1/4 fceEQ and exhibit smooth gradual frequency increase across their > 10 ms temporal duration. A second much stronger subpacket is seen at frequencies around the local value of 1/4 fce with small wave normal angle (< 10°) and steeply rising df/dt. Smooth frequency and phase variation across and between the initial subpackets support continuous phase trapping of resonant electrons and increased potential for MeV electron acceleration. The total energy gain for individual seed electrons with energies between 100 keV and 3 MeV ranges between 2 and 15%, in their nonlinear interaction with a single chorus element.

Discrete Rising Tone Elements of Whistler‐Mode Waves in the Vicinity of the Moon: ARTEMIS Observations

AbstractWe report on discrete rising‐tone elements of whistler‐mode waves observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) in the vicinity of the Moon. The two‐probe ARTEMIS observations suggest that a free energy source for the wave generation is provided by electron anisotropy resulting from lunar surface absorption and magnetic reflection. High time resolution dynamic spectra reveal that the waves consist of multiple rising tone elements, exhibiting striking similarities to the well‐known whistler‐mode chorus in planetary magnetospheres. The observed frequency sweep rates are generally consistent with those predicted by the nonlinear growth theory of chorus emissions by Omura et al. (2008, https://doi.org/10.1029/2007JA012622). These results imply that whistler‐mode waves can grow nonlinearly into chorus‐like emissions even around airless bodies without magnetospheres and that a well‐defined dipole field is not a prerequisite for the chorus generation.

Relativistic Electron Microbursts as High‐Energy Tail of Pulsating Aurora Electrons

AbstractIn this study, by simulating the wave‐particle interactions, we show that subrelativistic/relativistic electron microbursts form the high‐energy tail of pulsating aurora (PsA). Whistler‐mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few kiloelectron volts (PsA) to several megaelectron volts (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of subrelativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few hertz. The chorus bursts for a few seconds cause the microburst trains of subrelativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave‐particle interactions along the field line.

Direct Evidence of the Pitch Angle Scattering of Relativistic Electrons Induced by EMIC Waves

AbstractIn this study, we analyze an electromagnetic ion cyclotron (EMIC) wave event of rising tone elements recorded by the Van Allen Probes. The pitch angle distributions of relativistic electrons exhibit a direct response to the two elements of EMIC waves: at the intermediate pitch angle, the fluxes are lower; and at the low pitch angle, the fluxes are higher than those when no EMIC was observed. In particular, the observed changes in the pitch angle distributions are most likely to be caused by nonlinear wave‐particle interaction. The calculation of the minimum resonant energy and a test‐particle simulation based on the observed EMIC waves support the role of the nonlinear wave‐particle interaction in the pitch angle scattering. This study provides direct evidence for the nonlinear pitch angle scattering of electrons by EMIC waves.

Triggered Plasmaspheric Hiss: Rising Tone Structures

AbstractIn this study, a rare hiss event observed by Van Allen Probe is reported and the possible generation is investigated based on wave and plasma measurements. The results suggest that the normal hiss (from 0.05fce to 0.5fce) with dominantly equatorward Poynting fluxes is locally generated by plasma sheet electrons via cyclotron instability. The low‐frequency band (from 30 Hz to 0.05fce) with a mixture of equatorward and poleward Poynting fluxes is probably due to multiple reflections inside the plasmasphere. Such difference in the two bands is confirmed by the calculation of minimum energy of resonant electrons and local growth rate. Moreover, the analysis on the fine structures of normal hiss waves shows that besides the expected incoherent structure (below 1 kHz), several rising tone elements are measured above 1 kHz. The rising tone structures are probably triggered by the incoherent hiss part below 1 kHz, which is rarely reported before.

Observational evidence of the nonlinear wave growth theory of plasmaspheric hiss

AbstractWe test the recently developed nonlinear wave growth theory of plasmaspheric hiss against discrete rising tone elements of hiss emissions observed by the Van Allen Probes. From the phase variation of the waveforms processed by bandpass filters, we calculate the instantaneous frequencies and wave amplitudes. We obtain the theoretical relation between the wave amplitude and frequency sweep rates at the observation point by applying the convective growth rates and dispersion factors to the known relation at the equator. By plotting the theoretical relation over scatterplots of the wave amplitudes and the frequency sweep rates for rising tone elements, we find good agreement between the hiss observations and the nonlinear theory. We also find that the duration periods of the hiss elements are in good agreement with the nonlinear transition time necessary for the formation of a resonant current through coherent nonlinear wave‐particle interactions.

Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves

AbstractWe investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low‐altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified above ~3 keV. Moreover, stable precipitations at ~1 keV are found for the PsA. A “precipitation gap” is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave‐particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on‐off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at ~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.

On the origin of falling-tone chorus elements in Earth's inner magnetosphere

Abstract. Generation of extremely/very low frequency (ELF/VLF) chorus waves in Earth's inner magnetosphere has received increased attention recently because of their significance for radiation belt dynamics. Though past theoretical and numerical models have demonstrated how rising-tone chorus elements are produced, falling-tone chorus element generation has yet to be explained. Our new model proposes that weak-amplitude falling-tone chorus elements can be generated by magnetospheric reflection of rising-tone elements. Using ray tracing in a realistic plasma model of the inner magnetosphere, we demonstrate that rising-tone elements originating at the magnetic equator propagate to higher latitudes. Upon reflection there, they propagate to lower L-shells and turn into oblique falling tones of reduced power, frequency, and bandwidth relative to their progenitor rising tones. Our results are in good agreement with comprehensive statistical studies of such waves, notably using magnetic field measurements from THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. Thus, we conclude that the proposed mechanism can be responsible for the generation of weak-amplitude falling-tone chorus emissions.

Effect of the background magnetic field inhomogeneity on generation processes of whistler‐mode chorus and broadband hiss‐like emissions

By a series of self‐consistent electron hybrid code simulations, we study the effect of the background magnetic field inhomogeneity on the generation process of whistler‐mode chorus emissions. Chorus with rising tones are generated through nonlinear wave‐particle interactions occurring around the magnetic equator. The mirror force plays an important role in the nonlinear interactions, and the spatial inhomogeneity of the background magnetic field is a key parameter of the chorus generation process. We have conducted numerical experiments with different spatial inhomogeneities to understand properties of the chorus generation process. We assume the same initial condition of energetic electrons at the magnetic equator in all simulation runs. The simulation results reveal that the spectral characteristics of chorus significantly vary depending on the magnetic field inhomogeneity. Whistler‐mode emissions are generated and propagate away from the equator in all simulation runs, but distinct chorus elements with rising tones are only reproduced in the cases of small inhomogeneities. In the simulation that had the smallest inhomogeneity, we find excitation of broadband hiss‐like emission (BHE) whose amplitudes are comparable to discrete chorus elements found in other simulation runs. The BHE consists of many wave elements with rising tones nonlinearly triggered in the region close to the magnetic equator. We show that the small spatial inhomogeneity of the background magnetic field results in the small threshold amplitude for the nonlinear wave growth and allows the triggering process of rising tone elements to emerge easily in the equatorial region of the magnetosphere.

Relativistic electron microbursts associated with whistler chorus rising tone elements: GEMSIS‐RBW simulations

Relativistic electron microbursts, which are bursty enhancements of the precipitation of relativistic electrons, are often observed by low‐altitude satellite measurements. These microbursts are likely to play an important role in high‐energy electron flux loss in the outer radiation belt. Some observations suggest that whistler chorus waves are a cause of relativistic electron microbursts. First, we derived the relativistic time of flight model considering the propagation of whistler mode waves, and then investigated characteristics of the precipitations. We found that relativistic electron precipitation has a positive energy dispersion at low altitude. The duration of electron precipitation by one whistler chorus element decreases when the energy of the precipitated electrons is increased. We then performed three‐dimensional test particle simulation with a newly developed wave‐particle interaction model using realistic plasma parameters in the inner magnetosphere. The test particle simulation showed for the first time that the resonant interactions with whistler chorus elements at high‐latitudes produce bursty enhancements of relativistic electron precipitation, thus confirming the results of the TOF analysis. A few Hz modulations are embedded in the precipitating electron flux variations, which is associated with the repetition period of the whistler chorus elements. The simulation results indicate that microbursts of relativistic electrons of the outer belt are caused by chorus wave‐particle interactions at high latitudes and a series of rising tone elements of chorus waves produce a few Hz modulation of microbursts observed by the SAMPEX satellite.

PENGUIn/AGO and THEMIS conjugate observations of whistler mode chorus waves in the dayside uniform zone under steady solar wind and quiet geomagnetic conditions

We perform a case study of conjugate observations of whistler mode chorus waves on the dayside made on 26 July 2008 by three THEMIS spacecraft and ground‐based ELF/VLF receivers at the Automatic Geophysical Observatories (AGO) in Antarctica supported by the U.S. Polar Experiment Network for Geospace Upper‐atmosphere Investigations (PENGUIn) project. The dayside chorus waves were excited during a period of no substorm activity with geomagnetic indices indicating quiet conditions (Dst ∼ −10 nT; AE &lt; 200 nT). The solar wind dynamic pressure was almost constant during the chorus wave intensification. Conjugate observations in the outer magnetosphere confirm that the chorus intensification was localized within the radial distance R = 7–10 RE near noon (12.5 &lt; MLT &lt; 13.5 h). The waves persisted for at least 1.5 h in the same location, where field lines are not accompanied by off‐equatorial minimum‐B pockets but rather exhibit nearly zero dB/ds, the field‐aligned gradient in B‐magnitude, over a wide range of magnetic latitudes (∼±20°). The location did not seem to corotate with the Earth or drift with the energetic electrons. The chorus waves consisted of discrete, rising tone elements, propagating away from the magnetic equator, quasi‐parallel to the ambient magnetic field (wave‐normal angles &lt; 20°). We conclude that the long‐lasting, localized, quiet time dayside chorus amplification was due to the nearly zero dB/ds conditions that occur naturally in the dayside uniform zone (DUZ), the transition region between the near‐Earth dipole and the compressed, off‐equatorial double‐minimum field configuration found closer to the magnetopause. We thus suggest that the magnetic field configuration in the dayside outer magnetosphere plays a key role in the generation of dayside chorus waves under quiet geomagnetic conditions.

Full particle simulation of whistler‐mode rising chorus emissions in the magnetosphere

We perform an electromagnetic full particle simulation to study the generation mechanism of VLF whistler‐mode chorus emissions in the equatorial region of the magnetosphere. Parabolic variation of the static magnetic field is assumed as a model for the dipole magnetic field in the vicinity of the equator. We have cold thermal electrons and relatively low anisotropic hot electrons as plasma particles. In the initial phase, the amplitude growth of the incoherent whistler‐mode waves is determined by the linear growth rate. When the wave amplitude reaches a certain level, it begins to grow more rapidly with a series of rising tone emissions consisting of coherent phase structures in the vicinity of the magnetic equator, and their wave packets propagate away form the magnetic equator. The frequency sweep rates of the excited rising tone elements decrease gradually. We find a distinct threshold for such a nonlinear wave growth generating the rising chorus‐like elements. The relation between the wave amplitude and the frequency sweep rate of each element found in the simulation fully supports the nonlinear wave growth theory of chorus emissions.