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We have performed a series of simulation runs for whistler-mode wave-particle interaction in a parabolic magnetic field with 12 different frequencies of triggering waves and 3 different plasma frequencies specifying cold plasma densities. Under a given plasma condition, a specific frequency range of the triggering wave exists that can generate rising-tone emissions. The generation region of rising-tone emission shifts upstream. The velocity of the wave generation region is dependent on duration of the subpacket, which is controlled by the formation of the resonant current in the generation region. When the source velocity, which is a sum of the resonance and group velocities, is approximately the same as the velocity of the wave generation region, a long-sustaining rising-tone emission is generated. When the spatial and temporal gap between subpackets exists due to damping phase of short subpacket generation, resonant electrons in the gap of the subpackets are carried at the resonance velocity to the upstream region, and the velocity of the wave generation region becomes large in magnitude. When formation of resonant currents is delayed, the velocity of the generation region becomes smaller than the source velocity in magnitude. Below one quarter of the cyclotron frequency, coalescence of subpackets takes place, suppressing formation of the resonant current in the generation region. Since gradual upstream shift of the generation region is necessary for the wave to grow locally, the source velocity should be a small negative value.

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time.

Rapid radiation belt recovery following storm time depletion involves local acceleration of multi-MeV electrons in nonlinear interactions with VLF chorus waves. Previous studies of an apparent impe...

We have conducted a one‐dimensional electromagnetic particle simulation with a parabolic magnetic field to reproduce whistler‐mode hiss emissions in the plasmasphere. We assume a bi‐Maxwellian distribution with temperature anisotropy for energetic electrons injected into the plasmasphere and find that hiss emissions are generated with spectrum characteristics typical of those observed by spacecraft near the magnetic equator. The hiss emissions contain fine structures involving rising tone and falling tone elements with variation in frequencies. The amplitude profile of the spectra agrees with the optimum wave amplitude derived from the nonlinear wave growth theory. The simulation demonstrates that hiss emissions are generated locally near the magnetic equator through linear and nonlinear interactions with energetic electrons with temperature anisotropy. The coherent hiss emissions efficiently scatter resonant electrons of 2.5–80 keV into the loss cone.

We show the regions where nonlinear growth of whistler-mode chorus waves is preferred to occur in the inner magnetosphere. A global magnetohydrodynamics (MHD) simulation was used to obtain large-scale electric and magnetic fields under the southward interplanetary magnetic field condition. With the electric and magnetic fields obtained by the MHD simulation, we ran a comprehensive inner magnetosphere-ionosphere model to solve the evolution of phase space density of electrons. Hot electrons originating from the tail region drift sunward and penetrate deep into the inner region due to a combination of convection and substorm-associated electric fields. Cold electrons also drift sunward, resulting in a contraction of the plasmasphere. We obtained the following results. (1) The whistler waves can first grow due to the linear mechanism (pitch angle anisotropy) in the premidnight-prenoon region outside the plasmapause, followed by rapid, nonlinear mechanism accompanied with rising-tone chorus elements. (2) When the solar wind speed is high, the whistler waves grow more efficiently due to linear and nonlinear mechanisms over a wider area because of deep penetration of hot electrons and the large contraction of the plasmasphere. This is consistent with the observation that the outer belt electrons increase for the fast solar wind. (3) For slow solar wind, the linear growth is mostly suppressed, but the nonlinear growth can still take place when external seed waves are present. This may explain the persistence of dawn chorus and large-amplitude chorus waves that are often observed in the premidnight-postdawn region in relatively weak geomagnetic activities.

We carry out a series of self-consistent electron hybrid code simulations for the dependence of chorus generation process on the temperature anisotropy and density of energetic electrons in the Earth's inner magnetosphere. We use the same magnetic field gradient in the simulation system and different temperature anisotropy AT for the initial distribution of energetic electrons at the magnetic equator. We conduct 6 sets of simulations for different AT from 4 to 9, changing the initial number density N-h of energetic electrons at the equator in each set of simulations. By analyzing the spectra obtained in the simulation results, we identify chorus elements with rising tones in the results for higher Nh but no distinct chorus in smaller Nh. We compare the simulation results with estimations of the threshold and optimum amplitude proposed by the nonlinear wave growth theory. We find that the chorus generation processes reproduced in the simulation results are consistently explained by the theoretical estimates. We also compare the simulation results with linear growth rates for all simulation runs. We find clear disagreement between the spectral characteristics of reproduced chorus and the predictions by the linear theory. The present study clarifies that the spectra of chorus are essentially different from those predicted by the linear theory and are determined fully by nonlinear processes of wave-particle interactions in the chorus generation region.

A scheme is suggested and tested for forecasting severe space weather (SvSW) using solar wind velocity (V) and the north-south component (Bz) of the interplanetary magnetic field (IMF) measured using the ACE (Advanced Composition Explorer) satellite from 1998 to 2016. SvSW has caused all known electric power outages and telegraph system failures. Earlier SvSW events such as the Carrington event of 1859, Quebec event of 1989 and an event in 1958 are included with information from the literature. Dst storms are used as references to identify 89 major space weather events (DstMin ≤ -100 nT) in 1998-2016. The coincidence of high CME front (or CME shock) velocity ΔV (sudden increase in V over the background by over 275 km/s) and sufficiently large Bz southward at the time of the ΔV increase is associated with SvSW; and their product (ΔV x Bz) is found to exhibit a large negative spike at the speed increase. Such a product (ΔV x Bz) exceeding a threshold seems suitable for forecasting SvSW. However, the coincidence of high V (not containing ΔV) and large Bz southward does not correspond to SvSW, indicating the importance of the impulsive action of large Bz southward and high ΔV coming through when they coincide. The need for the coincidence is verified using the CRCM (Comprehensive Ring Current Model) model, that produces extreme Dst storms (

Prompt recovery of MeV electron populations in the post-storm core of the outer terrestrial radiation belt involves local acceleration of a seed population of energetic electrons in interactions with VLF chorus waves. Electron interactions during the generation of VLF rising tones are strongly non-linear, such that a fraction of the relativistic electrons at resonant energies are trapped by waves, leading to significant non-adiabatic energy exchange. Through detailed examination of VLF chorus and electron fluxes observed by Van Allen Probes, we investigate the efficiency of non-linear processes for acceleration of electrons to MeV energies. We find through subpacket analysis of chorus waveforms that electrons with initial energy 100s keV - 3 MeV can be accelerated by 50 keV - 200 keV in resonant interactions with a single VLF rising tone on a time scale of 10-100 msec.

By performing test particle simulations of relativistic electrons scattered by electromagnetic ion cyclotron (EMIC) rising tone emissions, we find a nonlinear scattering process named SLPA (Scattering at Low Pitch Angle) totally different from the nonlinear wave trapping. The nonlinear wave trapping, occurring for high pitch angles away from the loss cone, scatters some of resonant electrons to lower pitch angles, and a fraction of the electrons is further transported into the loss cone by SLPA after being released from the wave trapping. SLPA as well as the nonlinear wave trapping can work in any cases with proton band or helium band and inside or outside the plasmapause. We clarify that the combined scattering process causes significant depletion of the outer radiation belt. In the time evolution of an electron distribution observed locally in longitude, we find echoes of the electron depletion by the localized EMIC emissions. Monitoring fluxes of electrons being lost into the atmosphere in the wave generation region, we also find that efficient relativistic electron precipitation in several seconds. The characteristics of the precipitating electron flux as a function of kinetic energy vary significantly depending on the wave frequency range and the plasma density.

Efficient acceleration of relativistic electrons at Landau resonance with obliquely propagating whistler‐mode chorus emissions is confirmed by theory, simulation, and observation. The acceleration is due to the perpendicular component of the wave electric field. We first review theoretical analysis of nonlinear motion of resonant electrons interacting with obliquely propagating whistler‐mode chorus. We have derived formulae of inhomogeneity factors for Landau and cyclotron resonances to analyze nonlinear wave trapping of energetic electrons by an obliquely propagating chorus element. We performed test particle simulations to confirm that nonlinear wave trapping by both Landau and cyclotron resonances can take place for a wide range of energies. For an element of large amplitude chorus waves observed by the Van Allen Probes, we have performed detailed analyses of the wave form data based on theoretical framework of nonlinear trapping of resonant electrons. We compare the efficiencies of accelerations by cyclotron and Landau resonances. We find significant acceleration can take place both in Landau and cyclotron resonances. What controls the dynamics of relativistic electrons in the Landau resonance is the perpendicular field components rather than the parallel electric field of the oblique chorus wave. In evaluating the efficiency of nonlinear trapping, we have taken into account variation of the wave trapping potential structure controlled by the inhomogeneity factors.

We have studied the spectral properties of quiet-time electromagnetic ion cyclotron (EMIC) waves following a steady quiet condition, which is defined with Kp values 1 during 12h, using GOES 10, 11, and 12 magnetometer data for solar minimum years 2007-2008. We identified 6584 steady quiet-time EMIC wave samples using a semiautomated procedure. Approximately 82% of the samples were observed in the morning-to-early afternoon sector (0700-1500 magnetic local time) with a maximum occurrence near noon, and their peak frequencies were mostly in the He band. We found that the occurrence rate of steady quiet-time EMIC waves is higher than that of EMIC waves for all or quiet geomagnetic conditions (Dst > 0nT or AE < 100nT) reported in previous studies by a factor of 2 or more. The frequency ratio f(peak) (sample's peak frequency)/ fH+ (the local proton gyrofrequency) of the He-band waves (approximate to 0.11-0.16) under steady quiet conditions is lower than that (approximate to 0.14-0.24) in previous studies. These results may be due to the fact that the plasmasphere expanded more frequently to the geosynchronous region under extremely quiet geomagnetic conditions in 2007-2008 than the periods selected in previous studies. The amplitude and frequency of He-band EMIC waves for nonlinear wave growth are examined as changing cold plasma density at geosynchronous orbit. We confirm that the spectral properties of observed EMIC waves are in good agreement with the nonlinear theory.

Electromagnetic ion cyclotron (EMIC) waves with rising or falling frequency variations have been studied intensively because of their effects on energetic particles in the Earth's magnetosphere. We develop an automated classification method of EMIC events based on the characteristics of frequency variations. We report some basic statistical properties of frequency variations in EMIC waves observed over 5–10 RE by three Time History of Events and Macroscale Interactions during Substorms probes from January 2012 to December 2014. We clarify whether rising tones or falling tones are observed in each chosen 20 min time segment. In the present analysis, we find that the occurrence rate of EMIC rising or falling tone events is more than 30% of the total EMIC wave events. The dayside magnetosphere is a preferential region for the EMIC frequency variations. The occurrence rate of rising tone events is slightly greater than that of falling tone events. We examine the relation between the frequency characteristics and the magnetospheric conditions. The solar wind pressure strongly controls the occurrence rates of frequency variations. We also calculate ranges of frequency variations. Large-amplitude EMIC waves tend to have wider frequency variations, and the range of frequency variation is largest around the prenoon region. In addition, rapid variations in wave amplitudes called “subpacket structures” are found in 70% of the EMIC rising or falling tone events in the dayside region. Subpacket structures appear mainly in large-amplitude EMIC rising or falling tones. These features are consistent with nonlinear wave growth theory.

We modeled Cluster spacecraft observations of slow electrostatic solitary waves (SESWs) in the Earth's northern plasma sheet boundary layer (PSBL) region on the basis of nonlinear fluid theory and fluid simulation. Various plasma parameters observed by the Cluster satellite at the time of the SESWs were examined to investigate the generation process of the SESWs. The nonlinear fluid model shows the coexistence of slow and fast ion acoustic waves and the presence of electron acoustic waves in the PSBL region. The fluid simulations, performed to examine the evolution of these waves in the PSBL region, showed the presence of an extra mode along with the waves supported by the nonlinear fluid theory. This extra mode is identified as the Buneman mode, which is generated by relative drifts of ions and electrons. A detailed investigation of the characteristics of the SESWs reveals that the SESWs are slow ion acoustic solitary waves.

Accepted: 2015-10-19, 資料番号: SA1150223000

A theoretical model to study electromagnetic ion cyclotron (EMIC) waves in kappa-Maxwellian plasma is developed. The plasma is assumed to have five components, i.e., electrons, cool and hot protons, and singly charged helium and oxygen ions. The kappa-Maxwellian anisotropic particle distribution function is assumed for the hot protons. We use the Kyoto University Plasma Dispersion Analysis Package, a full dispersion solver developed at Kyoto University, to obtain the numerical results and delineate the oxygen, helium, and proton bands. Higher harmonics of the EMIC waves are also studied, and the effects of the kappa distribution on the growth of these waves are clearly demonstrated. Our results are applied to Cluster spacecraft observations of EMIC waves in the inner magnetosphere.

We report subpacket structures found in electromagnetic ion cyclotron (EMIC) rising tone emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. We investigate three typical cases in detail. The first case shows a continuous single rising tone with four obvious subpackets, and the second case is characterized by a patchy emission with multiple subpackets triggered in a broadband frequency. The third case looks like a smooth rising tone without any obvious subpacket in the fast Fourier transform spectrum, while its amplitude contains small peaks with increasing frequencies. The degree of polarization of each subpacket is generally higher than 0.8 with a left-handed polarization, and the wave direction of the subpackets is typically field aligned. We show that the time evolution of the observed frequency and amplitude can be reproduced consistently by nonlinear growth theory. We also compare the observed time span of each subpacket structure with the theoretical trapping time for second-order cyclotron resonance. They are consistent, indicating that an individual subpacket is generated through a nonlinear wave growth process which excites an element in accordance with the theoretically predicted optimum amplitude.

Accepted: 2015-08-28, 資料番号: SA1150170000

Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event.

We perform test particle simulations of relativistic electrons interacting with electromagnetic ion cyclotron (EMIC)-triggered emissions in the plasmasphere. EMIC-triggered emissions are generated by energetic protons injected into the inner magnetosphere. EMIC-triggered emissions are characterized by large wave amplitudes, rising-tone frequencies, and coherent left-hand circularly polarized waves. We study trajectories of relativistic radiation belt electrons drifting eastward interacting with longitudinally distributed EMIC-triggered emissions. Some electrons are trapped by wave potentials and efficiently guided down to lower pitch angles. Repeated interactions occur due to the mirror motion and result in the scattering of particles into the loss cone. We use two EMIC wave models for the test particle simulations. One assumes that the wave amplitude is constant, and the other assumes a time-dependent wave amplitude that characterizes subpackets. Both model waves are resonant with 0.5–6.0 MeV electrons and precipitate them. Electrons in the energy range 1.1–3.0 MeV are precipitated most efficiently. Approximately 50% of the total injected number of 1.1–3.0 MeV electrons are precipitated in a timescale of 2 min. We obtain the relativistic electron distribution in equatorial pitch angle and in pitch angle at the atmosphere. Further, we determine the timing and longitudinal location of the relativistic electron precipitation with respect to different particle energies.

Plasmaspheric hiss has been widely regarded as a broadband, structureless, incoherent emission. In this study, by examining burst-mode vector waveform data from the Electric and Magnetic Field Instrument Suite and Integrated Science instrument on the Van Allen Probes mission, we show that plasmaspheric hiss is a coherent emission with complex fine structure. Specifically, plasmaspheric hiss appears as discrete rising tone and falling tone elements. Our study comprises the analysis of two 1 h samples within which a total of eight 1 s samples were analyzed. By means of waveform analysis on two samples, we identify typical amplitudes, phase profiles, and sweep rates of the rising and falling tone elements. The exciting new observations reported here can be expected to fuel a reexamination of the properties of plasmaspheric hiss, including a further reanalysis of the generation mechanism for hiss.

Spacecraft observations revealed the presence of electrostatic solitary waves (ESWs) in various regions of the Earth's magnetosphere. Over the years, many researchers have attempted to model these observations in terms of electron/ion acoustic solitary waves by using nonlinear fluid theory/simulations. The ESW structures predicted by fluid models can be inadequate due to its inability in handling kinetic effects. To provide clear view on the application of the fluid and kinetic treatments in modeling the ESWs, we perform both fluid and particle-in-cell (PIC) simulations of ion acoustic solitary waves (IASWs) and estimate the quantitative differences in their characteristics like speed, amplitude, and width. We find that the number of trapped electrons in the wave potential is higher for the IASW, which are generated by large-amplitude initial density perturbation (IDP). The present fluid and PIC simulation results are in close agreement for small amplitude IDPs, whereas for large IDPs they show discrepancy in the amplitude, width, and speed of the IASW, which is attributed to negligence of kinetic effects in the former approach. The speed of IASW in the fluid simulations increases with the increase of IASW amplitude, while the reverse tendency is seen in the PIC simulation. The present study suggests that the fluid treatment is appropriate when the magnitude of phase velocity of IASW is less than the ion acoustic (IA) speed obtained from their linear dispersion relation, whereas when it exceeds IA speed, it is necessary to include the kinetic effects in the model.

Using the Geotail observation of upper band and lower band (dual-band) rising tone chorus emissions in the dayside outer magnetosphere, we evaluate the mechanism of creating their frequency gap bandwidths, on the basis of a nonlinear damping mechanism at half the local gyrofrequency proposed by Omura et al. (2009). For ELF dual-band chorus elements observed in the dayside of the equatorial outer magnetosphere (L ∼ 10.5), the lower cutoff frequency of the upper band elements follows half the local gyrofrequency. On the other hand, the upper cutoff of the lower band elements is almost consistent with half the gyrofrequency at a minimum-B pocket (a possible chorus generation region) along the field lines connecting Geotail according to the Tsyganenko geomagnetic TS05 model. This is consistent with the scenario that the rising tone wave packet initially excited in a wide frequency range suffers from damping at half the local gyrofrequencies during quasi-parallel propagation. Since the local gyrofrequency gradually increases away from the generation region, the upper cutoff of a lower band element should represent half the gyrofrequency at the generation region, whereas the lower cutoff of an upper band element should follow half the local gyrofrequency. We confirm that the frequency sweep rates and amplitudes of the observed chorus wave packets are consistent with those predicted by the nonlinear growth theory of chorus emissions, except for the frequency gap. This indicates an observational evidence of nonlinear chorus generation in the minimum-B pocket in the outer magnetosphere and creation of dual-band elements due to nonlinear damping through quasi-parallel wave propagation.

Accepted: 2014-04-10, 資料番号: SA1140197000

[1] A sub-Alfve´nic jet in the tailward outflow region near the separatrix of the magnetic reconnection is observed by Geotail on 9 February 1995. Several dozens of electrostatic solitary waves/pulses (ESWs) are observed, respectively, on the current sheet-side and the lobe-side of the separatrix. The ESWs on the current sheet-side are of type-B with direction outward (toward to the tailward) while on the lobe-side they are of type-A directed to X-line. The amplitude of ESWs on the current sheet-side is about 6 times more than those on the lobe-side, suggesting that energies flowing outward from the reconnection X-line are much larger than those flowing inward. Moreover, observations show, that electron beams associated with ESWs, which are parallel to the ambient magnetic field, are much stronger on the current sheet-side than on the lobe-side of the separatrix. Furthermore, the direction of the electron beam on the lobe-side of the separatrix is mainly antiparallel to the ambient magnetic field and it is mainly parallel on the current sheet-side. Both are consistent with the propagation of ESWs which is in agreement with the generation mechanism of ESWs, which is suggested to be related to electron beams. These results are helpful for solving the issue of ESWs' generation mechanism associated with magnetic reconnection. It also provides an important clue for understanding the fast energy release during the magnetic reconnection process.

[1] Spacecraft observations and simulations show generation of coherent electromagnetic ion cyclotron (EMIC) triggered emissions with rising tone frequencies. In the inner magnetosphere, the spontaneously triggered EMIC waves are generated by the energetic protons with large temperature anisotropy. We reproduced EMIC triggered emissions in the Earth's magnetosphere by real scale hybrid simulations with cylindrical magnetic geometry. We obtained spontaneously triggered nonlinear EMIC waves with rising frequencies in the H+band of the EMIC dispersion relation. The proton holes in the phase space are formed. We have also derived the theoretical optimum wave amplitude for triggering process of the EMIC nonlinear wave growth. The optimum wave amplitude and the nonlinear transition time show a good agreement with the present simulation result. The nonlinear wave growth over a limited time forms a subpacket structure of a rising tone emission. The formation process of a subpacket is repeated because of a new triggering wave generated by the phase-organized protons, which are released from the previous subpacket. Then, the EMIC triggered emission is formed as a train of subpackets generated at different rising frequencies.

[1] We show that the anomalous cyclotron resonance between relativistic electrons and electromagnetic ion cyclotron (EMIC) triggered emissions takes place very effectively near the magnetic equator because of the variation of the ambient magnetic field. Efficient precipitations are caused by nonlinear trapping of relativistic electrons by electromagnetic wave potentials formed by EMIC triggered emissions. We derive the necessary conditions of the wave amplitude, kinetic energies, and pitch angles that must be satisfied for the nonlinear wave trapping. We have conducted test particle simulations with a large number of relativistic electrons trapped by a parabolic magnetic field near the magnetic equator. In the presence of coherent EMIC-triggered emissions with increasing frequencies, a substantial amount of relativistic electrons is trapped by the wave, and the relativistic electrons at high pitch angles are guided to lower pitch angles within a short time scale much less than a second, resulting in rapid precipitation of relativistic electrons or relativistic electron microbursts.

[1] 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.

Electromagnetic ion cyclotron (EMIC) triggered chorus emissions have recently been a subject of several experimental, theoretical and simulation case studies, noting their similarities with whistler-mode chorus. We perform a survey of 8 years of Cluster data in order to increase the database of EMIC triggered emissions. The results of this is that EMIC triggered emissions have been unambiguously observed for only three different days. These three events are studied in detail. All cases have been observed at the plasmapause between 22 and 24 magnetic local time (MLT) and between − 15∘ and 15∘ magnetic latitude (λm). Triggered emissions are also observed for the first time below the local He gyrofrequency math formula. The number of events is too low to produce statistical results, nevertheless we point out a variety of common properties of those waves. The rising tones have a high level of coherence and the waves propagate away from the equatorial region. The propagation angle and degree of polarization are related to the distance from the equator, whereas the slope and the frequency extent vary from one event to the other. From the various spacecraft separations, we determine that the triggering process is a localized phenomenon in space and time. However, we are unable to determine the occurrence rates of these waves. Small frequency extent rising tones are more common than large ones. The newly reported EMIC triggered events are generally observed during periods of large AE index values and in time periods close to solar maximum.

We have performed computer simulations of the self-consistent nonlinear evolution of electrostatic and electromagnetic 2f p waves excited by electron beams with electromagnetic particle code. In both one- and two-dimensional periodic systems an electrostatic 2f p wave is generated at twice the wave number of forward propagating Langmuir waves by wave-wave coupling. This wave grows with the forward propagating Langmuir wave in the nonlinear stage of the simulations. The electrostatic 2f p wave in the simulations is saturated at about -20 ∼ -30 dB of that of the Langmuir waves. It is larger than the value expected from observations in the terrestrial electron foreshock. The electromagnetic 2f p wave is only excited in two-dimensional systems. The magnitude of the electromagnetic 2f p wave is correlated with the backward propagating Langmuir wave, not with the electrostatic 2f p wave. This result suggests that the electromagnetic 2f p wave is excited by the wave-wave coupling of forward and backward propagating Langmuir waves. The typical power density estimated from a reasonable amplitude of Langmuir wave is of the same order or much weaker than the value typically observed around the electron foreshock.