Spectrum characteristics of electromagnetic ion cyclotron triggered emissions and associated energetic proton dynamics (original) (raw)
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Theory and observation of electromagnetic ion cyclotron triggered emissions in the magnetosphere
Journal of Geophysical Research, 2010
1] We develop a nonlinear wave growth theory of electromagnetic ion cyclotron (EMIC) triggered emissions observed in the inner magnetosphere. We first derive the basic wave equations from Maxwell's equations and the momentum equations for the electrons and ions. We then obtain equations that describe the nonlinear dynamics of resonant protons interacting with an EMIC wave. The frequency sweep rate of the wave plays an important role in forming the resonant current that controls the wave growth. Assuming an optimum condition for the maximum growth rate as an absolute instability at the magnetic equator and a self-sustaining growth condition for the wave propagating from the magnetic equator, we obtain a set of ordinary differential equations that describe the nonlinear evolution of a rising tone emission generated at the magnetic equator. Using the physical parameters inferred from the wave, particle, and magnetic field data measured by the Cluster spacecraft, we determine the dispersion relation for the EMIC waves. Integrating the differential equations numerically, we obtain a solution for the time variation of the amplitude and frequency of a rising tone emission at the equator. Assuming saturation of the wave amplitude, as is found in the observations, we find good agreement between the numerical solutions and the wave spectrum of the EMIC triggered emissions.
Simulation of electromagnetic ion cyclotron triggered emissions in the Earth's inner magnetosphere
Journal of Geophysical Research, 2011
1] In a recent observation by the Cluster spacecraft, emissions triggered by electromagnetic ion cyclotron (EMIC) waves were discovered in the inner magnetosphere. We perform hybrid simulations to reproduce the EMIC triggered emissions. We develop a self-consistent one-dimensional hybrid code with a cylindrical geometry of the background magnetic field. We assume a parabolic magnetic field to model the dipole magnetic field in the equatorial region of the inner magnetosphere. Triggering EMIC waves are driven by a left-handed polarized external current assumed at the magnetic equator in the simulation model. Cold proton, helium, and oxygen ions, which form branches of the dispersion relation of the EMIC waves, are uniformly distributed in the simulation space. Energetic protons with a loss cone distribution function are also assumed as resonant particles. We reproduce rising tone emissions in the simulation space, finding a good agreement with the nonlinear wave growth theory. In the energetic proton velocity distribution we find formation of a proton hole, which is assumed in the nonlinear wave growth theory. A substantial amount of the energetic protons are scattered into the loss cone, while some of the resonant protons are accelerated to higher pitch angles, forming a pancake velocity distribution.
2011 XXXth URSI General Assembly and Scientific Symposium
In a recent observation by the Cluster spacecraft, electromagnetic ion cyclotron (EMIC) triggered emissions were discovered in the inner magnetosphere. We perform hybrid simulations to reproduce the EMIC triggered emissions. We develop a self-consistent one-dimensional (1D) hybrid code with a cylindrical geometry of the background magnetic field. We assume a parabolic magnetic field to model the dipole magnetic field in the equatorial region of the inner magnetosphere. Triggering EMIC waves are driven by a left-handed polarized external current assumed at the magnetic equator in the simulation model. Cold proton, helium, and oxygen ions, which form branches of the dispersion relation of the EMIC waves, are uniformly distributed in the simulation space. Energetic protons with a loss cone distribution function are also assumed as resonant particles. We reproduce rising tone emissions in the simulation space, finding a good agreement with the nonlinear wave growth theory. In the energetic proton velocity distribution we find formation of a proton hole, which is assumed in the nonlinear wave growth theory. A substantial amount of the energetic protons are scattered into the loss cone, while some of the resonant protons are accelerated to higher pitch angles, forming a pancake velocity distribution.
Journal of Geophysical Research: Space Physics, 2013
1] Understanding excitation of electromagnetic ion cyclotron (EMIC) waves remains a considerable scientific challenge in the magnetospheric physics. Here we adopt correlated data from the Thermal Emission Imaging System (THEMIS) spacecraft under low (K p = 1 + ) and medium (K p = 4) geomagnetic activities to investigate the favorable conditions for the excitation of EMIC waves. We utilize a sum of bi-Maxwellian components and kappa components to fit the observed ion (6-25 keV) distributions collected by the electrostatic analyzer (ESA) onboard the THEMIS spacecraft. We show that the kappa distribution models better and more smoothly with the observations. Then we evaluate the local growth rate and path-integrated gain of EMIC waves by bi-Maxwellian and kappa distributions, respectively. We demonstrate that the path-integrated wave gain simulated from the kappa distribution is consistent with observations, with intensities 24 dB in H + band and 33 dB in He + band. However, bi-Maxwellian distribution tends to overestimate the wave growth rate and path-integrated gain, with intensities 49 dB in H + band and 48 dB in He + band. Moreover, compared to the He + band, a higher proton anisotropy is needed to excite the H + band waves. The current study presents a further observational support for the understanding of EMIC wave instability under different geomagnetic conditions and suggests that the kappa-type distributions representative of the power law spectra are probably ubiquitous in space plasmas.
Journal of Geophysical Research: Space Physics, 2014
We report observations of electromagnetic ion cyclotron (EMIC) triggered emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes outside the plasmasphere. Although these phenomena have recently received much attention because of the possibility of strong interaction with particles, only a few events of EMIC triggered emissions have been reported near the equatorial plasmapause. We performed a survey of the THEMIS probe data and found various types of emissions mainly on the dayside at radial distances of 6–10 RE. We study three distinctive events in detail. The first is a typical event with an obvious rising tone emission in the afternoon sector. The emissions in the second event are simultaneously excited in different frequency bands separated by the cyclotron frequency of helium ions. In the third event, which occurred near local noon, rising tone emissions were excited in an extended region near the equator where the field‐ali...
Space Science Reviews
We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or DeltaLsim0.56\Delta L\sim 0.56DeltaLsim0.56 Δ L ∼ 0.56 ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or ...
A laboratory study of collisional electrostatic ion cyclotron waves
Journal of Geophysical Research, 1986
The effects of neutral-particle collisions on current-driven electrostatic ion cyclotron (EIC) waves are studied in a Q machine with a cesium (Cs +) plasma. We find that even when vin --0.312ci, EIC waves of substantial amplitude (/Xn/n of several percent) can be excited.
Nonlinear interaction of ion-cyclotron waves with fast protons in the magnetosphere
sta of1 lnv the magnetoýphe]e Se1 wit еп€ tiol ter ým of1 tha nat, mai the Tht ord finc whC flec rот .ра B(z and vaI 1. INTB,oDUCTIoN 1Ъе conseqrrenceý of t}e feaonant int€ ractlona Ьеhrееп waveв and parH,cleB in а plaвma confhed Ьу а magnetic fteld аrе регtiпепt to tЪе physicB of laboratory plaBmаs and plaBmaB in sрасе, as can Ье shоvrп Ьу Bimply cltiпg thе rQgеатсh оп instabilities in adiabatic confinement вуst€ mя,i the тевеаrсh оп cyclotTon heating (вее Ref. 2 Bnd the Цtвrаtчrе cited tbeTe), tlв elfoЁ to int€ rpret the паfirrаl and maJr-made vlf апd цlf гаdiаtiоп iп the magпеtоярhеrэ,8rа Bnd the рrоьlеm of int€ Ipr€ ting the fine ýtпrсtчrе of the вроrаФс воlдт radio еmlsвiоп.5 Tlmofeevz hаs studied in detail the попliпеаr tnteraction of сirсчlаrlу polarized waveB with сhаrgеd раrtlсlеs 1п cormectlon with the theoly of the сусlоtrоп heating of рlаsmав 1п magBetic conflnement sуst€ mý. Under cyclotron-heating condftions, the саsе of цовt intereBt 1в kпча << О (k rr iB the longttudinal wave пчmЬоr, апd v1, and Qс аr€ tb thеrmаl and cyclotron velocities of Фе раrticles of thе соrrевропФпg BlBcieB), and this is tЪе саsе dealt wtth 1п Ref. 2. In the рrФlеm of the interaction of cyclotron wачеs with fast раrfi,сlеs in Фе mцпеtоврhеrе, lt Ь песеssаry to trest the саве of large DoppleT BhlftB Вцч9 -Qg, пrhете vq is фе characteriýtic velocity of the faBt particles). Тhis сirсчmstапсе rафсаllу сhапgеg the пдfurе of the рrоЬlеm. Until rэсепtlу, this latter саве hаs been analyzed аввчmirц that the characterlstic tlmeB т of the рrосеsýеs чпdеr сопsldеrаff,оп (8ее the фsсчýslоп belщI for mоrе detatls) ýatbfy the сопфtiоп т << ,Q6{, whеr€ Qь is t}e сhаrасtеrlstiс bounce frequency of the partlcles between the magnetlc mirrоrs. Мчсh rроrk has Ьееп carried out чпdеr thls aBsumption (вее Ref. б and the litеrаtчrе cited there). For ýpical magnetospheric condltions, фе inequality , .. O5t соrrеsропds to rаthег large wave amplltudes (оr growth rаtеs).
Scientific Reports, 2021
A study using Arase data gives the first observational evidence that the frequency drift of electromagnetic ion cyclotron (EMIC) waves is caused by cyclotron trapping. EMIC emissions play an important role in planetary magnetospheres, causing scattering loss of radiation belt relativistic electrons and energetic protons. EMIC waves frequently show nonlinear signatures that include frequency drift and amplitude enhancements. While nonlinear growth theory has suggested that the frequency change is caused by nonlinear resonant currents owing to cyclotron trapping of the particles, observational evidence for this has been elusive. We survey the wave data observed by Arase from March, 2017 to September 2019, and find the best falling tone emission event, one detected on 11th November, 2017, for the wave particle interaction analysis. Here, we show for the first time direct evidence of the formation of a proton hill in phase space indicating cyclotron trapping. The associated resonance cu...