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Found 23 entries in the Bibliography.
Showing entries from 1 through 23
| 2020 |
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The Impenetrable Barrier: Suppression of Chorus Wave Growth by VLF Transmitters 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 impenetrable barrier at L ~ 2.8 focused on diffusion and precipitation loss mechanisms for an explanation of the sharp reduction of multi-MeV electron fluxes earthward of L ~ 3. Van Allen Probes observations for cases when the plasmasphere is contracted earthward of L ~ 3 indicate that strong coherent signals from VLF transmitters can play significant roles in the suppression of nonlinear chorus wave growth earthward of L ~ 3. As a result, local nonlinear acceleration of hundreds of keV electrons to MeV energies does not occur in this region. During the recovery of the outer radiation belt when the plasmasphere is significantly contracted, the suppression of chorus wave growth and local acceleration by the action of the transmitter waves at the outer edge of the VLF bubble contributes to the sharp inner edge of the new MeV electron population and the formation of the impenetrable barrier at L ~ 2.8. Foster, John; Erickson, Philip; Omura, Yoshiharu; Baker, Daniel; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA027913 Radiation belt; Plasmapause; VLF transmitters; wave-particle interactions; Electron acceleration; nonlinear VLF chorus; Van Allen Probes |
| 2019 |
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On 23 February 2014, Van Allen Probes sensors observed quite strong electromagnetic ion cyclotron (EMIC) waves in the outer dayside magnetosphere. The maximum amplitude was more than 14 nT, comparable to 7\% of the magnitude of the ambient magnetic field. The EMIC waves consisted of a series of coherent rising tone emissions. Rising tones are excited sporadically by energetic protons. At the same time, the probes detected drastic fluctuations in fluxes of MeV electrons. It was found that the electron fluxes decreased by more than 30\% during the 1 min following the observation of each EMIC rising tone emissions. Furthermore, it is concluded that the flux reduction is a nonadiabatic (irreversible) process since holes in the particle flux levels appear as drift echoes with energy dispersion. We examine the process of electron pitch angle scattering by nonlinear wave trapping due to anomalous cyclotron resonance with EMIC rising tone emissions. The energy range of precipitated electrons agrees with the presumed energy for the threshold amplitude for nonlinear wave trapping. This is the first report of rapid precipitation (<1 min) of relativistic electrons by EMIC rising tone emissions and their drift echoes in time observed by spacecraft. Nakamura, S.; Omura, Y.; Kletzing, C.; Baker, D.; Published by: Journal of Geophysical Research: Space Physics Published on: May-08-2020 YEAR: 2019 DOI: 10.1029/2019JA026772 EMIC waves; Magnetosphere; microburst; nonlinear; Radiation belt; Van Allen Probes; Wave-particle interaction |
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We report the electron flux modulations without corresponding magnetic fluctuations from unique multipoint satellite observations of the Arase (Exploration of Energization and Radiation in Geospace) and the Van Allen Probe (Radiation Belt Storm Probe [RBSP])-B satellites. On 30 March 2017, both Arase and RBSP-B observed periodic fluctuations in the relativistic electron flux with energies ranging from 500 keV to 2 MeV when they were located near the magnetic equator in the morning and dusk local time sectors, respectively. Arase did not observe Pc5 pulsations, while they were observed by RBSP-B. The clear dispersion signature of the relativistic electron fluctuations observed by Arase indicates that the source region is limited to the postnoon to the dusk sector. This is confirmed by RBSP-B and ground-magnetometer observations, where Pc5 pulsations are observed to drift-resonate with relativistic electrons on the duskside. Thus, Arase observed the drift-resonance signatures \textquotedblleftremotely,\textquotedblright whereas RBSP-B observed them \textquotedblleftlocally.\textquotedblright Teramoto, M.; Hori, T.; Saito, S.; Miyoshi, Y.; Kurita, S.; Higashio, N.; Matsuoka, A.; Kasahara, Y.; Kasaba, Y.; Takashima, T.; Nomura, R.; e, Nos\; Fujimoto, A.; Tanaka, Y.-M.; Shoji, M.; Tsugawa, Y.; Shinohara, M.; Shinohara, I.; Blake, J.; Fennell, J.F.; Claudepierre, S.G.; Turner, D.; Kletzing, C.; Sormakov, D.; Troshichev, O.; Published by: Geophysical Research Letters Published on: 11/2019 YEAR: 2019 DOI: 10.1029/2019GL084379 |
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Evaluation of Plasma Properties From Chorus Waves Observed at the Generation Region In this study we present an inversion method which provides thermal plasma population parameters from characteristics of chorus emissions only. Our ultimate goal is to apply this method to ground-based data in order to derive the lower-energy boundary condition for many radiation belt models. The first step is to test the chorus inversion method on in situ data of the Van Allen Probes in the generation region. The density and thermal velocity of energetic electrons (few kiloelectron volts to 100 keV) are derived from frequency sweep rate and starting frequencies of chorus emissions through analysis of wave data from the Electric and Magnetic Field Instrument Suite and Integrated Science on board the Van Allen Probes. The nonlinear wave growth theory of Omura and Nunn (2011, https://doi.org/10.1029/2010JA016280) serves as the basis for our inversion method, assuming that the triggering wave is originated by the linear cyclotron instability. We present 16 consecutive rising-tone emissions recorded in the generation region between 11 and 12 UT on 14 November 2012. The results of the inversion are compared with density and thermal velocities (parallel and perpendicular) of energetic electrons derived from the unidirectional flux data of the Helium, Oxygen, Proton, and Electron instrument, showing a good agreement: The normalized root-mean-square deviation between the measured and predicted values are less than \~15\%. We found that the theoretical amplitudes are consistent with the measured ones. The relation between linear and nonlinear wave growth agrees with our basic assumption; namely, linear growth is a preceding process of nonlinear wave growth. We analyze electron distributions at the relativistic resonant energy ranges. asz, Lilla; Omura, Yoshiharu; Lichtenberger, J\; Friedel, Reinhard; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2019 YEAR: 2019 DOI: 10.1029/2018JA026337 chorus inversion; Van Allen Probes; Wave-particle interaction |
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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. Omura, Yoshiharu; Hsieh, Yi-Kai; Foster, John; Erickson, Philip; Kletzing, Craig; Baker, Daniel; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2019 YEAR: 2019 DOI: 10.1029/2018JA026374 inner magnetosphere; nonlinear process; Radiation belts; relativistic electrons; Van Allen Probes; wave particle interaction; whistler-mode chorus |
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Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q=2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q=2 ions is ~10\% below 3000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere Miyoshi, Y.; Matsuda, S.; Kurita, S.; Nomura, K.; Keika, K.; Shoji, M.; Kitamura, N.; Kasahara, Y.; Matsuoka, A.; Shinohara, I.; Shiokawa, K.; Machida, S.; Santolik, O.; Boardsen, S.A.; Horne, R.B.; Wygant, J.F.; Published by: Geophysical Research Letters Published on: 04/2019 YEAR: 2019 DOI: 10.1029/2019GL083024 Arase; EMIC; M/Q=2 ions; Magnetsonic waves; plasmasphere; Van Allen Probes |
| 2018 |
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Fine structure of whistler-mode hiss in plasmaspheric plumes observed by the Van Allen Probes We survey 3 years (2013-2015) of data from the Van Allen Probes related to plasmaspheric plume crossing events. We detect 194 plume crossing events, and we find that 97\% of the plumes are accompanied by VLF hiss emissions. The plumes are mainly detected on the duskside or dayside. Careful examination of the hiss spectra reveals that all hiss emissions consist of obvious fine structure. Application of a band pass filter reveals that the fine structure is consistent with the occurrence of discrete wave packets. The hiss data display high coherency. The events are classified by location. Dusk side hiss and night side hiss tend to have extremely high polarization with no chorus at the high-frequency end of the dynamic spectrum. The dusk side hiss has a distinct upper frequency limit. On the other hand, the dawn side hiss has strong chorus elements at the upper hiss frequency which makes the upper frequency limit ambiguous. We show that the structure of whistler-mode hiss is different from artificial random noise. Although noise also has fine spectral characteristics, the polarization and waveform data are totally different from the hiss cases. Our results strongly suggest that whistle-mode hiss in plasmaspheric plumes universally possesses fine structure. Nakamura, S.; Omura, Y.; Summers, D.; Published by: Journal of Geophysical Research: Space Physics Published on: 10/2018 YEAR: 2018 DOI: 10.1029/2018JA025803 fine structure; hiss; nonlinear; plasmaspheric plume; Van Allen Probes |
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Simultaneous observations of the magnetic field and plasma waves made by the Arase and Van Allen Probe A satellites at different magnetic local time (MLT) enable us to deduce the longitudinal structure of an oxygen torus for the first time. During 04:00\textendash07:10 UT on 24 April 2017, Arase flew from L = 6.2 to 2.0 in the morning sector and detected an enhancement of the average plasma mass up to ~3.5 amu around L = 4.9\textendash5.2 and MLT = 5.0 hr, implying that the plasma consists of approximately 15\% O+ ions. Probe A moved outbound from L = 2.0 to 6.2 in the afternoon sector during 04:10\textendash07:30 UT and observed no clear enhancements in the average plasma mass. For this event, the O+ density enhancement in the inner magnetosphere (i.e., oxygen torus) does not extend over all MLT but is skewed toward the dawn, being described more precisely as a crescent-shaped torus or a pinched torus. e, M.; Matsuoka, A.; Kumamoto, A.; Kasahara, Y.; Goldstein, J.; Teramoto, M.; Tsuchiya, F.; Matsuda, S.; Shoji, M.; Imajo, S.; Oimatsu, S.; Yamamoto, K.; Obana, Y.; Nomura, R.; Fujimoto, A.; Shinohara, I.; Miyoshi, Y.; Kurth, W.; Kletzing, C.; Smith, C.; MacDowall, R.; Published by: Geophysical Research Letters Published on: 10/2018 YEAR: 2018 DOI: 10.1029/2018GL080122 Arase satellite; Geomagnetic storm; inner magnetosphere; oxygen torus; simultaneous observation; Van Allen Probes; Van Allen Probes satellite |
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In Earth\textquoterights inner magnetosphere, electromagnetic waves in the ultra-low frequency (ULF) range play an important role in accelerating and diffusing charged particles via drift resonance. In conventional drift-resonance theory, linearization is applied under the assumption of weak wave-particle energy exchange so particle trajectories are unperturbed. For ULF waves with larger amplitudes and/or durations, however, the conventional theory becomes inaccurate since particle trajectories are strongly perturbed. Here, we extend the drift-resonance theory into a nonlinear regime, to formulate nonlinear trapping of particles in a wave-carried potential well, and predict the corresponding observable signatures such as rolled-up structures in particle energy spectrum. After considering how this manifests in particle data with finite energy resolution, we compare the predicted signatures with Van Allen Probes observations. Their good agreement provides the first observational evidence for the occurrence of nonlinear drift resonance, highlighting the importance of nonlinear effects in magnetospheric particle dynamics under ULF waves. Li, Li; Zhou, Xu-Zhi; Omura, Yoshiharu; Wang, Zi-Han; Zong, Qiu-Gang; Liu, Ying; Hao, Yi-Xin; Fu, Sui-Yan; Kivelson, Margaret; Rankin, Robert; Claudepierre, Seth; Wygant, John; Published by: Geophysical Research Letters Published on: 08/2018 YEAR: 2018 DOI: 10.1029/2018GL079038 drift resonance; nonlinear process; Particle acceleration; Radiation belts; ULF waves; Van Allen Probes; wave-particle interactions |
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Generation process of large-amplitude upper band chorus emissions observed by Van Allen Probes We analyze large-amplitude upper-band chorus emissions measured near the magnetic equator by the EMFISIS (Electric and Magnetic Field Instrument Suite and Integrated Science) instrument package onboard the Van Allen Probes. In setting up the parameters of source electrons exciting the emissions based on theoretical analyses and observational results measured by the HOPE (Helium Oxygen Proton Electron) instrument, we calculate threshold and optimum amplitudes with the nonlinear wave growth theory. We find that the optimum amplitude is larger than the threshold amplitude obtained in the frequency range of the chorus emissions and that the wave amplitudes grow between the threshold and optimum amplitudes. In the frame of the wave growth process, the nonlinear growth rates are much greater than the linear growth rates. Kubota, Yuko; Omura, Yoshiharu; Kletzing, Craig; Reeves, Geoff; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2018 YEAR: 2018 DOI: 10.1029/2017JA024782 Chorus; energetic electrons; nonlinear wave-particle interaction; observation; Radiation belt; Van Allen Probes |
| 2017 |
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Prompt recovery of MeV (millions of electron Volts) electron populations in the poststorm 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 nonlinear, such that a fraction of the relativistic electrons at resonant energies are trapped by waves, leading to significant nonadiabatic energy exchange. Through detailed examination of VLF chorus and electron fluxes observed by Van Allen Probes, we investigate the efficiency of nonlinear processes for acceleration of electrons to MeV energies. We find through subpacket analysis of chorus waveforms that electrons with initial energy of hundreds of keV to 3 MeV can be accelerated by 50 keV\textendash200 keV in resonant interactions with a single VLF rising tone on a time scale of 10\textendash100 ms. Foster, J.; Erickson, P.; Omura, Y.; Baker, D.; Kletzing, C.; Claudepierre, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2017 YEAR: 2017 DOI: 10.1002/2016JA023429 nonlinear acceleration; Radiation belt; Van Allen Probes; VLF chorus; wave-particle interactions |
| 2016 |
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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. Foster, J.; Erickson, P.; Omura, Y.; Baker, D.; Kletzing, C.; Claudepierre, S.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2016 YEAR: 2016 DOI: 10.1002/2016JA023429 nonlinear acceleration; Radiation belt; Van Allen Probes; VLF chorus; wave particle interactions |
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Observational evidence of the nonlinear wave growth theory of plasmaspheric hiss We 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. Nakamura, Satoko; Omura, Yoshiharu; Summers, Danny; Kletzing, Craig; Published by: Geophysical Research Letters Published on: 09/2016 YEAR: 2016 DOI: 10.1002/2016GL070333 magnetospheric dynamics; nonlinear wave growth theory; plasma wave; Plasmaspheric Hiss; Van Allen Probes; whistler-mode chorus |
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To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator. Ozaki, M.; Shiokawa, K.; Miyoshi, Y.; Kataoka, R.; Yagitani, S.; Inoue, T.; Ebihara, Y.; Jun, C.-W; Nomura, R.; Sakaguchi, K.; Otsuka, Y.; Shoji, M.; Schofield, I.; Connors, M.; Jordanova, V.; Published by: Geophysical Research Letters Published on: 08/2016 YEAR: 2016 DOI: 10.1002/2016GL070008 fast modulation; Pc1 geomagnetic pulsations; pulsating proton aurora; subpacket structure; Van Allen Probes; wave-particle interactions |
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Pulsating proton aurora caused by rising tone Pc1 waves We found rising tone emissions with a dispersion of \~1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over \~30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7\textdegreeN, 246.7\textdegreeE, magnetic latitude 61.7\textdegreeN). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, \~10\% variations in intensity, and fine structures of 3\textdegree in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of \~1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time. Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2016 YEAR: 2016 DOI: 10.1002/2015JA021681 |
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Pulsating proton aurora caused by rising tone Pc1 waves We found rising tone emissions with a dispersion of \~1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over \~30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7\textdegreeN, 246.7\textdegreeE, magnetic latitude 61.7\textdegreeN). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, \~10\% variations in intensity, and fine structures of 3\textdegree in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of \~1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time. Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.; Published by: Journal of Geophysical Research: Space Physics Published on: 02/2016 YEAR: 2016 DOI: 10.1002/2015JA021681 |
| 2015 |
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We perform test particle simulations of energetic electrons interacting with whistler mode chorus emissions. We compute trajectories of a large number of electrons forming a delta function with the same energy and equatorial pitch angle. The electrons are launched at different locations along the magnetic field line and different timings with respect to a pair of chorus emissions generated at the magnetic equator. We follow the evolution of the delta function and obtain a distribution function in energy and equatorial pitch angle, which is a numerical Green\textquoterights function for one cycle of chorus wave-particle interaction. We obtain the Green\textquoterights functions for the energy range 10 keV\textendash6 MeV and all pitch angles greater than the loss cone angle. By taking the convolution integral of the Green\textquoterights functions with the distribution function of the injected electrons repeatedly, we follow a long-time evolution of the distribution function. We find that the energetic electrons are accelerated effectively by relativistic turning acceleration and ultrarelativistic acceleration through nonlinear trapping by chorus emissions. Further, these processes result in the rapid formation of a dumbbell distribution of highly relativistic electrons within a few minutes after the onset of the continuous injection of 10\textendash30 keV electrons. Omura, Yoshiharu; Miyashita, Yu; Yoshikawa, Masato; Summers, Danny; Hikishima, Mitsuru; Ebihara, Yusuke; Kubota, Yuko; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015 YEAR: 2015 DOI: 10.1002/2015JA021563 Chorus; nonlinear wave-particle interaction; Particle acceleration; Radiation belts; relativistic electrons; simulation |
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Sub-packet structures in EMIC rising tone emissions observed by the THEMIS probes We report sub-packet structures found in electromagnetic ion cyclotron (EMIC) rising tone emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probles. We investigate three typical cases in detail. The first case shows a continuous single rising tone with obvious four sub-packets, and the second case is characterized by a patchy emission with multiple sub-packets triggered in a broadband frequency. The third case looks like a smooth rising tone without any obvious sub-packet in the FFT spectrum, while its amplitude contains small peaks with increasing frequencies. The degree of polarization of each sub-packet is generally higher than 0.8 with a left-handed polarization, and the wave direction of the sub-packets 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 sub-packet structure with the theoretical trapping time for second-order cyclotron resonance. They are consistent, indicating that an individual sub-packet is generated through a nonlinear wave growth process which excites an element in accordance with the theoretically predicted optimum amplitude. Nakamura, Satoko; Omura, Yoshiharu; Shoji, Masafumi; e, Masahito; Summers, Danny; Angelopoulos, Vassilis; Published by: Journal of Geophysical Research: Space Physics Published on: 08/2015 YEAR: 2015 DOI: 10.1002/2014JA020764 EMIC wave; inner magnetosphere; The nonlinear wave growth; THEMIS |
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Although most studies of the effects of EMIC waves on Earth\textquoterights 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 February 23, 2014 that extended over 8 hours in UT and over 12 hours in local time, stimulated by a gradual 4-hour 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 hours 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 footpoint 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. Engebretson, M.; Posch, J.; Wygant, J.; Kletzing, C.; Lessard, M.; Huang, C.-L.; Spence, H.; Smith, C.; Singer, H.; Omura, Y.; Horne, R.; Reeves, G.; Baker, D.; Gkioulidou, M.; Oksavik, K.; Mann, I.; Raita, T; Shiokawa, K.; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015 YEAR: 2015 DOI: 10.1002/2015JA021227 EMIC waves; magnetospheric compressions; Radiation belts; Van Allen Probes |
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Most previous work on nonlinear wave-particle interactions between energetic electrons and VLF waves in the Earth\textquoterights magnetosphere has assumed parallel propagation, the underlying mechanism being nonlinear trapping of cyclotron resonant electrons in a parabolic magnetic field inhomogeneity. Here nonlinear wave-particle interaction in oblique whistlers in the Earth\textquoterights magnetosphere is investigated. The study is nonself-consistent and assumes an arbitrarily chosen wave field. We employ a \textquotedblleftcontinuous wave\textquotedblright wave field with constant frequency and amplitude, and a model for an individual VLF chorus element. We derive the equations of motion and trapping conditions in oblique whistlers. The resonant particle distribution function, resonant current, and nonlinear growth rate are computed as functions of position and time. For all resonances of order n, resonant electrons obey the trapping equation, and provided the wave amplitude is big enough for the prevailing obliquity, nonlinearity manifests itself by a \textquotedbllefthole\textquotedblright or \textquotedbllefthill\textquotedblright in distribution function, depending on the zero-order distribution function and on position. A key finding is that the n = 1 resonance is relatively unaffected by moderate obliquity up to 25\textdegree, but growth rates roll off rapidly at high obliquity. The n = 1 resonance saturates due to the adiabatic effect and here reaches a maximum growth at ~20 pT, 2000 km from the equator. Damping due to the n = 0 resonance is not subject to adiabatic effects and maximizes at some 8000 km from the equator at an obliquity ~55\textdegree. Nunn, David; Omura, Yoshiharu; Published by: Journal of Geophysical Research: Space Physics Published on: 04/2015 YEAR: 2015 DOI: 10.1002/2014JA020898 Chorus; nonlinear process; oblique propagation; simulation; Wave-particle interaction; whistler |
| 2014 |
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Fine structure of plasmaspheric hiss 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 (EMFISIS) 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 one-hour samples within which a total of 8 one-second 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 re-examination of the properties of plasmaspheric hiss, including a further re-analysis of the generation mechanism for hiss. Summers, Danny; Omura, Yoshiharu; Nakamura, Satoko; Kletzing, Craig; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2014 YEAR: 2014 DOI: 10.1002/2014JA020437 |
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First observation of rising-tone magnetosonic waves Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90\textdegree and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be \textquotedbllefttemporally continuous\textquotedblright in their time-frequency spectrogram. Here we show for the first time that MS waves actually have discrete wave elements with rising-tone features in their spectrogram. The frequency sweep rate of MS waves, ~1 Hz/s, is between that of chorus and electromagnetic ion cyclotron (EMIC) waves. For the two events we analyzed, MS waves occur outside the plasmapause and cannot penetrate into the plasmasphere; their power is smaller than that of chorus. We suggest that the rising-tone feature of MS waves is a consequence of nonlinear wave-particle interaction, as is the case with chorus and EMIC waves. Fu, H.; Cao, J.; Zhima, Z.; Khotyaintsev, Y.; Angelopoulos, V.; ik, O.; Omura, Y.; Taubenschuss, U.; Chen, L.; Huang, S; Published by: Geophysical Research Letters Published on: 11/2014 YEAR: 2014 DOI: 10.1002/grl.v41.2110.1002/2014GL061867 discrete; frequency sweep rate; magnetosonic wave; nonlinear wave-particle interaction; Plasmapause; rising tone |
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Ground-based ELF/VLF chorus observations at subauroral latitudes-VLF-CHAIN Campaign We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17\textendash25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions. Shiokawa, Kazuo; Yokoyama, Yu; Ieda, Akimasa; Miyoshi, Yoshizumi; Nomura, Reiko; Lee, Sungeun; Sunagawa, Naoki; Miyashita, Yukinaga; Ozaki, Mitsunori; Ishizaka, Kazumasa; Yagitani, Satoshi; Kataoka, Ryuho; Tsuchiya, Fuminori; Schofield, Ian; Connors, Martin; Published by: Journal of Geophysical Research: Space Physics Published on: 09/2014 YEAR: 2014 DOI: 10.1002/jgra.v119.910.1002/2014JA020161 Chorus; ELF/VLF; Radiation belts; subauroral latitudes; wave-particle interactions |
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