The Experts below are selected from a list of 258 Experts worldwide ranked by ideXlab platform
Yoshiharu Omura - One of the best experts on this subject based on the ideXlab platform.
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effect of the background magnetic field inhomogeneity on generation processes of whistler mode chorus and broadband hiss like emissions
Journal of Geophysical Research, 2013Co-Authors: Yuto Katoh, Yoshiharu OmuraAbstract:[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.
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nonlinear spatiotemporal evolution of whistler mode chorus waves in earth s inner magnetosphere
Journal of Geophysical Research, 2012Co-Authors: Danny Summers, Yoshiharu Omura, Y MiyashitaAbstract:[1] We analyze the nonlinear evolution of whistler mode chorus waves propagating along a magnetic field line from their Equatorial source. We solve wave evolution equations off the Equator for the wave magnetic field amplitude and wave frequency, subject to boundary conditions at the Equator comprising model “chorus equations” that describe the generation of a seed chorus element. The electron distribution function is assumed to evolve adiabatically along a field line. The wave profiles exhibit nonlinear convective growth followed by saturation. Convective growth is due to nonlinear wave trapping, and the saturation process is partly due to a combination of adiabatic effects and a decreasing resonant current with latitude. Notwithstanding computationally expensive full-scale kinetic simulations, our study appears to be the first to analyze the nonlinear evolution and saturation of whistler mode waves off the Equator.
Danny Summers - One of the best experts on this subject based on the ideXlab platform.
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nonlinear spatiotemporal evolution of whistler mode chorus waves in earth s inner magnetosphere
Journal of Geophysical Research, 2012Co-Authors: Danny Summers, Yoshiharu Omura, Y MiyashitaAbstract:[1] We analyze the nonlinear evolution of whistler mode chorus waves propagating along a magnetic field line from their Equatorial source. We solve wave evolution equations off the Equator for the wave magnetic field amplitude and wave frequency, subject to boundary conditions at the Equator comprising model “chorus equations” that describe the generation of a seed chorus element. The electron distribution function is assumed to evolve adiabatically along a field line. The wave profiles exhibit nonlinear convective growth followed by saturation. Convective growth is due to nonlinear wave trapping, and the saturation process is partly due to a combination of adiabatic effects and a decreasing resonant current with latitude. Notwithstanding computationally expensive full-scale kinetic simulations, our study appears to be the first to analyze the nonlinear evolution and saturation of whistler mode waves off the Equator.
Tsutomu Kondo - One of the best experts on this subject based on the ideXlab platform.
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On the formation of a fast thermospheric zonal wind at the magnetic dip Equator
Geophysical Research Letters, 2011Co-Authors: Tsutomu Kondo, Arthur D. Richmond, Shigeto WatanabeAbstract:[1] Simulations with the NCAR Thermosphere - Ionosphere - Electrodynamics General Circulation Model (TIE-GCM) have been carried out to understand the cause of strong thermospheric zonal wind at the magnetic dip Equator. The simulations show that the zonal winds blow strongly at the magnetic dip Equator instead of at the geographic Equator due to the latitude structure of ion drag. The fast winds at the dip Equator are seen throughout the altitude between 280 km and 600 km, and the wind above 400 km is mainly accelerated via viscosity. A test simulation without viscosity verifies that the extension of the fast Equatorial wind to heights above 400 km is maintained by viscous coupling with the winds at lower altitudes, in spite of there being an ion-drag maximum instead of relative minimum at the dip Equator at high altitudes. Basically, viscosity is not so large compared to the pressure gradient and ion drag, but dynamics causes the pressure gradient and ion drag approximately to balance, and viscosity becomes important. The simulation results are consistent with the observations by the DE-2 and CHAMP satellites. Therefore we suggest that the zonal wind velocity in the low latitude region is controlled by ion drag and viscosity.
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fast thermospheric wind jet at the earth s dip Equator
Geophysical Research Letters, 2009Co-Authors: Shigeto Watanabe, Tsutomu KondoAbstract:[1] The thermospheric zonal wind forms a fast wind jet at the Earth's dip Equator instead of the geographic Equator. This remarkable feature is revealed in two sets of independent observations made two decades apart. One is from the CHAMP satellite during the year of 2002 and the other is from the DE-2 satellite during Aug. 1981–Feb. 1983. Both observations show that this wind jet is eastward at night with speed reaching 150 ms−1, and westward around noon with speed over 75 ms−1. These fast wind jets are observed during local times of fully developed Equatorial ionization anomaly (EIA). On the other hand, a channel of slow wind is found on the dip Equator during the period of 05–08 MLT, which corresponds to local times before the EIA develops. These features strongly suggest the ion drag being the principle cause for shifting the wind jet from the geographic Equator to the dip Equator.
Y Miyashita - One of the best experts on this subject based on the ideXlab platform.
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nonlinear spatiotemporal evolution of whistler mode chorus waves in earth s inner magnetosphere
Journal of Geophysical Research, 2012Co-Authors: Danny Summers, Yoshiharu Omura, Y MiyashitaAbstract:[1] We analyze the nonlinear evolution of whistler mode chorus waves propagating along a magnetic field line from their Equatorial source. We solve wave evolution equations off the Equator for the wave magnetic field amplitude and wave frequency, subject to boundary conditions at the Equator comprising model “chorus equations” that describe the generation of a seed chorus element. The electron distribution function is assumed to evolve adiabatically along a field line. The wave profiles exhibit nonlinear convective growth followed by saturation. Convective growth is due to nonlinear wave trapping, and the saturation process is partly due to a combination of adiabatic effects and a decreasing resonant current with latitude. Notwithstanding computationally expensive full-scale kinetic simulations, our study appears to be the first to analyze the nonlinear evolution and saturation of whistler mode waves off the Equator.
Yuto Katoh - One of the best experts on this subject based on the ideXlab platform.
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a simulation study of the propagation of whistler mode chorus in the earth s inner magnetosphere
URSI General Assembly and Scientific Symposium, 2014Co-Authors: Yuto KatohAbstract:Whistler-mode chorus emissions are generated in the region close to the magnetic Equator outside the plasmapause during geomagnetically disturbed periods. Spacecraft observations near the magnetic Equator have revealed that chorus appear in the frequency range from 0.2 to 0.8 Ω e0 , where Ω e0 is the electron gyrofrequency at the magnetic Equator, while the frequency range of chorus is classified into the lower-band (0.2 to 0.5 Ω e0 ) and upper-band chorus (0.5 to 0.8 Ω e0 ) by a distinct gap at 0.5 Ω e0 [1]. Observations have revealed that chorus typically propagate along a magnetic field line in its source region and become oblique during their propagation away from the Equator. Propagation properties of chorus have been studied for a half of century (e.g., [2,3]). Theoretical estimations and results of ray-tracing studies have been compared with observations of chorus in space and at high-latitude ground stations. In the present study, we study the propagation of whistler-mode chorus in the magnetosphere by a spatially two-dimensional simulation code in the dipole coordinates [4]. We set the simulation system so as to assume the outside of the plasmapause, corresponding to the radial distance from 3.9 to 4.1 R E in the Equatorial plane and the latitudinal range from −15 to +15 degrees, where R E is the Earth's radius. We assume a model chorus element propagating northward from the magnetic Equator of the field line at L=4 with a rising tone from 0.2 to 0.7 Ω e0 in the time scale of 5000 Ω e0 −1. For the initial density distribution of cold electrons, we assume three types of initial conditions in the outside of the plasmapause: without duct (Run 1), a density enhancement duct (Run 2), and a density decrease duct (Run 3). In Run 1, the simulation result reveals that whistler-mode waves of different wave frequency propagate in the different ray-path in the region away from the magnetic Equator. In Runs 2 and 3, the model chorus element propagates inside the assumed duct with changing the wave normal angle. The simulation results show the different propagation properties of the chorus element in Run 2 and Run 3, and reveal that resultant wave spectra observed along the field line are different between the density enhancement and the density decrease duct cases. The spectral modification of chorus by the propagation effect should play a significant role in the precipitation of energetic electrons related to pulsating aurora through the interaction with chorus in the magnetosphere, particularly in the region away from the Equator. The present study clarifies that the variation of propagation properties of chorus should be taken into account for the thorough understanding of resonant interactions of chorus with energetic electrons in the inner magnetosphere.
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effect of the background magnetic field inhomogeneity on generation processes of whistler mode chorus and broadband hiss like emissions
Journal of Geophysical Research, 2013Co-Authors: Yuto Katoh, Yoshiharu OmuraAbstract:[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.
