Masafumi Shoji

Simulation of electromagnetic ion cyclotron triggered emissions in the Earth's inner magnetosphere

[1]xa0In 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.

Mirror instability and L-mode electromagnetic ion cyclotron instability: Competition in the Earth's magnetosheath

We performed both two- and three-dimensional hybrid simulations of the competing processes between the L-mode electromagnetic ion cyclotron (EMIC) and mirror instabilities, assuming anisotropic energetic ions with T ⊥ /T ∥ = 4.0. In the two-dimensional model, the energy of the EMIC waves is higher at the linear growth phase because its growth rate is larger than that of the mirror mode. In the three-dimensional model, however, the energy of the mirror mode waves is larger than that of the EMIC waves for all times because the wave number spectra of mirror mode waves form torus-like structures. We also theoretically derived a necessary condition for the dominance of the mirror instability

Akebono observations of EMIC waves in the slot region of the radiation belts

[1]xa0This paper describes a unique observation of electromagnetic ion cyclotron (EMIC) waves in the deep inner magnetosphere at L = 2.5 − 5 made by the Akebono satellite at altitudes of 3,300 − 8,700 km. The mode conversion, i.e., L mode (He+ band) → R mode (He+ band) → L mode (O+ band) was clearly identified from the equator to high latitudes. In addition, we found rising tone structures, recently identified as EMIC triggered emissions, which could lead to bursty precipitation of relativistic electrons. First, we estimated the ion composition ratio (H+, He+, O+) = (83%, 16%, 1%) from polarization analysis. Second, we estimated minimum resonant electron energies with the observed EMIC waves and triggered emissions to be ∼1–10 MeV. The satellite trajectory during the wave observation was primarily through the slot region of electron radiation belts. The collocation implies possible contribution of EMIC waves to formation of the slot region of radiation belts after a magnetic storm.

Precipitation of highly energetic protons by helium branch electromagnetic ion cyclotron triggered emissions

[1]xa0In the equatorial region of the Earths inner magnetosphere, the electromagnetic ion cyclotron (EMIC) triggered emissions are generated through interaction with energetic protons. We investigate the generation process of the EMIC triggered emissions in the He+branch and associated precipitation of the energetic protons using a one-dimensional hybrid simulation with a cylindrical parabolic magnetic geometry. The simulation results show a good agreement with the nonlinear wave growth theory. As the electron density becomes higher as in the plasmasphere or the plasmaplume, the wave amplitude thresholds for both H+ and He+ band triggered emissions become lower and their nonlinear growth rates become higher. The higher hot proton density also makes the thresholds lower. While the H+ branch triggered emissions interact with a few keV protons, the He+ branch triggered emissions interact with more energetic protons of a few hundred keV with a larger nonlinear growth rate.

Electromagnetic ion cyclotron waves in the helium branch induced by multiple electromagnetic ion cyclotron triggered emissions

[1]xa0Electromagnetic ion cyclotron (EMIC) triggered emissions with rising tones between the H+ and He+ cyclotron frequencies were found in the inner magnetosphere by the recent Cluster observations. Another type of EMIC wave with a constant frequency is occasionally observed below the He+ cyclotron frequency after the multiple EMIC triggered emissions. We performed a self-consistent hybrid simulation with a one-dimensional cylindrical magnetic flux model approximating the dipole magnetic field of the Earths inner magnetosphere. In the presence of energetic protons with a sufficient density and temperature anisotropy, multiple EMIC triggered emissions are reproduced due to the nonlinear wave growth mechanism of rising-tone chorus emissions, and a constant frequency wave in the He+ EMIC branch is subsequently generated. Through interaction with the multiple EMIC rising-tone emissions, the velocity distribution function of the energetic protons is strongly modified. Because of the pitch angle scattering of the protons, the gradient of the distribution in velocity phase space is enhanced along the diffusion curve of the He+ branch wave, resulting in the linear growth of the EMIC wave in the He+ branch.

Multidimensional nonlinear mirror-mode structures in the Earth's magnetosheath

[1]xa0We performed two-dimensional (2D) and three-dimensional (3D) hybrid simulations in open boundary models to study the nonlinear mirror-mode structures driven by the temperature anisotropy (T⊥/T∥> 1) of protons in the magnetosheath. In the open systems, because of the propagation of EMIC waves, we obtain the clearer non-propagating mirror-mode structures. We analyzed the relation between the mirror instability and the magnetic peaks and dips observed in the magnetosheath. In the 2D open boundary model with low beta (β∥ ≲ 1), we obtain fine structures of the magnetic dips at the nonlinear stage. In the 3D model, on the other hand, the mirror instability makes the magnetic peak structures with the same parameters. The parametric analysis indicates that the magnetic peaks also arise in both 2D and 3D high beta cases (β∥ ⪆ 1) as shown by the Cluster observations. In the high beta cases, the high mobility of the protons helps continuous coalescence of the diamagnetic currents inside the magnetic dips. The coalescence makes the magnetic dips larger and shallower. Between the large and shallow magnetic dips, the magnetic peaks appear in the high beta cases. In the 3D models, because degree of freedom increases in the perpendicular direction, the continuous coalescence can take place even in the low beta cases. Thus, the magnetic peaks appear in the 3D models in both cases.

Nonlinear mirror mode structures in multi-dimensional models

The temperature anisotropy (T⊥/T∥>1) of ions in the magnetosheath drives the mirror instability. We performed two-dimensional (2D) and three-dimensional (3D) hybrid simulations in open boundary models to study the nonlinear mirror mode structures. In the open boundary systems, because of the propagation of EMIC waves, we can obtain the clearer non propagating mirror mode structures. We analyzed the relation between the mirror instability and the magnetic peaks and dips which are peculiar magnetic structures observed in the magnetosheath. In the 2D open boundary model, we obtain the clear magnetic dips at the nonlinear stage. The magnetic structures become larger in the parallel directions rather than the perpendicular directions. In the 3D model, on the other hand, the mirror instability makes the magnetic peaks structures with the same parameters. The cigar-like magnetic peak structures are formed because of the nonlinear evolution of mirror instability and the symmetric structure in the perpendicular directions. We also performed parametric analyses on the ion beta in both 2D and 3D models. We find that the magnetic peaks also arise in both 2D and 3D high beta case as shown in the Cluster observations. We find the MHD equilibrium between the particle pressure and the magnetic field of these magnetic structures. Integrating the equilibrium equation with the assumption of the magnetic dips, we find that the integrated pressure becomes larger as the spatial size of the magnetic dips becomes larger. Between the large scale magnetic dips which are made through the coalescence of the magnetic structures, the magnetic peak appears. Thus, the magnetic peaks are excited in high particle beta conditions.

Pitch angle scattering by electromagnetic ion cyclotron triggered emissions in the inner magnetosphere: Hybrid simulations

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.