A new simulation method has been developed to investigate the excitation and saturation processes of toroidal Alfven eigenmodes (TAE modes). The background plasma is described by a magnetohydrodynamic (MHD) fluid model, while the kinetic evolution of energetic alpha particles is followed by the drift kinetic equation. The magnetic tluctuation of n=2 mode develops and saturates at the level of 1.8X 10m3 of the equilibrium field when the initial beta of alpha particles is 2% at the magnetic axis. after saturation, the TAE mode amplitude shows an oscillatory behavior with a frequency corresponding to the bounce frequency of the alpha particles trapped by the TAE mode. The decrease of the power transfer rate from the alpha particles to the TAE mode, which is due to the trapped particle effect of a finite-amplitude wave, causes the saturation. From the linear growth rate the saturation Ievel can be estimated. 0 1995 American Institute of Physics.
Extensive three-dimensional computer simulations of the magnetosphere-ionosphere (M-I) coupling are performed to study self-excitation of an auroral arclike structure with special emphasis on (1) nonlinear evolution of the feedback instability in the M-I coupling system, (2) controlling mechanisms of the arc structure, (3) formation of a field-aligned electric potential structure in association with the development of the feedback instability, and (4) effects of the parallel potential generation on the development of the arclike structure. The present study takes the first step toward the theoretical understanding of the M-I coupling system with parallel potentials. As was already shown by Sate [1978] and Watanabe and Sate [1988], it is reconfirmed that the feedback instability produces a longitudinally elongated, latitudinally striated structure where the upward field-aligned current and the ionospheric density are locally enhanced. On top of this the present extended study reveals the following important new features: The global distribution of the striation structure is primarily governed by the magnetospheric convection pattern and the ionospheric density distribution. There appears a significant dawn-dusk asymmetry in the arc formation, even though the apparent geometrical relationship is symmetric. This dawn-dusk asymmetry reflects the geometrical fact that the ionospheric Pedersen current closing the magnetospheric current is antisymmetric with respect to the noon-midnight plane, while the self-closed Hall current is symmetric. The recombination effect plays a significant role in the global, as well as local, development of the arc structure. The nonlinearity of recombination, in conjunction with the closure of an arc-associated local field-aligned current system, acts to destroy an old arc and creates a new arc in a different but adjacent position. This results in a peculiar dynamic evolution of the arclike structure. A V-shaped field-aligned potential structure is created in association with an arc structure, when we introduce the parallel anomalous resistivity. The nonlinear phase mismatching due to the parallel resistivity reduces the growth rate of the feedback instability, and suppresses the growth of the arc structure. When the effect of precipitating hot electrons is taken into account, the ionospheric density is considerably enhanced locally at the foot of the field lines where the field-aligned potential is generated. An oscillatory behavior is observed in the development of the ionospheric density, field-aligned current and potential. The period seems to be governed by the Alfv•n bounce time and the ionospheric density.
INTRODUCTIONAuroral phenomena in the Earth's polar region are considered to occur in the final stage of the energy transport processes from the solar wind into the ionosphere through the energy conversion processes in the magnetospheric boundary region, the magnetotail, and the acceleration region of auroral electrons. Auroras are, as a whole, classified, according to their acti...
Mach–Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p–n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p–n diode. This length is shorter than commercial LiNbO3 modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index ng. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.
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