Yoshiharu Omura 교수

The generation process of whistler‐mode chorus emissions is analyzed by both theory and simulation. Driven by an assumed strong temperature anisotropy of energetic electrons, the initial wave growth of chorus is linear. After the linear growth phase, the wave amplitude grows nonlinearly. It is found that the seeds of chorus emissions with rising frequency are generated near the magnetic equator as a result of a nonlinear growth mechanism that depends on the wave amplitude. We derive the relativi

We present computer experiments of electrostatic solitary waves (ESW) observed by Geotail in the magnetotail. ESW correspond to broadband electrostatic noise, and they are excited through electron two‐stream instabilities along a static magnetic field. We performed one‐dimensional electrostatic particle simulations involving two electron beams and an ion beam traveling along the static magnetic field. We vary the density ratio of the electron beams and the thermal velocities of the electron and

We develop a nonlinear wave growth theory of magnetospheric chorus emissions, taking into account the spatial inhomogeneity of the static magnetic field and the plasma density variation along the magnetic field line. We derive theoretical expressions for the nonlinear growth rate and the amplitude threshold for the generation of self‐sustaining chorus emissions. We assume that nonlinear growth of a whistler mode wave is initiated at the magnetic equator where the linear growth rate maximizes. Se

We report a very efficient process for accelerating high‐energy electrons by coherent whistler mode waves in the Earth's dipole magnetic field, which we have found in our recent test particle simulations. The efficient acceleration process takes place for weakly relativistic seed electrons of a few hundred kiloelectronvolts. Under an assumption that the whistler mode wave packets are excited near the equatorial plane of the inner magnetosphere and propagate away from the equator, the acceleratio

We present simulations of the electrostatic solitary waves (ESW) as observed by GEOTAIL which have been identified as broadband electrostatic noise (BEN) in previous studies. We have found that ESW are generated as a result of the nonlinear coalescence of strong electrostatic waves excited by an electrostatic beam instability. This instability is driven by an electron beam drifting relative to the ions and other electrons drifting with the ions. As a necessary condition for generation of ESW, th

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. Ass

[1] Chorus emissions are triggered from the linear cyclotron instability driven by the temperature anisotropy of energetic electrons (10–100 keV) in the magnetosphere. Chorus emissions grow as an absolute nonlinear instability near the magnetic equator because of the presence of an electromagnetic electron hole in velocity space. The transition process from the linear wave growth at a constant frequency to the nonlinear wave growth with a rising tone frequency is due to formation of a resonant c

Close examination of whistler mode chorus emissions reveals that a chorus emission is a coherent monochromatic wave typically with a fast rising tone. The frequency of the emission increases rapidly along with growth of the wave amplitude. We first consider the generation mechanism of whistler mode chorus emissions. The essential mechanism of the frequency change is critically related to the inhomogeneity of the geomagnetic field in the equatorial region. The rising tone emission is only possibl

Abstract 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 f

We derive the second‐order resonance condition for interaction between a relativistic electron and a coherent Electromagnetic Ion Cyclotron (EMIC) wave with a variable frequency. We perform test particle simulations of relativistic electrons interacting with EMIC waves with a fixed frequency and a rising‐tone frequency such as EMIC triggered emissions observed in the inner magnetosphere. Trapping of resonant electrons leads to rapid and efficient pitch angle scattering of relativistic electrons,

Abstract Nonlinear processes associated with the generation process of whistler-mode chorus emissions are summarized. The nonlinear dynamics of energetic electrons interacting with a coherent whistler-mode wave and the formation of electromagnetic electron holes or hills in the velocity phase space are described. The condition for resonant electrons to be free from the anomalous trapping at low pitch angles is obtained. In the presence of the inhomogeneity due to the frequency variation and the

Basic processes of coherent whistler mode wave‐particle interactions in the magnetosphere are studied by self‐consistent computer simulations. The wave equations used in the simulation to determine the wave amplitude and frequency are discussed and physical interpretations of them are given. Nonlinear processes of the wave growth in a uniform magnetic field are examined in detail. Difference between uniform (periodic) and nonuniform (encounter) interactions is studied. Taking into account the in

Heating of heavy ions is observed in the equatorial magnetosphere in conjunction with ion cyclotron waves generated by anisotropic hot protons (i.e., GEOS 1 and 2 and ATS 6 results). The mechanism of the heating is studied by a numerical simulation. The plasma parameters which have been chosen are those which prevail in the dayside magnetosphere at geostationary altitudes. The plasma consists of cold and isotropic H + and He + ions (κ T /2 ∼ 1.7 eV) with a small number of hot anisotropic protons

We show that the anomalous cyclotron resonance between relativistic electrons and electromagnetic ion cyclotron (EMIC) triggered emissions takes place very effectively near the magnetic equator because of the variation of the ambient magnetic field. Efficient precipitations are caused by nonlinear trapping of relativistic electrons by electromagnetic wave potentials formed by EMIC triggered emissions. We derive the necessary conditions of the wave amplitude, kinetic energies, and pitch angles th