Yoshiharu Omura

Electrostatic solitary waves (ESW) in the magnetotail: BEN wave forms observed by GEOTAIL

The authors report on broadband wave features captured by the wave form capture receiver on the GEOTAIL satellite. In the plasma sheet boundary layer, the broadband specta are composed of a series of pulses, which the authors term electrostatic solitary waves. They propose a model to account for these wave events as highly nonlinear instabilities of electron beams.

Electron beam instabilities as generation mechanism of electrostatic solitary waves in the magnetotail

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 ion beams. The values of these parameters strongly affect diffusion processes of the electron beams, and accordingly, different types of electrostatic waves are generated. We studied four different cases: cold bistream instability, weak-beam instability, bump-on-tail instability, and warm bistream instability. For these electron beam instabilities, we performed two different runs with cold and hot ions, respectively. The cold bistream instability gives ESW for hot ions and ion acoustic waves for cold ions. The weak-beam instability gives Langmuir waves, while the bump-on-tail instability gives ESW. The amplitudes of the waves excited by the weak-beam and bump-on-tail instabilities are small and do not induce nonlinear decay to ion acoustic waves even in the presence of cold ions. The warm bistream instability gives electron hole modes regardless of the value of the ion temperature. The electron hole mode is a normal mode in the presence of a two-hump electron distribution, and it is regarded as narrowband electrostatic noise. It also leads to formation of ESW, if the phase velocity of the electron hole mode is much larger than the ion drift velocity. A necessary condition for ESW formation through the bump-on-tail instability is derived theoretically, and its significance to Geotail observations is discussed.

A review of observational, theoretical and numerical studies of VLF triggered emissions

Abstract We review theoretical and numerical studies of VLF triggered emissions generated by manmade signals propagating in the whistler mode in the magnetosphere. Instead of listing all the past studies on the subject we select some of those works which give important ideas and the theoretical basis for future studies. The main purpose of the paper is to clarify what problems remain unresolved and to make suggestions for future studies. We first describe the phenomenon of VLF triggered emissions and give a brief review of experiments and observations. We then summarize the basic physics of VLF wave-particle interactions, which is indispensable for an understanding of the triggering mechanisms. We review important ideas and theories, as well as numerical models so far employed, summarize the present understanding of the phenomenon and consider briefly those problems which need further study. Especially, we focus our attention on numerical simulations whose capabilities in nonlinear physics are increasing with the increasing power of supercomputers.

The Numerical Simulation of VLF Chorus and Discrete Emissions Observed on the Geotail Satellite using a Vlasov Code

The work consdiers VLF chorus elements observed omn the Geotail satellite, which passes the equator at 10 earth radii. We have used a VHS Vlasov simulation code to model these emissions, employing all the ambient plasma data observed on Geotail. Excellent agreement with observation results, with steep risers and slow fallers being reproduced. The results confirm the overall validity of the non linear trapping theory of VLF emissions, and also confirm the efficacity of the Vlasov VHS code.

Computer simulation of chorus wave generation in the Earth's inner magnetosphere

A self-consistent particle simulation with a dipole magnetic field model is carried out, reproducing chorus emissions with rising tones successfully. We assume energetic electrons forming a highly anisotropic velocity distribution in the equatorial region. No initial wave is assumed except for electromagnetic thermal noise induced by the energetic electrons. In the early stage of the simulation, coherent whistler-mode waves are generated from the equator through an instability driven by the temperature anisotropy of the energetic electrons. During the propagation of the whistler-mode waves, we find formation of a narrowband emission with negative frequency gradient (NEWNFG) in the spatial distribution of the frequency spectrum in the simulation system. The trailing edge of NEWNFG is continuously created at increasing frequencies in the region close to the equator. Observed at a fixed point, the NEWNFG shows a frequency variation of a typical chorus emission.

Computer simulation of electrostatic solitary waves: A nonlinear model of broadband electrostatic noise

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, the ion thermal velocity must be large enough so as to prevent decay of the electrostatic waves to ion acoustic waves. Another condition is that the density of the electron beam drifting relative to the ions must be larger than 30% of the plasma density.

A new charge conservation method in electromagnetic particle-in-cell simulations

Abstract We developed a fast algorithm for solving the current density satisfying the continuity equation of charge in electromagnetic particle-in-cell (PIC) simulations. In PIC simulations of the charge conservation, a particle trajectory over one time step is conventionally assumed to be a straight line. In the present new scheme we assume that a particle trajectory is a zigzag line. Compared with the Villasenor–Buneman method and Esirkepovs method, the present scheme has an advantage in computation speed without any substantial distortion of physics.

Van Allen probes, NOAA, GOES, and ground observations of an intense EMIC wave event extending over 12 h in magnetic local time

Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earths 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 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h 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 h 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 foot point 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.

Two‐dimensional computer simulations of electrostatic solitary waves observed by Geotail spacecraft

We performed computer simulations of electrostatic solitary waves (ESW), which correspond to broadband electrostatic noise (BEN), observed by Geotail spacecraft in the magnetotail. According to Geotail/WFC data analysis, ESW are composed of sequences of impulsive solitary waves, and their potentials are revealed to have uniform structures in the direction perpendicular to the ambient magnetic field. We performed a series of two-dimensional electrostatic particle simulations with a cold electron beam drifting against background hot electrons along the ambient magnetic field. In these simulations, spatial potentials are excited by the initial electron beam instability. They coalesce with each other and form potential troughs uniform in the direction perpendicular to the ambient magnetic field, as predicted by the Geotail observations. We varied the magnitude of the ambient magnetic field and found that this parameter critically affects the ESW formation process. These simulation results indicate that ESW are excited by the electron beam instability in the presence of the static magnetic field with a certain magnitude. ESW are also subject to the enhanced thermal fluctuations due to the limited number of superparticles in the simulations.

Electrostatic solitary waves carried by diffused electron beams observed by the Geotail spacecraft

We study waveforms of electrostatic solitary waves (ESW) and corresponding electron velocity distribution functions observed by the Geotail spacecraft. When we observe a series of ESW in the plasma sheet boundary layer of the Earths magnetotail, we find enhanced fluxes of high-energy electrons flowing along the ambient magnetic field. We find good correlation between the propagation direction of the ESW and the direction of the enhanced high-energy electron flux. This supports the model proposed by computer simulations that ESW are formed by the electrons modulated through the bump-on-tail instability. The enhanced electron flux is regarded as the diffused electron beam after the saturation of the instability. From comparison between the variation of waveforms and the velocity of the electron beams, the spatial scales and depths of the ESW potentials and their propagation directions are estimated. We found both tailward and earthward ESW in the plasma sheet boundary layer, even in the deep magnetotail.