Convective generation of a vortex street in plasma shear flow

Yamamoto, Takashi; Yamashita, Miyako

doi:10.1063/1.866615

Cited by 4 publications

(1 citation statement)

References 8 publications

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“…The latter structure arises from secondary instabilities which are driven at the head of the instability train. This "convective generation" of instabilities is discussed in Yamamoto et al 1988. Indeed, the primary modes of the instability train can non-linearly trigger secondary KH modes when convected by the mean flow, since the downstream medium is taken to be linearly unstable.…”

Section: Overview Of a Transonic Runmentioning

confidence: 99%

Spatial simulations of the Kelvin-Helmholtz instability in astrophysical jets

Viallet

Baty

2007

A&A

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Aims. The long term magnetohydrodynamic stability of magnetized transonic/supersonic jets is numerically investigated using a spatial approach. We focus on two-dimensional linearly-unstable slab configurations where the jet is embedded in a flow-aligned uniform magnetic field of weak amplitude. We compare our results with previous studies using a temporal approach where longitudinally periodic domains were adopted. Methods. The finite-volume based versatile advection code is used to solve the full set of ideal compressible MHD equations. We follow the development of Kelvin-Helmholtz modes that are driven by a white noise perturbation continuously introduced at the jet inlet. Results. No noticeable difference is observed in spatial simulations versus analogous temporal ones during the linear and early nonlinear evolution of the configuration. However, in the case of transonic flows, a different long-term scenario occurs in our spatial runs. Indeed, after the large-scale disruption of the flow, a sheath region of enhanced magnetic field encompassing the jet core forms along the whole flow. This provides a partial stabilization mechanism leading to enhanced stability for later times, which is almost independent of the initial magnitude of the magnetic field. The implication of this mechanism for the stability of astrophysical jets is discussed.

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Section: Overview Of a Transonic Runmentioning

confidence: 99%

Spatial simulations of the Kelvin-Helmholtz instability in astrophysical jets

Viallet

Baty

2007

A&A

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A particle simulation of large‐amplitude undulations on the evening diffuse auroral boundary

Yamamoto¹,

Makita²,

Meng³

1991

J. Geophys. Res.

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The pattern of undulation on the equatorward boundary of the diffuse aurora, occasionally observed in the afternoon-evening sector, is studied by two-dimensional particle simulations for the motion of magnetospheric plasma perpendicular to the geomagnetic field. In the simulation model, the poleward electric field is assumed to be initially enhanced locally in latitude and independent of longitude, as suggested by the observations near the undulations of the diffuse auroral boundary. Such an electric field is assumed to be maintained by space charges carried by a cold plasma. The diffuse aurora is initially distributed over a longitudinally extended zone. Since this type of diffuse aurora is probably caused by precipitation of the energetic protons with energies in excess of a few keV, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. Sheared plasma flow arising from the electric field distribution drives the Kelvin-Helmholtz instability and causes undulations on the equatorward boundary of the diffuse aurora with wavelengths and amplitudes comparable to the typical observed values. Notably, the particle simulation can reproduce the outstanding indentations characteristic of large-amplitude undulations which have occasionally been observed. The good agreement between the simulation results and the observed undulations suggests that the Kelvin-Helmholtz shear-driven process is responsible for undulations on the equatorward boundary of the diffuse aurora in the afternoon-evening sector. It is also shown that the magnetic drift of the energetic protons producing the diffuse aurora is important in creating the characteristic pattern of the diffuse auroral boundary.

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Convective generation of “giant” undulations on the evening diffuse auroral boundary

Yamamoto¹,

Ozaki²,

Inoue³

et al. 1994

J. Geophys. Res.

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Convective generation of “giant” undulations on the equatorward boundary of an evening diffuse aurora is numerically simulated. A giant undulation is defined as a waveform having the crest‐to‐trough amplitude comparable to the wavelength. The two‐dimensional electrostatic particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. According to the simulation results by Yamamoto et al. [1993], the giant undulation is a manifestation of the Kelvin‐Helmholtz (K‐H) waves arising from the polarization of “an arc sheet” (dense plasma population of ionospheric origin) in the magnetosphere, which is assumed to be located just equatorward of the region of proton diffuse aurora. In this previous simulation, initially, irregularities are given evenly over the entire azimuthal length of the arc sheet so that the resulting undulations are periodic. The present simulation deals with a different situation, that a K‐H wave starts growing from local irregularities on the polarized arc sheet. The simulation results show that the disturbance propagates both westward and eastward (relative to the background flow), forming a series of K‐H waves along the arc sheet. As a consequence, giant undulations with spatially varying amplitudes are developed on the equatorward boundary of a diffuse aurora, which is located just poleward of the arc sheet. A series of giant undulations convectively produced in the simulation is remarkably similar to some auroral images photographed from the Defense Meteorological Satellite Program (DMSP) satellites. In addition, we show the direct evidence for the presence of an arc sheet, probably associated with the giant undulations, which is provided by the measurements of precipitating particles from the DMSP F7 satellite crossing the equatorward boundary of the diffuse aurora; the high‐density (∼10 cm−3) ions with energy of a few hundred electron volts are detected near the edge of the region of energetic (∼10 keV) ion precipitation. The primary cause of the formation of that arc sheet is thought to be escape of the oxygen and hydrogen ions from the topside ionosphere, due to the transverse acceleration by the ion‐cyclotron waves.

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Convective generation of a vortex street in plasma shear flow

Cited by 4 publications

References 8 publications

Spatial simulations of the Kelvin-Helmholtz instability in astrophysical jets

Spatial simulations of the Kelvin-Helmholtz instability in astrophysical jets

A particle simulation of large‐amplitude undulations on the evening diffuse auroral boundary

Convective generation of “giant” undulations on the evening diffuse auroral boundary

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