K. Kondoh

Spontaneous fast reconnection model in three dimensions

The spontaneous fast reconnection model is studied in a three-dimensional (3D) situation for different plasma parameter values. In any case, once a current-driven anomalous resistivity is ignited, magnetic reconnection explosively evolves as a nonlinear instability, and the 3D fast reconnection mechanism involving large-scale standing slow shocks is realized as an eventual solution on the nonlinear saturation phase. For the smaller plasma β, the reconnection evolution is more drastic, and the resulting fast reconnection mechanism becomes more powerful. In the fast reconnection configuration, the central 3D diffusion region becomes unstable against resistive tearing and is bifurcated into a pair of diffusion regions, which move away from each other. In the moving diffusion region, the locally enhanced anomalous resistivity is self-consistently sustained by the reconnection flow, and the slow shock stands between the 3D diffusion region and a large-scale 3D plasmoid. Since the plasmoid moves much more rapid...

Computer studies on the spontaneous fast reconnection evolution in various physical situations

The spontaneous fast reconnection evolution is studied in a long current sheet system in various physical situations, where the threshold of current-driven anomalous resistivity is assumed to increase with the thermal velocity. If the initial threshold VC0 is sufficiently large in a low-β plasma, the fast reconnection mechanism can fully be set up; on the other hand, if VC0 is so small that the anomalous resistivity can easily occur in the usual circumstances, the resulting diffusion region notably lengthens so that the reconnection process becomes much less effective. Also, the fast reconnection evolution is strongly influenced by plasma β in the ambient magnetic field region, and an essential condition for the fast reconnection mechanism to evolve explosively is that the plasma β is sufficiently small. In fact, only in a low-β plasma does the magnetic tension force involved play the dominant role in the overall system dynamics and in the drastic magnetic energy release. It is also demonstrated that the ...

Computer simulations on three-dimensional magnetic loop dynamics by the spontaneous fast reconnection model

Three-dimensional (3D) dynamics of a large-scale magnetic loop is studied by precise magnetohydrodynamic simulations on the basis of the spontaneous fast reconnection model. Once a (current-driven) anomalous resistivity is ignited, the fast reconnection mechanism drastically evolves by the positive feedback between the (3D) global reconnection flow and the anomalous resistivity; on the nonlinear saturation phase, the global reconnection flow has grown so that the reconnection (diffusion) region shrinks to a small extent, and the fast reconnection mechanism involving a pair of standing slow shocks is established in the finite extent. When the 3D plasmoid, formed ahead of the fast reconnection jet, collides with the mirror plane boundary, the reconnected field lines are piled up, leading to formation of a large-scale 3D magnetic loop. Since the resulting 3D fast reconnection jet becomes supersonic, a definite fast shock builds up at the interface between the magnetic loop top and the fast reconnection jet. ...

Computer studies on the three-dimensional spontaneous fast reconnection model as a nonlinear instability

The present paper studies the basic physics of the spontaneous fast reconnection model in a three-dimensional (3D) situation for different resistivity parameter values, where the threshold for occurrence of current-driven anomalous resistivity is allowed to increase with the thermal velocity (T), and the initial plasma density notably changes in space with the plasma pressure in the current sheet system. For any case, once the anomalous resistivity is ignited, the 3D fast reconnection mechanism explosively evolves as a nonlinear instability by the positive feedback between the anomalous resistivity and the reconnection flow, even if the threshold significantly increases with the thermal velocity; for the larger threshold values, the fast reconnection evolution becomes more drastic and the reconnection rate, finally attained on the nonlinear saturation phase, becomes larger. In the resulting 3D fast reconnection configuration, slow shocks stand and extend outwards in the finite extent; also, ahead of the f...

The first observation of sulfur in anomalous cosmic rays by the Geotail and the wind spacecrafts

The Geotail high-energy particle instruments have observed cosmic-ray particles in the energy range from 3 MeV n-1 to 150 MeV n-1 at 1 AU during the period 1992 September-1995 August. A remarkable enhancement of anomalous cosmic-ray (ACR) N, O, Ne, and C is observed during the period. A measurable enhancement of the sulfur flux below about 20 MeV n-1 was observed. This is the first evidence showing the existence of sulfur in the anomalous component. The flux increase of anomalous sulfur, with a first ionization potential (FIP) of 10.4 eV, is smaller than that of ACR carbon with an FIP of 11.3 eV and much smaller than those of high-FIP elements, which suggests that the fractions of neutral carbon and sulfur atoms are significantly low in the very local interstellar medium.

Numerical studies on three-dimensional earthward fast plasma flows in the near-Earth plasma sheet by the spontaneous fast reconnection model

[1] The spontaneous fast reconnection model is applied to the earthward fast flow events observed in the near-Earth plasma sheet. Here, the earthward fast flow events include both of bursty bulk flow events and flow burst events. In order to apply it directly to actual observations, virtual probes are located in the plasma sheet region in the three-dimensional simulation domain so that we can directly observe the temporal variations of plasma quantities in accordance with the growth and proceeding of the fast reconnection. In this model, magnetic reconnection drastically evolves and Alfvenic fast plasma jet flows in the very restricted narrow channel, and a large-scale plasmoid is formed ahead of the fast plasma jet. The results of virtual observation of these evolutions are found to be in good agreement with actual satellite observations. At the same time, in the lobe region, travelling compression regions (TCRs) are observed in connection with the fast flow events. The temporal profiles of magnetic fields detected by the virtual probes are also in good agreement with actual satellite observations. It is concluded that the earthward fast flow events and earthward TCR events result from the fast reconnection mechanism.

Evolution of magnetospheric current wedge by the spontaneous fast reconnection model

On the basis of the spontaneous fast reconnection model, the underlying physical mechanism of magnetospheric current wedge evolution is studied by magnetohydrodynamic simulations. It is demonstrated that when a three-dimensional magnetic loop top is compressed by the fast reconnection jet, field-aligned currents are suddenly generated by the resulting sheared fields inside the loop; simultaneously, a large-scale current wedge evolves to link, through the field-aligned currents, the sheetcurrent ahead of the magnetic loop to the current in the local loop footpoint of reconnected field lines. In accordance with the current-wedge evolution, the sheetcurrent, which initially flows ahead of the loop top in the middle of the system, is abruptly bifurcated and turns its direction toward the local loop footpoint, where strong currents are concentrated and intensified. Therefore, once the channel for the current wedge is realized, effective energy dissipation occurs, through the channel, in the local region of the loop footpoint connected to the separatrix, which bounds the reconnected field lines and the ambient (prereconnection) field lines.

Magnetohydrodynamic study of three-dimensional instability of the spontaneous fast magnetic reconnection

Three-dimensional instability of the spontaneous fast magnetic reconnection is studied with magnetohydrodynamic (MHD) simulation, where the two-dimensional model of the spontaneous fast magnetic reconnection is destabilized in three dimension. Generally, in two-dimensional magnetic reconnection models, every plasma condition is assumed to be uniform in the sheet current direction. In such two-dimensional MHD simulations, the current sheet destabilized by the initial resistive disturbance can be developed to fast magnetic reconnection by a current driven anomalous resistivity. In this paper, the initial resistive disturbance includes a small amount of fluctuations in the sheet current direction, i.e., along the magnetic neutral line. The other conditions are the same as that of previous two-dimensional MHD studies for fast magnetic reconnection. Accordingly, we may expect that approximately two-dimensional fast magnetic reconnection occurs in the MHD simulation. In fact, the fast magnetic reconnection acti...

Three-dimensional non-linear instability of spontaneous fast magnetic reconnection

Three-dimensional instability of spontaneous fast magnetic reconnection is studied using MHD (magnetohydro- dynamic) simulation. Previous two-dimensional MHD studies have demonstrated that, if a current-driven anomalous resistivity is assumed, two-dimensional fast magnetic reconnection occurs and two-dimensional largescale magnetic loops, i.e., plasmoids, are ejected from the reconnection region. In most two-dimensional MHD studies, the structure of the current sheet is initially one-dimensinal. On the other hand, in recent space plasma observations, fully three-dimensional magnetic loops frequently appear even in the almost one-dimensional current sheet. This suggests that the classical two-dimensional fast magnetic reconnection may be unstable to any three-dimensional perturbation, resulting in three-dimensional fast magnetic reconnection. In this paper, we show that a three-dimensional resistive perturbation destabilizes two-dimensional fast magnetic reconnection and results in three-dimensional fast magnetic reconnection. The resulting three-dimensional fast reconnection repeatedly ejects three-dimensional magnetic loops downstream. The obtained numerical results are similar to the pulsating downflows observed in solar flares. According to the Fourier analysis of the ejected magnetic loops, the time evolution of this three-dimensional instability is fully non-linear.

Composition and energy spectra of anomalous cosmic rays observed by the GEOTAIL satellite

Abstract The composition and energy spectra of anomalous cosmic-rays (ACRs) in the energies 6 – 200 MeV/n have been measured during the period from September 1992 to August 1995 using the High Energy Particle instrument onboard the GEOTAIL satellite orbiting at 1 AU. A remarkable enhancement of ACR N, O, Ne and Ar is observed during the period. A flux enhancement of anomalous argon at 1 AU is confirmed by the GEOTAIL satellite. The first possible evidence for the emergence of a measurable sulfur component in ACR below about 20 MeV/n is found. The flux increase of anomalous sulfur with 10.4 eV FIP is smaller than that of ACR carbon with 11.3 eV FIP, and much smaller than those of high FIP elements, which suggests that the fraction of neutral carbon and sulfur atoms is significantly lower in the very local interstellar medium.