Study of three dimensional structure of the fast convection flow in the plasma sheet by MHD simulations on the basis of spontaneous fast reconnection model

Three dimensional structure of the fast convection flow in the plasma sheet is examined using magnetohydrodynamic (MHD) simulations on the basis of spontaneous fast reconnection model. The fast flow observed in the near-Earth magnetotail is one of the key phenomena in order to understand the causal relationship between magnetic substorm and magnetic reconnection. In this paper, we focus on this earthward fast flow in the near-Earth magnetotail. Our previous studies have shown that the fast reconnection produces the Alfvénic fast reconnection outflow and drastic magnetic field dipolarization in the finite extent. In this paper, the results of our simulations are compared with those of the in-situ observations in the geomagnetotail. They have consistent temporal profiles of the plasma quantities. It is suggested that the fast convection flows are caused by spontaneous fast reconnection.     

Magnetic reconnection in a high-temperature plasma of solar flares

In the frame of a simple self-consistent model for high-temperature turbulent current sheet (HTCS) /1/, three effects are considered. (i) Gradient instabilities create anamalous plasma diffusion across magnetic field and increase the power of energy release in HTCS. (ii) Penetration of a small transverse component of magnetic field into HTCS also can significantly increase an energy output of HTCS. (iii) There appears electric current circulating around a current sheet at a compression of longitudinal magnetic field. This current induces a Joule heat; however, a total flux of the longitudinal field remains constant.     

Dynamical magnetic reconnection in Parker''s coronal heating model

The excitation (flares, ejections, heating, …) of the corona can be understood in terms of the dynamics of the confectively driven magnetized plasma. In particular, anomalous ohmic heating may be a consequence of the formation and rapid dissipation of small-scale magnetic fields in the corona. We have performed numerical simulations of the loop heating model proposed by Parker (1972, 1994), and have studied its dynamics and global power balance in order to assess its viability as a coronal heating candidate, with promising results. Our results suggest the following view of the small-scale dynamics of coronal loops. First of all, photospheric granular motions quasi-statically twist the magnetic field of the corona in a random-walk fashion. In topologically closed structures, the perpendicular magnetic energy increases, causing magnetic shear to build up at the quasi-separatrices of the resulting close-packed magnetic flux tubes. At some point, the boundary driving causes this stressed configuration to cross the threshold of an ideal time-scale MHD instability (possibly magnetic coalescence or resistive tearing) or a point of nonequilibrium and the field lines pinch toward a small-scale sheared configuration. It then becomes energetically favorable for dynamic reconnection to occur, producing narrow current sheets and an Ohmic heating rate sufficient to balance the input Poynting flux.     

Numerical simulation of three-dimensional reconnection due to the instability of collisionless current sheets

Based on analytical calculations we have currently argued that spontaneous reconnection through thin collisionless current sheets is an essentially three-dimensional (3 D) process (Büchner, 1996 a, b). Since 3 D kinetic PIC codes have become available, the three dimensional nature of the collisionless current sheet decay are now illustrated by numerical simulations (Büchner and Kuska, 1996; Pritchett and Coroniti, 1996; Zhu and Winglee, 1996). While the latter two claim a coupling to a longer wavelength kink mode as a main factor, destabilizing thin current sheets in 3 D, our simulations have revealed that even shorter scale perturbations in the current direction suffice to destabilize thin sheets very quickly. Since past simulation runs, however, were limited to mass ratios near unity, the influence of the electrons was not treated adequately. We have now investigated the stability of thin collisionless current sheets including 64 times lighter negatively charged particles. We can now show that while the two-dimensional tearing instability slows down for M = M_p/m_e = 64, the three-dimensional current sheet decay is a much faster process — practically as fast as the mass ratio M = 1 3 D sheet decay, even without kinking the sheet. We further conclude that, unlike the two-dimensional tearing instability, the three-dimensional decay of thin current sheets is not controlled by the electrons. For a sheet width comparable with the ion inertial length, we also recovered signatures of the Hall effect as predicted by Vasyliunas (1975) in the mass ratio M = 64 case. The ion inertial length seems to be the critical scale at which the sheet starts to decay.     

Large-scale reconnection in a large flare

Some specific features of the large-scale magnetic reconnection in large solar flares are briefly reviewed. In particular, the large-scale structure and dynamics of the 3B/X5.7 flare on 14 July 2000 are interpreted in terms of the topological model. The role of the betatron effect in collapsing magnetic traps, that are created by reconnection in the solar corona, is considered. We discuss some possibilities to observe the collapsing trap acceleration in solar flares.     

Numerical model of time-dependent hydrodynamic flows with mass loading

We study the influence of mass loading on the formation of shocks employing a time-dependent hydrodynamic model. Numerical examples illustrate in which way the formation and propagation of shocks in the plasma is affected by variations of the ionization rate (due to changes of solar wind parameters or of neutral gas production). It is found that mass loading may be an efficient mechanism to prevent the formation of discontinuities in hydrodynamic flows.     

Imaging of reconnection processes in hard X-rays

The Hard X-ray Spectrometer aboard the SMM detected several events of energy release late in the development of two-ribbon flares. One such event, at 21:12 UT on 21 May, 1980 (～ 20 min after the flare onset and 15 min after the peak of the impulsive phase) is studied in detail. The site of new brightening first became visible in hard X-rays (> 22 keV) and only afterwards showed up at lower energies. It was clearly located high in the corona so that one can identify it with energy release at the tops of newly formed post-flare loops. Thus, if the Kopp and Pneuman model of the loop formation is adopted, we may have imaged here a reconnection process in the solar corona. An attempt is made to estimate physical parameters at the reconnection site.     

Theory of magnetic reconnection in three dimensions

Magnetic reconnection in traditional, two-dimensional theory can be closely associated with topological properties of the magnetic field, separatrices, neutral lines, or separators. Such well-defined topological properties may no longer exist, or become hopelessly complicated, in more general three-dimensional configurations, that otherwise behave physically quite similarly to the two-dimensional configurations. We discuss generalized definitions of reconnection applied to such situations and illustrate typical properties with simulations of three-dimensional reconnection in the magnetotail.     

A transient MHD model applicable for the source of solar cosmic ray acceleration

A two-dimensional, time-dependent magnetohydrodynamic (MHD) model is used to describe the possible mechanisms for the source of solar cosmic ray acceleration following a solar flare. The hypothesis is based on the propagation of fast mode MHD shocks following a sudden release of energy. This model has already been used with some success for simulation of some major features of type II shocks and white light coronal transients. In this presentation, we have studied the effects of initial magnetic topology and strength on the formation of MHD shocks. We consider the plasma beta (thermal pressure/magnetic pressure) as a measure of the $\text{initial}$, relative strength of the field. During dynamic mass motion, the Alfvén Mach number is the more appropriate measure of the magnetic field's ability to control the outward motion. We suggest that this model (computed self-consistently) provides the shock wave and the disturbed mass motion behind it as likely sources for solar cosmic ray acceleration.     

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.     

Multi-point observations of earthward fast flow in the plasma sheet by virtual satellites located in the MHD simulation domain

Time profiles of some physical values in earthward fast flows in the plasma sheet are observed at three dimensionally different positions by employing virtual satellites located in the three-dimensional magnetohydrodynamic simulation domain, and these simulations are done on the basis of the spontaneous fast reconnection model. In the spontaneous fast reconnection evolution, the width of the flow channel is narrow in the dawn-dusk direction, and it does not spread until the plasma collides with the magnetic loop. The enhancements in B_z and V_x are larger at the center of the fast flow channel than those at its dawn and dusk edges, reflecting the differences in the reconnection rate in the diffusion region. The enhancement in V_x is shorter near the plasma sheet boundary layer than that near the neutral sheet, reflecting the changes in the thickness of the flow channel.     

On the diurnal variations in the temperature and composition: a three-dimensional model with superrotation

Based on a simplified theoretical interpretation of the composition measurements with the ONMS and OIMS experiments on Pioneer Venus, the conclusion was drawn that the rotation rate of the thermosphere should be close (within a factor of two) to that of the lower atmosphere. A more realistic three-dimensional model of the thermosphere dynamics is now being developed, considering non-linear processes, higher order modes and collisional momentum exchange between the major species CO₂, CO and O, which describes the diurnal variations in temperature and composition (Niemann et al., $\text{JGR}$, 1980). The computed horizontal winds are about 300 m/sec near the terminators and poles. Results are also presented from a two-dimensional (quasi-axisymmetric) spectral model which describes the four day superrotation in the lower atmosphere of Venus.     

Numerical simulation of reconnection in an emerging magnetic flux region

The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. The solution involves both ideal-MHD and resistive-MHD processes, and the solution shows an evolution which is remarkably suggestive of the preflare, impulsive, and main phases of the flare-cycle.     

Nonlinear development of MHD waves in coronal loops

The solar corona, modelled by a low β, resistive plasma slab sustains MHD wave propagations due to footpoint motions in the photosphere. The numerical simulation presents the evolution of MHD waves and the formation of current sheets. Steep gradients at the slab edges, which are signatures of resonance layers are observed. Singularities are removed by the inclusion of finite resistivity. The fast waves develop kink modes. As the plasma evolves the current sheets which provide heating at the edges fragment into two current sheets at each edge which in turn come closer when the twist is enhanced.     

Global MHD simulations for southward IMF: a pair of wings in the flanks

Global magnetohydrodynamic (MHD) simulations have shown that for southward interplanetary magnetic field nightside reconnection takes place only in a limited spatial scale in the cross-tail direction. Between the end of the nightside reconnection line and the flank magnetopause are regions of closed magnetic field lines which move relatively slowly tailward, compared with the magnetosheath flow on the far side and the jet flow produced by nightside reconnection on the side closer to the midnight. The magetosphere appears to have a pair of extended wings on the nightside. Further refinement of the simulations shows that the wings are not produced by the numerical effects. Similar features have been observed previously and also shown in different simulation models. The existence of the wings in the simulations indicates that the solar wind momentum is transferred to the closed magnetosphere if the simulations are relevant to reality.     

A solar wind driven model of geosynchronous plasma moments

We describe a tabular specification model of the density and temperature of ions and electrons at geosynchronous orbit as a function of magnetic local time and solar wind parameters. This model can be used to provide boundary conditions for numerical ring current models. Unlike previous specification models of geosynchronous plasma moments, this model is parameterized by upstream solar wind conditions. We find that solar wind parameters are a better predictor of geosynchronous ion density than magnetospheric indices, and as upstream parameters they are often more appropriate as model inputs since they causally precede the model outputs. Of the upstream parameters that were tested, the best predictors of geosynchronous conditions were the solar wind flow pressure and the magnitude and Z-component of the interplanetary magnetic field.     

Symmetric model of spacecraft charging and plasma emission effects

The electrostatic charging of satellites in space and the interactions with the plasma in the near surroundings are investigated by making use of symmetric models. In this case, the Vlasov-Poisson system describing the ambient plasma disturbances and the plasma emitted from the surface can be integrated self-consistently within a numerical iteration scheme, and the current balance yields the floating potential of the probe. The spacecraft charging and the potentials in its surroundings are investigated for the following plasma and emission conditions: (1) in the ionosphere in the case of very negative surface potentials, (2) in the solar wind with regard to the HELIOS mission and (3) in the near vicinity of the comet Halley, where a very strong plasma emission due to the impact of neutral gases onto the surface must be regarded. Finally, the importance of the shielding due to the ambient plasma is discussed.     

Excitation of electrostatic plasma waves by a moving charged source in a quantum plasma

Within a quantum hydrodynamic model and using the reductive perturbation technique, the nonlinear ion-acoustic wave (IAW) excitations due to a moving charged object in an electron-pair-ion quantum plasma are studied both analytically and numerically. In such quantum plasmas we have derived forced Korteweg-de Vries (fKdV) type equation for finite amplitude nonlinear IAWs. The effect of relevant plasma parameters on solitonic excitations is investigated. Numerical simulation shows the generation of advancing solitons ahead of the forcing term traveling at a faster rate with trailing wakes behind the forcing disturbance. It is found that propagation characteristics of nonlinear excitations are significantly affected by quantum parameter. Additionally, we have pursued our analysis by extending it to account for arbitrary amplitude IA solitons, and derived a system of nonlinear differential equations which are analyzed numerically to study the dynamics. Nonlinear analysis predicts the existence of periodic and quasiperiodic nature of the nonlinear system and reveals that the transition from quasiperiodic to periodic behavior occurs due to the variation of quantum diffraction.     

Numerical MHD modeling of the solar driver for the destabilization of a coronal helmet streamer

We investigate the forms of the solar driver which cause the destabilization of helmet streamers. Two forms of solar drivers are considered; (i) emergence of a flux-rope from sub-photospheric levels and (ii) application of a photospheric shear motion to a streamer-flux rope system. Numerical results showed that both cases exhibit the characteristics of commonly observed coronal mass ejections (CMEs), but the propagation speed of the CME is higher than the background solar wind speed when the solar driver is the emerging magnetic flux and is the same as the solar wind speed when the photospheric shear is used as the solar driver. The energy constraint allowing the magnetic field transition from a closed to an open configuration is also addressed.