Chengcai Shen

Numerical Experiments of Wave-like Phenomena Caused by the Disruption of an Unstable Magnetic Configuration

The origin of the Moreton wave observed in the chromosphere and the EIT wave observed in the corona during the eruption remains an active research subject. We investigate numerically in this work the evolutionary features of the magnetic configuration that includes a current-carrying flux rope, which is used to model the filament, after the loss of equilibrium in the system takes place in a catastrophic fashion. Rapid motions of the flux rope following the catastrophe invoke the velocity vortices behind the rope, and may also invoke slow-and fast-mode shocks in front of the rope. The velocity vortices at each side of the flux rope propagate roughly horizontally away from the area where they are produced, and both shocks expand toward the flank of the flux rope. The fast shock may eventually reach the bottom boundary and produce two echoes moving back into the corona, but the slow one and the vortices totally decay somewhere in the lower corona before arriving of the bottom boundary. The interaction of the fast shock with the boundary leads to disturbance that accounts for the Moreton wave observed in Ha, and the disturbance in the corona caused by the slow shock and the velocity vortices should account for the EIT wave whose speed is about 40% that of the Moreton wave. The implication of these results to the observed correlation of the type II radio burst to the fast-and the slow-mode shocks and that of EIT waves to coronal mass ejections and flares has also been discussed.

NUMERICAL EXPERIMENTS ON FINE STRUCTURE WITHIN RECONNECTING CURRENT SHEETS IN SOLAR FLARES

We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibrium that separates two regions of the magnetic field with opposite polarity which are line-tied at the lower boundary representing the photosphere. Reconnection commences gradually due to an initially imposed perturbation, but becomes faster when plasmoids form and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. Among these X-points, a principal one exists at which the reconnection rate reaches maximum. A fluid stagnation point (S-point) in the sheet appeared where the reconnection outflow bifurcates. The S-point and the principal X-point (PX-point) are not co-located in space though they are very close to one another. Their relative positions alternate as reconnection progresses and determine the direction of motion of individual magnetic islands. Newly formed islands move upward if the S-point is located above the PX-point, and downward if the S-point is below the PX-point. Merging of magnetic islands was observed occasionally between islands moving in the same direction. Reconnected plasma flow was observed to move faster than blobs nearby.

NON-EQUILIBRIUM IONIZATION MODELING OF THE CURRENT SHEET IN A SIMULATED SOLAR ERUPTION

The current sheet that extends from the top of flare loops and connects to an associated flux rope is a common structure in models of coronal mass ejections (CMEs). To understand the observational properties of CME current sheets, we generated predictions from a flare/CME model to be compared with observations. We use a simulation of a large-scale CME current sheet previously reported by Reeves et al. This simulation includes ohmic and coronal heating, thermal conduction, and radiative cooling in the energy equation. Using the results of this simulation, we perform time-dependent ionization calculations of the flow in a CME current sheet and construct two-dimensional spatial distributions of ionic charge states for multiple chemical elements. We use the filter responses from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory and the predicted intensities of emission lines to compute the count rates for each of the AIA bands. The results show differences in the emission line intensities between equilibrium and non-equilibrium ionization. The current sheet plasma is underionized at low heights and overionized at large heights. At low heights in the current sheet, the intensities of the AIA 94 angstrom and 131 angstrom channels are lower for non-equilibrium ionization than for equilibrium ionization. At large heights, these intensities are higher for non-equilibrium ionization than for equilibrium ionization inside the current sheet. The assumption of ionization equilibrium would lead to a significant underestimate of the temperature low in the current sheet and overestimate at larger heights. We also calculate the intensities of ultraviolet lines and predict emission features to be compared with events from the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory, including a low-intensity region around the current sheet corresponding to this model.

Statistical and spectral properties of magnetic islands in reconnecting current sheets during two-ribbon flares

We perform a set of two dimensional resistive magnetohydrodynamic simulations to study the reconnection process occurring in current sheets that develop during solar eruptions. Reconnection commences gradually and produces small-scale structures inside the current sheet, which has one end anchored to the bottom boundary and the other end open. The main features we study include plasmoids (or plasma blobs) flowing in the sheet, and X-points between pairs of adjacent islands. The statistical properties of the fine structure and the dependence of the spectral energy on these properties are examined. The flux and size distribution functions of plasmoids roughly follow inverse square power laws at large scales. The mass distribution function is steep at large scales and shallow at small scales. The size distribution also shows that plasmoids are highly asymmetric soon after being formed, while older plasmoids tend to be more circular. The spectral profiles of magnetic and kinetic energy inside the current sheet are both consistent with a power law. The corresponding spectral indices c are found to vary with the magnetic Reynolds number R-m of the system, but tend to approach a constant for large R-m (> 10(5)). The motion and growth of blobs change the spectral index. The growth of new islands causes the power spectrum to steepen, but it becomes shallower when old and large plasmoids leave the computational domain

The coupling mode between Kelvin–Helmholtz and resistive instabilities in compressible plasmas

Using a compressible magnetohydrodynamics (MHD) simulation method, the coupling mode between Kelvin–Helmholtz (K–H) and resistive instabilities has been investigated. It is found that, in compressible plasmas, the coupling mode can take place in a limited range of ambient shear flow velocity, beyond which the coupling mode is stabilized. The flow shear can significantly enhance the growth of the coupling mode. Only in a narrow range of plasma beta, could the coupling mode occur. It is deduced that this kind of coupling mode may appear at Earth’s magnetopause.

Tearing mode with strong flow shear in the viscosity-dominated limit

Using a standard boundary layer approach, the tearing mode with shear flow comparable with shear magnetic field and fluid viscosity much larger than resistivity has been studied analytically. The results show that, the growth rate in this case scales as ν2/3ν, where νν is the normalized viscosity, in agreement with the numerical results of Einaudi and Rubini [Phys. Fluids B 1, 2224 (1989)]. It is found analytically that large viscosity may have a destabilizing effect on the instability.

Properties of the neutral energetic atoms emitted from Earth’s ring current region

Simulations of energetic neutral atoms (ENA) during the geomagnetic storm main phase have been carried out to provide reliable theoretical foundations for the development of an ENA detector on board the polar satellite of the Chinese Double Star Program (DSP), and to make preparation for the future ENA observational data analyses. In this research, an approximate analytical model for the ring current particle distributions, including the ion loss due to charge exchange processes, has been developed. The simulations have shown that there are two maximum ENA flux regions: The ring current inner boundary region, and the particle precipitation region at the northern and southern poles. The stronger the storms, the lower the particle injection, and the larger the flux of ENA emitted from the ring current region. The ENA detector at advantageous positions can measure the inner boundary of the injection region or the injection front. The ENA detector is able to measure the inhomogeneity of the ring current ions....

Shocks associated with the Kelvin-Helmholtz-resistive instability

In this paper, a new type of shock associated with magnetic reconnection processes has been explored using a compressible magnetohydrodynamics simulation method. The simulations have shown that, when there are strong field-aligned shear flows at the two sides of a current sheet, the coupling mode between Kelvin–Helmholtz and resistive instabilities will appear; further, reflected shocks and incident shocks can be produced at both sides of the boundary layer. Both the reflected shocks and incident shocks are fast shocks, through which the magnetic field strength, density, and temperature all increase sharply, while the plasma velocity decreases steeply. It is expected that some inhomogeneous structures can be formed at plasma boundary layer regions due to the existence of fast field-aligned shear flow driven shocks.

Properties of the tearing mode in periodic current sheets

The tearing mode in multiple periodic current sheet systems with a hyperbolic tangent profile of magnetic shear has been investigated. It is found that the linear growth rate of the antisymmetric and symmetric mode tearing instability increases and decreases, respectively, with the current sheet separation decreasing, which is similar to the situation with the step function profile of current density [A. Otto and G. T. Birk, Phys Fluids B 4, 3811 (1992)]. But, the results here also show that the linear growth rate of the antisymmetric mode tearing instability maximizes at certain current sheet separation. The linear growth of the tearing instability in multiple current sheet systems is not enhanced as greatly as has usually been expected.