Influence of Turbulence on Magnetic Reconnection in Space Plasma
M.I. Pudovkin and A.V. Runov
St. Petersburg University
The fundamental physical mechanisms of fast large-scale restructuring of magnetic fields, that means of magnetic reconnection, as they occur in flares of stars like the sun, in the solar wind, in supernovae envelopes, in the earth's magnetosphere, and possibly also in protostars and accreation discs, are up to now almost unclear.
One of the problems consists in the fact that the electrical resistivity caused by direct particle interaction is to low to explain the obtained small time scales of the process. That is why one develops the theory of plasma instabilities further. Especially ion-acoustic, ion-cyclotron or gradient-driven instabilities like the lower-hybrid-drift instability are considered.
Instabilities may generate waves, which interact with the particles, and thus locally increase the resitivity by many orders. Thus, in cosmical current sheets, diffusion regions may occur, which partially destruct these layers. Then, during the reconnection process, a large part of the energy of the magnetic field is transformed into heat and acceleration energy of the particles.
In Figs. 1-6, results of two-dimensional magnetohydrodynamic modeling of reconnection processes are presented for systems with regions of initially antiparallel directed magnetic field lines and strong tail-like current sheets. The temporal and spatial evolution of resistivity, plasma density, and the electric and magnetic fields was investigated. It was assumed that at the magnetic neutral sheet ion-cyclotron turbulence is generated. Further, as a consequence of the enhancement of the pressure gradients by the excitation of hydrodynamic waves, at a certain distance from the neutral sheet also the lower-hybrid-drift instability is generated.
Fig. 1: Restructured magnetic field (blue), which initially had almost horizontal field lines directed at z>0 to the left and at z<0 to the right. Reconnection was initiated by wave excitation at z=0, x=5. L is the scale of the reconnection region. In the solar system, L amounts to 10^7 - 10^10 m. The red arrows mark the velocities of the accelerated particles which almost equal the Alfven velocity of the plasma of 5*10^4-10^6 m/s.
Fig. 2: During the reconnection process the region of increased resistivity, which was initially situated at z=0,x=5, propagates from the neutral sheet into the matter and mainly to the periphery of the reconnection region. The figure shows the resistivity at Ta. Ta is the Alfven time amounting to the Alfven velocity times the linear dimension L of the reconnection region.
Fig. 3: Contours of the vector potential of the magnetic field and of the inductive electric field at time t=0.25 Ta.
Fig. 4: Contours of the vector potential of the magnetic field and of the inductive electric field at time t=0.5 Ta.
Fig. 5: Contours of the vector potential of the magnetic field and of the inductive electric field at time t=1.0 Ta.
Fig. 6: Density variations in the reconnection region caused by the excitation of magnetohydrodynamic waves.
C.-V. Meister gratefully acknowledges financial support by the project 24-04/055-1999 of the Ministerium für Wissenschaft, Forschung und Kultur des Landes Brandenburg, and by the project ME 1207/7 of the Deutsche Forschungsgemeinschaft. M.I.Pudovkin and A.V.Runov would like to thank the organization Deutsche Forschungsgemeinschaft for financing their stay at the Astrophysical Institute Potsdam to work on the given topic within the frame of the DFG-RFBR project 436 RUS 113/77.