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Abstract:
Context. Motion of the photospheric plasma forces the footpoints of magnetic flux tubes to move. This can give rise to electric currents in the solar atmosphere. The dissipation of these electric currents and the consequent heating of the solar plasma may be responsible for the formation of Extreme-UltraViolet (EUV) and X-ray bright points. Earlier bright point models usually consider either the emergence or the canceling of photospheric magnetic features as being responsible for reconnection causing the bright point.
Aims. We investigate the consequences of different patterns of horizontal photospheric plasma motion for the generation of electric currents in the solar atmosphere and locate them with respect to an observed EUV bright point. The goal is to find out whether these currents might be responsible for the heating of bright points.
Methods. To perform this study we use a "data driven" three dimensional magnetohydrodynamic model. The model solves an appropriate set of magnetohydrodynamic equations and uses, as initial condition, the magnetic field extrapolated from the line-of-sight component of the photospheric magnetic field observed by MDI/SoHO and the height-stratified, equilibrium density and temperature of the solar corona. We apply different patterns of horizontal photospheric plasma motion, derived from the temporal evolution of the photospheric magnetic structures in the course of the bright point lifetime, as boundary conditions of the model.
Results. All applied patterns of horizontal photospheric plasma motion (shearing, convergence and fragmentation) lead to the formation of electric currents in the chromosphere, transition region and corona. Currents do not develop everywhere in the region where the motion is applied but in specific places where the magnetic field connectivity changes significantly. An important result is that the position where the electric currents develop is independent of the motion pattern used as boundary condition of the model. A comparison with data obtained by TRACE in the 1550 angstrom channel and by the EIT in the 195 angstrom channel shows that the region where the strongest current concentrations are formed coincides with the region where the EUV bright point appears.
