THE IMPORTANCE OF PHOTOSPHERIC INTENSE FLUX TUBES FOR CORONAL HEATING

Dissipation of magnetic energy in the corona requires the creation of Very fine scale-lengths because of the high magnetic Reynolds number o f the plasma. The formation of current sheets is a natural possible so lution to this problem and it is now known that a magnetic field that is stressed by continous photospheric motions through a series of equi libria can easily form such sheets. Furthermore, in a large class of 3 D magnetic fields without null points there are locations, called 'qua si-separatrix layers' (QSLs), where the field-line linkage changes dra stically. They are the relevant generalisation of normal separatrices to configurations without nulls: along them concentrated electric curr ents are formed by smooth boundary motions and 3D magnetic reconnectio n takes place when the layers are thin enough. With a homogenous norma l magnetic field component at the boundaries, the existence of thin en ough QSL to dissipate magnetic energy rapidly requires that the field is formed by flux tubes that are twisted by a few turns. However, the photospheric field is not homogeneous but is fragmented into a large n umber of thin flux tubes. We show that such thin tubes imply the prese nce of a large number of very thin QSLs in the corona. The main parame ter on which their presence depends is the ratio between the magnetic flux located outside the flux tubes to the flux inside. The thickness of the QSLs is approximately given by the distance between neighbourin g flux tubes multiplied by the ratio of fluxes to a power between two and three (depending on the density of flux tubes). Because most of th e photospheric magnetic flux is confined in thin flux tubes, very thin QSLs are present in the corona with a thickness much smaller than the flux tube size. We suggest that a turbulent resistivity is triggered in a QSL, which then rapidly evolves into a dynamic current sheet that releases energy by fast reconnection at a late that we estimate to be sufficient to heat the corona. We conclude that the fragmentation of the photospheric magnetic field stimulates the dissipation of magnetic energy in the corona.