Origin of resistivity in reconnection

Resistivity is believed to play an important role in reconnection leading t o the distinction between resistive and collisionless reconnection. The for mer is treated in the Sweet-Parker model of long current sheets, and the Pe tschek model of a small resistive region. Both models in spite of their dif ferent dynamics attribute to the violation of the frozen-in condition in th eir diffusion regions due to the action of resistivity, In collisionless re connection there is little consensus about the processes breaking the froze n-in condition. The question is whether anomalous processes generate suffic ient resistivity or whether other processes free the particles from slavery by the magnetic field. In the present paper we review processes that may c ause anomalous resistivity in collisionless current sheets. Our general con clusion is that in space plasma boundaries accessible to in situ spacecraft , wave levels have always been found to be high enough to explain the exist ence of large enough local diffusivity for igniting local reconnection. How ever, other processes might take place as well. Non-resistive reconnection can be caused by inertia or diamagnetism.