Magnetic reconnection in magnetotail and solar plasmas

The formation of current sheets which dominates the heating of the solar corona and the onset of substorms due to collisionless tearing instability in the magnetotail are investigated in the context of magnetic field line reconnection in space plasmas.; In Chapters II and III of this thesis, the collisionless tearing instability and current disruption of the magnetotail are considered. The linear collisionless tearing instability, with wavelengths of the order of 10 {dollar}Rsb{lcub}E{rcub}{dollar}, and with a growth rate {dollar}gammasim{dollar} 10{dollar}sp{lcub}-2{rcub}{dollar}/{dollar}sec{dollar}, is identified as a possible mechanism for the growth phase of a substorm. The linear analysis is carried out in the presence of an equilibrium {dollar}Bsb{lcub}y{rcub}{dollar}-field, neglected in other theories. The nonlinear theory of collisionless tearing mode is dominated by mode coupling effects. From the evolution equations for electro-magnetic field perturbations, we derived a nonlinear growth rate by generalizing the boundary layer techniques of linear theory. We find that the nonlinear growth is of the order of 1 {dollar}sec,{dollar} much faster than the linear growth. It is proposed that collisionless tearing modes provide a mechanism for current disruption observed by spacecrafts. The electrical field generated during the nonlinear evolution can cause particle acceleration in the earth-tail direction. Our estimates indicate ion energies up to 0.7 {dollar}MeV{dollar} and electron energies up to 8.1 {dollar}MeV,{dollar} which is not inconsistent with the observations.; In the Chapter IV it is shown that current sheets can be formed in Parker's model of the solar corona in the presence of smooth photospheric flows, despite van Ballegooijen and Field's proof that in an ideal plasma current sheets do not occur unless the boundary velocity field is discontinuous. The formation of current sheets is demonstrated from the requirement that magnetic helicity is conserved in the presence of a very small but finite plasma resistivity that permits slippage of field lines with respect to the plasma. The rapid reconnection involved in the formation of current sheets justifies the helicity conservation.