MHD and the Drift Model
The basic physical idea of Magnetohydrodynamic (MHD) is that a magnetized plasma can move quickly across the magnetic field, with flow velocities near the thermal speed. The resulting drift model is more complicated than MHD, including in particular terms of various orders in δ. It is also more realistic in many cases of interest. Magnetic reconnection is thought to be the cause of several observed phenomena such as coronal mass ejections, solar flares, magnetic storms, and disruptions in laboratory plasma. Thus resistive MHD should be viewed as a model which, without detailed mathematical justification, sheds light on real phenomena. Kinetic MHD is so-named because it uses the key physical assumptions of MHD, allowing the perpendicular electric field to be large as, and omitting all finite-gyroradius corrections. The perpendicular motion of magnetized electrons, because of their much smaller mass, is simpler than that of ions: such effects as electron gyroviscosity are proportional to ρe and consistently neglected.