A STUDY OF THE EFFECT OF PITCH-ANGLE AND SPATIAL DIFFUSION ON TEARINGINSTABILITY USING A NEW FINITE-ELEMENT BASED LINEAR CODE

Although the collisionless tearing mode has been a leading candidate a s the mechanism responsible for substorm expansion phase onset, it has not been established that the near-Earth plasma sheet can become unst able to ion tearing owing to electron compression stabilization. The o bservation that the electron orbits become stochastic when the current sheet thins prior to substorm onset has given impetus to a view that pitch angle diffusion might overcome electron compression sufficiently to establish an ion mode, Theoretical attempts to demonstrate that in trinsic stochasticity in the electron orbits can destabilize the teari ng mode have not been upheld without controversy, In order to examine the effect of pitch angle scattering and spatial diffusion on the tear ing mode, we have developed a new linear code based on the finite elem ent technique. This code incorporates numerical particle orbits and ov ercomes many of the usual difficulties associated With solving kinetic linear stability problems. We find that neither intrinsic stochastici ty in the particle orbits nor externally imposed pitch angle scatterin g can destabilize the ion mode, The electron mode, however; can be ree stablished by pitch angle diffusion, although only for values of the n ormal magnetic field component that are too small to be of physical si gnificance, We also show that for the observed values of the normal ma gnetic field, the magnetotail is linearly stable to ion tearing even i n the presence of a large spatial diffusion. The possibility of ion te aring due to nonlinear and/or three-dimensional effects cannot, howeve r, be ruled out at this time.