A study of the effect of pitch angle and spatial diffusion on tearing instability using a new finite element based linear code

Abstract

Although the collisionless tearing mode has been a leading candidate as the mechanism responsible for substorm expansion phase onset, it has not been established that the near‐Earth plasma sheet can become unstable to ion tearing owing to electron compression stabilization. The observation 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 intrinsic stochasticity in the electron orbits can destabilize the tearing mode have not been upheld without controversy. In order to examine the effect of pitch angle scattering and spatial diffusion on the tearing mode, we have developed a new linear code based on the finite element technique. This code incorporates numerical particle orbits and overcomes many of the usual difficulties associated with solving kinetic linear stability problems. We find that neither intrinsic stochasticity in the particle orbits nor externally imposed pitch angle scattering can destabilize the ion mode. The electron mode, however, can be reestablished by pitch angle diffusion, although only for values of the normal magnetic field component that are too small to be of physical significance. We also show that for the observed values of the normal magnetic field, the magnetotail is linearly stable to ion tearing even in the presence of a large spatial diffusion. The possibility of ion tearing due to nonlinear and/or three‐dimensional effects cannot, however, be ruled out at this time.