A Linear Analysis of Tearing Instability in Magnetotail Equilibria Using Simulation and Numerical Methods.
The mechanism that is responsible for the initiation of magnetospheric substorms has been sought for decades. Spacecraft observations support the view that substorm initiation is correlated with disruption of the near-Earth magnetotail current sheet and that magnetic reconnection may be involved. The collisionless tearing mode, which provides the dissipation necessary for reconnection, has been the leading candidate for tail disruption and substorm initiation, although not without controversy. The theoretical demonstration that electron compression is sufficient to stabilize the ion tearing mode for typical magnetotail parameters has been countered by predictions that pitch angle diffusion (PAD), caused by external waves or intrinsic chaos, can remove the electron stabilization. In this thesis, the formulation of the energy principle in the presence of PAD is re-evaluated since it appears to be the central issue of the controversy and the source of the divergent results. A new algorithm is introduced that is able to test these predictions and to determine what effect PAD might have on the stability of the mode. This algorithm features a finite element solution of the energy principle where the quantities that depend on the details of the particle orbits are determined by simulation methods. The result of a numerical investigation using this algorithm indicates that PAD does not remove the strong stabilizing effect of electron compression and that the ion mode is stable for a wide range of parameters typically observed in the pre-substorm near-Earth magnetotail current sheet. Another result that was not predicted previously, but has been demonstrated by this algorithm, is the destabilization of the electron tearing mode by PAD for extremely small values of the normal magnetic field component. In this limit, where electron compression is negligible, the electron resonance can be restored by diffusion across the magnetic field. A second algorithm that incorporates simulation methods in the solution of the dispersion relation has been applied in this work toward the development of the linear theory of tearing instability.
- Pub Date:
- January 1995
- Physics: Fluid and Plasma, Physics: Astronomy and Astrophysics