On nonlinear plasma instabilities during the substorm expansive phase onset

Abstract

Using a stability analysis based on a Lagrangian–Hamiltonian approach to magnetohydrodynamics, this paper addresses the stability of the Earth’s magnetotail during the later stages of the growth phase of the substorm. The analysis shows that the magnetotail at about 10–12RE is the likely region for instability. Furthermore, the analysis helps to explain why the substorm intensification begins on field line mapping from the Earthward edge of the substorm plasma sheet to auroral arcs near the equatorward edge of the auroral region. Our model and analysis provides a possible explanation for pseudobreakups as well as the initiation of more global substorm intensifications. A stability analysis of the evolving stretched field configurations of the growth phase shows that initially, at a certain level of stretching and in regions where the plasma β is 5–10, localized instabilities are possible. These localized instabilities saturate before producing a more global instability and the reconfiguration of the magnetic field topology of the magnetotail. Eventually the threshold for nonlinear instabilities is reached and a global, explosive instability is possible. Even though this paper looks only at the possibility of explosive instabilities and the initial stages of the substorm intensification, it provides an important first step in forming a complete picture of the substorm instabity and the subsequent return to a lower energy, metastable state through dipolarization of field lines in the magnetotail. A number of important features in the initiation of the substorm intensification can be explained by considering an explosive ballooning mode. Nevertheless, a more complete analysis of the plasma dynamics associated with the instability initiating the substorm intensification and the subsequent evolution of the substorm, including kinetic effects and reconnection, is needed for a more complete model of the substorm process.