Magnetospheric substorms: An inner-magnetospheric modeling perspective

Abstract

Abstract Kilovolt electrons cause satellite anomalies through surface charging and frequently interfere with the operation of geosynchronous spacecraft. The Magnetospheric Specification Model was designed to specify kilovolt electron fluxes at and near synchronous orbit, and while it has seen limited operational use, its accuracy is not high. One promising approach to improving the accuracy lies in assimilation of real-time-measured geosynchronous fluxes. An initial effort in that direction showed significant, though undramatic, improvements in accuracy. Since major changes in geosynchronous electrons are typically associated with magnetospheric substorms, accurate specification and forecast of geosynchronous electrons is likely to require understanding of the substorm phenomenon, particularly as it impacts the inner magnetosphere. Coupling of the Rice Convection Model (RCM) to a friction-code equilibrium solver now allows solution of a complete set of physical equations for the inner magnetosphere and inner plasma sheet, assuming adiabatic drift and isotropic pressure. In this formulation, the plasma is characterized by the distribution function f (λ,α,β), where λ= W k V 2/3 is the isotropic invariant, V is the flux-tube volume, and α and β are Euler potentials. Initial application of this coupled model to the substorm problem has produced the following results: o 1. More realistic calculation has verified the standard picture of the substorm growth phase. Enforcing a strong convection electric field across the magnetotail and assuming adiabatic convection (specifically conservation of f (λ,α,β) along a drift path) leads to storage of magnetic flux in the tail, development of a magnetic-field minimum in the inner plasma sheet, and a thinning and intensification of the current sheet there. Adiabatic convection by itself does not lead to injection of plasma into the inner magnetosphere. 2. From the viewpoint of the inner magnetosphere, the main effect of the substorm expansion phase is the creation of new closed plasma-sheet flux tubes having lower values of f (λ,α,β) and pV 5/3 than ordinary plasma-sheet flux tubes. These lower content flux tubes can more easily be injected into the geosynchronous-orbit region and inner magnetosphere. 3. Since the RCM is based on the assumption of adiabatic drift, it does not directly describe the nonadiabatic processes that are essential to the substorm expansion phase, but those processes can be parameterized in the RCM. Different assumptions about the non-adiabatic reduction of f on different flux tubes in the substorm expansion phase lead to different predicted ionospheric electric field patterns. These different patterns, along with differences in the particle distribution functions during injections, may prove useful in clarifying the physics of the expansion phase and choosing between competing substorm scenarios.