The Expansive Phase of the Magnetospheric Substorm

Abstract

Stress balance and current closure in the magnetosphere are equivalent representations of the same condition. The stresses and currents in the magnetosphere that determine the steady-state convective flow are reviewed. A thinner plasma sheet implies a stronger earthward force (more-dipolar field lines) in the plasma sheet and hence a greater stress in the magnetosphere driving convective flow. Many of the observed properties of substorms can be explained by simply assuming that tail current is diverted along magnetic field lines and through the ionosphere in some local time sector without knowing the cause of the diversion. In particular: i) the current system is capable of controlling the ionospheric conductivity by precipitation associated with the current and the collapse of field lines to a more dipolar form, ii) increased ionospheric conductivity requires more-dipolar field lines near midnight in the plasma sheet, iii) the westward electrojet acts to limit the inflow, also resulting in more-dipolar field lines, iv) the current system causes a significant redistribution of plasma if tail current is due to proton drifts and field-aligned currents are carried by electrons, v) a twist must appear in flux tubes as the upward current increases. It is not clear whether the substorm energy comes from stored energy in the magnetotail, or whether it flows more or less directly from the solar wind due to an increased efficiency of the magnetosphere in extracting energy from the solar wind. The latter can occur because an increase in ionospheric conductivity leads to a distorted magnetosphere which takes more energy from the solar wind. Evidence for this is found in the thinning of the plasma sheet, in the substorm-associated motion of the dayside auroras, and in the correlation between the AE index and solar-wind parameters. Three possible macroinstabilities might be the cause of the current diversion and hence of the substorm. The first is the formation of an X-type neutral line, the second is magnetosphere-ionosphere coupling and the control of currents by precipitation (and vice versa) resulting in a feedback system, and the third is an interchange instability which is decoupled from the ionosphere by parallel electric fields. A mathematical development related predominantly to the second of the above macro instabilities gives satisfactory instability growth times and propagation velocities, and further shows that the system is normally near the instability threshold. The most likely cause of the onset of instability is an increase in electric field due to either an increase in cross-polarcap potential or a decrease in the width of the auroral oval. A model in which the macroinstability is due to ionospheremagnetosphere coupling and in which the energy flow is more or less directly from the solar wind probably gives the best fit to observations at this time.