Solar-terrestrial coupling poleward of the auroral zone.

Abstract

Stresses of importance inside the magnetosphere include ionospheric ion drag, plasma pressure, inertia and viscous stresses. These, or the corresponding currents, must be combined with boundary conditions of plasma inflow, tangential electric fields and either magnetic fields or currents to solve for the internal convection. The convection of Bipolar flux tubes is best understood, the weakest point being the boundary conditions. Convection in the plasma sheet is poorly understood since neither the plasma input nor the energy loss mechanism are clearly understood. On open flux tubes, the ionosphere and outer boundary conditions determine the convection. Understanding here is, at best, qualitative. The solar wind interacts with the magnetosphere to twist flux tubes about their axes (producing distributed Birkeland currents) or about each other (producing current sheets). Time constants involved include the untwisting of flux tubes (decay of Birkeland current) by convection antisunward of the connection point of the flux tube to the solar wind (2000 sec), and by convection of the flux tube feet in the ionosphere (a few hours). The presence of tangential discontinuities and also of X lines and separatrices should be indicated by Birkeland current sheets, since both occur at discontinuities in the topological connection of field lines to the outer regions. If, as the IMF rotates in the yz plane, the merging sites move from closed to open field lines as predicted by the antiparallel merging model, then the hierarchy of convection cells: viscous, merging, lobe and reclosure appears to be plausible. The reclosure cell also requires nightside merging and closed field lines at high latitudes. However, these are not the only possibilities and much work on merging models and topology remains to be done.