Duality of the magnetic flux tube and electric current descriptions of magnetospheric plasma and energy flow

Abstract

The duality between electric current and magnetic flux tubes is outlined for the magnetosphere. Magnetic flux tubes are regarded as fluid elements subject to various stresses. Current closure then becomes the dual of stress balance, and Poynting vector energy flow a dual of J · E dissipation. The stresses acting on a flux tube are magnetic stresses, which correspond to currents at a distance, and plasma stresses, which correspond to local currents. The duality between current and stress is traced for ionospheric ion drag forces, solar wind stresses at the magnetopause, inertial effects, and the effects of energetic plasma on flux tubes. The stress balance and dual current systems are outlined for idealized magnetospheres of increasing complexity. For a simple magnetosphere with no convective flow, the balanced stresses are solar wind pressure and neutral sheet plasma pressure. The corresponding current systems are the Chapman‐Ferraro magnetopause currents and the magnetotail current system. The introduction of convective flow introduces further stresses: ionospheric ion drag, Alfvén layer shielding, and an imbalance in day‐night magnetic stresses due to transport of flux tubes to the nightside by the solar wind. These stresses balance, and hence the corresponding additional currents (the ionospheric Pedersen current and the electrojets, the partial ring current, and two other current systems from the magnetopause and tail) must form a closed current system and do so by the region I and II field‐aligned currents of Iijima and Potemra. The energy flow in the above models is described in terms of both Poynting vectors and the above current systems. Temporal variations examined are (1) an increase in dayside merging and/or nightside reconnection, (2) an increase in the energy density of plasma in the plasma sheet, (3) an increase in ionospheric conductivity, and (4) an increase in solar wind pressure. Each of the first three requires an increase in all the currents and stresses associated with convective flow. The symptoms of variation 3 are very similar to many of those observed during magnetospheric substorms, and it is concluded that ionospheric conductivity enhancement is important in substorms, since it represents a major change in the forces affecting the flow and, in the dual picture, of the currents associated with the flow. Finally, the effect of aurora‐associated magnetic‐field‐aligned electric fields are considered, and the variation in the flows when the east‐west component of electric field parallel to the arc is included. Quite different plasma transport may result even though the north‐south meridian cross sections are the same.