Auroral Substorms: Search for Processes Causing the Expansion Phase in Terms of the Electric Current Approach

Auroral substorms are mostly manifestations of dissipative processes of electromagnetic energy. Thus, we consider a sequence of processes consisting of the power supply (dynamo), transmission (currents/circuits) and dissipations (auroral substorms-the end product), namely the electric current line approach. This work confirms quantitatively that after accumulating magnetic energy during the growth phase, the magnetosphere unloads the stored magnetic energy impulsively in order to stabilize itself. This work is based on our result that substorms are caused by two current systems, the directly driven (DD) current system and the unloading system (UL). The most crucial finding in this work is the identification of the UL (unloading) current system which is responsible for the expansion phase. A very tentative sequence of the processes leading to the expansion phase (the generation of the UL current system) is suggested for future discussions. The solar wind-magnetosphere dynamo enhances significantly the plasma sheet current when its power is increased above 10 18 erg / s ( 10 11 w). The magnetosphere accumulates magnetic energy during the growth phase, because the ionosphere cannot dissipate the increasing power because of a low conductivity. As a result, the magnetosphere is inflated, accumulating magnetic energy. When the power reaches 3 – 5 × 10 18 erg / s ( 3 – 5 × 10 11 w) for about one hour and the stored magnetic energy reaches 3 – 5 × 10 22 ergs ( 10 15 J), the magnetosphere begins to develop perturbations caused by current instabilities (the current density ≈ 3 × 10 − 12 A / cm 2 and the total current ≈ 10 6 A at 6 Re). As a result, the plasma sheet current is reduced. The magnetosphere is thus deflated. The current reduction causes ∂ B / ∂ t > 0 in the main body of the magnetosphere, producing an earthward electric field. As it is transmitted to the ionosphere, it becomes equatorward-directed electric field which drives both Pedersen and Hall currents and thus generates the UL current system. A significant part of the magnetic energy is accumulated in the main body of the magnetosphere (the inner plasma sheet) between 4 Re and 10 Re, because the power (Poynting flux [ E × B ] ) is mainly directed toward this region which can hold the substorm energy. The substorm intensity depends on the location of the energy accumulation (between 4 Re and 10 Re), the closer the location to the earth, the more intense substorms becomes, because the capacity of