THE AVERAGE IONOSPHERIC ELECTRODYNAMICS FOR THE DIFFERENT SUBSTORM PHASES

The average patterns of the electrostatic potential, current vectors, and Joule heating in the polar ionosphere, as well as the associated f ield-aligned currents, are determined for a quiet time, the growth pha se, the expansion phase, the peak epoch, and the recovery phase of sub storms. For this purpose, the Kamide-Ricbmond-Matsushita magnetogram-i nversion algorithm is applied to a data set (for March 17, 18, and 19, 1978) from the six meridian magnetometer chains (the total number of magnetometer stations being 71) which were operated during the period of the International Magnetospheric Study (IMS). This is the first att empt at obtaining, on the basis of individual substorms, the average p attern of substorm quantities in the polar ionosphere for the differen t epochs. The main results are as follows: (1) The substorm-time curre nt patterns over the entire polar region consist of two components. Th e first one is related to the two-cell convection pattern, and the sec ond one is the westward electrojet in the dark sector which is related to the wedge current. (2) Time variations of the two components for t he four substorm epochs are shown to be considerably different. (3) Th e dependence of these differences on the ionospheric electric field an d the conductivities (Hall and Pedersen) is identified. (4) It is show n that the large-scale two-cell pattern in the electric potential is d ominant during the growth phase of substorms. (5) The expansion phase is characterized by the appearance of a strong westward electrojet, wh ich is added to the two-cell pattern. (6) The large-scale potential pa ttern becomes complicated during the recovery phase of substorms, but the two-cell pattern appears to be relatively dominant again during th eir late recovery as the wedge current subsides. These and many other earlier results are consistent with the present ones, which are more q uantitatively and comprehensively demonstrated in our global study. Th us the two components are tentatively identified as the directly drive n and the unloading components, respectively, although there is some i ndication that both components are actually coupled in the ionosphere. In the present paper we show that the directly driven component is pr esent throughout the lifetime of substorms, becoming a dominant featur e during the recovery phase of substorms as the unloading component wa nes. The fact that the two components exist and that their time variat ions are different indicates that different processes are in progress even for the same value, say, -500 nT, of AL and that we must be cauti ous in using a single electrojet index, such as AL, in ordering satell ite data.