Where is the magnetic energy for the expansion phase of auroral substorms accumulated?

Characteristics of the ionospheric current system (the directly driven (DD) / unloading (UL) currents) and the power ε during auroral substorms are examined semiquantitatively in terms of input‐output relationship. The study confirms (a) The magnetic energy for the expansion phase is accumulated with the rate of a little more than 1018 erg/s during the growth phase which lasts for about 1 h (because the ionosphere cannot dissipate the input power). (b) The global energy dissipation rate in the ionosphere (including the Joule heat production and the particle precipitation) is more than 3.3 × 1018 erg/s. (c) Thus, the total energy consumption by the ionosphere during the impulsive expansion phase (lasting for about 1 h) is 1.2 × 1022 erg or up to 1023 erg for medium substorms. (d) However, it is shown that even the total magnetic energy in the magnetotail at a distance between 10 RE and 20 RE (6.5 × 1021 ergs = 6.5 × 1014 J) cannot supply the energy for the expansion phase. This estimate is consistent with the most recent satellite observations (4.3 × 1013 J (= 4.3 × 1020 ergs)). Thus, the following conclusions may be applicable for medium intensity substorms: (i) The major part of the magnetic energy for the expansion phase must be accumulated within a distance of 10 RE and be released almost totally during the expansion phase (because the duration of the growth phase and the expansion phase is about the same). (ii) This suggests that there is an upper limit of the energy which the magnetosphere can accumulate even at distances less than 10 RE. Further, it is confirmed that: (a) The expansion phase is characterized by a single cell current system UL (not a twin cell current system DD which is driven by the earthward plasma convection from the magnetotail). (b) The UL current is independent of the DD current, requiring a different origin. (c) During the recovery phase, the UL current is absent and the DD current approximately follows ε (no accumulation of the energy), because the ionosphere is capable of dissipating the power even if the power is moderate). (d) Based on these results, as an example, one possible chain of processes within 10 RE is proposed as the release process of the accumulated magnetic energy by taking into account the fact that the frozen‐in field condition is observed to be broken down. Key Points Magnetic energy for expansion phase must be accumulated before substorm onset Major part accumulated within distance of 10 RE and released du