Where is the magnetic energy for the expansion phase of auroral substorms accumulated? 2. The main body, not the magnetotail

Abstract

It is suggested that the magnetosphere tries to stabilize itself by quickly unloading the magnetic energy accumulated within its main body, when the accumulated magnetic energy exceeds a limited amount, which can be identified as the energy for the expansion phase. It is this process which manifests as the impulsive expansion phase, during which auroral arcs advance well beyond the presubstorm latitude in the midnight sector. It was shown in the previous paper that the magnetotail does not have enough magnetic energy for a medium substorm (energy 5 × 1015 J; AE = 1000 nT). In this paper, it is shown that (1) the reason of the short lifetime (1–1.5 h) of the expansion phase is due to the fact that a limited amount of magnetic energy accumulated during the growth phase is dissipated in a period similar to the duration of the growth phase (1–1.5 h); the accumulation rate is similar to the dissipation rate during the expansion phase: (2) when the main body of the magnetosphere accumulates the magnetic energy, it is inflated; β (= (nkT/B2/8π)) even at XGSM = −6 RE becomes close to 1.0 for magnetic energy (2.9 × 1014 J) which is less than the amount consumed by a medium intensity substorm. (3) As a result, the plasma sheet current and thus the magnetosphere are expected to become unstable, unloading the accumulated excess magnetic energy and resulting in current reduction and deflation. (4) The resulting deflation can cause an earthward electric field of 5–50 mV/m, which can generate Bostrom's current system, which is mainly responsible in producing various phenomena of the expansion phase. (5) The large range of substorm intensity (AE = 100–2000 nT) is likely to be due to the location where the energy is accumulated; the closer is the distance to the Earth (XGSM between −10 RE and −4 RE), the more intense the substorm intensity is.