Signatures for substorm development of the growth phase and expansion phase

Abstract

The development of a polar substorm consists of two distinctly different phases; i.e. the growth phase lasting 1 or 2 hr prior to the well-defined breakup, and the subsequent expansion phase. The essential signatures for the growth phase are a gradual development of the polar vortex type current without remarkable current enhancement in the auroral belt (DP 2 field), and the asymmetric growth of the low latitude field-depression in the eveningmidnight-early morning sector ( DRT field). After a distinct DP 2 and DRT fields, drastic changes occur in the substorm development and the expansion phase starts. The expansion phase is characterized by a rapid expansion of the auroral electrojet disturbance ( AEJ or DP- B field) in the high latitudes, and a rapid recovery of the field-depression near the midnight meridian and the subsequent asymmetric development of the disturbance field in the low latitudes. Considering the close associations of the interplanetary magnetic disturbance field with the magnetospheric field and particle disturbances which have been confirmed by various authors, it may be concluded that the growth phase is the precursory excitation of the magnetospheric background and the energy-storage or the energy-conversion stage in the magnetosphere due to the non-equilibrium magnetospheric convection, while the expansion phase is the energy-diversion stage in the magnetosphere. The disturbance field in the growth phase is a nucleus-like, fundamental element for the substorm breakup and it is essentially different from the expansion phase disturbance in quality and in quantity.