The substorm as an internal magnetospheric instability: Substorms and their characteristic time scales during intervals of steady interplanetary magnetic field

Examples of magnetospheric substorms occurring during intervals when the interplanetary magnetic field is steady are used to illustrate the natural, internal instability of the magnetosphere, apart from the recently emphasized classes of substorms more closely associated with interplanetary medium variations. Such substorms have characteristic “expansion” phase and “recovery” phase times such that in virtually all cases T exp < T rec . The average time scales for the substorms examined are 〈 T exp 〉 = 13 min and 〈 T rec 〉 = 41 min. These time scales are tentatively identified as indicative of the natural, internal time scales for the basic magnetospheric substorm instability.