On the conditions preceding sudden magnetotail magnetic flux unloading

According to the substorm paradigm, the tail magnetic flux F is accumulated during the growth phase and suddenly released afterward. However, only 33 out of 142 isolated midtail sudden unloading events identified by Geotail in 1995–1998 demonstrated monotonous F increase (type 1 substorms). In 32 cases, balanced tail flux (BTF) period (F ~constant) with enhanced dissipation and pseudobreakup activity during 0.5–2 h before unloading was registered (type 2, a new substorm class). The rest 77 substorms lie between types 1 and 2. Superposed epoch analysis revealed that during ~40 min before unloading, both groups demonstrate almost identical plasma sheet thinning and Bz decrease at Geotail. By the sudden unloading time, both groups have nearly identical Bz values in the midtail as well as at 6.6 RE. We conclude that pre‐onset midtail configuration changes are common for all substorm types irrespective of flux growth.