ИСТИНА

Substorms and similar transients developing without evident external trigger in the interplanetary space are analyzed. “Non-triggered” substorms are classified in accordance with space weather conditions into two groups. The substorms of the first group are actually caused by a trigger in the interplanetary space, caused by a jump in parameters of fluctuations of IMF and solar wind dynamic pressure. The other group can be notated as really non-triggered substorms, as the develop from intra-magnetospheric processes. Specific pre-substorrm ULF geomagnetic variations in 1-4 mHz (Pc5/Pi3), frequency range, substorm precursors, are analyzed for both types of substorms and non-substorm days.