On the threshold for triggering substorms

The substorm-neutral-line model of Hones (1984) is extended in order to interpret substorm-related effects that have not previosly been linked to model. It is proposed that the level of stress at which the substorm expansion starts is controlled by the tail field geometry and that substorms most easily initiate when the bending of the magnetotail is most extreme. Using this 'bent-tail' (BT) hypothesis, a new interpretation is developed for the annual and diurnal variations of the level of geomagnetic activity, that are independent of the polarity of the IMF but are due to the BT effect. The BT effect leads to predictions regarding annual and diurnal signatures of substorm occurrence frequency and magnitude that can be tested.