On the threshold for triggering substorms

Many features of substorms are satisfactorily described by a phenomenological model in which the substorm onset is related to the formation of a neutral line within the plasma sheet close to the Earth. However, the model does not account for the fact that the amount of tail stress released in different substorms is highly variable and that the intensity of global substorm-related signatures can differ greatly. Here we propose that the level of stress at which the substorm expansion starts is controlled by the tail field geometry and remark that the field line curvature required for the formation of a near-Earth neutral line is already present when the dipole is tilted towards or away from the Sun. Assuming that substorms are most readily initiated when the tail field geometry is favourable, we develop a new interpretation of the aspects of the annual and diurnal variation of the level of geomagnetic activity that are independent of the polarity of the interplanetary magnetic field. We attribute the variations to the ''bent tail'' (BT) effect. We believe that the BT effect provides a more reasonable interpretation of the observed modulations than does the previously-proposed Kelvin-Helmholtz mechanism. The BT effect leads to predictions regarding annual and diurnal signatures of substorm occurrence frequency and magnitude that can be tested. (author)