Reasons why some solar wind changes do not trigger substorms

Previous studies have shown that a northward turning of the interplanetary magnetic field (IMF) or a reduction in the magnitude of IMF By component can trigger a substorm. Also a solar wind dynamic pressure enhancement has been found to trigger a substorm under strongly southward IMF conditions. Solar wind changes directed opposite to those of, or not satisfying the conditions of, these known triggers may be considered as inherently nontriggers. In the present paper we have examined 12 events where more than one solar wind quantity change simultaneously and no substorm is triggered. We suggest that nontriggering by the solar wind changes studied in the present work can be explained by the following three reasons: (1) there is a lack of, or insufficient, substorm growth phase development prior to the potential triggering change; (2) the solar wind change is an inherently nontriggering change; (3) the nullifying effect occurs, where one quantity changes in the direction of a substorm trigger and simultaneously another quantity changes in the direction opposite to that of a trigger. The nontriggering types found in this study are consistent with the suggestion that variations of the solar wind that do not reduce the convection strength within the inner plasma sheet do not trigger substorms, which is opposite to what is expected from a triggering solar wind variation.