Triggering of expansive phase intensifications of magnetospheric substorms by northward turnings of the interplanetary magnetic field

Over the past decade evidence has arisen suggesting that northward turnings of the interplanetary magnetic field after an interval of sustained southward IMF can trigger the onset of major substorm expansive phase effects. However, there has been some question in the past as to just how many substorms are triggered and whether, indeed, the correlation of substorm onsets and northward turnings was just a matter of chance. One reason for doubt was that the examples presented in the literature appeared during disturbed periods where the IMS was fluctuating significantly on a time scale of less than one hour and many substorm expansive phase intensifications were observable in the time frame where the correlation between the IMF and ground data was reported. The purpose of this paper is to present three clear‐cut examples of isolated substorms where the reader may be able to convince himself of the validity of the reported triggering effect. For these events, it will be clear that the substorm current wedge triggered by the northward turning of the IMF carries substantially larger currents than the driven system associated with the overall substorm event. The role of the IMF in externally triggering episodes of Joule heating and particle precipitation within the substorm process will be discussed.