Substorm Triggering by the Solar Wind

Abstract

AbstractThe coupling function monitors solar wind driving of geomagnetic activity. Magnetic indices, including the polar cap (PC) index, the auroral lower (AL) index, and the midlatitude positive bay index (MPB), provide measures of the magnetospheric response. Sharp changes in these are associated with the onset of a substorm and a maximum in coupling. Many reinterpret this association as evidence that northward turnings of the interplanetary magnetic field (IMF) trigger onset. We investigate this hypothesis using a list of over 100,000 negative bay onsets. When data are available, we calculate coupling and its time derivatives. From these, we construct ensembles of data segments centered on the onsets.`For all lists, a pulse occurs 20 min before onset and appears to trigger onset. By correlating the average derivative with every event in the ensemble, we conclude that a pulse of either sign precedes 40% of all onsets. More than half of the triggered onsets display a positive pulse, and slightly fewer show a negative pulse. Substorms following a positive pulse are stronger than those after a negative pulse. The pulse is usually hidden by noise, but ensemble averaging and correlation analysis can detect the pulse in the coupling data.