Predictions of substorms following northward turnings of the interplanetary magnetic field

Substorms are often observed to occur when the interplanetary magnetic field (IMF) turns northward at the end of intervals of southward IMF. Despite the significant correlation between northward turnings and substorms, a direct causal relationship between northward turnings and substorms has not been demonstrated. If such a causal relationship exists, then we should be able to predict that substorms will occur within a particular interval following the observation of a northward turning in the IMF. We have searched Wind IMF data and identified 16 strong northward turnings that following intervals of steady, southward IMF. To enhance the chances that the selected northward turnings impacted the magnetosphere, we required that each of them be also observed at either Geotail or IMP 8 while the separation of the second spacecraft from Wind was more that 10 RE in the y direction. Additionally, we required that either the second satellite was on the opposite side of the y = 0 plane from Wind or that one of the satellites was near y = 0. These two spacecraft observations also allowed us to more accurately predict the arrival times of the northward turnings at the Earth than we could by using data only from one spacecraft. Of the predicted substorms, 10 predictions were clearly successful to within ±8 min. Five predictions failed, but these failures reveal important new criteria that must be satisfied for a northward turning to cause a substorm. Three of the failures were caused by an increase in |ByGSM| that occurred simultaneously with the northward turning. This suggests that the triggering is associated with sharp reductions in the magnetospheric electric field, since increases in |ByGSM| negate the convection reduction associated with northward turnings. Two of the failures occurred for cases with the weakest southward IMF preceding the northward turning, which improves our criteria for the strength of the southward IMF that must precede a substorm. The final northward turning resulted in unusual auroral activity. These results establish the predictability of substorms following sharp northward turnings of the IMF and demonstrate that a direct causal relationship between northward turnings and substorm onset exists.