Comparison of substorm onsets during different levels of interplanetary magnetic field $$B_{z}$$ B z

Abstract

The magnetospheric response during the substorm events to solar wind driving, as determined by the level and sign of the interplanetary magnetic field (IMF) \(B_{z}\), is analysed. Using the superposed epoch analysis, solar wind and geomagnetic conditions under three levels of \(B_{z}\) are characterised, i.e. northward or BZN (\(B_{z} > 0~\hbox {nT}\), 75 events), very weak or BZ0 (\(B_{z } \sim 0~\hbox {nT}\), 78 events) and southward or BZS (\(B_{z} < 0~\hbox {nT}\), 80 events). No northward turning is observed during BZS, while northward turning occurs 20 and 50 min prior to onset, during the BZN and BZ0 classes, respectively. IMF has a strong duskward component and the solar wind speed is also the fastest during BZN onsets. Auroral activity, as measured by the AL index, takes a longer time to decay to preonset values during BZ0 and BZS onsets compared to the BZN onsets. The level of IMF \(B_{z}\) does not seem to influence the oval thickness in the noon sector. The oval is the thinnest during BZ0 events in all the sectors. The rate of auroral widening in the dawn sector is found to be evidently slower than in the dusk sector during the BZS group.