SAPS onset timing during substorms and the westward traveling surge

We present multispacecraft observations in the magnetosphere and conjugate ionosphere of the onset time of subauroral polarization streams (SAPS) and tens of keV ring current injections on the duskside in three individual substorms. This is probably the first unequivocal determination of the substorm SAPS onset timing. The time lag between the SAPS and substorm onsets is much shorter than the gradient‐curvature drift time of ∼10 keV ions in the plasmasphere. It seemingly depends on the propagation time of substorm‐injected plasma from the dipolarization onset region to the plasmasphere, as well as on the SAPS position. These observations suggest that fast onset SAPS and ring current injections are causally related to the two‐loop system of the westward traveling surge.