Response of the polar electrojets in the evening sector to polar magnetic substorms

Several studies have established the existence of an eastward electrojet in the evening sector during periods of magnetospheric activity. It is shown that this electrojet may exist in such a state that it does not fluctuate in conjunction with the substorm-associated westward electrojet. However, it is also seen that the eastward electrojet may intensify at the time of the onset of a polar magnetic substorm. Studies of the evening sector auroral zone reveal that it is generally dominated by the eastward electrojet and that during substorm activity the westward electrojet in the midnight sector intensifies and expands into the evening sector and occupies a narrow latitudinal regime north of the poleward border of the eastward jet. The exact configuration of the electrojets in the evening sector is confirmed by using model current system calculations. It is shown that the growth of the eastward electrojet in the evening sector may be the signature of the onset of the expansive phase of a substorm in the midnight sector. Thus it is not possible to consider the growth of the eastward electrojet as a unique signature of the growth phase of a substorm.