Enhancements in auroral drift velocity in the dusk sector associated with a small substorm in the midnight sector.

Temporal and spatial variations of the magnetospheric electric field are studied during a small isolated substorm, by measuring drifts of auroral patches on the basis of all-sky TV records of aurora. An increase in westward drift velocity of many non-pulsating auroral patches was observed in the dusk sector associated with the substorm which broke out at 3:26 UT on February 17, 1980 in the midnight sector. It was shown that the drift velocity increased and decreased with a time lag of about ten minutes behind the development and decay, respectively, of the westward electrojet centered at midnight meridian. The drift velocity increased more evidently in higher latitude so that a distinct latitudinal dependence in intensity was seen, particularly at its maximum stage. Two mechanisms are discussed in order to explain these observations; the first one considers the electric field modified by injected particles, while the second one is propagation of the induced electric field associated with the substorm evolution.