Drifts of auroral structures and their relationship to geomagnetic activity

The paper investigates the accurate motion of some auroral structures, such as pulsating and nonpulsating patches and discrete auroral arc fragments, on the basis of all-sky TV observations of auroras. It is shown that the general drift pattern of these auroral structures correspond, on a gross scale, to magnetospheric conversion, namely, westward in the evening sector and eastward in the morning sector. The drift velocity ranges from 50 to 2000 m/s, corresponding to an electric field intensity from 3 to 120 mV/m at the ionospheric level and from 0.1 to 4 mV/m in the magnetospheric equatorial region at an L value of about 6.5. Temporal and spatial changes in the drift of the patches are related to the evolution of a substorm. Clear enhancement in the drift velocity associated with increases in the westward electrojet is observed at a different local time region from the center of the electrojet. It is concluded that the convection electric field changes temporally and spatially when a new substorm expansion of aurora occurs in the midnight sector.