The development of the substorm expansive phase: The “eye” of the substorm

The explosive onset and development of a clearly defined auroral substorm has been analysed using data from the ultraviolet imager aboard the VIKING satellite. These data permit the evolution of the substorm to be traced with a time resolution as high as 40 seconds. The envelope of the disturbed region ‐ the "eye" ‐ expands both poleward and azimuthally to the east and west reaching an azimuthal extent of almost six hours of magnetic local time after about 15 minutes. The poleward expansion involves irregular poleward motion accompanying intensifications within the eye, with the maximum poleward displacement being at the meridian of the center of the eye. Within the "eye", discrete regions of enhanced luminosity develop with a tendency for each newly disturbed region to be to the west of the previous ones. It is, however, observed that a newly excited region can be to the east of the pre‐existing activated region. Each new intensification is accompanied by a brief further poleward expansion of the high latitude edge of the "eye". Each region of enhanced luminosity within the "eye" remains relatively stationary and fluctuates in intensity over its lifetime. We identify these regions of enhanced luminosity with westward traveling surges as defined from ground observations in the earlier literature. Our UV imager data demonstrate that these surges do not necessarily propagate although the overall envelope of the substorm disturbed region expands both eastward and westward during the lifetime of the event.