PHOTOMETRIC STUDIES OF THE POST-NOON DAYSIDE CLEFT REGION

Ground-based photometric observation of the postnoon dayside cleft region was carried out at Sachs Harbour ( LAMBDA equals 76. 7 degree ) during the period November 30 - December 19, 1977, by using a 6-channel meridian-scanning photometer. Continuous observation of the 6300-A O I emission permitted the mapping of the postnoon cleft morphology. The cleft was located at 77 degree - 78 degree LAMBDA on geomagnetically quiet days and farther equatorward by 2 degree - 3 degree on the most active days (Kp equals 4). During each afternoon an equatorward movement of the cleft was observed after about 1400 MLT. The peak intensity of the 6300-A emission was usually 1 - 2 kR. A distinction is made between 'soft' cleft characteristics identified early on several afternoons, when the 6300-A emission rate is always greater than that at 5577-A, and a more structured character seen at all other times. The structure takes the form of transient enhancements of the auroral emissions, particularly 5577 A. The enhancements have a duration of about 1 min and tend to occur as sequences. On none of the 10 clear days of observations was there optical evidence of energetic electron precipitation equatorward of the cleft.