Photometric studies of the postnoon dayside cleft region

Ground‐based photometric observation of the postnoon dayside cleft region was carried out at Sachs Harbour (Λ=76.7°) during the period November 30–December 19, 1977, by using a 6‐channel meridian‐scanning photometer. Continuous observation of the 6300‐Å O I emission permitted the mapping of the postnoon cleft morphology. The cleft was located at 77°–78°Λ on geomagnetically quiet days and farther equatorward by 2°–3° on the most active days (Kp=4). During each afternoon an equatorward movement of the cleft was observed after about 1400 MLT. The peak intensity of the 6300‐Å emission was usually 1'2 kR. A distinction is made between ‘soft’ cleft characteristics identified early on several afternoons, when the 6300‐Å emission rate is always greater than that at 5577‐Å, and a more structured character seen at all other times. The structure takes the form of transient enhancements of the auroral emissions, particularly 5577 Å. 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.