Radar observations of cleft dynamics

The ionospheric F region is perturbed by low‐energy particle precipitation in the cleft. On January 28–29, 1977, UT an ionospheric density enhancement associated with the cleft was monitored continuously by the Chatanika incoherent scatter radar during a 6‐hour period including an onset and recovery of global substorm activity. A strongly southward interplanetary magnetic field (IMF) brought the cleft into the field of view of the radar (invariant latitudes less than 75°). The observed motions of the cleft included a rapid equatorward excursion (∼4° Λ/h) to 70°Λ before occurrence of a major substorm intensification, continued location at low magnetic latitude during substorm expansion, and a poleward retreat to ∼73.5°Λ during substorm recovery in response to a northward turning of the IMF. The latitudinal width of the cleft density enhancement varied from less than 1° during the presubstorm equatorward motion to ∼4° after substorm recovery. F region densities immediately poleward of the cleft were reduced by a factor of 10 from their quiet time value.