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 ¿&Lgr;/h) to 70 ¿ before occurrence of a major substorm intensification, continued location at low magnetic latitude during substorm expansion, and a poleward retreat to ~73.5 ¿&Lgr; 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.