Plasma convection in the vicinity of the dayside cleft

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Abstract
Experiments conducted with the Chatanika, Alaska, incoherent scatter radar observed the ionospheric convection pattern equatorward of the noontime cleft. There plasma transport is characterized by low‐speed convergence toward a meridian centered somewhat before noon and rotation of the flow into a poleward direction at higher latitudes. For undisturbed conditions there is only weak convection at noon at these latitudes, but the eastward electric field reaches a maximum of 15 mV/m 2–4 hours before and after noon at latitudes between 70°Λ and 74°Λ. During active conditions at the peak of the solar cycle the ionospheric cleft was located within the Chatanika field of view on several occasions. A longitudinally narrow region of rapid poleward convection near noon was not observed; rather, poleward flow occurred over a 3–4 hour spread of local time. For disturbed conditions, the prenoon and postnoon regions of plasma entry into the polar cap were enhanced, and an eastward electric field of 25 mV/m was seen across the 2 hours of local time around noon. High‐density F region plasma was observed convecting poleward through the cleft from a source at lower latitudes in the afternoon sector. Such plasma, seen at very high latitudes within the polar cap, serves as a tracer of the convection pattern away from the cleft.