Electron Concentration Profiles in E- & F-regions during Sunrise & Sunset

132-134Electron concentration profiles during sunrise and sunset periods, observed with the Arecibo incoherent scatter radar, are presented. The measurements are made in the height range of 100-500 km, using short pulses resulting in a height resolution of 15 km. Electron concentrations are observed to change by more than an order of magnitude during these periods in the height range of 120-200 km. The changes are rather small in the Es and in the F2 region and thus a large valley is developed between 100 and 200 km, with a minimum of electron concentration at about 140 km. The fast decay of electron concentration during sunset is used to estimate the effective recombination coefficient which is then compared with the experimental values