Abstract
The effects of sudden ionospheric disturbances (S.LD.'s) on the complicated structure ()f ionospheric echoes obtained with moderately high sensitivity at 2�28 Mcls are described. The observations indicate that flares of classes 2 and 3 can produce high values of electron density near the base of the ionosphere. Mean electron density can exceed 1000/cm3 over the height range 60-75 km, where the greatest relative increase in ionization occurs. Nevertheless, the greater part of the S.LD. absorption of waves reflected from the E region or above still occurs above 85 km.

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