Abstract Seasonal variations of ionospheric absorption are studied from the mid-day routine tabulations of the parameter ƒ min , from eighteen ionospheric stations. A quantity, B = ƒ min : ƒ 0E , is defined and its variations in both geographic and geomagnetic latitudes are studied. It is shown that the value of B varies about a steady value, the amplitude and sense of the variation being related to the intensity of the current in the S q current system and the latitude of the station. A lunar semi-diurnal variation in the quantity B is found for Kihei. The ratio of the solar and lunar variations in B is found to compare favourably with the ratio of the total currents in the S q and lunar current systems. A recent model of the E-layer is used to define the upper and lower limits of the absorbing region, and it is concluded that the absorbing region occupies the whole of the dynamo-current region below the reflection height of the exploring waves. The control of the absorption process is related to the vertical drift of neutral ionisation resulting from east-west current flow. A mechanism is proposed in which the scale size of scattering irregularities in the E-layer is changed by the direction and velocity of the vertical drift of ionisation. A number of observed absorption effects are discussed and it is shown that the theory describes the qualitative behaviour observed in these effects. Reference is made to the likely importance of the process of control of absorption in the auroral zones.