Autumn and winter anomalies in ionospheric absorption as measured by riometers

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Seasonal variation of ionospheric absorption has been studied on the basis of riometer (A2) measurements obtained over a wide latitude range. In agreement with the results of earlier studies of A1 radio wave absorption, equinoctial maxima of approximately equal amplitude are observed in the auroral zone and near the equator. At mid-latitudes riometer absorption maximizes during the fall season, whereas the A1 data show a semi-annual variation with maxima occurring in summer and winter. The autumn maximum or autumn anomaly, in riometer absorption is observed at much higher geographic latitude in the southern than in the northern hemisphere but at comparable geomagnetic latitudes in both hemispheres. The secondary maximum in riometer absorption at mid-latitudes during the winter months, the winter anomaly, is seen only if absorption is calculated at constant solar zenith angle. In the appendix the different methods to determine a quiet-day curve are discussed.