‘Winter Anomaly’ in Ionospheric Absorption and Stratospheric Warmings

Abstract

IT has long been known that in middle latitudes the ionospheric absorption of radio waves shows a distinct ‘winter anomaly’. This takes the form of a very marked day-to-day variability coupled, on many days, with an excessively large absorption to the extent that on days of ‘winter anomaly’ the absorption is often considerably larger than that found on a normal summer day. It has been established that this abnormal winter absorption is due to certain changes in the lower ionosphere—in the D and lower E region—but the precise cause of this phenomenon has not yet been identified. Investigations of the winter anomaly have established certain facts, such as that it is only weakly correlated with solar or magnetic activity and that it occurs more or less simultaneously over large areas. At the same time there appears to be a peculiar inverse relationship between its incidence in different longitudes in that a period of marked winter anomaly in one longitude zone seems to be associated with an absence of the phenomenon in another longitude sector. In a recent paper, Bossolasco and Elena¹ have produced evidence for a correlation between the winter anomaly in ionospheric absorption and temperature in the upper stratosphere.