Geomagnetic control of the winter anomaly in absorption of radio waves at mid‐latitudes

Abstract

Morphological features of the winter anomaly in radio wave absorption, i.e. anomalous and large excessive absorptions of MF and HF waves occuring on groups of winter days at mid‐latitudes, are examined based on absorption data sets of MF waves at three ionospheric stations and fmin data sets at 35 stations in the northern and southern hemispheres. It is found that the winter anomaly occurs simultaneously in an extensive daytime area, at least scatteringly, mostly several hours to 1–3 days following the onset of a sc (or si) geomagnetic storm or storms. The magnitude, duration and occurrence area of the winter anomaly seem to depend on geomagnetic activity. It is also shown that fmin enhancements occur simultaneously in the daytime at geomagnetically conjugate stations in both hemispheres in association with a storm, irrespective of season, and they are closely correlated to cosmic noise absorption enhancements. These results suggest that the winter anomaly observed at a point in one hemisphere is linked to an excessive absorption event at the conjugate point in the other (summer) hemisphere and that they would be caused by ionization enhancements due to precipitating electrons associated with a sc storm. It is therefore proposed that the winter anomaly is not the peculiar event implying that anomalously large excessive absorptions of radio wave arise solely in winter, but the phenomenon indicating that excessive wave absorptions appearing throughout the year associated with sc storms are abnormally enhanced in winter. An explanation for the large seasonal variation of excessive wave absorption is also presented.