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. lation to the winter anomaly. Gregory and Manson [1970] and Kawahira [1970] have also suggested that the planetary wave, which would be related to variations of temperature and air pressure or to redistributions of ions and neutral particles in the mesosphere and stratosphere, seems to be responsible for the winter anomaly, and this interpretation has been supported by Deland and Cavalieri [1973], Manson [1976], Rose and Widdel [1977], and Labitzke et al. [1979]. On the other hand, the correlation between NO (nitric oxide) and the win-sources, Lauter [1965], Lauter and Knuth [1967], and Maehlum [1967] have demonstrated an alternative hypothesis that ionization enhancements responsible for the winter anomaly would be caused by precipitating charged particles, probably energetic electrons. In his extensive analysis Maehlum [1967] indicated that fluxes on daytime of precipitating electrons during and after a storm are well correlated to magnitudes of wave absorption. This explanation has been developed by Gough and Coilin [1973], Wratt [1976], Miircz [1976], and Sato [1980]. These authors have shown that the electron precipitation occurs usually after a geomagnetic storm or disturbance and the electrons dump down mainly from the slot region between the inner and outer radiation belts. Belrose and Thomas [1968], Hargreaves [1973], Marcz [1976], and Aikin et al. [1977] have also pointed out that some correlat...

Delayed traveling ionospheric disturbances, especially in equatorial regions, following geomagnetic activity

“…The Irkutsk plot has delays which are as long as 10 days. Sato (1981) was able to measure absorption at stations as equatorward as Wakkanai. Wakkanai and Brisbane (which have similar latitudes) have been investigated for delayed absorption by a method similar to that for Christchurch and Irkutsk.…”

“…One proposal has suggested that AGWs and related MS-TIDs are generated as a result of some of the D-region precipitation events responsible for the absorption (Bowman, 2000). Sato (1981) investigated this delayed absorption over a range of stations and detailed the literature on the subject. One result (Sato, 1981), which is relevant here, indicated the measurement of absorption of "-long duration, sometimes more than a fortnight".…”

“…Sato (1981) investigated this delayed absorption over a range of stations and detailed the literature on the subject. One result (Sato, 1981), which is relevant here, indicated the measurement of absorption of "-long duration, sometimes more than a fortnight".…”

“…This paper will also consider delays after GA of D-region absorption because of some similarities with the spread-F delays. The literature on the subject is summarised by Sato (1981). In addition satellite particle-precipitation events reported by Nagata et al (1988) have been analysed for evidence of delays after GA.…”

Latitudinal Extent of the f-min Winter Anomaly Region during the DYANA Campaign.

“…The generation mechanisms of winter anomaly are still controversial (e.g., OFFERMANN, 1979;SATO, 1981). OGAWA et al (1992) have reported the winter anomaly over Japan during the DYANA campaign in 1990.…”