One of the major features of the mid-latitude ionosphere is the anomalous enhancements that often either occur before or after the more usual post mid-day peak. The structure in the total electron content comprising a pre- or post-midday peak and another enhancement is referred to by Pi et al. (1993) as a diurnal double maxima (DDM). DDM structures were sometimes observed during daytime in a long time-series of mid-latitude slant total electron content (sTEC) measurements collected in Italy between 1975 and 1991. From a total of 1366 daily TEC observations, DDM structures were observed on 302 days. The first peak usually occurred between the hours of 10-11. UT, and the second peak usually occurred either between the hours of 13-14. UT or 18-19. UT, where UT. ≈. LT. -. 1. hour. The times of occurrence of each peak demonstrated a seasonal variation, with the second peak appearing later in the day during summer. The rate of occurrence of the DDM structure was higher in winter and summer than in autumn and spring. Independent observations were sought by using foF2 and hmF2 measurements from an ionosonde in Rome. Some evidence shows that vertical plasma motions in the F-region can cause DDM, however the cause of many other DDM events is still not clear. Furthermore, a comparison of DDM structures across different geomagnetic and auroral activities indicated that the structures were not necessarily related to geomagnetic storms or substorm events.
|Journal||Journal of Atmospheric and Solar-Terrestrial Physics|
|Publication status||Published - 1 Nov 2012|