THE WINTER ANOMALY AT TROMSO

A database of average electron densities at several heights above Tromsø, in northern Norway, has been constructed using EISCAT Common Programme data. These data cover more than half a solar cycle, and are binned by height, time of year and time of day. A comparison of the summer and winter electron density variations of any height band above 250 km shows that the F-region winter anomaly exists at Tromsø latitudes at solar maximum, as it does at mid-latitudes, and that, also like mid-latitudes, it is largely absent at solar minimum. However, upon closer examination it is seen that the average daytime electron density maximum is larger a month or so each side of the winter solstice than it is at mid-winter, and there, even at solar minimum, this density maximum is anomalously high compared to the summer. This effect is unlikely to be due simply to the variation of solar illumination. Numerical modelling of the mid- and high-latitude F-region has so far concentrated on explaining the winter anomaly. It is clear, though, that this off-solstice or 'semi-annual' effect can be of as great, or greater, importance, and more work is needed to explain it fully. © 1990.