abstract
The rapid transition between LT -controlled and UT-controlled behaviour of f0F2 which is observed at some Antarctic stations is studied here. The dates of transition have been determined for seven stations (six Antarctic and one sub-Antarctic) for a number of years. At the six Antarctic stations the changeover is very rapid; in addition the average changeover date for the transition from LT-to UT-control and the average changeover date for the reverse transition are symmetrical about one of the solstices. For the other station the changeover is much less sudden and the changeover dates are not symmetrical about the solstices. This may be due to a difference in the factors causing the UT peak at different stations.
