[en] The physical explanation of the changes of the maximum electron concentration of the F2 layer, NmF2, as a result of geomagnetic disturbances has proved elusive even after several decades of study. In this paper, we concentrate upon the main phase of the F-region storm which usually begins a few hours after the onset of a significant increase of geomagnetic activity. We present some new analysis of ionosonde data from the South Atlantic and Antarctica. There are major differences in storm phenology in the South compared with the North. In winter in the South, NmF2 is usually increased as the result of a geomagnetic storm whereas in the North it is depressed. In summer, both hemispheres normally show reduced values of NmF2 as a result of storm activity. It is these features which provide new insight into the problem of F-region storms. We interpret the observed seasonal and diurnal variation of NmF2 in terms of a mean (DC) effect and a diurnal variation (AC) effect, and discuss possible explanations for both. (author)