The greatest frequency on an O-mode trace is considered
to be the critical (or penetration) frequency of the ionospheric F2 region, which is termed
foF2. In this paper, we examine the foF2 (in MHz) recorded every 15-minute by an ionosonde
at Chung-Li (25.0°N, 121.0°E) during 1997 - 1999 (Fig. 1).