Fig. 5

Time variations in a solar X-ray flux, b fmin, c dfmin, and d maximum ionosonde signal at each frequency during the longest SWF event on April 3, 2001, observed at Kokubunji. The fmin and dfmin values of the blackout period are shown as “B” in red in b and c. dfmin ≥ 2.5 MHz and dfmin ≥ 3.5 MHz values are shown in blue and light blue, respectively, in c. Black pluses in d show fmin values. This event was recorded by the ionosonde system with an intensity resolution of 1 bit. The smoothing procedure in the analysis provides an arbitrary color scale with increasing signal intensity from blue to red