Fig. 1

Time variations in a solar X-ray flux, b fmin, c dfmin, and d ionograms during solar flare events on November 10, 2004. The fmin and dfmin values of the blackout period are shown as “B” in red in b, c. dfmin ≥ 2.5 MHz and dfmin ≥ 3.5 MHz values are shown in blue and light blue, respectively, in c. The weak echo signal at ~ 3 MHz at altitudes of 100–150 km is surrounded by red circles in d at LT 11:00 and 12:15