Altitude distribution of the attenuation rate and cumulative attenuation for 3 MHz in the ionosphere for all events. The left (a, b, c) and right panels (d, e, f) show the HF radio wave’s attenuation rate and cumulative attenuation, respectively. Upper panels (a, d) are the results when dfmin are less than 2.5 MHz, middle panels (b, e) are the results when dfmin are 2.5 MHz or more, and lower panels (c, f) are the results at the blackouts.