HF Spectrum during a large solar flare

If you look at the Culgoora HF Spectrum shown below, from around the time of the flare, you can see the effect of the flare on the HF band. The colored horizontal lines running from the left of the image correspond to groups of HF transmissions between about 7 and 23 MHz. From the point marked '25' on the x-axis (0225 UT), the radio frequency radiation from the solar flare which has come directly from the sun is clearly visible as the vertical streaks running from the top of the image.

From the time of the solar flare onwards, most of the horizontal lines stop because HF transmissions are being completely absorbed by the ionosphere. See HF Fadeout. The signals which keep going are due to ground-wave propagation of short-range (local) RF signals.

During December 2006 there were a large number of significant solar X-ray flares. The class X3.4 flare depicted above (which commenced at 0225 UT on 13 December), caused a major Short-wave or HF Fadeout across the entire Australasian region, as shown in the map below.