M class flares, with power levels over 1 X 10E-5 Watts per square meter can cause HF radio blackouts on the earth's sunward side. X class flares, with power levels over 1 X 10E-4 Watts per square meter will certainly cause HF blackouts and may cause enough noise in the SHF / EHF spectrum to disrupt satellite downlinks in those bands.

Magnetic field activity at GOES-12 (over the eqator at 75 degrees west) and GOES-11 (over the eqator at 135 degrees west). When the plot is near zero, or choppy, there is plenty of geomagnetic activity going on. Expect poor HF conditions and plenty of auroral activity.

Magnetig field activity at Boulder, Colorado. The H component is aligned north-south, and the D component is aligned east-west. When the plot is choppy, expect poor HF conditions and plenty of auroral sctivity.

This chart predicts D layer absorbtion on a global basis. When colors for a region are orange or red, expect conditions from poor to full HF blackout..