What are Radio Blackouts?

A radio blackout refers to a major signal loss.

Under normal conditions, HF radio waves (3–30 MHz) provide long-distance communication by reflecting off the higher regions of the ionosphere. However, there are times when radio blackouts occur due to high solar activity, causing ionospheric disturbances that block HF bands.

The solar flares reach Earth at the speed of light, after 8 minutes and 20 seconds.
The ejected particles however reach earth after few hours up to four days, depending on particles' speed.

Both events can produce increased ionization of the D-region of the ionosphere, which blocks HF radio transmissions.

The bursts on the surface of the Sun are unexpected, causing on Earth a "Sudden Ionospheric Disturbance" (SID). A recovery may take from minutes to hours.

See below a screenshot of NOAA's spaceweather forecast website:

The last remarkable radio blackout event
May 11, 2024

The geomagnetic storm was the most powerful to affect Earth since March 1989, and produced aurorae at far more equatorial latitudes than usual in both the Northern and Southern Hemispheres.

Extremely strong solar flares were recorded during the 25th solar cycle: X-class flare, X6.3, from Region 3590 peaked on Feb. 22, 2024. The largest flare from Region 3664 reached X8.7, on May 14, 2024

* A link to the recent reports and prediction of blackouts (fadeouts)

* Stay up to date with the latest forecast discussions.

References:
1. Communications blackout. ^Wikipedia
2. Radio Blackout ^UNDRR
3. May 2024 Solar Storms ^Wikipedia
4. Geomagnetic Storms May 2024 ^Duckduckg
5. Search the term "Blackout" at NOAA website.

_{The Understanding HF Propagation Project provides radio amateurs with a detailed overview and tutorials on several aspects of HF propagation.}