↑ What are Radio Blackouts?

By Doron Tal, 4X4XM

Under normal conditions, HF radio waves (3–30 MHz) provide long-distance communication by reflecting off the higher layers of the ionosphere.

However, there are times when radio blackouts occur due to extreme solar activity causing ionospheric changes that block high-frequency bands.

It happens when the Sun releases "Solar Flares" (soft X-Ray radiation) and/or "Solar Particle Events" (SPE).
Solar Flares and Coronal Mass Ejections are the components of solar storms are that affect skywave radio propagation.

The Solar Flares reach Earth at the speed of light, after 8 minutes and 20 seconds. The SPE reach earth after few hours up to four days, depending on particles' weight and speed. Both events can produce increased ionization of the D-layer of the ionosphere, which blocks HF radio transmissions.

The bursts on the surface of the Sun are sudden and in most cases unexpected, causing a Sudden Ionospheric Disturbance (SID). All of them increase ionization in the D region of the ionosphere. A recovery may take minutes or hours. See below a screenshot courtesy NOAA:


↑ The remarkable radio blackout
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 fadeouts & blackouts

* 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.


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