What are Radio Blackouts?

1. Radio blackout phenomenon
2. Current and predicted fadeouts
3. The lastest significant blackout event

Under normal conditions, HF radio waves from 3 to 30 MHz provide long-distance communication by reflecting off the ionosphere.

An artist view illustration by Doron, 4X4XM

A radio blackout or fadeout refers to a major signal loss.

Current and predicted fadeouts as reported by ASWFC
Flare alarm	HF fadeout current	Fadeouts Expected fadeout

The drop in signal affects in particular the lower HF bands.

Fadeout signal strength vs. time, courtesy of Australian Space Weather Services

Blackouts happen when the Sun releases "Solar Flares" (soft X-Ray radiation) and / or "Solar Particle Events" (SPE).

Solar Flares and Coronal Mass Ejections
are components of suden solar storms that may
affect skywave radio propagation on Earth.

Flashes of radiation

A Solar Flare courtesy of NOAA, May 2023

Solar flares reach Earth at the speed of light, taking approximately 8 minutes and 20 seconds. However, the ejected particles can take anywhere from a few hours to up to four days to reach Earth, depending on their speed.

Ejection of solar matter

A CME is a shock-wave of highly charged particles emitted by the sun.

When a CME enters the magnetosphere, it causes a Geomagnetic Storm

Major magnetic storms can block HF propagation from 3 to 30 MHz by altering the distribution of free electrons in the ionosphere. These storms can increase ionization in the D-region of the ionosphere, which blocks HF radio transmissions. Unexpected bursts on the Sun's surface cause Sudden Ionospheric Disturbances (SIDs) on Earth, with recovery times ranging from minutes to hours.

Solar events in the last three days Recent 3 days of Space Weather - RSG Scales Published by NOAA SWPC services Reference: Space Weather images at (US) NOAA SWPC

See below a screenshot of NOAA's spaceweather center:

The lastrest significant radio blackout event
May 10-11, 2024

NASA’s Solar Dynamics Observatory captured images of the two solar flares on May 10 and May 11, 2024. These flares were part of a series of strong solar activities, classified as X5.8 and X1.5-class flares, respectively.

There were previous flares during the 25th solar cycle: the X6.3 flare from region 3590 peaked on February 22, 2024. Another flare from region 3664 reached X8.7 on May 14, 2024.

This 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.

A indices recorded 30 days before that blackout event

K indices before and after this blackout event

Kp index 3 days after that 11-May-2024 blackout event

* Read more about radio blackouts, SID phenomena, and how they affect lower HF bands, as well as the prediction model.

* Stay up to date with the latest forecast discussions.

References:
1. Communications blackout. ^Wikipedia
2. Radio Blackout ^UNDRR
3. Search the term "Blackout" at NOAA website.
4. Strong Solar Flare Activity: May 10-14, 2024 ^NASA
5. May 2024 Solar Storms ^Wikipedia
6. Geomagnetic Storms May 2024 ^Duckduckg