What Is The Radio Blackout Scale And When Is It Needed?

Earlier this week, the Sun released two large solar flares in quick succession. Associated coronal mass ejections headed close to Earth may still make direct hits. While skywatchers anticipated a high latitude auroral treat for the eyes, R3 radio blackouts occurred across half the planet, raising the questions: why do these radio blackouts occur, and how are they measured?

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What causes radio blackouts?

Solar flares involve large, sometimes enormous, bursts of electromagnetic radiation. These briefly bump up the Sun’s usual output, and the energy released is much more concentrated at higher frequencies, including X-rays, than normal sunlight. The radiation reaches Earth in eight minutes, traveling at the speed of light, because it is light, or higher-energy photons that are part of the same spectrum.

On arrival, the X-rays and some ultraviolet light from the flare tear electrons off atoms and molecules in the Earth’s ionosphere, ionizing them. Long-distance radio communication relies on the ionosphere to bend signals around the curve of the Earth. (No, we don’t know how flat Earthers explain this either). Passage through all these excess loose electrons degrades the radio signals until the electrons recombine with atoms. At best, it becomes hard to make out the message the radio waves are carrying. At worst, the signal is lost entirely, creating a radio blackout. 

Flares can also release high-energy particles, which take longer to reach Earth, but not a lot. So, within an hour of the flare occurring, charged particles can deliver an additional hit to the atmosphere, deepening the disruption.

The effects are most acute in the high frequency (3-30 MegaHertz) band, which lies between those used for AM and FM radio, but inevitably, the effects are felt at other frequencies. This band was once popular for shortwave international radio broadcasting, but is now primarily used for air-to-ground aviation communication, particularly over oceans. The systems for transmitting radio signals can even be directly affected by the radio burst released by the flare itself, although this is rarer. 

The consequences of flares are different from, but connected to, the triggers for geomagnetic storms that cause auroras. Flares sometimes, although not always, lift coronal mass ejections (CMEs), composed of heavily magnetized plasma, off the Sun. Depending on circumstances, CMEs take between 15 hours and four days to cross the gap between Earth and the Sun. When they do, they encounter the Earth’s magnetic field, which funnels the magnetized particles towards the poles, creating geomagnetic storms, and the particles from the plasma interact with molecules in the upper atmosphere to trigger auroras. 

How are radio blackouts measured?

The National Oceanic and Atmospheric Administration (NOAA) uses a five-level scale to measure blackouts, which will presumably survive even if NOAA is dismantled. To distinguish the blackouts from other effects of solar activity, they’re referred to as R 1-5, whereas the geomagnetic storms that cause auroras (and sometimes crash electricity systems) are G 1-5, and solar radiation storms of high-energy particles are S 1-5.

According to NOAA, R3 events such as the recent one involve a “Wide area blackout of high-frequency radio communication, loss of radio contact for about an hour on the sunlit side of Earth.” Meanwhile, the low-frequency radio signals used for navigation are “degraded for about an hour”. R3’s are triggered by X1-9 flares. 

Naturally, R4s (described as “severe”) are more serious, with a blackout of high-frequency radio communication for 1-2 hours for most of the daylight side of the planet, and errors in navigation position for a similar time. They’re the product of X10-X19 flares.

Extreme events (R5s) can involve the whole daylight side of the planet losing high-frequency radio communication for many hours. This can cause transport authorities to lose contact with ships and planes, which may also experience loss of positioning based on satellite navigation.

R3s are not rare. NOAA reports they occur around 140 times each 11-year solar cycle. Clearly, the world gets through them fine, and few people outside specific fields even notice. R4s are much less common, happening around eight times a cycle, while there is less than one R5 per cycle.

Radio blackouts are more severe at high latitudes, so some commercial flights on polar routes will not travel when R3 or greater flares are anticipated. However, while observations of solar activity have increased our capacity to predict solar flares, they can still sometimes take us by surprise. Unlike geomagnetic storms, flares reach us so quickly that there’s no time to ground planes that might be in danger. Currently, critical systems rely on redundancy, such as transmission in different bands and through multiple routes to maintain connections, but that doesn’t work if the blackout is extreme enough.

With R5s being so rare, you can understand why the scale has not been extended to include higher values, but it is worth considering what something like the Carrington Event would do to radio transmission. The threats such an event would pose to satellites and electricity grids get discussed a fair bit, but the consequences for flights stuck in the air and unable to communicate with air traffic controllers get less publicity.