HF Radio Blackout: Solar Flares, the R-Scale and D-RAP

You switch on at noon, the bands were full of stations an hour ago, and now there is nothing but a low hiss from 80 to 10 metres. Your antenna has not failed and your receiver is fine. What you have just experienced is an HF radio blackout: a sudden loss of high frequency propagation on the sunlit side of the Earth, caused by a burst of X-rays from a solar flare. Blackouts are short, usually minutes to a couple of hours, but they arrive without warning and they can ruin a contest hour or a DX opening. This guide explains how flares cause blackouts, what the scale published by NOAA means for the amateur bands, how to check whether a blackout is under way and how to plan around them.

What Causes an HF Radio Blackout

The NOAA Space Weather Prediction Center describes the process in its page on solar flares and radio blackouts. Solar flares are large eruptions of electromagnetic radiation from the Sun, lasting from minutes to hours. The burst of electromagnetic energy travels at the speed of light, so any effect on the sunlit side of the Earth’s upper atmosphere occurs at the same time that the event is observed. The extra X-ray and extreme ultraviolet radiation ionises the lower layers of the ionosphere on the sunlit side.

Under normal conditions, high frequency signals travel long distances because the upper layers of the ionosphere refract them back to Earth. When a strong flare ionises the lower, denser D layer, radio waves that pass through it collide more often with electrons in the higher-density environment, and the waves lose energy. The signals become degraded or are completely absorbed. NOAA describes the result as a radio blackout, the absence of HF communication, primarily affecting the 3 to 30 MHz range. This is the same D layer that makes the lower bands poor in the daytime, and a flare simply makes it far more absorbing than usual. The general phenomenon is also known as a sudden ionospheric disturbance, as described in the reference article on sudden ionospheric disturbances.

Two features of a blackout matter for operators. It affects only the sunlit side of the Earth, so a path that is entirely in darkness is not touched. And it arrives with no warning, because the X-rays travel at light speed. That is very different from a geomagnetic storm, which takes a day or more to arrive after a solar eruption and which is a different problem. Our article on solar flux and the A and K indices covers the slower space weather that you can see coming.

Flare Classes and the NOAA R-Scale

Flares are classified by the peak X-ray flux that they produce in the soft X-ray band, between 0.1 and 0.8 nanometres, measured by the NOAA GOES satellites. The classes start at A and rise by factors of ten through B, C, M and X. NOAA translates the strongest of them into a five-level scale of radio blackouts, labelled R1 to R5, which relate directly to the flare’s peak. The scales are explained in more detail on the NOAA Space Weather Scales page, and the table below summarises the radio blackout part for HF.

The frequency of events falls as the level rises. NOAA’s table lists R1 events as about 2,000 per solar cycle and R2 as about 350, then R3 as about 175 per cycle, R4 as about 8 and R5 as fewer than one per cycle, where a cycle is 11 years. So the weak events happen often, and the extreme ones are rare, but an R1 or R2 event can still spoil a morning if it arrives at the wrong moment.

The same page gives the effect on other systems too. Low-frequency navigation signals can be degraded, and the stronger events add errors to satellite navigation on the sunlit side. Those are of less direct interest to the amateur, but they show how far-reaching the underlying disturbance is.

What You Hear on the Bands

A blackout has a recognisable signature. Signals that were strong a minute ago fade down, usually from the highest frequencies first, into the noise, and the whole sunlit part of the band becomes quiet. Often, you can still hear local stations, because a short path through the ionosphere suffers less absorption, and the lowest frequencies are affected most. You may also notice that the noise floor changes and that the usual crowd of FT8 decodes vanishes.

• Sudden onset. Within minutes, there are no more distant signals on the day side.
• Worse on the lower bands. The absorption is greater at lower frequencies, so 80 and 40 metres suffer first and most, and the effect fades towards 10 metres, although a strong event can silence all of them.
• Only the sunlit part of the path. A path that is mostly in darkness, such as a night-time path across the ocean, may work well in the middle of a blackout.
• Gradual recovery. After the flare peaks, the ionisation decays and signals return gradually, from the higher bands back down, over minutes to hours.

If the band suddenly goes dead and you know that your equipment is working, it is worth checking the space weather before you start taking your station apart.

Checking for a Blackout: D-RAP and Other Tools

NOAA publishes a product designed for exactly this purpose: the D Region Absorption Predictions (D-RAP) page. It addresses the operational impact of solar X-ray flux and energetic particle events on HF communication. It shows a global map of the highest frequency affected by one decibel of absorption, an attenuation bar graph, status messages and an estimated recovery clock. The components update continuously, driven by one-minute GOES X-ray flux data and five-minute proton flux data. Polar maps show the highest frequency affected by ten decibels of absorption, which is mainly caused by energetic particle events.

1. Open the D-RAP map. If the map shows a high affected frequency on the part of the world where your path lies, a blackout is under way.
2. Read the recovery clock. It gives an estimate of when the absorption will fall, so you know whether to wait or to switch to a path in darkness.
3. Check the X-ray flux. A sudden rise in the X-ray flux plot is the signature of a flare, and its level tells you the R-scale value.
4. Look at the forecast. NOAA forecasts the probability of C, M and X-class flares and relates them to the probability of R1 to R2 events and R3 or greater events, as part of its three-day forecast. It also issues an alert when an M5 flare, which corresponds to R2, occurs.

For a quick check on the air, the beacon network is useful. The international beacons transmit in a fixed rotation on several bands, and if you can hear none of them from the sunlit part of the world, a blackout is a likely cause. Our guide to NCDXF beacons explains how to use them, and the article on what decides whether you are heard on HF gives the background on the layers.

Planning Around Blackouts

You cannot predict an individual flare, but you can reduce its effect on your plans.

• Know the activity level. When sunspot groups are large and complex, the chance of an M or X flare is higher. NOAA’s forecast gives the probability for the next three days.
• Keep a night path in reserve. If a blackout hits at midday, move to a band and a path that is in darkness. The lower bands at your night time or a path across the dark side of the Earth will probably still work.
• Do not give up on a long contest. Most blackouts last minutes to hours. Take a break, check the recovery clock and resume when the signals return.
• Avoid blaming your station. Before you start a long search for a fault, check the space weather page, the beacons and a second receiver, if you have one.
• Use weak-signal modes for the recovery. After a blackout, the bands return gradually, and weak-signal modes can show the first returning signals before the bands sound alive on voice. Our article on the WSPR beacon system explains one of the best tools for this.

It is also worth knowing that the effect is a daytime effect, which is one more reason why the lower bands at night and the higher bands at midday behave so differently when the Sun is active.

What to Do During a Blackout

When you suspect a blackout, a short routine saves time and worry. First, confirm it: check the beacons, a second band and the D-RAP map. Second, note the time in your log, because a gap in your contacts is easier to explain in a contest log if you know what caused it. Third, decide whether to wait or to switch. If the recovery clock suggests a short event, use the time for a break, a snack or a check of your logging. If the blackout is long, change to a path that is in darkness, to a regional net on a band that still works or to a local VHF contact. Fourth, keep an eye on the flare activity, because strong flares often come in groups from the same active region. After the recovery, expect the bands to be a little unsettled for a while, with signals gradually strengthening, and use that time to look for stations that you could not hear before.

Flares and the Solar Cycle

Flares follow the solar cycle. Near the peak of an 11-year cycle there are many active regions on the Sun and more flares, and near the minimum there are few. The cycle also brings the best propagation on the higher bands, so there is a trade-off: the same activity that opens 10 and 12 metres can also produce blackouts at midday. The experienced operator learns to use the good hours and to be calm about the dropouts.

One more point is easy to miss: some flares come with energetic particles that affect the polar regions. These particle events can cause polar cap absorption, which is why NOAA’s D-RAP product includes separate polar maps. If your path crosses the polar regions, check those maps in addition to the global one.

The Bottom Line

An HF radio blackout is a quick, daytime-only loss of propagation caused by X-rays from a solar flare, and it is graded by NOAA from R1 to R5. It arrives at light speed, so it cannot be avoided, but it can be recognised and planned for. Check the D-RAP map and the X-ray flux, use the beacons to confirm what you hear, move to a path in darkness when a blackout hits and wait for the recovery. Understanding what is happening turns a frustrating dead band into a short break in operating.

Frequently Asked Questions

What is an HF radio blackout?

It is a loss of high frequency radio communication on the sunlit side of the Earth, caused by extra ionisation of the D layer after a solar flare. It mainly affects frequencies from 3 to 30 MHz.

How long does a radio blackout last?

From several minutes to a couple of hours for most events. According to NOAA, an R3 event causes loss of contact for about an hour, while the strongest events can last a number of hours.

Which bands are affected most?

The lower HF bands suffer the most absorption, but a strong event can affect the whole range from 3 to 30 MHz on the sunlit side.

What do R1 to R5 mean?

They are the five levels of the NOAA radio blackout scale. R1 corresponds to an M1 flare and a minor effect, and R5 corresponds to an X20 flare and a complete HF blackout on the sunlit side for hours.

Can a night path still work during a blackout?

Yes. The blackout affects the sunlit side of the Earth, so a path that is entirely in darkness is generally not affected.

Where can I check for a blackout?

The NOAA D-RAP page shows the highest affected frequency and a recovery estimate, and the X-ray flux plot shows the flare itself. Beacons on the air are a useful confirmation.