Radio Blackout

At ~11 p.m. EST on January 22, 2012, a radio blackout x-ray flare erupted on the Sun, emitting a burst of electromagnetic radiation toward Earth. Solar flares release fast-traveling streams of ions, protons, and electrons outward into the solar system. The solar flare emissions can reach Earth in just eight minutes. When they come into contact with the Earth’s ionosphere, they can cause radio degradation or blackouts, particularly at high frequencies on the daylit side of Earth. Low-frequency navigation signals can also be impacted.

To help stakeholders be aware of the impacts of a radio blackout, the Space Weather Prediction Center (SWPC) uses an “R” scale to rate the severity of such an event.

The impacts expected for the various event levels are described in the following table. On the left is the category of the event based on severity; the next column describes possible impacts; and the last two columns discuss the physical strength and frequency of events.

Use the R scale in the next image to answer the question below:

SWPC's radio blackout scale showing the impacts, physical measurement, and frequency of R1 (minor) to R5 (extreme) events.

Click to view larger version (opens in a new tab or window).


At which R category do wide-area blackouts of high frequency communications begin to occur? (Select the best answer.)

The correct answer is c).

While occasional loss of radio contact can occur with all categories, wide-area blackouts of high frequency (HF) communications typically occur at R3, which signifies a “strong” event. Random, smaller areas at or near the sub-solar point (the location on the planet where the Sun is directly overhead, or at zenith) may experience communications degradation at R2, or even R1 levels. The flare that headed towards Earth on January 22, 2012 was rated a moderate (R2) event and disrupted HF communications over areas of the Pacific Ocean, Alaska, and Asia.

The effects of a radio blackout can seriously impact aircraft and controllers relying on HF communications. Radio blackouts affect the portion of Earth facing the Sun and experiencing daylight (referred to as the sunlit side of Earth). Extreme events can blackout HF signals for a number of hours. All HF radio contact will be lost, which can pose difficulties for aviation operators as well as mariners. Low-frequency navigation signals are also degraded or lost entirely during radio blackouts, and satellite positioning errors can propagate to Earth’s night side as well. Misdirection or absorption of very high frequency (VHF) signals used at mid and low latitudes is less common, with effects generally limited to the sunlit side of Earth. During the winter in high latitudes, the absence of sunlight can mitigate radio blackout effects; however, solar radiation storms (discussed in the next section) will still be an issue.

As you can observe from the table, radio blackouts causing significant disruptions are relatively rare. Severe radio blackouts (R4) have a frequency of occurrence of 8 events per solar cycle, or 8 days out of 11 years. On average, an extreme event (R5) occurs less than once per solar cycle. However, minor to strong events can occur much more often: an 11-year solar cycle might experience over 2500 R1 to R3 events amounting to approximately 1400 days of potential concern. The number of events often increases near solar maximum, but in general, the possibility of these events and their impacts is something you should be aware of every day.

Exercise: While forecasting for aviation operations in the Anchorage, Alaska area you receive word that a large solar emission was just observed. SWPC has issued the following product:

Space Weather Message Code: ALTXMF
Serial Number: 169
Issue Time: 2012 Mar 13 1727 UTC
ALERT: X-Ray Flux exceeded M5
Threshold Reached: 2012 Mar 13 1726 UTC
NOAA Scale: R2 - Moderate
Potential Impacts: Area of impact centered on sub-solar point on the sunlit side of Earth. Extent of blackout of HF (high frequency) radio communication dependent upon current X-ray Flux intensity. For real-time information on affected area and expected duration please see



Using the NOAA Space Weather Scale for Radio Blackouts, what are the potential impacts to your aviation partners? (Select all that apply.)

The correct answers are b), c), and e).

An R2 event is considered “moderate” and could cause radio losses and navigation errors on the sunlit side of Earth.


What step will you take? (Select the best answer.)

Answer the question and click Done to view more content.

The correct answer is a).

Radio blackouts have the potential to disrupt HF communication and increase positioning errors for time periods of a few hours. Because the solar flare emissions will have already reached Earth by the time the alert is issued, you should take immediate action to pass on this alert.

In Summary:

  • Radio blackouts result from solar flares whose X-ray emissions disturb the ionosphere.
  • HF communications and to an extent VHF channels, as well as satellite navigation signals are affected, primarily on the sunlit side of Earth.
  • The effects begin to be observed within minutes after the flare and can persist several hours.
  • Radio blackout alerts warrant attention. Caution is necessary in areas where HF channels are the predominant communications. Effects are most likely to be observed where the Sun is almost directly overhead (referred to as the subsolar point).