Radio Blackout

Unique identifier / Notation

ET0006

Synonyms

D region absorption.

Radio blackout is a prolonged period of fading or faded radio communications, primarily in the high frequency range from ionospheric changes because of increased solar activity, in particular solar flares of C-class level or higher on the sunlit side of Earth (AMS, 2018).

Primary reference(s)

AMS, 2018. Radio blackout. American Meteorological Society (AMS). Accessed 15 October 2020.

Additional scientific description

Radio blackouts due to solar flares can last from minutes to hours. Solar proton events can also cause long-term radio blackouts over the polar regions for days; these are known as polar cap absorption events (PCAs). Radio blackouts due to solar flares of the M-class level and higher are classified using the NOAA R-Scale (AMS, 2018).

Radio blackouts due to solar flares of the M-class level and higher are classified using the National Oceanic and Atmospheric Administration R-Scale (AMS, 2018).

Solar flare intensities cover a large range and are classified in terms of peak emission in the 0.1–0.8 nm spectral band (soft X-rays) (NOAA, 2019). The X-ray flux levels start with the ‘A’ level (nominally starting at 10–8 W/m2). The next level, ten times higher, is the ‘B’ level (≥10–7 W/m2); followed by ‘C’ flares (10–6 W/m2), ‘M’ flares (10–5 W/m2), and finally ‘X’ flares (10–4 W/ m2).

The UK Civil Aviation Authority reported that during moderate and above solar storms, high frequency communications on the sunlit side of the Earth are prejudiced through radio blackouts associated with sudden ionospheric disturbances due to the flare (UK CAA, 2020). They noted that at very high latitudes high frequency communications can be prejudiced as a consequence of the radiation storm which causes polar cap absorption, and at auroral latitudes rapid fading and further absorption can occur as a secondary effect associated with the geomagnetic storm. They reported that the various events can last for periods of minutes to hours. As a consequence, aircraft crossing the Atlantic have well established procedures for coping with a loss of high frequency communications which allows aircraft to continue their intended flight plan (UK CAA, 2020).

Metrics and numeric limits

Radio blackouts are classified using a five-level scale by the United States National Oceanic and Atmospheric Administration (NOAA, no date; see chart below). Each level is directly related to the flare’s maximum peak in soft X-rays reached or expected (NOAA, 2019).

The Space Weather Prediction Center currently forecasts the probability of C, M, and X-class flares and relates it to the probability of an R1-R2, and R3 or greater event as part of its three-day forecast and forecast discussion products. The Space Weather Prediction Center also issues an alert when an M5 (R2) flare occurs (NOAA, 2019).

Scale	Description	Effect	Physical measure	Average Frequency (1 cycle = 11 years)
R 5	Extreme	HF Radio: Complete HF (high frequency) radio blackout on the entire sunlit side of the Earth lasting for a number of hours. This results in no HF radio contact with mariners and en route aviators in this sector. Navigation: Low-frequency navigation signals used by maritime and general aviation systems experience outages on the sunlit side of the Earth for many hours, causing loss in positioning. Increased satellite navigation errors in positioning for several hours on the sunlit side of Earth, which may spread into the night side.	X20 (2 x 10-3)	Less than 1 per cycle.
R 4	Severe	HF Radio: HF radio communication blackout on most of the sunlit side of Earth for one to two hours. HF radio contact lost during this time. Navigation: Outages of low-frequency navigation signals cause increased error in positioning for one to two hours. Minor disruptions of satellite navigation possible on the sunlit side of Earth.	X10 (10-3)	8 per cycle (8 days per cycle).
R 3	Strong	HF Radio: Wide area blackout of HF radio communication, loss of radio contact for about an hour on sunlit side of Earth. Navigation: Low-frequency navigation signals degraded for about an hour.	X1 (10-4)	175 per cycle (140 days per cycle).
R 2	Moderate	HF Radio: Limited blackout of HF radio communication on sunlit side, loss of radio contact for tens of minutes. Navigation: Degradation of low-frequency navigation signals for tens of minutes.	M5 (5 x 10-5)	350 per cycle (300 days per cycle).
R 1	Minor	HF Radio: Weak or minor degradation of HF radio communication on sunlit side, occasional loss of radio contact. Navigation: Low-frequency navigation signals degraded for brief intervals.	M1 (10-5)	2000 per cycle (950 days per cycle).

Key relevant UN convention / multilateral treaty

Not applicable.

Examples of drivers, outcomes and risk management

Solar flares are large eruptions of electromagnetic radiation from the Sun lasting from minutes to hours. The sudden outburst of electromagnetic energy travels at the speed of light, therefore any effect upon the sunlit side of Earth’s exposed outer atmosphere occurs at the same time the event is observed. The increased level of X-ray radiation results in ionisation in the lower layers of the ionosphere on the sunlit side of Earth.

Under normal conditions, high frequency (HF) radio waves are able to support communication over long distances by refraction via the upper layers of the ionosphere. When a strong enough solar flare occurs, ionisation is produced in the lower, more dense layers of the ionosphere (the D-region), and radio waves that interact with electrons in layers lose energy due to the more frequent collisions that occur in the higher density environment of the D-region.

This can cause HF radio signals to become degraded or completely absorbed. This results in a radio blackout – the absence of HF communication, primarily impacting the 3 to 30 MHz band (NOAA, 2019).