What is a radio blackout?

A radio blackout (R-scale event) happens when a solar flare's X-rays ionize Earth's upper atmosphere, degrading or blocking high-frequency (HF) radio signals on the sunlit side of Earth. NOAA classifies these R1 (Minor) through R5 (Extreme) based on GOES X-ray flux.

What does the R-scale measure?

The NOAA R-scale is based on the peak long-wavelength (0.1–0.8 nm) X-ray flux measured by GOES satellites, in watts per square meter. R1 begins at an M1-class flare (10^-5 W/m^2); R5 begins at an X20-class flare.

Radio Blackout Tracker — Live Solar X-ray Activity

Current radio blackout level (NOAA official scale)

R0 — None

Below R1 — no significant radio blackout.

C2.2 flare

GOES long-channel X-ray flux

6.68e-7W/m²

QUIET

NOAA GOES

Current flare class

C2.2

ACTIVE

NOAA GOES

X-ray flux — last 12 hours

Affected side of Earth

Radio blackout effects occur primarily on the sunlit side of Earth — the hemisphere currently facing the Sun. X-rays travel at the speed of light, so any effect on HF radio propagation begins the moment a flare's peak X-ray emission reaches Earth's atmosphere, with no meaningful transit delay. Duration depends on flare evolution and ionospheric conditions, and typically resolves within tens of minutes to a few hours as the flare subsides.

Communication impacts

Aviation

HF communications can be degraded or disrupted, especially on routes relying on HF radio, including polar and long-range oceanic routes.

Maritime

HF communications used by ships at sea may experience disruption during significant radio blackout events.

Emergency / long-distance HF

Amateur radio and other long-distance HF communications can experience reduced range or complete signal loss during strong events.

Active NOAA radio blackout alerts

R2 — R2 Radio Blackout Summary

Radio blackout level 2. HF radio (used by pilots and ham operators) may be degraded on the sunlit side of Earth. Area of impact centered primarily on sub-solar point on the sunlit side of Earth.

R2 — R2 Radio Blackout Alert

Radio blackout level 2. HF radio (used by pilots and ham operators) may be degraded on the sunlit side of Earth. 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 http://www.swpc.noaa.gov/products/d-region-absorption-predictions-d-rap.