What are Radio Blackouts?

Explanatory Tutorial

Radio blackout phenomenon

Why and How does a fadeout happen?

Current and predicted fadeouts

The most significant blackout event during the 25th solar cycle

Radio blackouts occur when solar flares release bursts of X-ray and severe ultraviolet radiation that ionize the Earth's upper atmosphere (D region). This ionization has the potential to block or degrade high-frequency radio transmissions, which has a significant impact on aviation and maritime communications.

An artist view

Figure 1: A radio blackout or fadeout refers to a major signal loss

The drop in signal affects in particular the lower HF bands.

Figure 2: Fadeout signal strength vs. time, courtesy of Australian Space Weather Services

Under normal conditions, HF radio waves from 3 to 30 MHz provide long-distance communication by refracting off the ionosphere. However, suddenly the signal vanishes, and you hear nothing but static. This sudden loss of signal is what's known as a radio blackout.

Figure 3: Blackout occurs when the LUF-MUF window is closed

During a strong sudden ionospheric disturbance (SID), the LUF will increase to a frequency higher than the MUF, thus closing the usable frequency window, an event called a fadeout or blackout.

Effects of Radio Blackouts

Communication Disruption: Radio blackouts can interrupt communication, especially affecting airplanes, ships, and emergency services that rely on certain radio frequencies.

Navigation Issues: GPS signals might become unreliable, which can be a problem for navigation systems.

Why and How does a fadeout happen?

Blackouts happen as a result of "solar flares" (X-ray radiation) and "solar ejection of matter (energetic charged particles)."

Figure 4: Solar Flares and Coronal Mass Ejections
are components of suden solar storms that may
affect skywave radio propagation on Earth.
Image credit: NASA, Aug. 2012; captions added by 4x4xm.

Flashes of intense X-Rays

Figure 5: A Solar Flare courtesy of NOAA, May 2023

Solar flares reach Earth at the speed of light, taking approximately 8 minutes and 20 seconds.

Ejection of solar matter

Solar energetic particles (SEPs) are high-energy, charged particles ↗ from the solar atmosphere, including electrons, protons, alpha particles, and heavy ions, with energies ranging from a few tens of keV to many GeV. They are accelerated during solar particle events (SPEs) either at solar flare sites or through shock waves from coronal mass ejections (CMEs) ↗. Ejected particles can reach Earth within 20 to 30 minutes (relativistic electrons) or up to four days later (heavier charged particles).

Figure 6: Solar wind, CME and Earth magnetosphere
When a CME enters the magnetosphere, it causes a Geomagnetic Storm

Figure 7: An artist view of a Geomagnetic Storm

Major magnetic storms can disrupt HF propagation between 3 and 30 MHz by affecting the distribution of free electrons in the ionosphere. These storms can increase ionization in the D-region of the ionosphere, which blocks HF radio transmissions. Unexpected bursts on the Sun's surface cause Sudden Ionospheric Disturbances (SIDs) on Earth, with recovery times ranging from minutes to hours.