Space weather

Solar Flares and X-Ray Flux: The Sudden Ionospheric Disturbance

A solar flare is an abrupt release of energy from the sun’s atmosphere, often near an active region. Flares emit X-rays that travel to earth in eight minutes and ionize the D layer of the ionosphere as they arrive. The result is a sudden ionospheric disturbance (SID) on the sunlit side of earth, which can absorb HF signals for periods ranging from minutes (small flares) to hours (large flares). Flares are classified by peak X-ray flux into A, B, C, M, and X classes, with each step a tenfold increase in intensity.

Why it matters for HF operating

A short-wave fadeout (SWF) is the operating consequence of a solar flare. During an M-class or X-class flare, HF on the sunlit side of earth can drop into the noise within minutes. The lower bands (80 meters, 40 meters) suffer first because D-layer absorption scales as 1 / frequency squared. The upper bands recover faster because they are less affected by D-layer attenuation. Operators who hear bands suddenly go quiet should check the X-ray flux feed before assuming their station is broken.

Key values to know

Class Peak X-ray flux Effect on sunlit-side HF
A < 10⁻⁷ W/m² Negligible
B 10⁻⁷ to 10⁻⁶ W/m² None operationally
C 10⁻⁶ to 10⁻⁵ W/m² Minor degradation on lower bands
M 10⁻⁵ to 10⁻⁴ W/m² Lower-band fadeouts for 10 to 60 minutes
X 10⁻⁴+ W/m² Sunlit-side blackouts; full HF recovery in hours
  • Sunlit side only. Flares affect HF on the sunlit half of earth. Stations on the night side see little to no immediate impact.
  • Lag from solar activity. Flare frequency tracks sunspot activity. During solar maximum, M-class events occur every few days and X-class events occur weekly to monthly.
  • CME risk. Some flares are accompanied by coronal mass ejections, which arrive 1 to 4 days later and can produce geomagnetic storms.

Common misconceptions

A solar flare is not the same event as a coronal mass ejection (CME), though they often happen together. Flares produce immediate X-ray effects on the ionosphere; CMEs produce delayed geomagnetic storms when the ejected material arrives. Operators sometimes blame a quiet HF day on “the storm last week,” when the more recent issue is actually a fresh flare or unrelated geomagnetic activity.

SkyWave reads X-ray flux from the NOAA Space Weather Prediction Center (SWPC) and flags conditions during active flare periods on the Go screen. See it in the app →