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SESK: Stuff Everyone Should Know · sesk.info/hame-r3-blackout

A solar flare of what class produces an R3 (Strong) radio blackout?

X1, causing about an hour of lost HF contact on the sunlit side.

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  1. X1 ✔
  2. M5
  3. X10

Why: NOAA's R scale ties R3 (Strong) to an X1 flare. It describes a wide-area HF blackout with loss of radio contact for about an hour on the sunlit side of Earth.

Huh, didn't know that: R5 (Extreme) means a complete HF blackout on the entire sunlit side for a number of hours, tied to an X20 flare.

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References

Answer: X1. NOAA's R scale ties R3 (Strong) to an X1 flare. It describes a wide-area HF blackout with loss of radio contact for about an hour on the sunlit side of Earth.

  1. NOAA SWPC Solar Flares / Radio Blackouts www.spaceweather.gov/phenomena/solar-flares-radio-blackouts
    R3 X1
    Checked 2026-10-11.
  2. NOAA Space Weather Scales www.spaceweather.gov/noaa-scales-explanation
    loss of radio contact for about an hour
    Checked 2026-10-11.
  3. NOAA SWPC Solar Flares / Radio Blackouts www.spaceweather.gov/phenomena/solar-flares-radio-blackouts
    R2 M5
    Checked 2026-10-11.
  4. NOAA SWPC Solar Flares / Radio Blackouts www.spaceweather.gov/phenomena/solar-flares-radio-blackouts
    R5 X20
    Checked 2026-10-11.

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