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Answer: A solar flare is a burst of electromagnetic radiation from the Sun lasting minutes to hours. Because it travels at light speed, it affects Earth's sunlit side as soon as it is seen, ionizing the lower ionosphere and blacking out high-frequency (HF) radio. NOAA rates these on an R1 to R5 scale.

Where flares come from

Flares usually happen in active regions, areas of strong magnetic fields often tied to sunspot groups. When those fields become unstable they release energy.

Flare classes

Flares are classed by peak soft X-ray flux: A, B, C, M and X, each ten times stronger than the last. X flares start at 10^-4 W/m2.

Why radio fails

Normally HF radio travels long distances by bouncing off upper ionosphere layers. A strong flare ionizes the lower, denser D-layer, where radio waves lose energy to collisions.

NOAA scales

NOAA's space weather scales describe effects at each level, from minor HF degradation to widespread outages; the same scale family covers geomagnetic storms (G) and radiation storms (S).

Claims

• id: C1 claim: Solar flares are large eruptions of electromagnetic radiation lasting minutes to hours. source: S1 evidence: "Solar flares are large eruptions of electromagnetic radiation from the Sun lasting from minutes to"
• id: C2 claim: Flares usually occur in active regions associated with sunspot groups. source: S1 evidence: "Solar flares usually take place in active regions"
• id: C3 claim: Radio blackouts are classified on a five-level NOAA scale. source: S1 evidence: "Radio blackouts are classified using a five-level NOAA Space Weather Scale"
• id: C4 claim: At severe levels, HF radio propagation may be impossible in many areas for one to two days. source: S2 evidence: "HF (high frequency) radio propagation may be impossible in many areas for one to two"

Sources

• [S1] Solar Flares (Radio Blackouts). NOAA. https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts . License: US government work, public domain. Accessed 2026-11-04.
• [S2] NOAA Space Weather Scales. NOAA. https://www.swpc.noaa.gov/noaa-scales-explanation . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_ekrgq8c7,
  author = {{iinventedlight}},
  title = {{Solar flares and radio blackouts}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/solar-flares-radio-blackouts/},
  note = {Permanent ID iil:note:ekrgq8c7. License: CC-BY-4.0.},
}

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