When the Sun Fires Twice: How Consecutive Flares Affect HF
In May 2024, the Sun produced 13 major X-class flares in just one week — one of the most flare-active periods in decades. For radio amateurs, this meant sudden HF blackouts, unpredictable recovery times, and in some cases lingering absorption. A new study by Wang et al. (2025) analyzed this event with ionosonde data from Europe and beyond, showing exactly how consecutive flares behave and what it means for us on the bands.
What the Study Looked At
- 13 major X-class flares between May 8–15, 2024, including four within ~24 hours.
- Used ionosonde stations (Athens, Dourbes, Juliusruh, Prague, Sopron, San Vito, Ascension Island) to track fmin (minimum reflection frequency) and foE (E-layer critical frequency).
- Compared single vs consecutive flares, and checked the role of solar energetic protons (SEPs).
Main Findings
- All X-flares raised D/E-layer absorption — fmin spiked, HF traces vanished on ionograms, typical blackout lasted ~1 hr.
- Consecutive flares don’t add up linearly — the first flare depletes neutral atoms, so the second flare often has less impact if it follows closely.
- Protons make things worse — if a flare is followed by a solar proton event, the blackout can linger much longer than 1 hr (classic “polar cap absorption”).
- Only on the daylit side — nightside stations showed no major effect; flare impacts are hemisphere-local to where the Sun is up.
HF & VHF Band Impact
| Condition | HF Bands | VHF/UHF |
|---|---|---|
| Single X-class flare | HF blackout ~1 hr on sunlit side, strongest on 80–20 m. Recovery is quick. | Negligible direct effect, but some auroral enhancement possible. |
| Consecutive flares (no SEPs) | First flare worst; second flare weaker due to prior ionization. Recovery shorter. | Minimal. |
| Flares + SEP event | Prolonged HF blackout, especially over polar paths. Absorption persists hours. | Polar VHF paths enhanced, aurora scatter more likely. |
What This Means for Hams
- During daytime X-flares, expect sudden loss of HF — not equipment failure, just the D-layer “switching off” the bands.
- If another flare follows quickly, don’t panic — the second may be less severe unless protons are involved.
- Polar DX paths are the most at risk when SEPs follow a flare. Non-polar paths often recover faster.
Mini-FAQ
- How long does an HF blackout from a flare last? — Usually ~1 hour, but longer if solar protons arrive.
- Do multiple flares stack effects? — Not always; the first flare often reduces the impact of the next.
- Does it affect nighttime HF? — No, only the sunlit side of Earth is hit.
- Are VHF bands affected? — Only indirectly: polar protons may boost aurora and scatter, but HF takes the main hit.
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