When the Sun Fires Twice: How Consecutive Flares Affect HF
When the Sun Fires Twice: How Consecutive Flares Affect HF
A second X-class flare does not simply add another identical blackout. The ionosphere remembers what happened before it—but that memory is local, temporary and entangled with sunlight, chemistry and energetic particles.
The useful question is not whether an X-class flare is “big enough to kill HF.” It is which radiation or particles are reaching which part of the atmosphere, where the path is sunlit, and what the lower ionosphere was already doing. That is why I found the May 2024 analysis by Wang and colleagues worth reading: it compares four closely spaced X-class flares instead of treating every event as a clean laboratory reset.
Between 8 and 15 May 2024, thirteen X-class flares were recorded during an exceptionally active interval. Wang et al. concentrated on four flares on 14 and 15 May and compared satellite measurements of X-rays, extreme-ultraviolet radiation and energetic protons with ionosonde observations. Their paper offers a valuable event study. It does not establish a fixed blackout duration, a guaranteed “first flare is worst” rule or a band-by-band promise for every station.
What the May 2024 Study Actually Measured
The four selected flares were an X1.20 and X8.79 event on 14 May, followed by X3.48 and X2.90 events on 15 May. The authors used ionosonde records from several European stations and Ascension Island while geomagnetic conditions were comparatively quiet. That helped them examine the lower-ionosphere response without treating every disturbance as a geomagnetic-storm effect.
Two ionogram parameters carry much of the argument:
- fmin is the lowest frequency at which a usable ionospheric echo can be identified. When lower-ionosphere absorption rises, weaker low-frequency echoes disappear and the reported value often rises.
- foE is the E-layer critical frequency for vertical incidence. During strong absorption, the E trace can become unreadable, so “not interpretable” is not the same as a direct measurement of zero E-layer ionisation.
An ionosonde is therefore not a universal blackout meter. Its fmin also depends on transmitter power, receiver sensitivity, interference, noise and the trace-scaling method. What makes the Wang et al. comparison useful is the time correlation between the space-weather measurements and multiple ground stations—not a single threshold value lifted from one ionogram.
Why X-Rays Can Silence a Sunlit HF Path
Solar X-rays and extreme-ultraviolet radiation reach Earth at light speed. The practical radio response follows within minutes because the radiation increases ionisation in the lower ionosphere, especially the D region on the sunlit side. At those altitudes, electron-neutral collisions convert part of an HF wave’s energy into heat. A signal may still refract in the upper ionosphere yet become unusable because it suffers heavy absorption while passing through the lower layers on the way up and again on the way down.
The absorption is generally stronger at lower HF frequencies, but there is no universal boundary such as “80 through 20 metres black out for one hour.” Solar zenith angle, flare spectrum, path geometry, season, background ionisation, operating frequency and required signal-to-noise ratio all affect what an operator experiences. A circuit crossing the dayside can be impaired even when one endpoint is in darkness; a completely nightside path need not share the immediate X-ray response.
When the Second Flare Meets an Ionosphere With Memory
The most interesting result is that consecutive flares did not produce a simple linear sum. In the 14 May pair, the much larger X8.79 flare did not create a proportionally larger increase in the reported lower-ionosphere response than the earlier X1.20 flare. The paper discusses two relevant differences: the solar zenith angle had changed, and the first event had already altered the local ionisation, neutral density and thermal state.
That is a physically plausible memory effect, but it should remain bounded to the observed event and the authors’ interpretation. A close second flare can encounter fewer available neutral constituents and a chemically disturbed lower ionosphere, reducing the incremental electron-density change. Another sequence, location or time separation can behave differently. Flare class measures peak soft X-ray flux near Earth; it is not a complete specification of the radiation spectrum, duration or received radio-path loss.
So the operating rule is not “the second flare will be weaker.” It is: do not predict the next HF response from flare class alone. Watch the actual X-ray trace, the absorption map and the paths you use. The atmosphere has state, and close events do not start from identical initial conditions.
X-Rays, Protons and Geomagnetic Storms Are Three Clocks
Radio discussions often collapse a flare, a solar radiation storm and a geomagnetic storm into one event. They are related when produced by the same active region, but they arrive on different timescales and affect radio by different mechanisms.
- Flare X-rays and EUV: arrive with the light from the Sun and can produce a rapid dayside D-region absorption increase. NOAA classifies the associated HF effect with the R-scale for radio blackouts.
- Solar energetic protons: arrive later and can maintain strong ionisation and absorption, especially on high-latitude and polar paths. NOAA uses the S-scale for solar radiation storms; polar-cap absorption can outlast the X-ray pulse by many hours and sometimes longer.
- CME-driven geomagnetic disturbance: may arrive roughly one to several days later if an Earth-directed ejection is present. Changes in the upper ionosphere, auroral zone and usable HF frequencies then belong to the geomagnetic response, not the immediate flare blackout.
Wang et al. found about a one-hour lower-ionosphere X-ray response in the cases they studied. On 15 May, energetic protons accompanied the sequence and the observed response was more persistent and cumulative. That does not turn one hour into a standard recovery time. A proton event can be much longer lived, and a later geomagnetic storm can move the usable frequencies and paths again after the immediate X-ray absorption has faded.
The paper also reports a sunward lower-ionosphere response associated with high-energy protons and discusses direct penetration and secondary-particle production. That is more specific than the familiar polar-cap-absorption picture, and it should be read as the authors’ interpretation of this event—not as permission to label every proton-related HF loss a global D-layer blackout.
What an Operator Can Expect Without Pretending to Forecast
During an active X-ray flare, the first symptom can be an abrupt drop in daytime HF signals, strongest on frequencies and paths suffering the most lower-ionosphere absorption. Raising frequency may help if the upper ionosphere still supports the path, but moving above the absorption does not help if the chosen frequency exceeds the path’s usable limit. Trying a different azimuth or a path with less dayside exposure can be more useful than marching through bands by habit.
After the X-ray flux falls, recovery is something to observe, not schedule. Compare the D-region Absorption Prediction map with ionograms, propagation beacons, Reverse Beacon Network reports and your own reference signals. If proton flux is elevated, give special attention to high-latitude paths and expect a different clock from the flare’s X-ray pulse. If geomagnetic activity follows, re-evaluate the path’s maximum usable frequency and auroral-zone conditions instead of calling the later change a lingering flare blackout.
VHF needs the same discipline. An X-ray radio blackout is fundamentally an HF absorption problem. Auroral propagation can occur during disturbed space weather, but a flare or proton alert does not guarantee useful auroral VHF enhancement. It needs the appropriate geomagnetic and auroral conditions, path geometry, frequency and scattering region.
A Practical Space-Weather Check for the Shack
- Start with the time: did the loss coincide with an X-ray rise or an R-scale alert?
- Map the path: determine which portions were sunlit and whether the circuit crossed a high-latitude absorption region.
- Check D-region absorption: use NOAA’s D-RAP product as a modelled estimate, then compare it with real signals and ionograms.
- Separate the proton clock: inspect energetic-proton flux and S-scale alerts when polar or persistent absorption is suspected.
- Separate the geomagnetic clock: use geomagnetic observations and forecasts for the later upper-ionosphere response.
- Keep a station reference: log a few known beacons, broadcast carriers or automated reports so a propagation change can be distinguished from a receiver, feedline or local-noise fault.
The May 2024 sequence is a good reminder that the ionosphere is not a switch with “flare” and “normal” positions. It is a chemical and electrodynamic system with memory. Consecutive events can interact, yet the size and sign of that interaction depend on what the atmosphere, radiation field and particle environment are doing at that place and time. That is a better operating model than assuming two X-class labels must produce twice the blackout.
Primary Technical References
- Wang et al.: Analysis of the Consecutive X-Ray Flares Effects on the Lower Ionosphere
- NOAA Space Weather Prediction Center: NOAA Space Weather Scales
- NOAA Space Weather Prediction Center: D-Region Absorption Predictions
- NOAA Space Weather Prediction Center: GOES X-Ray Flux
- NOAA Space Weather Prediction Center: GOES Proton Flux
Mini-FAQ
- Does a second flare always produce a weaker HF blackout? — No. The May 2024 study found a reduced incremental response in one close sequence, but sunlight, flare spectrum, timing and the ionosphere’s prior state all change the outcome.
- Does an X-class flare black out every HF band for one hour? — No. The study observed about a one-hour lower-ionosphere response in its cases; frequency, path, daylight, flare spectrum, absorption and required SNR determine the practical result.
- Why does a flare hurt HF on the sunlit side? — Flare radiation rapidly increases lower-ionosphere ionisation. Electron-neutral collisions then absorb more HF energy as the wave crosses the D region.
- Are a radio blackout and polar-cap absorption the same event? — No. Immediate flare X-rays drive dayside radio blackouts, while later energetic protons can cause prolonged absorption that is especially important on polar paths.
- Can a solar flare improve VHF auroral propagation? — A flare alert alone is not a forecast of usable auroral propagation. The scattering opportunity depends on geomagnetic and auroral conditions, path geometry and frequency.
- How can I separate propagation loss from a station fault? — Compare several bands and known signals, check current X-ray and D-RAP products, inspect ionograms or beacon reports, and test another receive path where practical.