Data Lab / WSPR Solar Cycle Modulation — How the 11-Year Cycle Reshapes HF Propagation

Figure 1: band correlation bars

Fig. 1: band correlation bars

Figure 2: detrended vs raw

Fig. 2: detrended vs raw

Figure 3: lag analysis

Fig. 3: lag analysis

Figure 4: paper figure1

Fig. 4: paper figure1

Figure 5: ssn scatter grid

Fig. 5: ssn scatter grid

Figure 6: timeseries overlay

Fig. 6: timeseries overlay

Open: band correlation barsOpen: detrended vs rawOpen: lag analysisOpen: ssn scatter gridOpen: timeseries overlay

WSPR Solar Cycle Modulation: How the 11-Year Cycle Reshapes HF Propagation

Status: Complete (R1 draft) Author: TerraPulse Lab Created: 2026-04-06

We aggregated 10.94 billion raw WSPR spots (258 monthly files, Nov 2004 to Mar 2026) into monthly mean signal-to-noise ratio (SNR) per band, then correlated them against SILSO sunspot numbers across two complete solar cycles (24 and 25).

The Surprise

10 m and 12 m show negative correlation with sunspot number, opposite to standard high-frequency (HF) propagation theory.

Band Pearson r p-value Detrended r Cohen's d (max vs. min) N months Survives Bonf
80 m -0.24 3.3e-04 +0.24 -0.26 217 *
40 m +0.39 3.1e-09 +0.63 +0.96 219 *
30 m +0.30 5.3e-06 +0.63 +0.77 217 *
20 m +0.27 5.6e-05 +0.52 +0.65 217 *
17 m +0.03 0.62 +0.34 +0.16 217
15 m +0.10 0.14 +0.37 +0.29 211
12 m -0.44 1.6e-11 -0.35 -0.88 210 *
10 m -0.40 8.6e-10 -0.31 -0.80 217 *

Fisher z contrast 10 m vs. 40 m: z = -8.64, p < 10^-17. The bands respond in opposite directions.

Permutation test on 10 m: observed r = -0.40, permutation p = 0.001, robust against chance.

The Likely Explanation: Selection Effect

This is not a refutation of solar physics. It is an artifact of how WSPR mean SNR is computed:

  1. 40 m, 30 m, and 20 m: F-layer enhancement during solar maximum lets more signals get through with higher SNR, producing the positive correlation. Standard physics.

  2. 10 m and 12 m: These bands are mostly dead during solar minimum. The few stations operating then are short-distance ground-wave or sporadic-E contacts with strong signals (SNR around -10 dB). During solar maximum, the band opens, and hundreds of stations attempt DX contacts (over 10,000 km), most of which are weak (SNR around -25 dB). More attempts at long DX produce lower mean SNR, even though more total contacts complete.

  3. 80 m: Slight negative correlation (-0.24). D-layer absorption during high solar activity slightly degrades nighttime sky-wave.

Validation

  • N = 217 months (18 years of monthly data)
  • All five significant bands (40 m, 30 m, 20 m, 12 m, and 10 m) survive Bonferroni correction (alpha = 0.00625).
  • Detrended correlations (linear trend removed) preserve the directions but change magnitudes. The solar cycle is not just a secular trend.
  • Permutation test (1000 shuffles) confirms the 10 m result: p = 0.001.
  • Total data: 10,935,947,917 spots.

What This Means for Future Analyses

The raw mean SNR metric on bands that toggle between dead and alive (10 m and 12 m) is biased by selection. To measure the actual ionospheric response on these bands, we must do one of the following:

  1. Filter by station consistency (same TX-RX pairs across the cycle).
  2. Condition on path geometry (only short paths, to remove DX selection).
  3. Use spot count as the primary signal rather than mean SNR.

This is a methodological finding that should inform any future WSPR-versus-solar-cycle analysis.

Reproducibility

cd workspaces/wspr-solar-cycle-modulation
python scripts/analyze.py    # ~40 minutes for 10.9B spots

Data: /mnt/ursa/data/terrapulse/wspr/raw/wspr_raw_YYYYMM.parquet (258 files, 213.8 GB) SSN: sunspot_number metric in PostgreSQL (SILSO daily, 1818-present)

Author: PMA

Published: 2026-04-06 · Updated: 2026-04-06

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