Correspondences

What is linked, what is not

Each arc joins two phenomena tested on this site. Green and animated: clear link (correlation significant at 5 % after correcting for autocorrelation, or a response outside chance in an event study). Grey: inconclusive test. A statistical link alone does not establish causation.

Clear link
Inconclusive

Hypotheses tested

Clear links

correlation p < 0.05 or epochs outside chance

Causation demonstrated

a correlation demonstrates none

Arc width from |r| of the stored result; several tests between the same phenomena are drawn as stacked arcs.

Hypothesis tested: Coronal holes last several solar rotations and send fast wind towards the Earth about every 27 days, the Sun’s synodic rotation: activity should repeat with that period (Bartels, 1932).

Pearson correlation r (lag 27 days)

0.12

95% interval [0.00; 0.00] (block bootstrap)

Aligned points

22,541

effective n ≈ 15,033 (autocorrelation)

p-value

0.00000

two-sided, Fisher z transform, effective n

Correlation at 13 days

0.002

at 40 days: -0.024

Period analysed

1965-01-01 → 2026-10-15

computed on Oct 23, 2026 · engine v1.0

Correlation by lag (days)

Show the data as a table

Pearson coefficient for each lag; a positive lag means the second series follows the first. The table gives n, effective n and p for each lag.

Aligned series (CSV)

Method and sources

Daily mean Ap index (OMNI2), with a 365-day running mean removed (to take out the solar cycle), then correlated with itself for each lag from 1 to 60 days. The peak is sought between 24 and 30 days; the effective sample size accounts for autocorrelation.

Computation parameters

{
    "lags": "1–60 days",
    "n_eff": "Bretherton et al. (1999)",
    "series": "daily mean Ap (OMNI2)",
    "peak_window": "24–30 days",
    "high_pass_days": 365
}

Inputs : omni.ap.3h (186,392 pts, run #133)

Reading this result

The correlation is small (about 0.12) but clear against neighbouring lags: the recurrence is strong in the declining phase of cycles, when coronal holes dominate, and weak at maximum, when unpredictable coronal mass ejections take over. Correlation is not causation: common factors (seasons, cycles, trends) or coincidence can produce it. The p-values account for autocorrelation (effective n) but not for preprocessing choices; the confidence interval comes from a block bootstrap. The limits of the source data apply.

SOURCES OF THIS PAGE

OMNI2 hourly near-Earth solar wind and geomagnetic indices: King, J. H., & Papitashvili, N. E. (2005). Solar wind spatial scales in and comparisons of hourly Wind and ACE plasma and magnetic field data. J. Geophys. Res., 110, A02104. OMNI2 data from NASA/GSFC SPDF OMNIWeb; Kp/ap from GFZ Potsdam, Dst from WDC Kyoto, F10.7 from NRC Canada (Penticton). · doi:10.1029/2004JA010649 · source page ↗
Licence : NASA data, public domain; acknowledge SPDF/OMNIWeb and the index providers · retrieved on Oct 23, 2026 · SHA-256 6de9b217bd4b…
Bartels (1932): Bartels, J. (1932). Terrestrial-magnetic activity and its relations to solar phenomena. Terrestrial Magnetism and Atmospheric Electricity, 37(1), 1–52. · doi:10.1029/TE037i001p00001
Licence : Published literature; cited
Richardson et al. (2012): Richardson, I. G., & Cane, H. V. (2012). Solar wind drivers of geomagnetic storms during more than four solar cycles. Journal of Space Weather and Space Climate, 2, A01. · doi:10.1051/swsc/2012001
Licence : Published literature; cited

The 27-day recurrence Geomagnetic activity follows the Sun’s rotation

What is linked, what is not