A Jupiter System III–Locked Modulation in Global Ground Magnetometer Data

We report evidence for a nanotesla-scale component in 2020 ground Z-magnetometer data that is phase-locked to Jupiter’s System III rotation. Using ephemeris-locked mixing, Welch PSD, extensive surrogate/null tests, and quiet-time AE/Dst/Kp gating, we find a persistent spectral envelope consistent with a System III reference.Robustness (new in v4.3): comparisons to randomized ephemerides; sensitivity to AE/Dst/Kp thresholds; Welch segment/overlap/taper variants; X/Y vs Z controls; station-level consistency and out-of-sample checks.Reproducibility: package includes station manifest, cohort-windows.csv (set→months), scripts, and checksums plus Supplements A1–A4. Licensing: Text/figures CC BY 4.0. Code/scripts Apache-2.0 (see LICENSE in the ZIP).Data citation: INTERMAGNET; WDC Kyoto AE/Dst (DOIs: 10.17593/15031-54800, 10.17593/14515-74000); GFZ Kp (DOI: 10.5880/Kp.0001); NASA/JPL HORIZONS (Giorgini et al., 1996).ORCID: https://orcid.org/0009-0003-5655-0052