Geomagnetic storm suppression of photographic plate transient detections in the POSS-I archive: an independent physical variable strengthening the nuclear test correlation

Bruehl & Villarroel (2025) reported a correlation (p = 0.008, 2.6 sigma) between atmospheric nuclear weapon tests and photographic plate transient detection rates in the Palomar Observatory Sky Survey (POSS-I) archive, independently replicated by Doherty (2026) using negative binomial regression with weather controls. I identify geomagnetic storm activity, measured by the planetary Kp index, as an additional independent variable modulating transient rates in the same dataset. Transient detection rates follow a monotonic dose-response across five Kp intensity bins, from 17.4% during geomagnetically quiet periods to 2.4% at Kp 8-9 (Cochran-Armitage trend: Z = -3.391, p = 0.0007). Nuclear test days are not geomagnetically quieter than the baseline; they are slightly more storm-influenced. A multivariate logistic regression including Kp and lunar-phase controls strengthens the nuclear-transient correlation from 2.6 sigma (p = 0.009, OR = 1.53) to 3.1 sigma (p = 0.002, OR = 1.70). The dose-response rules out emulsion defects and spectrally inert orbital debris as the primary transient source, indicating a population physically coupled to the radiation belt environment at geosynchronous altitude. A self-contained reproduction script is provided as supplementary material.