Plate Sensitivity Is Invariant Across Geomagnetic Storm Intensity at Harvard and Palomar: A Protocol for Artifact Control in Historical Plate Archive Studies

Abstract

Historical photographic plate archives anchor a growing body of time-domain astronomy, but time-domain claims drawn from them are vulnerable to plate-sensitivity variations correlated with environmental modulators that can mimic real astrophysical signals. I present a simple, broadly applicable protocol for testing such artifact hypotheses: regress catalog-aggregate reference-population metrics (stellar detection counts or plate limiting magnitudes) against the suspected modulator. Under the artifact hypothesis, the reference metric varies systematically; under the null hypothesis, it does not. I apply the protocol to test whether geomagnetic storm activity, measured by the planetary Kp index, modulates plate sensitivity at two independent observatories. At Harvard College Observatory, the DASCH DR7 archive provides limiting magnitudes for 12,510 exposures across 500 sky positions: no significant trend across Kp bins (Spearman rho = -0.011, p = 0.234). At Palomar, the MAPS Catalog of POSS-I records stellar detection counts for 638 fields: no significant trend (Spearman rho = 0.017, p = 0.673). Plate sensitivity is invariant across the full range of geomagnetic activity at both sites. The principal airglow-based artifact objection to recent claims of Kp-dependent transient suppression in the POSS-I archive is directly falsified at two observatories.