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

TL;DR

This study introduces a protocol to test if environmental factors like geomagnetic storms affect photographic plate sensitivity, finding no such effect at Harvard and Palomar observatories, thus supporting data reliability.

Contribution

The paper presents a simple, general protocol for artifact testing in historical plate archives, applied here to demonstrate invariance of plate sensitivity across geomagnetic storm intensities.

Findings

No significant correlation between geomagnetic activity and plate sensitivity at Harvard.

No significant correlation at Palomar observatory.

Plate sensitivity remains invariant across geomagnetic storm intensities.

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.