Bidirectional Modulation of Global Seismic Coherence by Geomagnetic Storms: Evidence from Five Events Across Two Solar Cycles

We report bidirectional modulation of global inter-station seismic coherence in the Pc5 pulsation band (1.5-5 mHz) during geomagnetic storms. Analyzing 19-22 Global Seismographic Network stations across five storms spanning Solar Cycles 24 and 25 (2015-2026), we find all events produce highly significant coherence changes (Fisher combined p ~ 0). The direction of modulation correlates with storm driver composition: pure CME-driven storms during SC25 ascending phase disrupt coherence (Cohen's d = -0.59 to -0.73), while storms with HSS/CIR components or occurring during SC24 enhance it (d = +0.52 to +1.73). NASA OMNI solar wind data reveal that the January 2026 G4 storm -- initially predicted to disrupt coherence based on its SC25 timing -- instead enhanced it, consistent with its mixed CME-HSS driver signature. Storm driver composition, modulated by heliospheric state, predicts modulation direction more accurately than solar cycle phase alone.