Electric Field Response to Geyser Fountain Dynamics

Geyser eruptions generate transient water fountains that interact with the atmospheric electric field under repeatable and well-constrained conditions. These systems provide a natural setting to study coupled fluid flow and electrical processes, yet the mechanisms controlling the observed signals remain unclear. We present the first systematic investigation of short-term electric field fluctuations during geyser eruptions, combining electrical measurements with high-speed video observations at Strokkur geyser, Iceland. Using a Biral Thunderstorm Detector and an Electric Field Mill, we recorded electrical signals during eruption events in October 2025 and relate them directly to the evolving fountain geometry. We observe that the polarity and amplitude of electrical signals correlate strongly with the ambient atmospheric potential gradient. We develop a finite-element electrostatic model (FEMM) to quantify the partial electrostatic shielding effect of a grounded water fountain on the ambient electric field. We derive empirical equations relating fountain height, radius, and potential gradient to induced electrical currents, allowing for a direct comparison between modeled and observed signals. By inverting the electrical signal, we reconstruct time-varying fountain radii that closely match independent video observations. This result demonstrates that electrical measurements can be used to infer the geometry and temporal evolution of water jets. Our findings show that the dominant electrical response arises from the conductive water column modifying the surrounding electric field during rapid vertical growth. More broadly, this approach provides a new, non-intrusive method to quantify fountain geometry and outflow in dynamic hydrothermal systems, linking electrical observations to their physical behaviour.