An Analysis of the Anomalous Response of the Van Allen Probes EFW Instrument

Recent in situ measurements of electric fields from missions such as Polar, DEMETER, and the Van Allen Probes have reported anomalies in double‐probe electric field measurements, attributed to unintended coupling between the sensor and nearby driven stub surfaces through the ambient plasma. The Van Allen Probes Electric Field and Waves (EFW) instrument observed anomalous gain and phase responses under specific plasma conditions, with densities between 30 and 2,000 cm −3 and frequencies up to a few kHz. Although commonly observed in on‐orbit measurements, the physical cause of these anomalies remains poorly understood. Here we present a model of the feedback interaction between the EFW sensor and the driven stub surface mediated by the surrounding plasma. We used a numerical simulation of conductive surfaces immersed in a warm magnetized plasma to calculate the mutual impedance between the sensor and stub across a range of plasma environments. By incorporating a simplified model of the stub‐biasing electronics, we directly compare model predictions with the anomalous gain and phase responses observed on the orbit. The modeled feedback response agrees well with the EFW observations. We suggest that feedback involving kinetic H + lower‐hybrid modes explains the observed anomalies. This effect is strongest in dense plasmas where electrons become partially demagnetized, and the Debye length approaches the stub length. These results will help inform the interpretation and design of future spaceborne electric field measurements.