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An Analysis of the Anomalous Response of the Van Allen Probes EFW Instrument
Journal article   Open access   Peer reviewed

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

K. Greene, J. W. Bonnell, K. A. Goodrich, J. L. Bowman, E. M. Tejero, D. P. Hartley and W. E. Amatucci
Journal of geophysical research : Space physics (2013 - Present), Vol.131(5), e2026JA035217
05/2026
DOI: 10.1029/2026JA035217
url
https://doi.org/10.1029/2026JA035217View
Published (Version of record) Open Access

Abstract

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. Plain Language Summary Spacecraft often measure electric fields using pairs of sensors mounted on long booms. Several missions, including the Van Allen Probes, have observed unexpected changes in signal strength and timing in these measurements. These anomalies are thought to result from unintended interactions between the sensors in the surrounding plasma and nearby spacecraft, but the physical cause remains unclear. In this study, we develop a new computer model to investigate how electric field sensors interact with their plasma environment. The model calculates how electrical signals can feed back between these components through the plasma under different space conditions. By including a simplified representation of the instrument electronics, we directly compared the model results with measurements from the Van Allen Probes. The model successfully reproduces the unusual gain and phase behavior seen in flight data. Our results indicate that the anomalies are caused by plasma‐mediated feedback involving plasma waves and are strongest in dense plasma environments where key plasma length scales become comparable to spacecraft dimensions. These findings improve confidence in interpreting past measurements and will help guide the design of future electric field instruments in space. Key Points The interaction between the electric field and waves (EFW) probe surfaces and the ambient plasma is modeled using a numerical simulation of a warm, magnetized plasma The simulated feedback response is in good agreement with the observed anomalous response of the Van Allen Probes EFW Instrument We suggest that this anomalous response is due to feedback between the driven stub and the sensor through ambient plasma's lower hybrid mode
 double probe electric field instrument instrumental noise instrument‐plasma interactions numerical modeling Van Allen probes

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