What We Can Learn From the Sensor-to-Plasma Coupling Resistance Measurements in the Earth Ionosphere

Measurements of electric potentials and currents in the Earth’s ionosphere are primarily influenced by the thermal plasma properties and the intensity of photoelectron emission from sunlit spacecraft surfaces. These measurements can be significantly affected by a plasma sheath that forms around the satellite to neutralize surface electric charging in contact with the ambient plasma. In the ionosphere, this sheath typically ranges from a few to several tens of centimeters in thickness and contains plasma that slightly differ from the surrounding undisturbed environment. Therefore, accurately characterizing the electrical properties of the sheath is essential for reliable data interpretation. In this study, we analyze ion plasma and electric field measurements taken at an altitude of 700 km by the Centre National d’Etudes Spatiales (CNES) detection of electro-magnetic emissions transmitted from earthquake region (DEMETER) microsatellite. The data were collected during specific orbital passes when onboard instruments operated in calibration mode. During these sequences, electric currents were intentionally injected from the electric field sensors into the surrounding plasma, enabling estimation of the sensor-to-plasma coupling resistance across the sheath. Plasma parameters obtained by fitting the resistance–current curve with a standard orbital-motion-limited (OML) model are compared with those derived from the ion instrument on the same spacecraft. The dataset confirms the consistency of plasma measurements performed by the two instruments.