Observing the Earth's Plasmasphere and Ionosphere From the Lunar Surface

We present the analysis of the first lunar‐based observational characterization of the Earth'splasmasphere and ionosphere using Global Navigation Satellite Systems signals tracked from the lunar surfaceby the Lunar GNSS Receiver Experiment (LuGRE). The Earth‐Moon geometry enables limb sounding of theplasmasphere at altitudes exceeding 3,000 km, bridging a critical observational gap. We compared TotalElectron Content measurements from GPS and Galileo satellites' signals with predictions from the Global CorePlasma Model. While the model captures the general morphology of the plasmasphere, significant discrepanciesemerge in the ionosphere/plasmasphere transition region. Specifically, LuGRE data reveal a overestimation ofelectron density by the model during the dayside phase indicating a lower plasma refilling efficiency thancurrently parameterized and an underestimation during the nightside phase. These results demonstrate thecapability of lunar‐based GNSS measurements to continuously monitor the global plasma environment, pavingthe way for future permanent observatories on the Moon.