Constraint on the lunar core size from electromagnetic sounding based on magnetic field observations by an orbiting satellite

We have attempted to constrain the lunar core size from electrical conductivity sounding by using magnetic field data from the Kaguya (KG) and Lunar Prospector (LP) satellites. As suggested by previous studies, the signature of induction in the core can sometimes be detected by the satellites as an internal dipole field when the Moon enters from the magnetosheath to the tail lobe of the Earth’s magnetosphere. Since the magnetic anomaly field is up to about 2 nT at the orbital altitude (∼100 km), we removed the anomaly field from the observed magnetic field on the basis of the magnetic anomaly analysis. A spherical harmonic analysis tuned for the satellite observations was applied for separation of the internal and external fields, and the internal to external dipole ratio was used to estimate the size of the core. We estimated the effect of lunar mantle induction due to the external field fluctuation in the tail lobe region for two typical conductivity models of the lunar interior. Simulation results show that mantle induction fields seriously disturb the core signal. However, data selection made with reference to statistical distribution of the dipole ratio is effective to minimize the influence of the mantle conductivity on the core size estimates. The internal to external dipole ratio obtained for the selected period of KG observation was 0.0023 ± 0.0019. The mean value corresponded to a lunar core size of 290 km, and the upper bound of the lunar core size was estimated to be about 400 km with a 95% confidence limit. On the other hand, it was not possible to select a time interval that was suitable for successful application of the method adopted in the present work to the LP dataset. Larger external magnetic field variation during the period of LP observation caused this, although the stronger magnetic field at the LP observation period was expected to be suitable for the core size estimation. The larger external field variation during the period of LP observation was a consequence of moderately active solar conditions in 1998 relative to the deep solar minimum conditions in 2008.