ARTEMIS observations of electromagnetically induced fields from the lunar interior

The lunar interior is comprised of a crust, mantle, and inner and outer cores, and is sufficiently conductive to sustain observable electromagnetic induction by the solar wind. Various hybrid plasma simulations have simulated the effects of a conductive lunar interior on the Moon's interaction with the solar wind and predicted that electromagnetic (EM) fields originating from induced eddy currents within the Moon should manifest as disturbances in the equilibrium magnetic field distribution outside the Moon, particularly downstream in the lunar wake. Case-by-case analyses of potential induced field signatures observed by the ARTEMIS mission during single transient events have proven difficult to interpret due to complications arising from spacecraft motion, changing upstream solar wind conditions, and other dynamic plasma phenomena.

Using a statistical approach, we have for the first time detected EM induced fields from the lunar interior using observations from the ARTEMIS mission. We spatially mapped approximately eight years of ARTEMIS magnetic field measurements around the Moon, separated these data into times when the upstream interplanetary magnetic field (IMF) was relatively quiet versus times when the upstream magnetic field was dynamic, and took the ratio of these two maps. During active IMF periods, perturbations to the magnetic fields are clearly seen both directly downstream of the Moon and along the boundaries of the lunar wake. In particular, the ARTEMIS data show enhanced field strengths very close (<1 lunar radius) to the nightside lunar surface during active magnetic field times, which we interpret as direct detection of induced EM fields. We have also explored the effects of these induced fields on the structure of the wake as a function of the IMF magnitude, solar wind density, velocity, and pressure, Alfvén Mach number, and plasma beta. We have compared ARTEMIS observations with hybrid plasma models of the solar wind interaction with the Moon with the presence of induced fields in the interior and generally found good agreement. We use the comparison of ARTEMIS observations and hybrid plasma simulations to place estimates on the strength of the induced magnetic moment versus the upstream change in the magnetic field, which may allow estimates of the lunar interior conductivity.