Distortion of Induced Magnetic Fields on the Nightside of the Moon and Implications for Electromagnetic Sounding

Abstract:

Nightside Time Domain Electromagnetic Sounding was performed during the Apollo era using transient discontinuities within the Interplanetary Magnetic Field (IMF) occurring when the Lunar Surface Magnetometer was well within the wake cavity. This analysis assumes that induced fields, generated by conducting layers within the interior of the Moon, respond as an undisturbed spherically symmetric dipole in vacuum. In actuality, the magnetohydrodynamic plasma environment interacts with the induced field. On the dayside, the induced field is confined to the surface by the dynamic pressure of the solar wind. On the nightside, Apollo era theories predicted the induced field would expand into the diamagnetic wake cavity. We present the first hybrid model studying the interaction of the wake fields with an induced dipole field at the Moon. Contrary to the assumptions during the Apollo soundings, we have found that the nightside dipole fields at the Moon are not confined within the wake cavity. The induced fields penetrate out of the rarefraction regions of the wake boundary. Furthermore, within the deep wake, near-vacuum region, distortion of the induced dipole fields due to the interaction with the wake varies between a few percent to more than 15% at the surface depending on the magnitude of the induced field, the geometry of the upstream fields, and the upstream plasma parameters. Thus, our results indicate that under cer tain conditions the assumption of a vacuum dipolar response is reasonable. For the application of this method, a two point measurement is required with altitudes below approximately 400km. Future Electromagnetic Sounding of the interior structure of the Moon with ARTEMIS will provide further insights into the lunar interior structure. These results are applicable to any airless body with a conducting interior, interacting directly with the solar wind in the absence of a parent body magnetic field.