Using the Moon to probe the geomagnetic tail lobe plasma

We have detected the presence of plasma in the lobes of the geomagnetic tail from observations of magnetic induction in the Moon forced by time variations of the Earth's magnetotail lobe field. The magnitude of the Moon's tangential electromagnetic transfer function when the Moon is in the lobes of the geomagnetic tail is less than that when the Moon is in the solar wind or geomagnetic tail plasma sheet. The tangential transfer function when the Moon is in the magnetotail lobes decreases at frequencies above about 8 milliHz due to finite wavelength effects. This shows that the waves in the magnetotail lobes which drive the lunar magnetic induction must have speeds far less than the speed of light and wavelengths comparable to the size of the Moon.