Atomic Magnetometry in the Mesospheric Sodium Layer

Within the Earth's mesosphere a band of free sodium atoms exists at altitudes of 90--100 km. This mesospheric sodium layer is the basis for ``laser guide stars'' employed in observational astronomy [1]. We will outline an experiment to use the $^{23}$Na atoms in this layer for high-precision atomic magnetometry; such a measurement would yield geomagnetic data on a previously unexplored length scale. A schematic of the proposed experiment will be presented, as well as some interesting challenges inherent in performing an atomic physics experiment outside the confines of the laboratory.\newline \newline [1] W.~Happer \emph{et al.}, J. Opt Soc. Am. A \textbf{11}, 263 (1994)