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We have demonstrated a remote magnetometer based on sodium atoms in the Earth's mesosphere, at a 106‐km distance from our instrument. A 1.33‐watt laser illuminated the atoms, and the magnetic field was inferred from backscattered light collected by a telescope with a 1.55‐m‐diameter aperture. We theoretically predict a shot noise limited measurement sensitivity of 19nT/Hz. The measured sensitivity was 162nT/Hz due to a smaller returned intensity and smaller resonance strength than expected. The value of magnetic field inferred from our measurement is consistent with several models of the Earth's field shape to within a fraction of a percent. Projected improvements in optics, plus the use of advanced lasers or a large telescope, could result in 1‐nT/Hz sensitivity. Plain Language Summary: The upper end of a spinning top will slowly revolve around a vertical axis, a phenomenon known as precession. The precession frequency of the top is proportional to the strength of the gravitational field in which the top sits. This work uses the precession of atomic spins to analogously measure the strength of a magnetic field. The interaction of light with the atoms is sensitive to this precession and allows existing devices to observe the precessing atoms within an instrument. We have, for the first time, measured the magnetic field 100 km from our instrument using naturally occurring sodium atoms in the Earth's mesosphere. Atomic‐based measurements, such as optical magnetometers, are typically performed in well‐controlled environments. Here we demonstrate the ability to initialize quantum spin states and readout the evolution of the state at a significant standoff distance using a modified laser guidestar. The technique provides unique information about the mesospheric region of the atmosphere that is inaccessible by either aircraft or satellites. The small amount of light collected by our telescope creates a much less sensitive measurement than typical optical magnetometers, but methods are discussed that can improve the performance of this initial setup to reach a sensitivity needed for advancing understanding of geomagnetic field fluctuations such as those due to solar eruptions. This novel measurement technique is also an example of feasibility for other remote quantum measurements in the ambient atmosphere. Key Points: Remotely measured the geomagnetic field at a 90‐km altitude, this is an altitude that was previously difficult to monitorMeasured geomagnetic field was 45,441 nT, comparable to geomagnetic models, with a sensitivity of 162 nT/√HzSeveral improvements to the present measurements are discussed; the sensitivity could be improved to better than 1 nT/√Hz [ABSTRACT FROM AUTHOR]

By means of laser optical pumping and Larmor-resonance detection, it is possible to use the naturally occurring sodium layer in the mesosphere to measure Earth's scalar magnetic field at ~90 km above ground. This is an altitude otherwise only accessible by rockets, which only will provide point measurements of very short time scales. During the winter of 2018-19 we have installed a cw sum-frequency laser for probing the sodium-atom Larmor resonance at the Artic Lidar Observatory for Mesospheric Research (ALOMAR) at Andøya in northern Norway, to measure and monitor the magnetic field in-situ in the high latitude mesosphere over longer time scales. The technique, which has been proved earlier at mid-latitudes, will in our project be applied to high latitudes in the auroral zone. This opens for a completely new domain of measurements of externally generated geomagnetic variations related to currents in the magnetosphere-ionosphere system.Here we report on the instrumental setup and present initial measurements of the mesospheric magnetic field. Furthermore, we discuss future improvements and plans. [ABSTRACT FROM AUTHOR]