Magnetometry with mesospheric sodium

Measurement of magnetic fields on the few 100-km length scale is significant for many geophysical applications including mapping of crustal magnetism and ocean circulation measurements, yet available techniques for such measurements are very expensive or of limited accuracy. We propose a method for remote detection of magnetic fields using the naturally occurring atomic sodium-rich layer in the mesosphere and existing high-power lasers developed for laser guide star applications. The proposed method offers a dramatic reduction in cost and opens the way to large-scale, parallel magnetic mapping and monitoring for atmospheric science, navigation, and geophysics. atomic physics | geomagnetism | optical pumping M easurements of geomagnetic fields are an important tool for peering into the Earth's interior, with measurements at differing spatial scales giving information about sources at corresponding depths. Mapping of fields on the few meter scale can locate buried ferromagnetic objects (e.g., unexploded ordnance or abandoned vessels containing toxic waste), whereas maps of magnetic fields on the kilometer scale are used to locate geological formations promising for mineral or oil extraction. On the largest scale, the Earth's dipole field gives information about the geodynamo at depths of several thousand kilometers. Magneticfield variations at intermediate length scales, in the range of several tens to several hundreds of kilometers likewise offer a window into important scientific phenomena, including the behavior of the outer mantle, the solar quiet dynamo in the ionosphere (1), and ionic currents as probes of ocean circulation (2), a major actor in models of climate change. To avoid contamination from local perturbations, measurements of such slowly varying components of the magnetic field must typically be made at a significant height above the Earth's surface (e.g., measurements of components with a spatial-variation scale of 100 km require an altitude of approximately 100 km) and with high sensitivity (on the order of 1 nT). Though magnetic mapping at high altitude has been realized with satellite-born magnetic sensors (3-5), the great expense of multisatellite missions places significant limitations on their deployment and use. Here, we introduce a high-sensitivity ground-based method of measuring magnetic fields from sources near Earth's surface with 100 km spatial resolution.* The method exploits the naturally occurring atomic sodium layer in the mesosphere and the significant technological infrastructure developed for astronomical laser guide stars (LGS). This method promises to enable creation of geomagnetic observatories and of regional or global sensor arrays for continuous mapping and monitoring of geomagnetic fields without interference from ground-based sources. Overview of Technique The measurement we envisage is a form of atomic magnetometry, adapted to the conditions of the mesosphere. The principle is to measure spin precession of sodium atoms by spin-polarizing them, allowing them to evolve coherently in the magnetic field, and determining the postevolution spin state. Spin polarization of mesospheric sodium is achieved by optical pumping, as proposed in the seminal paper on sodium LGS by Happer et al. (6). In the simplest realization, the pumping laser beam is circularly polarized and is launched from a telescope at an angle nearly perpendicular to the local magnetic field, as shown i

Laser Guide Stars for Optical Free-Space Communications

The German Aerospace Center (DLR) and the European Southern Observatory (ESO) performed a measurement campaign together in April and July 2016 at Teide-Observatory (Tenerife), with the support of the European Space Agency (ESA), to investigate the use of laser guide stars (LGS) in ground to space optical communications. Atmospheric turbulence causes strong signal fluctuations in the uplink, due to scintillation and beam wander. In space communications, the use of the downlink channel as reference for pointing and for pre-distortion adaptive optics is limited by the size of the isokinetic and isoplanatic angle in relation to the required point-ahead angle. Pointing and phase errors due to the decorrelation between downward and upward beam due to the point-ahead angle may have a severe impact on the required transmit power and the stability of the communications link. LGSs provide a self-tailored reference to any optical ground-to-space link, independently of turbulence conditions and required point-ahead angle. In photon-starved links, typically in deep-space scenarios, LGSs allow dedicating all downlink received signal to communications purposes, increasing the available link margin. The scope of the joint DLR-ESO measurement campaign was, first, to measure the absolute value of the beam wander (uplink-tilt) using a LGS, taking a natural star as a reference, and, second, to characterize the decrease of correlation between uplink-tilt and downlink-tilt with respect to the angular separation between both sources. This paper describes the experiments performed during the measurement campaigns, providing an overview of the measured data and the first outcomes of the data post-processing

Laser Pointing Camera: a valuable tool for the LGS-AO operations

Every observatory using LGS-AO routinely has the experience of the long time needed to bring and acquire the laser guide star in the wavefront sensor field of view. This is mostly due to the difficulty of creating LGS pointing models, because of the opto-mechanical flexures and hysteresis in the launch and receiver telescope structures.The launch telescopes are normally sitting on the mechanical structure of the larger receiver telescope. The LGS acquisition time is even longer in case of multiple LGS systems. In this framework the optimization of the LGS systems absolute pointing accuracy is relevant to boost the time efficiency of both science and technical observations.In this paper we describe the design and functionalities of the Laser Pointing Camera (LPC)3, developed at OAR for the 4LGSF of the ESO Adaptive Optics Facility. The LPC allows to have a fast pointing of the multiple LGS on the AO WFS during the initial acquisition phase of the telescope preset, thus reducing considerably the overheads currently experienced in most LGS-AO systems in operation.By recognizing via astrometric software the field stars as well as the multiple LGS, LPC is insensitive to flexures of the laser launch telescope or of the receiver telescope opto-mechanics. Moreover, LPC gives regularly the photometry and fwhm of the LGS, as well as the scattering of the uplink beams at the height of 10-15km, thus monitoring the presence and evolution of cirrus clouds. We present the first Commissioning results of the Laser Pointing Camera, obtained at the ESO VLT during the 4LGSF first Laser Guide Star Unit Commissioning, and will discuss its possible extension for the ELT operations

Laser Pointing Camera: a valuable tool for the LGS-AO operations

Every observatory using LGS-AO routinely has the experience of the long time needed to bring and acquire the laser guide star in the wavefront sensor field of view. This is mostly due to the difficulty of creating LGS pointing models, because of the opto-mechanical flexures and hysteresis in the launch and receiver telescope structures.The launch telescopes are normally sitting on the mechanical structure of the larger receiver telescope. The LGS acquisition time is even longer in case of multiple LGS systems. In this framework the optimization of the LGS systems absolute pointing accuracy is relevant to boost the time efficiency of both science and technical observations.In this paper we describe the design and functionalities of the Laser Pointing Camera (LPC)3, developed at OAR for the 4LGSF of the ESO Adaptive Optics Facility. The LPC allows to have a fast pointing of the multiple LGS on the AO WFS during the initial acquisition phase of the telescope preset, thus reducing considerably the overheads currently experienced in most LGS-AO systems in operation.By recognizing via astrometric software the field stars as well as the multiple LGS, LPC is insensitive to flexures of the laser launch telescope or of the receiver telescope opto-mechanics. Moreover, LPC gives regularly the photometry and fwhm of the LGS, as well as the scattering of the uplink beams at the height of 10-15km, thus monitoring the presence and evolution of cirrus clouds. We present the first Commissioning results of the Laser Pointing Camera, obtained at the ESO VLT during the 4LGSF first Laser Guide Star Unit Commissioning, and will discuss its possible extension for the ELT operations

Remote sensing of geomagnetic fields and atomic collisions in the mesosphere

Remote sensing of geomagnetic fields in mesosphere is both challenging and interesting to explore the magnetic field structures and atomic collision processes. Here the authors demonstrate an atomic magnetometer that utilizes the Larmor frequency in sodium atoms and operates in kilometers range

Polarization-driven spin precession of mesospheric sodium atoms

We report experimental results on the first on-sky observation of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. The magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 km and 100 km altitude. The experiment was performed at La Palma, and the uncertainty in the measured Larmor frequency ($\approx$260 kHz) corresponded to an error in the geomagnetic field of 0.4 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere

Field tests of elongated Na LGS wavefront sensing for the E-ELT

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Toward an on-sky ELT-scale sodium LGS wavefront sensing experiment

International audienceThe two first light adaptive optics (AO) instruments for the European Extremely Large Telescope (E-ELT) will both rely on several Sodium Laser Guide Stars (LGS). In using this technology, the E-ELT diameter comes with new challenges mainly raised by spot elongation. Before the final design studies of the E-ELT instruments, a Sodium LGS wavefront sensing (WFS) on-sky experiment at this scale is mandatory to provide meaningful spatial and temporal measurement error and performance evaluation. For this purpose, we propose to use CANARY, the Multi-Object AO demonstrator installed at the WHT (4.2m). CANARY is now undergoing a Rayleigh LGS upgrade and also provides natural guide star WFS. It could easily be adapted to the Sodium LGS case. Additionally, a transportable laser system, such as the one developed at ESO, positioned at a varying distance from the WHT could be used to provide off-axis launching (up to 40m), simulating the whole range of spot elongations that will be obtained on the E-ELT. Full scale simulations of a Sodium LGS WFS on the E-ELT have guided us in the definition of the experiment we are proposing. Previous simulation results in the literature have stressed that an estimate of the Sodium profile is necessary to reduce measurement error to an affordable level. Hence parallel Sodium profiling is mandatory in the proposed experiment. In this paper, we present the simulation results as well as the specifications for both the Sodium LGS WFS and profiler to be used on Canary