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... > Instruments > Earth Remote Sensing Instruments > Passive Remote Sensing > Magnetic Field/Electric Field Instruments > MAGNETOMETERS > ASM

Preferred term

ASM  

Definition

  • ASM is provided by CNES (French Space Agency) and CEA-LETI (French Atomic Energy Commission - Laboratoire d'Electronique de Technologie et d'Instrumentation), Grenoble, France. The objective is to measure magnetic field strength and to calibrate the VFM device to maintain the absolute accuracy during the multi-year mission. ASM is positioned at the very tip of the boom. The required main performance characteristics of the ASM are: absolute accuracy of < 0.3 nT (2σ), resolution < 0.1 nT within its full-scale range of 15000-65000 nT. Measurement concept: To overcome the limitations of the OVM (Overhauser Magnetometers) identified during the Oersted and Champ programs, a new magnetometer has been designed for the Swarm mission. The ASM pumped helium magnetometer relies on a low pressure helium vapor as the sensing medium (Figure 26), with the optical pumping process the counterpart of the dynamic nuclear polarization. One important difference is however due to the fact that the optical pumping is a much more efficient polarization method, leading to an almost complete polarization. As a consequence, the signal amplitude does no longer depend on the magnetic field strength and a resolution of 1 pT/ (Hz)1/2 is now obtained over the complete measurement range. As compared to most optically pumped magnetometers, the ASM operates with linearly polarized pumping light instead of circularly polarized light. The main reasons for that choice are the following: - the strong interaction between the laser pumping beam and the helium atoms can in general affect their energy level and result in so-called light shifts whenever the pumping light wavelength is detuned from the helium transition center wavelength. Now using linearly polarized light suppresses this effect, thus significantly increasing the instrument’s accuracy. - the key parameter governing the optical pumping angular dependence is then the direction of the laser polarization, whereas it is the propagation direction of the pumping beam that matters in circularly polarized light. Now when trying to design an isotropic instrument, i.e an instrument whose performances are independent of the sensor attitude, it is obviously easier to control the direction of the linear polarization than to rotate the whole sensor in order to align it properly with respect to the magnetic field direction. In our case the isotropy is thus simply achieved thanks to the use of an amagnetic piezoelectric motor which permanently controls the laser polarization and the RF magnetic field directions so that they are both perpendicular to the static magnetic field. (en)

Broader concept

Change note

  • 2018-07-11 07:32:34.0 [mmorahan] Insert Concept add broader relation (ASM [b7bc737c-15de-4b67-9bc3-5fa7c2b651d5,367827] - MAGNETOMETERS [deac2632-5c17-4d15-ae92-c61ebc5a405a,348905]);
  • 2018-07-11 07:40:18.0 [mmorahan] insert AltLabel (id: null category: primary text: Absolute Scalar Magnetometer language code: en); insert Definition (id: null text: ASM is provided by CNES (French Space Agency) and CEA-LETI (French Atomic Energy Commission - Laboratoire d'Electronique de Technologie et d'Instrumentation), Grenoble, France. The objective is to measure magnetic field strength and to calibrate the VFM device to maintain the absolute accuracy during the multi-year mission. ASM is positioned at the very tip of the boom. The required main performance characteristics of the ASM are: absolute accuracy of < 0.3 nT (2σ), resolution < 0.1 nT within its full-scale range of 15000-65000 nT. Measurement concept: To overcome the limitations of the OVM (Overhauser Magnetometers) identified during the Oersted and Champ programs, a new magnetometer has been designed for the Swarm mission. The ASM pumped helium magnetometer relies on a low pressure helium vapor as the sensing medium (Figure 26), with the optical pumping process the counterpart of the dynamic nuclear polarization. One important difference is however due to the fact that the optical pumping is a much more efficient polarization method, leading to an almost complete polarization. As a consequence, the signal amplitude does no longer depend on the magnetic field strength and a resolution of 1 pT/ (Hz)1/2 is now obtained over the complete measurement range. As compared to most optically pumped magnetometers, the ASM operates with linearly polarized pumping light instead of circularly polarized light. The main reasons for that choice are the following: - the strong interaction between the laser pumping beam and the helium atoms can in general affect their energy level and result in so-called light shifts whenever the pumping light wavelength is detuned from the helium transition center wavelength. Now using linearly polarized light suppresses this effect, thus significantly increasing the instrument’s accuracy. - the key parameter governing the optical pumping angular dependence is then the direction of the laser polarization, whereas it is the propagation direction of the pumping beam that matters in circularly polarized light. Now when trying to design an isotropic instrument, i.e an instrument whose performances are independent of the sensor attitude, it is obviously easier to control the direction of the linear polarization than to rotate the whole sensor in order to align it properly with respect to the magnetic field direction. In our case the isotropy is thus simply achieved thanks to the use of an amagnetic piezoelectric motor which permanently controls the laser polarization and the RF magnetic field directions so that they are both perpendicular to the static magnetic field. language code: en);
  • 2019-02-22 17:19:12.0 [mmorahan] insert WeightedRelation (id: null related concept uuid: 054787a6-0c47-43af-a4ee-05c572dd1705 relationship type: null relationship value: null generated by: null); insert WeightedRelation (id: null related concept uuid: ab7f9a64-ca5d-4795-94ff-fd5367d39f9f relationship type: null relationship value: null generated by: null); insert WeightedRelation (id: null related concept uuid: 769a52d4-7db1-4b8e-8d39-6fee4e74d34f relationship type: null relationship value: null generated by: null);

URI

https://gcmd.earthdata.nasa.gov/kms/concept/b7bc737c-15de-4b67-9bc3-5fa7c2b651d5

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