Measuring the Earth’s Magnetic Field from Space: Concepts of Past, Present and Future Missions

Olsen, Nils; Hulot, Gauthier; Sabaka, Terence J.

doi:10.1007/s11214-010-9676-5

Cited by 42 publications

(22 citation statements)

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“…These should be put in perspective with the typical observation errors present in geomagnetic field modelling, which amount for instance to tens to hundreds of nanoteslas in the historical period (Bloxham and Jackson, 1989) and a few nanoteslas in the satellite observation period (Olsen et al, 2010). An assimilation performed on real data will thus need to include a proper estimation of the observation error for the assimilated quantities.…”

Section: Discussionmentioning

confidence: 99%

“…Because we are dealing with the equivalent of global field models at the core-mantle boundary, and given their current resolution limits (e.g. Olsen et al, 2010), we define the observable part of the state vector as the poloidal magnetic field B p lm (r o ) at the outer boundary up to degree and order 13. The corresponding observation operator contains ones in the entries corresponding to an observed quantity and zeros otherwise.…”

Section: Best Linear Unbiased Estimate Of the Internal Structure Frommentioning

confidence: 99%

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Inferring internal properties of Earth's core dynamics and their evolution from surface observations and a numerical geodynamo model

Aubert

Fournier

2011

Nonlin. Processes Geophys.

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Abstract. Over the past decades, direct three-dimensional numerical modelling has been successfully used to reproduce the main features of the geodynamo. Here we report on efforts to solve the associated inverse problem, aiming at inferring the underlying properties of the system from the sole knowledge of surface observations and the first principle dynamical equations describing the convective dynamo. To this end we rely on twin experiments. A reference model time sequence is first produced and used to generate synthetic data, restricted here to the large-scale component of the magnetic field and its rate of change at the outer boundary. Starting from a different initial condition, a second sequence is next run and attempts are made to recover the internal magnetic, velocity and buoyancy anomaly fields from the sparse surficial data. In order to reduce the vast underdetermination of this problem, we use stochastic inversion, a linear estimation method determining the most likely internal state compatible with the observations and some prior knowledge, and we also implement a sequential evolution algorithm in order to invert time-dependent surface observations. The prior is the multivariate statistics of the numerical model, which are directly computed from a large number of snapshots stored during a preliminary direct run. The statistics display strong correlation between different harmonic degrees of the surface observations and internal fields, provided they share the same harmonic order, a natural consequence of the linear coupling of the governing dynamical equations and of the leading influence of the Coriolis force. Synthetic experiments performed with a weakly nonlinear model yield an excellent quantitative retrieval of the internal structure. In contrast, the use of a strongly nonlinear (and more realistic) model results in less accurate static estimations, which in turn fail to constrain the unobserved small scales in the time integration of the Correspondence to: J. Aubert (aubert@ipgp.fr) evolution scheme. Evaluating the quality of forecasts of the system evolution against the reference solution, we show that our scheme can improve predictions based on linear extrapolations on forecast horizons shorter than the system e-folding time. Still, in the perspective of forthcoming data assimilation activities, our study underlines the need of advanced estimation techniques able to cope with the moderate to strong nonlinearities present in the geodynamo.

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Section: Discussionmentioning

confidence: 99%

Section: Best Linear Unbiased Estimate Of the Internal Structure Frommentioning

confidence: 99%

Inferring internal properties of Earth's core dynamics and their evolution from surface observations and a numerical geodynamo model

Aubert

Fournier

2011

Nonlin. Processes Geophys.

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“…Many magnetometer applications, such as searches for permanent electric dipole moments [5], detection of NMR signals [6], and low-field magnetic resonance imaging [7], require sensitive magnetic measurements in a finite magnetic field. In addition, scalar magnetometers measuring the Zeeman frequency are unique among magnetic sensors in being insensitive to the direction of the field, making them particularly suitable for geomagnetic mapping [8] and field measurements in space [9,10]. The sensitivity of scalar magnetometers has been relatively poor, as summarized recently in [11].…”

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confidence: 99%

Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells

et al. 2013

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Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multi-pass optical cells. We use a pump-probe measurement scheme to suppress spin-exchange relaxation and two probe pulses to find the spin precession zero crossing times with a resolution of 1 psec. We realize magnetic field sensitivity of 0.54 fT/Hz 1/2 , which improves by an order of magnitude the best scalar magnetometer sensitivity and surpasses the quantum limit set by spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime. PACS numbers: 07.55.Ge, 42.50.Lc, 32.30.Dx Alkali-metal magnetometers can surpass SQUIDs as the most sensitive detectors of magnetic field, reaching sensitivity below 1 fT/Hz 1/2 [1, 2], but only if they are operated near zero magnetic field to eliminate spin relaxation due to spin-exchange collisions [3,4]. Many magnetometer applications, such as searches for permanent electric dipole moments [5], detection of NMR signals [6], and low-field magnetic resonance imaging [7], require sensitive magnetic measurements in a finite magnetic field. In addition, scalar magnetometers measuring the Zeeman frequency are unique among magnetic sensors in being insensitive to the direction of the field, making them particularly suitable for geomagnetic mapping [8] and field measurements in space [9,10]. The sensitivity of scalar magnetometers has been relatively poor, as summarized recently in [11]. The best directly measured scalar magnetometer sensitivity is equal 7 fT/Hz 1/2 with a measurement volume of 1.5 cm 3 [12], while estimates of fundamental sensitivity per unit measurement volume for various types of scalar alkali-metal magnetometers range from several fT cm 3/2 /Hz 1/2 [13, 14] to about 1 fT cm 3/2 /Hz 1/2 [12]. Here we describe a new type of scalar atomic magnetometer using multi-pass vapor cells [15,16] and operating in a pulsed pump-probe mode [17] to achieve magnetic field sensitivity of 0.54 fT/Hz 1/2 with a measurement volume of 0.66 cm 3 in each multi-pass cell. The magnetometer sensitivity approaches, for the first time, the fundamental limit set by Rb-Rb collisions. We also develop here a quantitative method to analyze significant effects of atomic diffusion on the spectrum of the spin-projection noise in vapor cells with buffer gas using a spin time-correlation function.The sensitivity of an atomic magnetometer, as any other frequency measurement, is fundamentally limited by spin projection noise and spin relaxation [18]. For N spin-1/2 atoms with coherence time T 2 the sensitivity after a long measurement time t ≫ T 2 is given by δB = 2e/N T 2 t/γ, where γ is the gyromagnetic ratio. Spin squeezing techniques can reduce this uncertainty by a factor of √ e, but do not change the scaling with N [18-20]. The number of atoms can be increased until collisions between them start to limit T 2 . Writing T −1 2 = nσv, where n is the density of atoms, σ is the spin relaxation c...

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“…Ionospheric contributions, however, can be both internal and external fields, and ionospheric currents, which are especially strong in the polar regions, complicate magnetic field modelling. Multisatellite missions including the upcoming Swarm mission are subject of Olsen et al (2010b), this issue.…”

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confidence: 99%

Geomagnetic Observations for Main Field Studies: From Ground to Space

et al. 2010

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Direct measurements of the geomagnetic field have been made for more than 400 years, beginning with individual determinations of the angle between geographic and magnetic North. This was followed by the start of continuous time series of full vector measurements at geomagnetic observatories and the beginning of geomagnetic repeat stations surveys in the 19th century. In the second half of the 20th century, true global coverage with geomagnetic field measurements was accomplished by magnetometer payloads on lowEarth-orbiting satellites. This article describes the procedures and instruments for magnetic field measurements on ground and in space and covers geomagnetic observatories, repeat J. Matzka ( )

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Measuring the Earth’s Magnetic Field from Space: Concepts of Past, Present and Future Missions

Cited by 42 publications

References 80 publications

Inferring internal properties of Earth's core dynamics and their evolution from surface observations and a numerical geodynamo model

Inferring internal properties of Earth's core dynamics and their evolution from surface observations and a numerical geodynamo model

Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells

Geomagnetic Observations for Main Field Studies: From Ground to Space

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