Scalar Magnetometer Research Articles

Alkali-vapor magnetic resonance driven by fictitious radiofrequency fields

We demonstrate an all-optical 133Cs scalar magnetometer, operating in nonzero magnetic field, in which the magnetic resonance is driven by an effective oscillating magnetic field provided by the AC Stark shift of an intensity-modulated laser beam. We achieve a projected shot-noise-limited sensitivity of 1.7fT/Hz and measure a technical noise floor of 40fT/Hz. These results are essentially identical to a coil-driven scalar magnetometer using the same setup. This all-optical scheme offers advantages over traditional coil-driven magnetometers for use in arrays and in magnetically sensitive fundamental physics experiments, e.g., searches for a permanent electric dipole moment of the neutron.

Fluxgate Offset Study

Offset and its long-term stability is a weak point of fluxgate sensors. Even the ultrastable sensors kept at no vibrations and stable temperature at magnetic observatories show offset drift. Such drift of the fluxgate triaxial sensor can only be partly corrected by the scalar resonance magnetometer. Periodical calibration of absolute reading should be made using nonmagnetic theodolite. In this paper, we study the origin of fluxgate offset. We distinguish the real magnetic sensor offset from the offset contributions originating in the false second harmonics signal that leaks to the sensor output from the distortion in the excitation signal, or which is borne as harmonic distortion when the signal processing electronics are subjected to the large first harmonic signal leaking from the excitation. We analyze the offset dependence on the angular position of the sensor core and its response to large field shocks. The experiments give an indication that only a part of the magnetic offset stems from a remanence of magnetically hard core regions. The residual part may be caused by a magnetostrictive signal, belonging to false signal contributions, but not considered in previous studies.

Filtration of one-second observations from vector and scalar magnetometers based on approximation piecewise linear models

Joint filtration of one-second observations on a system of vector and scalar magnetometers is considered. The filtration provides a reduction of errors in estimates for components of geomagnetic field strength vectors. A formulation for the problem of joint data filtration is proposed based on using approximation piecewise linear models, and a higher speed filtration algorithm is developed. The results of joint filtration of one-second observations are presented. Errors in the proposed filtration on the basis of statistical modeling are estimated.

A method of in-orbit calibration of fluxgate magnetometer based on the measurement of absolute scalar magnetometer

卫星在轨进行磁场测绘及监测时，需要高精度的磁场值，但由于磁通门磁强计三轴非正交性以及零点漂移所引起的误差大大降低了磁强计的测量精度，因而有必要对此进行校正.本文利用绝对标量磁强计，采用一种改进的最小二乘标定算法，通过不断迭代实现对磁通门三轴正交性和零点漂移的修正.结果表明，在考虑了磁强计本身噪声水平的实际情况下，磁通门磁强计的误差可由100nT左右修正到0.2nT以内.该算法为卫星在轨高精度磁场测量提供了一种可行的方法.

Highly responsive ac scalar atomic magnetometer with long relaxation time

We propose an operating condition for an optically pumped scalar atomic magnetometer with a unique band-pass frequency response. Under this operating condition, the signal response of the magnetometer is increased threefold around a nonzero resonance frequency. The resonance frequency is equal to the detuning between the Larmor frequency and the frequency of the oscillating magnetic field used for excitation. The resonance is explained in terms of the characteristics of the first-order perturbation expansion of the Bloch equation for a modulated bias magnetic field. The scalar magnetometer provides an enhanced magnetic response around its resonance frequency even beyond the cutoff frequency, which is defined as the inverse of the phase relaxation time of the spin polarization.

Nonlinear filtering of observations of a system of vector and scalar magnetometers

Nonlinear filtering of the observations of a system of vector and scalar magnetometers is considered. A general formulation of the problem is proposed and algorithms for single-step and multistep nonlinear filtering are developed. Statistical modeling is carried out, and the errors of these algorithms for model observations are estimated. The results of nonlinear filtering of observation data, obtained from a system of vector and scalar magnetometers in a magnetic observatory, are presented.

Suppression of Spin-Exchange Relaxation Using Pulsed Parametric Resonance

We demonstrate that spin-exchange dephasing of Larmor precession at near-Earth-scale fields is effectively eliminated by dressing the alkali-metal atom spins in a sequence of ac-coupled 2π pulses, repeated at the Larmor precession frequency. The contribution of spin-exchange collisions to the spectroscopic linewidth is reduced by a factor of the duty cycle of the pulses. We experimentally demonstrate resonant transverse pumping in magnetic fields as high as 0.1G, present experimental measurements of the suppressed spin-exchange relaxation, and show enhanced magnetometer response relative to a light-narrowed scalar magnetometer.

Characterization of CHAMP magnetic data anomalies: magnetic contamination and measurement timing

The CHAMP (CHAllenging Minisatellite Payload) mission ended after more than ten years in space on 19 September 2010. For achieving a high measurement accuracy of the magnetometers on CHAMP, detailed analyses of spacecraft magnetic characteristics in orbit are required. A decade of continuous magnetometer and housekeeping data are a good basis for evaluating some of the effects of variable spacecraft magnetic fields on the ambient field determination. It was found that some perturbations of FGM (FluxGate vector Magnetometer) or OVM (OVerhauser scalar Magnetometer) measurements are caused by stray fields induced by the power system, the ASC (advanced stellar compass) instrument or magneto-torquer currents. The magnetic effect of solar currents on FGM measurements varies with the local time of the orbit and amounts to 0.2 nT. In cases when one head of the ASC instrument was blinded by the sun, sometimes transient drops in instrument current strength occur, which were accompanied by magnetic disturbance signals (∼0.3 nT) in FGM measurements. The magnetic residual contamination of OVM data by the torquer currents was of order 0.1 nT but still detectable. An improved torquer correction matrix is derived which eliminates this effect. In-flight scalar calibration parameters revealed some of the effects of timing anomalies. Time lags between FGM and OVM readings are misinterpreted by the scalar calibration as variations of the angles between some of the sensor axes. The resulting amplitudes of the anomalies presented here lie in the range of some 0.1 nT, but they are systematic in nature.

Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells

Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multipass 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 a magnetic field sensitivity of 0.54 fT/Hz(1/2), which improves by an order of magnitude the best scalar magnetometer sensitivity and exceeds, for example, the quantum limit set by the spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime.

High-bandwidth optical magnetometer

We demonstrate a scalar Rb87 optical magnetometer that retains magnetic-field sensitivities below 10pT/√Hzover 3 dB bandwidths of 10 kHz in an ambient field Bo=11.4 μT and using a measurement volume of 1 mm3. The magnetometer operates at high atomic densities where both the sensitivity and the bandwidth are limited by spin-exchange collisions between the alkali atoms. By operating in this regime, our measurements show that the bandwidth of the magnetometer can be increased without a significant degradation in its sensitivity.

Calibration of three-axis fluxgate magnetometers with nonlinear least square method

The accuracy of three-axis magnetometers is limited by different scales, bias of each axis and nonorthogonality between axes, which is usually lower than that of scalar magnetometers. In this paper, the nonlinear least square method is proposed to calibrate three-axis magnetometers. The validity of this method was proved by simulation, in which the estimated parameters of the error model are close to prearranged parameters. In experiment, a three-axis fluxgate magnetometer (DM-050), a two dimensional nonmagnetic rotation equipment and a proton magnetometer (GSM-19T) were used. The scalar value of magnetic field was obtained by proton magnetometer and considered to be the true value. The calibration performance of unscented Kalman filter (UKF), two-step algorithm and nonlinear least square were compared. Experimental results show that the error average and standard deviation of nonlinear least square are the least among the three methods. After calibration, the average of scalar error is reduced from −76.2nT to −0.00093nT and the standard deviation is reduced from 10.832nT to 4.298nT. The results suggest an effective way for the calibration of three-axis fluxgate magnetometers.

Determining the orthogonality error of coil systems by means of a scalar magnetometer: application to delta inclination–delta declination (dIdD) magnetometers

In this note, a simple method is introduced for the determination of the pitch angle between two coil axes by means of a total field magnetometer. This method is applicable when the homogeneous volume inside the coils is large enough to accommodate the sensor of a total field magnetometer. The orthogonality of calibration coil systems used for calibration of vector magnetometers can be calibrated by this procedure. The method can be easily automated and applied for the calibration of delta inclination–delta declination (dIdD) magnetometers.

Scalar, vector, tensor magnetic anomalies: measurement or computation?

ABSTRACTMagnetic surveys for geophysical interpretation will most usefully furnish estimates of the three components of the magnetic field vector. We review methods for obtaining this information based on scalar and tensor magnetic field measurements and point out the advantages of fluxgate vector measurements. Fluxgate vector magnetometers can be powerful instruments in magnetic mapping. The main problems in using fluxgate magnetometers arise from calibration errors and drift but these can be overcome using a quick and simple method of calibration. This method also has the advantage of compensating permanent and induced magnetic fields generated by the airplane. This is illustrated by a new aeromagnetic survey flown in the Vosges area (France). Measurement accuracy is shown to be similar to that obtained with scalar magnetometers. We take advantage of this accuracy to calculate in the Fourier domain other magnetic functions from the total‐field anomaly, in particular, the magnetic gradient tensor is obtained without using any superconducting quantum devices. A similar approach is used to introduce a new magnetic anomaly tensor that is the equivalent of the pseudo‐gravity tensor. Maps presented in the last sections serve as an example to illustrate the various functions, the goal of the paper being to obtain magnetic vector data from the observations without first postulating the detailed nature of the sources.

An all-optical scalar and vector spin-exchange relaxation-free magnetometer employing on–off pump modulation

It is demonstrated that a spin-exchange relaxation-free (SERF) atomic magnetometer can be used for scalar measurements with no additional hardware. Because of relaxation processes, an ensemble of alkali atoms needs a constant supply of polarized photons by a pump beam to maintain a polarized state. If the pump beam is shuttered off, the system decays to its equilibrium state. For a low enough relaxation rate and with a magnetic field present, the system will exhibit oscillations at its natural frequencies. In a SERF magnetometer, it happens at the Zeeman resonance frequency of the atoms (Larmor frequency). Thus, shuttering off the pump beam reveals oscillations at the Larmor frequency. From this frequency, one can deduce the scalar value of the applied magnetic field. As a result, all-optical scalar measurements can be performed. At the same time, either one or two vector components of the applied field can be measured by using one or two orthogonal probe beams, respectively. In a low-polarization SERF regime, the ground state can be well described by the Bloch equations for the electron spin polarization. By solving the time-dependent Bloch equations [neglecting the diffusion term and assuming that the nuclear slowing-down factor q(P) is constant], the oscillation frequency of the system is obtained. From this frequency, the scalar value of the applied magnetic field is derived. It is shown that applied fields down to 1 nT can be measured with a 0.1% relative uncertainty. Fields down to 50 pT can be measured with a 10% relative uncertainty. The time dependence acquired in the “off” periods is strongly correlated with the Zeeman sublevels population of the atomic ground state and reveals its spin dynamics.

DI3 - A New Procedure for Absolute Directional Measurements

The standard observatory procedure for determining a geomagnetic field's declination and inclination absolutely is the DI-flux measurement. The instrument consists of a non-magnetic theodolite equipped with a single-axis fluxgate magnetometer. Additionally, a scalar magnetometer is needed to provide all three components of the field. Using only 12 measurement steps, all systematic errors can be accounted for, but if only one of the readings is wrong, the whole measurement has to be rejected. We use a three-component sensor on top of the theodolites telescope. By performing more measurement steps, we gain much better control of the whole procedure: As the magnetometer can be fully calibrated by rotating about two independent directions, every combined reading of magnetometer output and theodolite angles provides the absolute field vector. We predefined a set of angle positions that the observer has to try to achieve. To further simplify the measurement procedure, the observer is guided by a pocket pc, in which he has only to confirm the theodolite position. The magnetic field is then stored automatically, together with the horizontal and vertical angles. The DI3 measurement is periodically performed at the Niemegk Observatory, allowing for a direct comparison with the traditional measurements.

Extraction of the anomaly magnetic field of the earth from stratospheric balloon magnetic surveys at altitudes of 20–40 km

The solution to the problem of extraction of the anomaly Earth’s magnetic field (EMF) from stratospheric balloon magnetic surveys with the help of global analytical models of the normal EMF is proposed. In the problem solution, errors for the analytical models of the normal EMF and its secular variation at a set moment of time are assessed; the found error is introduced as a correction to the extracted anomaly EMF. The error of the model is determined in the places where significant magnetic anomalies are absent. In this case, the error of the model corresponds to deviations of the normal EMF components, synthesized by coefficients of analytical models, and to deviations of the EMF secular variations from the measured values at quite a low value of the variable EMF or one being taken into account. These places are determined when carrying out additional measurements in vertical gradients of the EMF with the use of scalar magnetometers at the gauge length of 6 km. It has been shown that the found places can be considered as nonanomaly, if the difference of values of the anomaly EMF at the gauge length of 6 km does not exceed 1.5 nT within the profile’s portion of about 100 km in length. An experiment in nature has revealed that errors for the IGRF-2005 and IGRF-2010 models, corrected for secular variation of the EMF, can reach 200 and 140 nT, respectively, within the limits of the territory where the Kama-Emba magnetic anomaly is located; these errors are determined by the considered causes. Comparison of aerostatic profiles of magnetic anomalies with data on the anomaly EMF, derived from the maps, has shown that the realizations derived from the maps contain overestimated negative values of the anomaly EMF, because they reflect processes in the near-surface layer of the Earth’s crust. This fact causes the situation when attempts to recalculate the anomaly EMF into the upper half-space by the near-surface data still have not been successful. Only realizations derived at the altitudes comparable to the thickness of the Earth’s crust can give an adequate model of the anomaly EMF in the circumterrestrial space and enable us to recalculate magnetic anomalies reliably into any altitude levels.

A new aeromagnetic survey of the North Pole and the Arctic Ocean north of Greenland and Ellesmere Island

We present a preliminary analysis of more than 50,000 km of aeromagnetic survey lines flown in the Arctic Ocean, acquired in 2009 with an optically pumped scalar magnetometer as part of the airborne geophysical survey ‘LOMGRAV’. From the observations we removed main and magnetospheric fields as given by the CHAOS-3 field model (Olsen et al., 2010) and remaining external fields as monitored by the Canadian magnetic observatory Alert. The reduced data were levelled based on cross-over differences at line intersections. Finally, a grid was computed, upward continued by 3500 m and compared with the EMAG2 grid (Maus et al., 2009), showing a good general agreement but also areas with systematic differences. The obtained data are expected to be part of the next version of the World Digital Magnetic Anomaly Map (WDMAM).

Stray magnetic field compensation with a scalar atomic magnetometer

We describe a system for the compensation of time-dependent stray magnetic fields using a dual channel scalar magnetometer based on nonlinear Faraday rotation in synchronously optically pumped Cs vapor. We detail the active control strategy, with an emphasis on the electronic circuitry, based on a simple phase-locked-loop integrated circuit. The performance and limits of the system developed are tested and discussed. The system was applied to significantly improve the detection of free induction decay signals from protons of remotely magnetized water precessing in an ultralow magnetic field.

GEOSTAR deep seafloor missions: magnetic data analysis and 1D geoelectric structure underneath the Southern Tyrrhenian Sea

From 2000 to 2005 two geophysical exploration missions were undertaken in the Tyrrhenian deep seafloor at depths between -2000 and -3000 m in the framework of the European-funded GEOSTAR Projects. The considered missions in this work are GEOSTAR-2 and ORION-GEOSTAR-3 with the main scientific objective of investigating the deep-seafloor by means of an automatic multiparameter benthic observatory station working continuously from around 5 to 12 months each time. During the two GEOSTAR deep seafloor missions, scalar and vector magnetometers acquired useful magnetic data both to improve global and regional geomagnetic reference models and to infer specific geoelectric information about the two sites of magnetic measurements by means of a forward modelling.

Low-noise high-density alkali-metal scalar magnetometer

We present an experimental and theoretical study of a scalar atomic magnetometer using an oscillating field-driven Zeeman resonance in a high-density optically-pumped potassium vapor. We describe an experimental implementation of an atomic gradiometer with a noise level below 10 fT/Hz^{1/2}, fractional field sensitivity below 10^{-9}/Hz^{1/2}, and an active measurement volume of about 1.5 cm^3. We show that the fundamental field sensitivity of a scalar magnetometer is determined by the rate of alkali-metal spin-exchange collisions even though the resonance linewidth can be made much smaller than the spin-exchange rate by pumping most atoms into a stretched spin state.