Magnetic Field Fluctuations Research Articles

Lower‐Hybrid Wave Structures and Interactions With Electrons Observed in Magnetotail Reconnection Diffusion Regions

AbstractWe investigate waves close to the lower‐hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near‐zero to 30%. In about half of the events, the wave vector has a small component along the current sheet normal, consistent with known lower‐hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4 of the events, the term has a significant contribution to the wave electric field, possibly a feature of lower‐hybrid waves more likely to exist in the diffusion region than further away from the X‐line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti‐correlation between parallel and perpendicular temperature components. Using four‐spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations in and . The lower‐hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid‐plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.

Modern Faraday Rotation Studies to Probe the Solar Wind

For decades, observations of Faraday rotation have provided unique insights into the plasma density and magnetic field structure of the solar wind. Faraday rotation (FR) is the rotation of the plane of polarization when linearly polarized radiation propagates through a magnetized plasma, such as the solar corona, coronal mass ejection (CME), or stream interaction region. FR measurements are very versatile: they provide a deeper understanding of the large-scale coronal magnetic field over a range of heliocentric distances (especially ≈1.5 to 20 R⊙) not typically accessible to in situ spacecraft observations; detection of small-timescale variations in FR can provide information on magnetic field fluctuations and magnetohydrodynamic wave activity; and measurement of differential FR can be used to detect electric currents. FR depends on the integrated product of the plasma density and the magnetic field component along the line of sight to the observer; historically, models have been used to distinguish between their contributions to FR. In the last two decades, though, new methods have been developed to complement FR observations with independent measurements of the plasma density based on the choice of background radio source: calculation of the dispersion measure (pulsars), measurement of Thomson scattering brightness (radio galaxies), and application of radio ranging and apparent-Doppler tracking (spacecraft). New methods and new technology now make it possible for FR observations of solar wind structures to return not only the magnitude of the magnetic field, but also the full vector orientation. In the case of a CME, discerning the internal magnetic flux rope structure is critical for space weather applications.

Remote Sensing of Mid‐Latitude Ionospheric Magnetic Field Fluctuations Using Cosmic Radio Sources

AbstractThis paper describes a new method for remote sensing of magnetic field fluctuations at ionospheric altitudes using a relatively long‐baseline interferometer and exceptionally bright cosmic radio sources at 35 MHz. The technique uses sensitive measurements of the difference in phase between two phased array telescopes separated by about 75 km and between the right and left circular polarizations to measure the amount of differential Faraday rotation. Combined with estimates of the background magnetic field and total electron content, these can be converted to measurements of fluctuations in the differential magnetic field parallel to the line of sight, ΔB‖. The temporal gradient in ΔB‖ roughly follows the diurnal pattern expected for B‖ due to the vertical gradient in the background electric field, but at roughly 25% the magnitude and offset by ∼50 nT hr−1. This suggests that the diurnal variation in the electric fields observed by the two telescopes are similar but slightly different (|ΔE| ≲ 0.1 mV m−1). Fluctuations in ΔB‖ were typically ∼10–30 nT with wavelike fluctuations often apparent. These typically have oscillation periods of about 10–30 min, similar to traveling ionospheric disturbances (TIDs). Simultaneous observations toward two sources separated by 25.4° on the sky (∼140 km in the F‐region) show a few detections of wavelike disturbances with lags of ±10–30 min between them. These imply speeds on the order of 100–200 m s−1, also similar to TIDs. We estimate that gravity waves with amplitudes within the dynamo region of ∼10 m s−1 could generate the observed fluctuations in ΔB‖.

Markovian Features of the Solar Wind at Subproton Scales

Abstract The interplanetary magnetic field carried out from the Sun by the solar wind displays fluctuations over a wide range of scales. While at large scales, say at frequencies lower than 0.1–1 Hz, fluctuations display the universal character of fully developed turbulence with a well-defined Kolmogorov-like inertial range, the physical and dynamical properties of the small-scale regime as well as their connection with the large-scale ones are still a debated topic. In this work we investigate the near-Sun magnetic field fluctuations at subproton scales by analyzing the Markov property of fluctuations and recovering basic information about the nature of the energy transfer across different scales. By evaluating the Kramers–Moyal coefficients we find that fluctuations in the subproton range are well described as a Markovian process with Probability Density Functions (PDFs) modeled via a Fokker–Planck (FP) equation. Furthermore, we show that the shape of the PDFs is globally scale-invariant and similar to the one recovered for the stationary solution of the FP equation at different scales. The relevance of our results on the Markovian character of subproton scale fluctuations is also discussed in connection with the occurrence of turbulence in this domain.

Conservation of currents in reduced full-F electromagnetic kinetic and fluid models

In this paper, we present an analysis of the conservation of currents in a full-F electromagnetic gyro-kinetic model in the long-wavelength limit. This equation corresponds to what is usually called the ‘vorticity equation’, which is not strictly correct as it cannot be formulated as the curl of a velocity equation. In the paper, we will therefore use the term ‘current conservation equation’ instead. Our results are relevant to reduced plasma descriptions like gyro-kinetic, drift-kinetic, gyro-fluid and drift-fluid models for tokamaks and stellarators. The equation describes the change of the polarization charge density (often called ‘vorticity’) in terms of the polarization stress due to the flow, external sources and three currents: the parallel current, the curvature current and a current related to the magnetic field fluctuations. We compare this equation with previous drift- and gyro-fluid equations and find general agreement, except in the vorticity source terms where previous drift-fluid models fail to capture the heating and density sources. We discuss the role of currents in the dynamics of diamagnetic and flow shear. The possible connection between these currents with phenomena observed in experiments that influence the radial electric field in the edge of tokamak plasmas, like resonant magnetic perturbations, and different magnetic field configurations and shapes, is presented.

Investigation of 5-Year Interconnections between Local Earth Magnetic Field Fluctuations and Acute Myocardial Infarction in Lithuania

The impact of the local Earth magnetic field (LEMF) on cardiovascular events has been studied recently. Data gathered during past years encouraged us to conduct this epidemiological analysis evaluating the association between changes in LEMF and hospital admissions due to AMI in Lithuania between August 2014 and September 2019. This study is unique due to its coverage of all Lithuanian patients. The frequency of morbidity of AMI was compared with the intensity of the LEMF and correlation coefficient was evaluated. The LEMF was measured by the Global Coherence Monitoring Network magnetometer located in Lithuania. LEMF was measured by pikotesla square (pT²). The LEMF was analized in five frequency ranges [Hz], generally called between Schumann resonance, which overlap with the human brain activity waves on electroencefalogram (EEG) frequency ranges (here, they are named as SDelta (0-3.5Hz), STheta (3.5-7Hz), SAlpha (7-15Hz), SBeta (15-32Hz) and SGamma (32-65Hz) to distinguish from the EEG bands). Significant correlations between weekly admissions of AMI cases and the weekly LEMF strength in five frequency ranges and in total range was found. A clear negative correlation was observed between cases of AMI in female group and LEMF frequency ranges SDelta (0-3.5Hz), STheta (3.5-7Hz), SAlpha (7-15Hz), SBeta (15-32Hz) and in total range. In the second half of the year the number of AMI is lower, therefore negative correlations between SDelta (0-3.5Hz), STheta (3.5-7Hz), SAlpha (7-15Hz) and SBeta (15-32Hz) ranges are stronger than in the first one. This is particularly noticeable in 2016 and 2018 years.

Synthetic solar cycle for active regions violating the Hale’s polarity law

ABSTRACTLong observational series for bipolar active regions (ARs) provide significant information about the mutual transformation of the poloidal and toroidal components of the global solar magnetic field. The direction of the toroidal field determines the polarity of leading sunspots in ARs in accordance with the Hale’s polarity law. The vast majority of bipolar ARs obey this regularity, whereas a few per cent of ARs have the opposite sense of polarity (anti-Hale ARs). However, the study of these ARs is hampered by their poor statistics. The data for five 11-yr cycles (16–18 and 23, 24) were combined here to compile a synthetic cycle of unique time length and latitudinal width. The synthetic cycle comprises data for 14838 ARs and 367 of them are the anti-Hale ARs. A specific routine to compile the synthetic cycle was demonstrated. We found that, in general, anti-Hale ARs follow the solar cycle and are spread throughout the time-latitude diagram evenly, which implies their fundamental connection with the global dynamo mechanism and the toroidal flux system. The increase in their number and percentage occurs in the second part of the cycle, which is in favour of their contribution to the polar field reversal. The excess in the anti-Hale ARs percentage at the edges of the butterfly diagram and near an oncoming solar minimum (where the toroidal field weakens) might be associated with the strengthening of the influence of turbulent convection and magnetic field fluctuations on the arising flux tubes. The evidence of the misalignment between the magnetic and heliographic equators is also found.

Global distribution of magnetic ripples and electron density fluctuations as observed by the Swarm satellites on the dayside and their relation to the rainfall estimated by the GSMaP

In mid- and low-latitude ionospheric F-region on the dayside, magnetic field and electron density (Ne) fluctuations with amplitude smaller than a few nT and 1–2% of Ne, respectively, are commonly observed. Their spatial scale along satellite orbit is around 70–250 km. It is presumed that they are generated by the waves propagated from lower atmosphere. However, the mode of waves (acoustic wave or internal gravity wave) and their source are not yet clear. Among the possible sources, cumulus convection and/or associated rainfall are considered to be the strong candidates for the atmospheric wave generation. We use the rainfall estimated by the hourly Global Satellite Mapping of Precipitation (JAXA/GSMaP) as a proxy of lower atmospheric disturbance as the wave source, and compare the rainfall with the amplitude of magnetic fluctuations (magnetic ripples) and electron density fluctuations observed by the Swarm satellites. The data from April 2014 to July 2020 are used. The global distribution of rainfall estimated by the GSMaP and its seasonal variation have similarities with amplitude distribution of magnetic ripples and electron density fluctuations on the dayside. We calculate the ratio of their magnitude, i.e., amplitude of magnetic ripples or electron density fluctuations in rainfall cases to those in no-rainfall cases. Although the longitudinally averaged ratio is not very large but around 1.1–1.2 in ± 10– ± 50° Apex latitudes, it is clearly larger than 1.0. The ratio increases when the intensity of rainfall (mm/h) increases. These results indicate that a cumulous convection which causes rainfall is one of the main sources of atmospheric waves that produce magnetic ripples and electron density fluctuations commonly observed in the dayside ionosphere. Anticipating acoustic waves as the driver of magnetic ripples and electron density variations, a difference in the generation mechanism of electron density fluctuations from that of magnetic ripples is suggested even if their sources are common.Graphical abstract

Scaling properties of magnetic field fluctuations in the quiet Sun

Context. The study of the dynamic properties of small-scale magnetic fields in the quiet photosphere is important for several reasons: (i) it allows us to characterise the dynamic regime of the magnetic field and points out some aspects that play a key role in turbulent convection processes; (ii) it provides details of the processes and the spatial and temporal scales in the solar photosphere at which the magnetic fields emerge, vary, and eventually decay; and (iii) it provides physical constraints on models, improving their ability to reliably represent the physical processes occurring in the quiet Sun. Aims. We aim to characterise the dynamic properties of small-scale magnetic fields in the quiet Sun through the investigation of the scaling properties of magnetic field fluctuations. Methods. To this end, we applied the structure functions analysis, which is typically used in the study of complex systems (e.g. in approaching turbulence). In particular, we evaluated the so-called Hölder-Hurst exponent, which points out the persistent nature of magnetic field fluctuations in the field of view targeted at a whole supergranule in the disc centre. Results. We present the first map of a solar network quiet region as represented by the Hölder-Hurst exponent. The supergranular boundary is characterised by persistent magnetic field fluctuations, which indicate the occurrence of longer-memory processes. On the contrary, the regions inside the supergranule are characterised by antipersistent magnetic field fluctuations, which suggest the occurrence of physical processes with a short memory. Classical Kolmogorov homogeneous and isotropic turbulence, for instance, belongs to this class of processes. The obtained results are discussed in the context of the current literature.

Dynamo chiral plasma instabilities from charges and oscillations driven by torsion fluctuations

Recently, Barrow et al. showed that chaotic dynamical systems may be obtained from small fluctuations of torsion in cosmological plasmas. Following this idea our goal in this paper is to show analytically that torsion fluctuations in primordial plasmas induce chiral fluctuations, magnetic field fluctuations, dynamo plasma instabilities, as well as rapid oscillations in the chiral plasma. These phenomena are important in plasma relaxation to allow for a plasma dynamo instability to induce a lower electric voltage in plasmas. This phenomenon is induced in Minkowskian plasma physics, but to our knowledge it is proved for the first time in Einstein–Cartan sort of cosmology. In cosmological models where torsion gravity and electromagnetic fields are non-minimally coupled, dynamo instability is obtained when conductivity fluctuations are driven by torsion. Moreover, symmetry breaking of plasmas is shown here to be obtained from conductivity torsion fluctuations, which serves to bypass Zeldovich anti-dynamo theorem, inducing a dynamo onset, as shown recently in domain planar walls [L. Garcia de Andrade, Einstein–Cartan spin-polarised nucleons in domain wall dynamos, Ann Phys. (N.Y.) 432 (2021) 168558.]. Moreover, chiral charge fluctuations are shown to be induced by torsion fluctuations. It is shown that chiral charges may vanish, whereas the effective chiral charges may be driven by torsional fluctuations. It is also shown that dynamo electric field induces a magnetic helicity which oscillates and dynamo action is shown to dominate oscillations. This agrees with the results obtained from loop voltage. Chiral effective charges’ fluctuations are derived to be proportional to the product of anisotropic conductivity and magnetic field fluctuations.

Non-locality of the turbulent electromotive force

ABSTRACTThe generation of large-scale magnetic fields ($\overline{\boldsymbol {B}}$) in astrophysical systems is driven by the mean turbulent electromotive force ($\overline{\rm{\boldsymbol {\cal E}} {}}$), the cross-correlation between local fluctuations of velocity and magnetic fields. This can depend non-locally on $\overline{\boldsymbol {B}}$ through a convolution kernel Kij. In a new approach to find Kij, we directly fit the time-series data of $\overline{\rm{\boldsymbol {\cal E}} {}}$ versus $\overline{\boldsymbol {B}}$ from a galactic dynamo simulation using singular value decomposition. We calculate the usual turbulent transport coefficients as moments of Kij, and show the importance of including non-locality over eddy length-scales to fully capture their amplitudes and that higher order corrections to the standard transport coefficients are small in this case.

Generic profile of a long-lived corotating interaction region and associated recurrent Forbush decrease

Context.Corotating interaction regions (CIRs), formed by the interaction of slow solar wind and fast streams that originate from coronal holes (CHs), produce recurrent Forbush decreases, which are short-term depressions in the galactic cosmic ray (GCR) flux.Aims.Our aim is to prepare a reliable set of CIR measurements to be used as a textbook for modeling efforts. For that purpose, we observe and analyse a long-lived CIR, originating from a single CH, recurring in 27 consecutive Carrington rotations 2057–2083 in the time period from June 2007–May 2009.Methods.We studied the in situ measurements of this long-lived CIR as well as the corresponding depression in the cosmic ray (CR) count observed by SOHO/EPHIN throughout different rotations. We performed a statistical analysis, as well as the superposed epoch analysis, using relative values of the key parameters: the total magnetic field strength,B, the magnetic field fluctuations,dBrms, plasma flow speed,v, plasma density,n, plasma temperature,T, and the SOHO/EPHIN F-detector particle count, and CR count.Results.We find that the mirrored CR count-time profile is correlated with that of the flow speed, ranging from moderate to strong correlation, depending on the rotation. In addition, we find that the CR count dip amplitude is correlated to the peak in the magnetic field and flow speed of the CIR. These results are in agreement with previous statistical studies. Finally, using the superposed epoch analysis, we obtain a generic CIR example, which reflects the in situ properties of a typical CIR well.Conclusions.Our results are better explained based on the combined convection-diffusion approach of the CIR-related GCR modulation. Furthermore, qualitatively, our results do not differ from those based on different CHs samples. This indicates that the change of the physical properties of the recurring CIR from one rotation to another is not qualitatively different from the change of the physical properties of CIRs originating from different CHs. Finally, the obtained generic CIR example, analyzed on the basis of superposed epoch analysis, can be used as a reference for testing future models.

The Extended Field-aligned Suprathermal Proton Beam and Long-lasting Trapped Energetic Particle Population Observed Upstream of a Transient Interplanetary Shock

Abstract The properties of the suprathermal particle distributions observed upstream of interplanetary shocks depend not only on the properties of the shocks but also on the transport conditions encountered by the particles as they propagate away from the shocks. The confinement of particles in close proximity to the shocks, as well as particle scattering processes during propagation to the spacecraft, lead to the common observation of upstream diffuse particle distributions. We present observations of a rare extended anisotropic low-energy (≲30 keV) proton beam together with a trapped ≳500 keV proton population observed in association with the arrival of an oblique interplanetary shock at the Advanced Composition Explorer, the Interplanetary Monitoring Platform-8, and the Wind spacecraft on 2001 January 31. Continuous injection of particles by the traveling shock into a smooth radial magnetic field region formed in the tail of a modest high-speed solar wind stream produced an extended foreshock region of energetic particles. The absence of enhanced magnetic field fluctuations upstream of the shock results in the observation of a prolonged anisotropic field-aligned beam of ≲30 keV protons as well as a population of higher-energy (≳500 keV) protons with small pitch-angle cosine (μ ∼ 0) extending far from the shock.

Prediction of Cylindrical Magnetic Shielding Performance by Considering the Magnetic Field Strength Inside the Material

Magnetoencephalography (MEG) is highly sensitive to even a small amount of magnetic noise, and thus, a high-performance magnetically shielded room (MSR) is essential. The shielding effect of a novel MSR should be predicted to verify whether it satisfies the required specifications of the MEG at the design stage. Therefore, an accurate prediction method based on the properties of shielding materials and the shape of the MSR is necessary. In this article, we proposed a prediction method using a simple formula and permeability measured against very small magnetic field fluctuations. Using a ring-shaped test piece, permeability was measured against a very small magnetic field corresponding to the Rayleigh region, and the initial permeability and Rayleigh constant were estimated to determine the permeability calculated by the formula. The magnetic field generated inside the material was estimated by magnetic analysis, and it was confirmed that the permeability of the Rayleigh region should be used in the formula. To verify the prediction accuracy, we examined a simple example of a cylindrical shell by comparing the calculated results with the experimental results. It was confirmed that the proposed method is

Magnetic stop signs signal a European songbird's arrival at the breeding site after migration.

Although it is known that birds can return to their breeding grounds with exceptional precision, it has remained a mystery how they know when and where to stop migrating. Using nearly a century's worth of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether fluctuations in Earth's magnetic field predict variation in the sites to which birds return. Ringing recoveries suggest that magnetic inclination is learned before departure and is subsequently used as a uni-coordinate "stop sign" when relocating the natal or breeding site. However, many locations have the same inclination angle. Data from populations with different migratory directions indicate that birds solve this ambiguity by stopping at the first place where the right inclination is encountered on an inherited return vector.

Properties of Magnetic Field Fluctuations in Long-Lasting Radial IMF Events from Wind Observation

Long-lasting radial interplanetary magnetic field (IMF) intervals in which IMF points along the solar wind velocity for several hours have many interesting properties. We investigate the average parameters and the behavior of magnetic field fluctuations within 419 such radial intervals. The power spectral density (PSD) calculated over 1-h intervals of a radial IMF is compared with PSDs in adjacent regions prior to and after the radial IMF. We concentrate on (1) the power of IMF fluctuations, (2) the median slopes of PSDs in both inertial and kinetic ranges, (3) the proton temperature and its anisotropy, and (4) the occurrence rate of wavy structures and their polarization. We have shown that the fluctuation amplitude is low in the radial IMF intervals in both magnetohydrodynamic (MHD) and kinetic ranges, and the spectral power increases with the cone angle in the MHD range. We discuss this effect in the light of present knowledge on plasma turbulence and peculiarities of observations of magnetic field variations under the radial background magnetic field. We found that in the radial IMF events, the proton temperature is more isotropic, the occurrence rate of waves is higher, and the waves have no preferred polarization in the frequency range from 0.1 to 1 Hz. It suggests that the radial IMF structure leads to a different development of turbulence than the typical Parker-spiral structure.

Organic molecular dynamics and charge-carrier lifetime in lead iodide perovskite MAPbI3

The long charge carrier lifetime of the hybrid organic-inorganic perovskites (HOIPs) is the key for their remarkable performance as a solar cell material. The microscopic mechanism for the long lifetime is still in debate. Here, by using a muon spin relaxation technique that probes the fluctuation of local magnetic fields, we show that the muon depolarization rate (Δ) of a prototype HOIP methylammonium lead iodide (MAPbI3) shows a sharp decrease with increasing temperature in two steps above 120 K and 190 K across the structural transition from orthorhombic to tetragonal structure at 162 K. Our analysis shows that the reduction of Δ is quantitatively in agreement with the expected behavior due to the rapid development of methyl ammonium (MA) jumping rotation around the C 3 and C 4 symmetry axes. Our results provide direct evidence for the intimate relation between the rotation of the electric dipoles of MA molecules and the charge carrier lifetime in HOIPs.

Three-Dimensional Anisotropy and Scaling Properties of Solar Wind Turbulence at Kinetic Scales in the Inner Heliosphere: Parker Solar Probe Observations

Abstract We utilize the data from the Parker Solar Probe mission at its first perihelion to investigate the three-dimensional (3D) anisotropies and scalings of solar wind turbulence for the total, perpendicular, and parallel magnetic-field fluctuations at kinetic scales in the inner heliosphere. By calculating the five-point second-order structure functions, we find that the three characteristic lengths of turbulence eddies for the total and the perpendicular magnetic-field fluctuations in the local reference frame ( L ˆ ⊥ , l ˆ ⊥ , l ˆ ∣ ∣ ) defined with respect to the local mean magnetic field B local feature as l ∣∣ > L ⊥ > l ⊥ in both the transition range and the ion-to-electron scales, but l ∣∣ > L ⊥ ≈ l ⊥ for the parallel magnetic-field fluctuations. For the total magnetic-field fluctuations, the wave-vector anisotropy scalings are characterized by l ∣ ∣ ∝ l ⊥ 0.78 and L ⊥ ∝ l ⊥ 1.02 in the transition range, and they feature as l ∣ ∣ ∝ l ⊥ 0.44 and L ⊥ ∝ l ⊥ 0.73 in the ion-to-electron scales. Still, we need more complete kinetic-scale turbulence models to explain all these observational results.

Characteristics of Turbulence Driven by Transient Magnetic Reconnection in the Terrestrial Magnetotail

Abstract This paper investigates the evolution of turbulence within the magnetotail reconnection exhaust observed by the Magnetospheric Multiscale spacecraft. The reconnection was in an unsteady state that caused significant temporal variations in the outflow speed. By dividing the exhaust into nine fast flows, we analyzed and compared the characteristics of turbulence in these nine flows. We find that the strength of the intermittency has a good relationship with the peak speed of the fast flows. The higher-order analysis of magnetic field fluctuations reveals that the turbulence is multifractal in the inertial range for these flows except one with the highest peak speed. Moreover, the turbulence is monofractal on kinetic scales in all of these fast flows. The magnetic energy was intermittently dissipated in these turbulent flows, predominantly occurred in the coherent structures. Since the coherent structures with the largest energy dissipation in these flows are different, we suggest that the mechanism of energy dissipation may be different among these flows.

A Highly Sensitive In Situ Magnetic Field Fluctuation Measurement Method Based on Nuclear-Spin Depolarization in an Atomic Comagnetometer

In this article, a highly sensitive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> magnetic field fluctuation measurement method is proposed based on nuclear-spin depolarization in a spin-exchange relaxation-free (SERF) atomic comagnetometer (ACM). The steady-state model of the hybrid atomic spin ensemble is systematically studied. According to the theoretical model, two different states of nuclear-spin polarization and unpolarization lead to different sensitivities of the system to rotation and magnetic fields. The gradient magnetic field (GMF) is employed to change the state of nuclear-spin polarization of the noble gas, transforming the ACM into an atomic magnetometer (AM). The polarization and relaxation rates of the electron spin and nuclear spin under the two measurement modes are measured and compared by the magnetic resonance method and transient response method. At the same time, the performances in the ACM and AM are tested. Detailed experiments verify that the AM can achieve sensitivity for the magnetic field measurement with an average value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$53.6~\mathbf {fT/Hz^{1/2}}$ </tex-math></inline-formula> in the frequency range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10\sim 30$ </tex-math></inline-formula> Hz. The Allan variance method is adopted to evaluate the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -axis magnetic field stabilities of the ACM, and the magnitudes are 61.6 and 55.9 fT at 100 s, contributing about 4.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-3}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> and 2.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-4}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> to the long-term stability of the ACM. This method can not only realize the high-sensitivity <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurement of the close-to-zero magnetic field and its fluctuation but also realize the switching of the two measurement modes of rotation and magnetic field measurements, which is essential for the evaluation of magnetic field error.