Magnetic Field Observations Research Articles

Global evolution of solar magnetic fields and prediction of activity cycles

AbstractPrediction of solar activity cycles is challenging because physical processes inside the Sun involve a broad range of multiscale dynamics that no model can reproduce and because the available observations are highly limited and cover mostly surface layers. Helioseismology makes it possible to probe solar dynamics in the convective zone, but variations in differential rotation and meridional circulation are currently available for only two solar activity cycles. It has been demonstrated that sunspot observations, which cover over 400 years, can be used to calibrate the Parker-Kleeorin-Ruzmaikin dynamo model, and that the Ensemble Kalman Filter (EnKF) method can be used to link the modeled magnetic fields to sunspot observations and make reliable predictions of a following activity cycle. However, for more accurate predictions, it is necessary to use actual observations of the solar magnetic fields, which are available only for the last four solar cycles. In this paper I briefly discuss the influence of the limited number of available observations on the accuracy of EnKF estimates of solar cycle parameters, the criteria to evaluate the predictions, and application of synoptic magnetograms to the prediction of solar activity.

Time variation of Jupiter’s internal magnetic field consistent with zonal wind advection

Determination of the time dependency (secular variation) of a planet’s magnetic field provides a window into understanding the dynamo responsible for generating its field. However, of the six Solar System planets with active dynamos, secular variation has been firmly established only for Earth. Here, we compare magnetic field observations of Jupiter from the Pioneer 10 and 11, Voyager 1 and Ulysses spacecraft (acquired 1973–1992) with a new Juno reference model (JRM09). We find a consistent, systematic change in Jupiter’s field over this 45-year time span, which cannot be explained by changes in the magnetospheric field or by changing the assumed rotation rate of Jupiter. Through a simplified forward model, we find that the inferred change in the field is consistent with advection of the field by Jupiter’s zonal winds, projected down to 93–95% of Jupiter’s radius (where the electrical conductivity of the hydrogen envelope becomes sufficient to advect the field). This result demonstrates that zonal wind interactions with Jupiter’s magnetic field are important and lends independent support to atmospheric and gravitational-field determinations of the profile of Jupiter’s deep winds.

Machine learning reveals systematic accumulation of electric current in lead-up to solar flares

Solar flares-bursts of high-energy radiation responsible for severe space weather effects-are a consequence of the occasional destabilization of magnetic fields rooted in active regions (ARs). The complexity of AR evolution is a barrier to a comprehensive understanding of flaring processes and accurate prediction. Although machine learning (ML) has been used to improve flare predictions, the potential for revealing precursors and associated physics has been underexploited. Here, we train ML algorithms to classify between vector-magnetic-field observations from flaring ARs, producing at least one M-/X-class flare, and nonflaring ARs. Analysis of magnetic-field observations accurately classified by the machine presents statistical evidence for (i) ARs persisting in flare-productive states-characterized by AR area-for days, before and after M- and X-class flare events; (ii) systematic preflare buildup of free energy in the form of electric currents, suggesting that the associated subsurface magnetic field is twisted; and (iii) intensification of Maxwell stresses in the corona above newly emerging ARs, days before first flares. These results provide insights into flare physics and improving flare forecasting.

The GOES-16 Spacecraft Science Magnetometer

Since their inception in the 1970s, the NOAA Geostationary Operational Environmental Satellite (GOES) system has monitored the sources of space weather on the sun and the effects of space weather at Earth. These observations are important for providing forecasts, warnings and alerts to many customers, including satellite operators, the power utilities, and NASA’s human activities in space. The GOES magnetometer provides observations of the geomagnetic field, which can be the first indication that significant space weather has reached Earth. In addition, the magnetic field observations are used to identify and forecast the severity of the space weather activity. This paper reviews the capabilities of the GOES-16 magnetometer (MAG) and presents initial post-launch calibration/validation results including issues found in the data. The GOES-16 MAG requirements and capabilities are similar to those for previously flown instruments, measuring three components of the geomagnetic field but with an improved sampling rate of 10 samples/second. The MAG data are low-pass filtered with a 2.5 Hz cutoff compared to the 0.5 Hz cutoff of previous GOES magnetometers. The MAG is composed of two magnetometers, an inboard (closer to spacecraft bus) and outboard (on tip of boom) magnetometer. Presented are the science and instrument requirements, ground and initial on-orbit instrument calibration and data validation. The on-orbit analysis found magnetic contamination along with temperature dependency effects that resulted in unexpected instrument noise and decreased accuracy, with the issues generally more significant on the inboard magnetometer. The outboard sensor was used for initial analysis of MAG performance. Preliminary comparison, excluding arcjet firing periods, between the outboard magnetometer and the GOES-14 magnetometer found a statistical difference of 5 nT at $3\sigma $ for the total field. This comparison does not consider inaccuracies in the GOES-14 magnetometer. Future studies will focus on optimizing the outboard sensor performance.

Average motion of emerging solar active region polarities

Aims. Our goal is to constrain models of active region formation by tracking the average motion of active region polarity pairs as they emerge onto the surface. Methods. We measured the motion of the two main opposite polarities in 153 emerging active regions using line-of-sight magnetic field observations from the Solar Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR) survey. We first measured the position of each of the polarities eight hours after emergence, when they could be clearly identified, using a feature recognition method. We then tracked their location forwards and backwards in time. Results. We find that, on average, the polarities emerge with an east-west orientation and the separation speed between the polarities increases. At about 0.1 days after emergence, the average separation speed reaches a peak value of 229 ± 11 ms−1, and then starts to decrease. About 2.5 days after emergence the polarities stop separating. We also find that the separation and the separation speed in the east-west direction are systematically larger for active regions that have higher flux. The scatter in the location of the polarities increases from about 5 Mm at the time of emergence to about 15 Mm at two days after emergence. Conclusions. Our results reveal two phases of the emergence process defined by the rate of change of the separation speed as the polarities move apart. Phase 1 begins when the opposite polarity pairs first appear at the surface, with an east-west alignment and an increasing separation speed. We define Phase 2 to begin when the separation speed starts to decrease, and ends when the polarities have stopped separating. This is consistent with a previous study: the peak of a flux tube breaks through the surface during Phase 1. During Phase 2 the magnetic field lines are straightened by magnetic tension, so that the polarities continue to move apart, until they eventually lie directly above their anchored subsurface footpoints. The scatter in the location of the polarities is consistent with the length and timescales of supergranulation, supporting the idea that convection buffets the polarities as they separate.

The Role of Current Sheet Scattering in the Proton Isotropic Boundary Formation During Geomagnetic Storms

AbstractThere is considerable evidence that current sheet scattering (CSS) plays an important role in isotropic boundary (IB) formation during quiet time. However, IB formation can also result from scattering by electromagnetic ion cyclotron waves, which are much more prevalent during storm time. The effectiveness of CSS can be estimated by the parameter , the ratio of the field line radius of curvature to the particle gyroradius. Using magnetohydrodynamic and empirical models, we estimated the parameter K associated with storm time IB observations on the nightside. We used magnetic field observations from spacecraft in the magnetotail to estimate and correct for errors in the K values computed by the models. We find that the magnetohydrodynamic and empirical models produce fairly similar results without correction and that correction increases this similarity. Accounting for uncertainty in both the latitude of the IB and the threshold value of K required for CSS, we found that 29–54% of the IB observations satisfied the criteria for CSS. We found no correlation between the corrected K and magnetic local time, which further supports the hypothesis that CSS played a significant role in forming the observed IBs.

Degree of electric current neutralization and the activity in solar active regions

Abstract Using time-sequence vector magnetic field observation from Helioseismic and Magnetic Imager, we examined the connection of non-neutralized currents and the observed activity in 20 solar active regions (ARs). The net current in a given magnetic polarity is algebraic sum of direct current (DC) and return current (RC) and the ratio |DC/RC| is a measure of degree of net current neutralization (NCN). In the emerging ARs, the non-neutrality of these currents builds with the onset of flux emergence, following the relaxation to neutrality during the separation motion of bipolar regions. Accordingly, some emerging ARs are source regions of CMEs occurring at the time of higher level non-neutrality. ARs in the post-emergence phase can be CME productive provided they have interacting bipolar regions with converging and shearing motions. In these cases, the net current evolves with higher level (>1.3) of non-neutrality. Differently, the |DC/RC| in flaring and quiet ARs vary near unity. In all the AR samples, the |DC/RC| is higher for chiral current density than that for vertical current density. Owing to the fact that the non-neutralized currents arise in the vicinity of sheared polarity-inversion-lines (SPILs), the profiles of the total length of SPIL segments and the degree of NCN follow each other with a positive correlation. We find that the SPIL is localized as small segments in flaring-ARs, whereas it is long continuous in CME-producing ARs. These observations demonstrate the dividing line between the CMEs and flares with the difference being in global or local nature of magnetic shear in the AR that reflected in non-neutralized currents.

Structure and evolution of the photospheric magnetic field in 2010–2017: comparison of SOLIS/VSM vector field and BLOS potential field

Context. The line-of-sight (LOS) component of the large-scale photospheric magnetic field has been observed since the 1950s, but the daily full-disk observations of the full vector magnetic field started only in 2010 using the SOLIS Vector Stokes Magnetograph (VSM) and the SDO helioseismic and magnetic imager (HMI). Traditionally, potential field extrapolations are based on the assumption that the magnetic field in the photosphere is approximately radial. The validity of this assumption has not been tested yet. Aims. We investigate here the structure and evolution of the three components of the solar large-scale magnetic field in 2010–2017, covering the ascending to mid-declining phase of solar cycle 24, using SOLIS/VSM vector synoptic maps of the photospheric magnetic field. Methods. We compare the observed VSM vector magnetic field to the potential vector field derived using the VSM LOS magnetic field observations as an input. The new vector field data allow us to derive the meridional inclination and the azimuth angle of the magnetic field and to investigate their solar cycle evolution and latitudinal profile of these quantities. Results. SOLIS/VSM vector data show that the photospheric magnetic field is in general fairly non-radial. In the meridional plane the field is inclined toward the equator, reflecting the dipolar structure of the solar magnetic field. Rotationally averaged meridional inclination does not have significant solar cycle variation. While the vector radial component Br and the potential radial component BPFSSr are fairly similar, the meridional and zonal components do not agree very well. We find that SOLIS/VSM vector observations are noisy at high latitudes and suffer from the vantage point effect more than LOS observations. This is due to different noise properties in the LOS and transverse components of the magnetic field, which needs to be addressed in future studies.

Interferometric Observations of Magnetic Fields in Forming Stars

The magnetic field is a key ingredient in the recipe of star formation. Over the past two decades, millimeter and submillimeter interferometers have made major strides in unveiling the role of the magnetic field in star formation at progressively smaller spatial scales. From the kiloparsec scale of molecular clouds down to the inner few hundred au immediately surrounding forming stars, the polarization at millimeter and submillimeter wavelengths is dominated by polarized thermal dust emission, where the dust grains are aligned relative to the magnetic field. Interferometric studies have focused on this dust polarization and occasionally on the polarization of spectral-line emission. We review the current state of the field of magnetized star formation in the context of several questions that continue to motivate the studies of high- and low-mass star formation. By aggregating and analyzing the results from individual studies, we come to several conclusions: (1) Magnetic fields and outflows from low-mass protostellar cores are randomly aligned, suggesting that the magnetic field at ~1000 au scales is not the dominant factor in setting the angular momentum of embedded disks and outflows. (2) Recent measurements of the thermal and dynamic properties in high-mass star-forming regions reveal small virial parameters, challenging the assumption of equilibrium star formation. However, we estimate that a magnetic field strength of a fraction of a mG to several mG in these objects could bring the dense gas close to a state of equilibrium. Finally, (3) We find that the small number of sources with hourglass-shaped magnetic field morphologies at 0.01 -- 0.1 pc scales cannot be explained purely by projection effects, suggesting that while it does occur occasionally, magnetically dominated core collapse is not the predominant mode of low- or high-mass star formation. [Abridged]

On the Accuracy of Adiabaticity Parameter Estimations Using Magnetospheric Models

AbstractRecent studies have found that even during quiet times, observed proton isotropic boundaries (IBs) are often projected to the region of high adiabaticity parameter (K≈30), where is the ratio of magnetic field line radius of curvature to the particle gyroradius. This contradicts the accepted hypothesis that current sheet scattering (CSS) is the dominant mechanism of IB formation because K≈8 would be expected for this mechanism. We used magnetohydrodynamic simulations and empirical models to compute K for 30‐keV proton IB observations within 3 hr of local midnight. We found that neither class of model reliably estimates K unless supported by magnetic field observations in the current sheet. magnetohydrodynamic simulations produced higher K values than expected for CSS (K = 15–30), and empirical models gave lower values (K < 4). We obtained reliable estimates of K by controlling for the accuracy of the normal component and the gradient of the radial component in the neutral sheet, using observations from three Time History of Events and Macroscale Interactions during Substorms satellites. For the first time, we demonstrated that both these variables should be taken into account for the accurate estimation of the curvature radius. This greatly reduced the spread of K values, indicating that much of the previous spread was due to errors in the magnetic field but also that these errors can be controlled. Most of the corrected values fall within the expected range for CSS, supporting the hypothesis that the IB's were formed by CSS. Accounting for all model results, we obtain an average corrected value of K = 6.0.

A physical approach to modelling large-scale galactic magnetic fields

Context.A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies.Aims.We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data.Methods.Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation.Results.The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software packageGALMAGwhich allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model’s magnetic field.Conclusions.The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. It can also be used for a simple simulation of a time-dependent magnetic field generated by dynamo action.

Eruptions from quiet Sun coronal bright points

Context. Our recent observational study shows that the majority of coronal bright points (CBPs) in the quiet Sun are sources of one or more eruptions during their lifetime. Aims. Here, we investigate the non-potential time-dependent structure of the magnetic field of the CBP regions with special emphasis on the time-evolving magnetic structure at the spatial locations where the eruptions are initiated. Methods. The magnetic structure is evolved in time using a non-linear force-free field (NLFFF) relaxation approach based on a time series of helioseismic and magnetic imager (HMI) longitudinal magnetograms. This results in a continuous time series of NLFFFs. The time series is initiated with a potential field extrapolation based on a magnetogram taken well before the time of the eruptions. This initial field is then evolved in time in response to the observed changes in the magnetic field distribution at the photosphere. The local and global magnetic field structures from the time series of NLFFF field solutions are analysed in the vicinity of the eruption sites at the approximate times of the eruptions. Results. The analysis shows that many of the CBP eruptions reported in a recent publication contain a twisted flux tube located at the sites of eruptions. The presence of flux ropes at these locations provides in many cases a direct link between the magnetic field structure, their eruption, and the observation of mini coronal mass ejections (mini-CMEs). It is found that all repetitive eruptions are homologous. Conclusions. The NLFFF simulations show that twisted magnetic field structures are created at the locations hosting eruptions in CBPs. These twisted structures are produced by footpoint motions imposed by changes in the photospheric magnetic field observations. The true nature of the micro-flares remains unknown. Further 3D data-driven magnetohydrodynamic modelling is required to show how these twisted regions become unstable and erupt.

On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study*

Abstract Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron–Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L < 5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

Estimating Total Open Heliospheric Magnetic Flux

Over the solar-activity cycle, there are extended periods where significant discrepancies occur between the spacecraft-observed total (unsigned) open magnetic flux and that determined from coronal models. In this article, the total open heliospheric magnetic flux is computed using two different methods and then compared with results obtained from in-situ interplanetary magnetic-field observations. The first method uses two different types of photospheric magnetic-field maps as input to the Wang Sheeley Arge (WSA) model: i) traditional Carrington or diachronic maps, and ii) Air Force Data Assimilative Photospheric Flux Transport model synchronic maps. The second method uses observationally derived helium and extreme-ultraviolet coronal-hole maps overlaid on the same magnetic-field maps in order to compute total open magnetic flux. The diachronic and synchronic maps are both constructed using magnetograms from the same source, namely the National Solar Observatory Kitt Peak Vacuum Telescope and Vector Spectromagnetograph. The results of this work show that the total open flux obtained from observationally derived coronal holes agrees remarkably well with that derived from WSA, especially near solar minimum. This suggests that, on average, coronal models capture well the observed large-scale coronal-hole structure over most of the solar cycle. Both methods show considerable deviations from total open flux deduced from spacecraft data, especially near solar maximum, pointing to something other than poorly determined coronal-hole area specification as the source of these discrepancies.

Magnetic Fields and the Supply of Low-frequency Acoustic Wave Energy to the Solar Chromosphere

Abstract The problem of solar chromospheric heating remains a challenging one with wider implications for stellar physics. Several studies in the recent past have shown that small-scale inclined magnetic field elements channel copious energetic low-frequency acoustic waves, which are normally trapped below the photosphere. These magnetoacoustic waves are expected to shock at chromospheric heights, contributing to chromospheric heating. In this work, exploiting simultaneous observations of photospheric vector magnetic field, Doppler, continuum, and line-core intensity (of Fe i 6173 Å) from the Helioseismic and Magnetic Imager and lower-atmospheric UV emission maps in the 1700 and 1600 Å channels of the Atmospheric Imaging Assembly, both on board the Solar Dynamics Observatory of NASA, we revisit the relationships between magnetic field properties (inclination and strength) and the propagation of acoustic waves (phase travel time). We find that the flux of acoustic energy, in the 2–5 mHz frequency range, between the upper photosphere and lower chromosphere is in the range of 2.25–2.6 kW m−2, which is about twice the previous estimates. We identify that the relatively less inclined magnetic field elements in the quiet Sun channel a significant amount of waves of frequency lower than the theoretical minimum acoustic cutoff frequency due to magnetic inclination. We also derive indications that these waves steepen and start to dissipate within the height ranges probed, while those let out due to inclined magnetic fields pass through. We explore connections with existing theoretical and numerical results that could explain the origin of these waves.

Emplacement mechanism of the Tafresh granitoids, central part of the Urumieh–Dokhtar Magmatic Arc, Iran: evidence from magnetic fabrics

AbstractGranitoid stocks crop out in the Ghahan and Sarbadan areas near Tafresh city, which is situated in the central part of the Urumieh–Dokhtar Magmatic Arc, Iran. The stocks, consisting of porphyritic and sub-granular diorite and granular granodiorite, intruded into Eocene volcano-sedimentary units. Normalized multi-element diagrams indicate that the analysed rocks are enriched in large-ion lithophile elements and depleted in high field strength elements. These geochemical features are typical of subduction-related calc-alkaline arc magmas. The stocks belong to the ferromagnetic and I-type granitoid series. Anisotropy of magnetic susceptibility provides information about the internal fabric of the granitoids. Susceptibility values range from 5.6 × 10−3 to more than 71.6 × 10−3, averaging 27.9 × 10−3 SI. Relatively low anisotropy values (P%) rarely exceed 10 %. Shape parameters (T) vary between −0.48 and +0.74, averaging + 0.2. Each stock is interpreted to contain a distinct feeder zone in which magnetic lineation plunges steeply (> 60°), suggesting that the magma ascended mainly in a NW–SE conduit and, to a lesser extent, in an E–W direction. Integration of magnetic fabric data, field observations and tectonic setting indicates that the shear zone that was developed between the Indes and Talkhab faults had created an opening into which the Ghahan and Sarbadan stocks were emplaced by way of creating a suitable tensional space for the ascent of magma.

Storm Time EMIC Waves Observed by Swarm and Van Allen Probe Satellites

AbstractThe temporal and spatial evolution of electromagnetic ion cyclotron (EMIC) waves during the magnetic storm of 21–29 June 2015 was investigated using high‐resolution magnetic field observations from Swarm constellation in the ionosphere and Van Allen Probes in the magnetosphere. Magnetospheric EMIC waves had a maximum occurrence frequency in the afternoon sector and shifted equatorward during the expansion phase and poleward during the recovery phase. However, ionospheric waves in subauroral regions occurred more frequently in the nighttime than during the day and exhibited less obvious latitudinal movements. During the main phase, dayside EMIC waves occurred in both the ionosphere and magnetosphere in response to the dramatic increase in the solar wind dynamic pressure. Waves were absent in the magnetosphere and ionosphere around the minimum SYM‐H. During the early recovery phase, He+ band EMIC waves were observed in the ionosphere and magnetosphere. During the late recovery phase, H+ band EMIC waves emerged in response to enhanced earthward convection during substorms in the premidnight sector. The occurrence of EMIC waves in the noon sector was affected by the intensity of substorm activity. Both ionospheric wave frequency and power were higher in the summer hemisphere than in the winter hemisphere. Waves were confined to an MLT interval of less than 5 hr with a duration of less than 186 min from coordinated observations. The results could provide additional insights into the spatial characteristics and propagation features of EMIC waves during storm periods.

Dynamics of Ionospheric Alfvén Resonances from the End of Cycle 21 through Cycle 24 of Solar Activity

The results of the study of the dynamics of ionospheric Alfven resonances (IARs) in a range of 0–10 Hz based on data from magnetic field observations at the midlatitude Borok observatory (L = 2.8) from the end of solar cycle 21 through solar cycle 24 are presented. It is shown that IARs in 30% of events are accompanied by the simultaneous observation of structured Pc1 geomagnetic pulsations (“pearls”) and IARs are recorded in 70% of events without Pc1 excitation. A specific feature of pearls is that they are observed predominantly at a frequency of the IAR first resonance band. The qualitative coincidence of the dynamics of frequencies of IAR and Pc1 wave packets is detected in 80% of events. The probability maximum of IAR observation falls in the hours before midnight (2000–2200 MLT). IAR seasonal variation is characterized by the presence of two equinoctial maxima. It is shown that the 11-year variation in the IAR emission is controlled by the dynamics of some parameters of the solar wind and IMF. The probability of IAR observation is maximal (in years of solar activity minima) when the ratio of the proton density to the helium ion (α particle) density Np/Na (we note that it is customary to use the inverse, i.e., Na/Np, for the solar wind) and parameter β (which characterizes the ratio of thermal pressure to magnetic pressure) reach the maximum values, while the dynamic pressure of solar wind Pdyn (which controls the magnetosphere compression) is decreased. The coincidence of the dynamics of the frequencies of the IAR first resonance band and pearls, as well as of their seasonal and cyclic variations, may be evidence of the interrelation of these oscillatory processes and the possible common mechanism of their generation.

Alternating magnetic force microscopy: simultaneous observation of static and dynamic magnetic field in three-dimensional space

In the scanning magnetic domain by using the conventional magnetic force microscopy (MFM), a laser beam reflection is used to detect the static magnetic force between probe and sample. Therefore, for the MFM, it is a challenge to directly detect the dynamic magnetic force between probe and sample under an external alternating-current (AC) magnetic field. In this study, it is proved that in an alternating magnetic force microscopy (A-MFM) a sensitive Co-GdO<sub><i>x</i></sub> superparamagnetic probe can be usedto detect the dynamic magnetic force under an external AC magnetic field (frequency <i>ω</i><sub>m</sub>). In the present method, the magnetization of Co-GdO<sub><i>x</i></sub> probe is modulated by an external AC magnetic field. Collecting <i>ω</i><sub>m</sub> and 2<i>ω</i><sub>m</sub> signals by using the combination of phase-locked loop (PLL) and lock in amplifiers can accurately represent the static (DC, which stands for direct current) magnetic field areas (the external AC magnetic field has no effect on the magnetized status of the sample) and dynamic (AC) magnetic field areas (the external AC magnetic field changes the magnetized status of the sample) of an anisotropic Sr ferrite sintered magnet at the same time, respectively. The Sr ferrite sample is a single-domain-type magnet where magnetization mainly changes via magnetic rotation. The A-MFM method can measure the strength and identify the polarities of the static magnetic field of sample with a DC demagnetized state. By modifying the traditional tapping-lift mode into a tapping-multiply lift mode, the A-MFM by using superparamagnetic tips can measure the static and dynamic magnetic field distribution in three-dimensional (3D) space. It is proved that the static and dynamic magnetic field as a function of the distance <i>z</i> between probe and sample are both expressed as <i>H<sub>z</sub></i>(<i>z</i>) = <i>H<sub>z</sub></i>(0)·exp(–<i>kz</i>). The experimental data are consistent with the previous theoretical calculations. The A-MFM can be used to study the dynamic magnetization process and to evaluate the magnetic homogeneity (microstructural homogeneity) of magnetic materials.

Stereoscopic observations of the Sun

This article analyzes key problems in solar physics and thecorresponding development trends in observation, and explains thenecessity of stereoscopic observation, especially for the solar magneticfield. It can be expected that such kind of observations will have asignificant impact on solar physics. At the same time, the articleintroduces the future (2035/2050) layout for solar stereoscopicexploration reached by the solar physics community in China. The planincludes stereoscopic observation of solar magnetic field, stereoscopicobservation of high-energy radiation in solar activities, andstereoscopic of solar flares and coronal mass ejections inmulti-wavelengths.