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Magnetic flux leakage (MFL) testing is a widely used nondestructive testing (NDT) method for the inspection of ferromagnetic materials. This review paper presents the basic principles of MFL testing and summarizes the recent advances in MFL. An analytical expression for the leakage magnetic field based on the 3D magnetic dipole model is provided. Based on the model, the effects of defect size, defect orientation, and liftoff distance have been analyzed. Other influencing factors, such as magnetization strength, testing speed, surface roughness, and stress, have also been introduced. As the most important steps of MFL, the excitation method (a permanent magnet, DC, AC, pulsed) and sensing methods (Hall element, GMR, TMR, etc.), have been introduced in detail. Finally, the algorithms for the quantification of defects and the applications of MFL have been introduced.

An ongoing magnetic reconnection event was detected in the Mercury's high latitude magnetopause during a northward interplanetary magnetic field. The reconnection X‐line region was revealed in the Mercury's magnetopause based on the encountered flux ropes ejected away from this region both planetward and tailward. A series of magnetic flux ropes, known as flux transfer event shower were observed tailward of this X‐line region. These flux ropes were probably expanding and deflected as they were ejected away tailward from the X‐line region. Large‐amplitude variations in all three components of the magnetic field and a few small‐scale flux ropes were observed inside the X‐line region, which could be the seed of the flux rope shower at the magnetopause. The observations suggest that magnetic reconnection is highly dynamic and persistent in Mercury's magnetosphere. Plain Language Summary Magnetic reconnection has been regarded as the most important process for dynamics of the Mercury's magnetosphere and for the interaction between the solar wind and the Mercury's magnetosphere also. Although magnetic flux ropes and flux transfer events (FTEs) resulting from magnetic reconnection have been extensively observed in the Mercury's magnetosphere, the key region of magnetic reconnection, namely the X‐line region, has never been reported so far by the spacecraft. Here, we present the first evidence of the reconnection X‐line region in the Mercury's magnetosphere. A few small‐scale magnetic flux ropes are observed inside the reconnection X‐line region, which could be the seed of the observed magnetic FTE shower. Furthermore, the evolution of these flux ropes is addressed also based on the spacecraft observations. Key Points A reconnection X‐line region is first observed in the Mercury's magnetopause during the northward interplanetary magnetic field The small‐scale magnetic flux ropes in the X‐line region could be the seed of the flux transfer event shower The flux ropes probably expand and is deflected after they are ejected away from the X‐line region

In a Josephson junction (JJ) at zero bias, Cooper pairs are transported between two superconducting contacts via the Andreev bound states (ABSs) formed in the Josephson channel. Extending JJs to multiple superconducting contacts, the ABSs in the Josephson channel can coherently hybridize Cooper pairs among different superconducting electrodes. Biasing three-terminal JJs with antisymmetric voltages, for example, results in a direct current (DC) of Cooper quartet (CQ), which involves a four-fermion entanglement. Here, we report half a flux periodicity in the interference of CQ formed in graphene based multi-terminal (MT) JJs with a magnetic flux loop. We observe that the quartet differential conductance associated with supercurrent exhibits magneto-oscillations associated with a charge of 4 e , thereby presenting evidence for interference between different CQ processes. The CQ critical current shows non-monotonic bias dependent behavior, which can be modeled by transitions between Floquet-ABSs. Our experimental observation for voltage-tunable non-equilibrium CQ-ABS in flux-loop-JJs significantly extends our understanding of MT-JJs, enabling future design of topologically unique ABS spectrum. Here, the authors perform measurements of the interference effects of Cooper Quartets (CQ), observed in a multi-terminal graphene Josephson junction where two terminals are tied by a flux loop. By biasing the superconducting contacts, they identify a superconducting branch attributed to CQ currents, and present evidence for interference between different CQ processes.

We report in situ observation of magnetic reconnection between magnetic flux rope (MFR) and magnetic hole (MH) in the magnetosheath by the Magnetospheric Multiscale mission. The MFR was rooted in the magnetopause and could be generated by magnetopause reconnection therein. A thin current sheet was generated due to the interaction between MFR and MH. The sub‐Alfvénic ion bulk flow and the Hall field were detected inside this thin current sheet, indicating an ongoing reconnection. An elongated electron diffusion region characterized by non‐frozen‐in electrons, magnetic‐to‐particle energy conversion, and crescent‐shaped electron distribution was detected in the reconnection exhaust. The observation provides a mechanism for the dissipation of MFRs and thus opens a new perspective on the evolution of MFRs at the magnetopause. Our work also reveals one potential fate of the MHs in the magnetosheath which could reconnect with the MFRs and further merge into the magnetopause. Plain Language Summary Magnetic flux rope (MFR) is a kind of helical magnetic field structure that is frequently observed in the Earth's magnetosphere. At the dayside magnetopause, MFRs are generally generated by the reconnection of the Earth's intrinsic magnetic field and the interplanetary magnetic field, especially when the interplanetary magnetic field points southward. These MFRs tend to grow larger after they are expelled from the reconnection sites and then travel along the magnetopause, and ultimately disintegrate into the cusp. In this study, we provide another potential fate of these magnetopause MFRs. They can interact with the magnetosheath magnetic holes and dissipate through reconnection with multiple magnetic holes. Based on the Magnetospheric Multiscale observation, we provide direct evidence of reconnection between the MFR and the magnetic hole, which has a pivotal role in this scenario. Our results give new insights into the evolution of MFRs at the magnetopause and further the coupling between the solar wind and the Earth's magnetosphere. Key Points First observation of magnetic reconnection between magnetic flux rope (MFR) and magnetic hole (MH) in the magnetosheath A thin current sheet with typical reconnection signatures was formed at the interface of MFR and MH due to their interaction An elongated electron diffusion region was detected in the reconnection exhaust

Wire rope inspection by nondestructive testing methods, sensors and signal processing techniques are mainly reviewed in this paper. Owing to the difference of physical mechanism and testing principles, magnetic flux leakage, eddy current, acoustic emission and ultrasonic guide wave testing as well as other inspection methods for steel wire rope are summarized. Then, the commonly and frequently used testing sensors of inductive coil, hall element, magnetoresistive sensors and others are compared in the perspective of their corresponding operating principles, development situation, advantages and disadvantages. Furthermore, signal processing techniques including the signal filtering techniques such as the time and frequency analysis methods, quantitative data processing methods such as the machine learning and defect classification are studied. Finally, the challenges and future developing trends of wire rope inspection in practical applications are discussed.

Reconstruction of the magnetospheric magnetic field using swarms of virtual spacecraft provided by data mining confirms seminal in situ evidence (Angelopoulos et al., 2008, https://doi.org/10.1126/science.1160495) that on 26 February 2008 an X‐line emerged in the region between two distant Time History of Events and Macroscale Interactions during Substorms probes at the time of the substorm activation in the magnetotail. It also shows that the X‐line formation was preceded by rapid current decay that happened 15 min earlier. The current was built up earthward of the pre‐existing X‐line formed prior to the previous substorm activation 45 min before. The most pronounced effect of the tail reconfiguration at the moments of two substorm activations and the current disruption is the rapid earthward redistribution of the magnetic flux. Comparison of low‐altitude mapping of the magnetotail structure with all‐sky imager data shows that these rapid reconfigurations might be triggered by plasma flows whose source was farther from the Earth than the resolved X‐lines.

Coronal mass ejections（CMEs） and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1–3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope（MFR）, of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.

Solar magnetic activity drives the dominant 11-year cyclic variability of different space environmental indices, but they can be delayed with respect to the original variations due to the different physical processes involved. Here, we analyzed the pairwise time lags between three global solar and heliospheric indices: sunspot numbers (SSN), representing the solar surface magnetic activity, the open solar flux (OSF), representing the heliospheric magnetic variability, and the galactic cosmic-ray (GCR) intensity near Earth, using the standard cross-correlation and the more detailed wavelet-coherence methods. All the three indices appear highly coherent at a timescale longer than a few years with persistent high coherence at the timescale of the 11-year solar cycle. The GCR variability is delayed with respect to the inverted SSN by about eight 27-day Bartels rotations on average, but the delay varies greatly with the 22-year cycle, being shorter or longer around positive A + or negative A − solar polarity epochs, respectively. The 22-year cyclicity of the time lag is determined by the global heliospheric drift effects, in agreement with theoretical models. The OSF lags by about one year behind SSN, and is likely determined by a combination of the short lifetime of active regions and a longer (≈3 years) transport time of the surface magnetic field to the poles. GCRs covary nearly in antiphase with the OSF, also depicting a strong 22-year cycle in the delay, confirming that the OSF is a good index of the heliospheric modulation of GCRs. This provides an important observational constraint for solar and heliospheric physics.

The combination of magnetoresistive (MR) element and magnetic flux concentrators (MFCs) offers highly sensitive magnetic field sensors. To maximize the effect of MFC, the geometrical design between the MR element and MFCs is critical. In this paper, we present simulation and experimental studies on the effect of the geometrical relationship between current-in-plane giant magnetoresistive (GMR) element and MFCs made of a NiFeCuMo film. Finite element method (FEM) simulations showed that although an overlap between the MFCs and GMR element enhances their magneto-static coupling, it can lead to a loss of magnetoresistance ratio due to a magnetic shielding effect by the MFCs. Therefore, we propose a comb-shaped GMR element with alternate notches and fins. The FEM simulations showed that the fins of the comb-shaped GMR element provide a strong magneto-static coupling with the MFCs, whereas the electric current is confined within the main body of the comb-shaped GMR element, resulting in improved sensitivity. We experimentally demonstrated a higher sensitivity of the comb-shaped GMR sensor (36.5 %/mT) than that of a conventional rectangular GMR sensor (28 %/mT).

This paper proposes a semi-supervised learning model for detecting multi-defect classification and localization on the steel surface for industries with limited labeled datasets. This study uses 1-D data from magnetic flux leakage (MFL) testing, a powerful and cost-effective nondestructive inspection method for steel bars. Most steel surface defect systems are based on supervised learning classification with 2-D image datasets. However, acquiring labeled datasets for developing supervised learning models is practically limited in the actual steel manufacturing process. Furthermore, due to the frequent occurrence of multiple defect classes on the same steel bar, the problem of multi-defect classification and localization needs to be addressed. Therefore, this paper proposes a steel bar surface inspection system for multi-defect classification and localization based on a semi-supervised learning model and MFL signals. The proposed system solves the multi-defect classification and localization problem by reducing the feature dimension with an autoencoder. Then, it classifies the defects based on the semi-supervised support vector machines that require only a small portion of the labeled dataset. Also, the classification process is repeated on the overlapped small steel section to address the multi-defect classification and localization issue. When it was evaluated on an industry MFL inspection dataset, the accuracy ranged from 81% to 90% when the labeled data ratio varied from 2% to 90%.