Planar Magnetic Structures Research Articles

Similarities and Differences between ICME-driven Shocks Observed by VEX (∼0.72 au) and WIND (∼1.0 au)

Abstract The features of interplanetary shocks driven by interplanetary coronal mass ejections (ICMEs) observed by WIND (∼1.0 au) and Venus Express (VEX; ∼0.72 au) during the same period are statistically analyzed by comparing their similarities and differences. It is found that the proportion of ICME-driven shocks in all shocks decreases slightly from ∼0.72 to ∼1 au. The yearly occurrence of ICME-driven shocks at both ∼0.72 and ∼1 au roughly follows the sunspot cycle, while the magnetic field ratio does not show such a correspondence. In each year, the annual medians of the shock angle for ICME-driven shocks at ∼1 au are consistently larger than those at ∼0.72 au, and the annual medians of the magnetic field ratio for events at ∼1 au are slightly smaller than those at ∼0.72 au. Planar magnetic structures (PMSs) downstream of ICME-driven shocks are also analyzed. Approximately 28.57% of the detected PMS events from VEX observations and 28.84% from WIND observations cover the entire 2 hr intervals downstream of the shocks, which are referred to as full PMS events. Through comparative analysis for VEX and WIND observations, it is found that strong and quasi-perpendicular ICME-driven shocks are the most preferable conditions for full PMS formation.

Analytical Verification of the Normal Vector Determination Methods for Planar Magnetic Structures

Abstract Minimum variance analysis of magnetic fields (MVAB) is conventionally implemented to determine the normal vector of a planar magnetic structure (PMS) for which the average of the normal magnetic field component vanishes (i.e., B n = 0). A more suitable normal vector determination method for single-spacecraft measurements, based on the minimum variance concept, is recommended for PMS studies, namely the MVAB method associated with the constraint B n = 0 (MVABC). An analytical PMS model is employed to verify the performance of the MVAB and MVABC methods for three different types of PMSs, under the circumstances that the B n values are highly fluctuated but the B n remains zero. The results show that MVABC outperforms MVAB in estimating the PMS normal vector. By analyzing a large number of test data sets randomly selected from the original data set, the conclusion remains unchanged. The normal of a PMS discontinuity, which is associated with significant B n fluctuations, is accurately predicted by MVABC and the cross-product method, as compared to MVAB. Moreover, the normal of the discontinuity aligns with the PMS normal, as expected. Two PMS events observed by the Advanced Composition Explorer spacecraft in the sheath of an interplanetary coronal mass ejection are presented. From the MVAB perspective, these two events are not classified as PMSs. The estimated normals of the in situ PMS discontinuities from the cross-product method form angles between 10° and 20° with the predicted PMS normals from MVABC. Discussions are given regarding the failure of the MVAB method in PMS identification.

Maximum Aligned Directional Derivative (MADD) Technique for Planar Structure Analysis in Space

Interplanetary discontinuities are the most common intermittent structures in the solar winds, whose jump conditions conform to the Rankine–Hugoniot relations. The identification and analysis of such planar structures rely on an accurate determination of the normal vectors. In this study, we perform a benchmark test of a technique designed to estimate the dimensionality and orientation of the magnetic fields based on four spacecraft measurements. Such a technique operates on the fact that in a strictly planar magnetic field, three columns of ∇ B are all aligned and the characteristic direction corresponding to the maximum aligned directional derivative (MADD; i.e., the maximum of ∣∇B·r^∣ ) can be considered the normal vector of the structure. A benchmark test of this technique, using both the simulation data and numerical models of planar structures, shows that the MADD technique can identify planar magnetic field structures efficiently. For a discontinuity with translational and rotational motion, MADD is able to estimate its normal vector with second-order precision and the speed of motion with first-order precision. Two demonstrations of the MADD application to the tangential discontinuity and shock structures are presented to show its efficiency and superiority. In principle, MADD is suitable for all types of planar structures in space. Compared with the minimum variance analysis and timing techniques, MADD is superior in the study of intermediate shocks that exhibit magnetic coplanarity and rapid spatial evolution.

Statistical Study of Geo-Effectiveness of Planar Magnetic Structures Evolved within ICME’s

Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent drivers of space weather disturbances, especially intense/extreme geomagnetic storms. Recent studies by our group showed that ICME sheaths and/or magnetic clouds (MC) could be transformed into a planar magnetic structure (PMS) and speculate that these structures might be more geo-effective. Thus, we statistically investigated the geo-effectiveness of planar and non-planar ICME sheaths and MC regions. We analyzed 420 ICME events observed from 1998 to 2017, and we found that the number of intense (−100 to −200 nT) and extreme (<−200 nT) geomagnetic storms are large during planar ICMEs (almost double) compared to non-planar ICMEs. In fact, almost all the extreme storm events occur during PMS molded ICME crossover. The observations suggest that planar structures are more geo-effective than non-planar structures. Thus, the current study helps us to understand the energy transfer mechanism from the ICME/solar wind into the magnetosphere, and space-weather events.

Planar Magnetic Structures Downstream of Coronal Mass Ejection–driven Shocks in the Inner Heliosphere

Planar magnetic structures (PMSs), characterized by interplanetary magnetic field vectors remaining parallel to a specific plane, are commonly observed in the solar wind, especially in the sheath region of interplanetary coronal mass ejections (ICMEs). In this study, PMS events in the 2 hr regions downstream of ICME-driven shocks were investigated to reveal the relationship between PMS formation and shock environment using data collected by the Parker Solar Probe, Solar Orbiter, and Venus Express spacecraft in the inner heliosphere. PMS events are identified in the majority (around 93%) of the postshock 2 hr regions, with transit times ranging from 10 to 120 minutes, which demonstrates their common occurrence associated with ICME-driven shocks. About 33% of the detected PMS events cover the whole 2 hr intervals, called full PMS events. Most of the full PMS events are observed in the downstream region of quasi-perpendicular shocks. In addition, statistical results show that full PMS events occurring in the downstream region of quasi-perpendicular shocks are generally associated with higher magnetic compression ratios, which implies that full PMS events are more likely to be formed in the downstream region of strong quasi-perpendicular shocks.

Intense ([formula omitted]) geomagnetic storms induced by planar magnetic structures in co-rotating interaction regions

Generally, Co-rotating Interaction Region (CIR) induced geomagnetic storms are weak to moderate in magnitude. Moreover, CIRs can also drive intense storms when they are coupled with an ICME or other interplanetary structures. Here, we investigated all intense (SYM-H⩽-100nT) geomagnetic storms caused by CIRs from the years 1996 to 2016. We found 12 intense storms that were solely caused by CIRs, i.e., without any component of ICME. We observed the evolution of a planar magnetic structure (PMS) in 10 CIR events i.e. 83% during the storm’s main phase. It suggests that PMS-like quasi-2D magnetic structures might play a significant role in strengthening the Bz component, which causes the intensification of the storm during CIR transit.

Novel Magnetic Properties in Curved Geometries

The expanding of planar magnetic structures into three dimensions (3D) creates the possibility of tuning the conventional magnetic textures or producing novel effects and functionalities by tailoring their curvature [...]

Magnetic Skyrmion Tubes as Nonplanar Magnonic Waveguides

Various latest experiments have proven the theoretical prediction that domain walls in planar magnetic structures can channel spin waves as outstanding magnonic waveguides, establishing a superb platform for building magnonic devices. Recently, three-dimensional nanomagnetism has been boosted up and become a significant branch of magnetism, because three-dimensional magnetic structures expose a lot of emerging physics hidden behind planar ones and will inevitably provide broader room for device engineering. Skyrmions and antiSkyrmions, as natural three-dimensional magnetic configurations, are not considered yet in the context of spin-wave channeling and steering. Here, we show that skyrmion tubes can act as nonplanar magnonic waveguides if excited suitably. An isolated skyrmion tube in a magnetic nanoprism induces spatially separate internal and edge channels of spin waves; the internal channel has a narrower energy gap, compared to the edge channel, and accordingly can transmit signals at lower frequencies. Additionally, we verify that those spin-wave beams along magnetic nanoprism are restricted to the regions of potential wells. Transmission of spin-wave signals in such waveguides results from the coherent propagation of locally driven eigenmodes of skyrmions, i.e., the breathing and rotational modes. Finally, we find that spin waves along the internal channels are less susceptible to magnetic field than those along the edge channels. Our work will open a new arena for spin-wave manipulation and help bridge skyrmionics and magnonics.

Halbach Effect at the Nanoscale from Chiral Spin Textures

Mallinson's idea that some spin textures in planar magnetic structures could produce an enhancement of the magnetic flux on one side of the plane at the expense of the other gave rise to permanent magnet configurations known as Halbach magnet arrays. Applications range from wiggler magnets in particle accelerators and free electron lasers to motors and magnetic levitation trains, but exploiting Halbach arrays in micro- or nanoscale spintronics devices requires solving the problem of fabrication and field metrology below a 100 μm size. In this work, we show that a Halbach configuration of moments can be obtained over areas as small as 1 μm × 1 μm in sputtered thin films with Néel-type domain walls of unique domain wall chirality, and we measure their stray field at a controlled probe-sample distance of 12.0 ± 0.5 nm. Because here chirality is determined by the interfacial Dyzaloshinkii-Moriya interaction, the field attenuation and amplification is an intrinsic property of this film, allowing for flexibility of design based on an appropriate definition of magnetic domains. Skyrmions (<100 nm wide) illustrate the smallest kind of such structures, for which our measurement of stray magnetic fields and mapping of the spin structure shows they funnel the field toward one specific side of the film given by the sign of the Dyzaloshinkii-Moriya interaction parameter D.

Magnetosheath High‐Speed Jets: Internal Structure and Interaction With Ambient Plasma

AbstractFor the first time, we have studied the rich internal structure of a magnetosheath high‐speed jet. Measurements by the Magnetospheric Multiscale (MMS) spacecraft reveal large‐amplitude density, temperature, and magnetic field variations inside the jet. The propagation velocity and normal direction of planar magnetic field structures (i.e., current sheets and waves) are investigated via four‐spacecraft timing. We find structures to mainly convect with the jet plasma. There are indications of the presence of a tangential discontinuity. At other times, there are small cross‐structure flows. Where this is the case, current sheets and waves overtake the plasma in the jet's core region; ahead and behind that core region, along the jet's path, current sheets are overtaken by the plasma; that is, they move in opposite direction to the jet in the plasma rest frame. Jet structures are found to be mainly thermal and magnetic pressure balance structures, notwithstanding that the dynamic pressure dominates by far. Although the jet is supermagnetosonic in the Earth's frame of reference, it is submagnetosonic with respect to the plasma ahead. Consequently, we find no fast shock. Instead, we find some evidence for (a series of) jets pushing ambient plasma out of their way, thereby stirring the magnetosheath and causing anomalous sunward flows in the subsolar magnetosheath. Furthermore, we find that jets modify the magnetic field in the magnetosheath, aligning it with their propagation direction.

Electrical model of giant magnetoimpedance sensors based on continued fractions

In this paper electrical models of magneto-impedance (MI) sensors based on a continued fractions is introduced. The models can be easily applied to cylindrical and planar magnetic structures via the proper mapping of Bessel and coth functions to an approximated passive linear RL network, respectively. This is done representing those functions as continued fractions and identifying the topology and the passive lineal electrical elements as Cauer’s type 2 networks. The equivalent circuits can be conveniently exploited for conditioning electronic purposes, or the characterization of the basis of the MI effect through the estimation of the sensor effective magnetic permeability. Simulation vs measurement results are presented to validate and demonstrate the scope of the model.

Planar magnetic structures in coronal mass ejection-driven sheath regions

Abstract. Planar magnetic structures (PMSs) are periods in the solar wind during which interplanetary magnetic field vectors are nearly parallel to a single plane. One of the specific regions where PMSs have been reported are coronal mass ejection (CME)-driven sheaths. We use here an automated method to identify PMSs in 95 CME sheath regions observed in situ by the Wind and ACE spacecraft between 1997 and 2015. The occurrence and location of the PMSs are related to various shock, sheath, and CME properties. We find that PMSs are ubiquitous in CME sheaths; 85 % of the studied sheath regions had PMSs with the mean duration of 6 h. In about one-third of the cases the magnetic field vectors followed a single PMS plane that covered a significant part (at least 67 %) of the sheath region. Our analysis gives strong support for two suggested PMS formation mechanisms: the amplification and alignment of solar wind discontinuities near the CME-driven shock and the draping of the magnetic field lines around the CME ejecta. For example, we found that the shock and PMS plane normals generally coincided for the events where the PMSs occurred near the shock (68 % of the PMS plane normals near the shock were separated by less than 20° from the shock normal), while deviations were clearly larger when PMSs occurred close to the ejecta leading edge. In addition, PMSs near the shock were generally associated with lower upstream plasma beta than the cases where PMSs occurred near the leading edge of the CME. We also demonstrate that the planar parts of the sheath contain a higher amount of strong southward magnetic field than the non-planar parts, suggesting that planar sheaths are more likely to drive magnetospheric activity.

Voltage controlled modification of flux closure domains in planar magnetic structures for microwave applications

Voltage controlled modification of the magnetocrystalline anisotropy in a hybrid piezoelectric/ferromagnet device has been studied using Photoemission Electron Microscopy with X-ray magnetic circular dichroism as the contrast mechanism. The experimental results demonstrate that the large magnetostriction of the epitaxial Fe81Ga19 layer enables significant modification of the domain pattern in laterally confined disc structures. In addition, micromagnetic simulations demonstrate that the strain induced modification of the magnetic anisotropy allows for voltage tuneability of the natural resonance of both the confined spin wave modes and the vortex motion. These results demonstrate the possibility for using voltage induced strain in low-power voltage tuneable magnetic microwave oscillators.

Radial evolution of the three‐dimensional structure in CIRs between Earth and Ulysses

The radial alignment of ACE and Ulysses in February 2004 and August 2007 provided us with unprecedented opportunities to study the radial evolution of planar magnetic structures (PMSs) in three corotating interaction regions (CIRs). The in situ observations are compared with results from an analytical and a numerical model of CIRs. We find that (1) all three CIRs' meridional tilt retained its North/South orientation at ACE and Ulysses, but the evolution was not systematic. Further, the model results of CIR meridional tilt do not agree with observations. (2) All three CIRs rotated azimuthally with the Parker spiral as expected; however, model results only describe this behavior quantitatively for one CIR. (3) For all three CIRs, the solar wind deflection angles were predicted by the coupled solar corona‐solar wind models, Wang‐Sheely‐Arge (WSA)‐Enlil and MHD Around a Sphere (MAS)‐Enlil, but neither model reproduced the observed planar magnetic structures. (4) The WSA‐Enlil results of azimuthal magnetic field orientation are in better agreement with observations than those based on the MAS‐Enlil. We suggest that the evolution of meridional tilt from ACE to Ulysses does not agree with projections because the parent coronal holes were highly structured. We also suggest that observations of azimuthal tilt do not agree with the model results because the models may underestimate transverse flows, whereas in reality, these flows could affect the observed azimuthal tilt of the CIR. The local orientation of PMSs within CIRs may also be distorted by transients, but their effect is unclear.

Formation, shape, and evolution of magnetic structures in CIRs at 1 AU

We have surveyed the properties of 153 co‐rotating interaction regions (CIRs) observed at 1 AU from January, 1995 through December, 2008. We identified that 74 of the 153 CIRs contain planar magnetic structures (PMSs). For planar and non‐planar CIRs, we compared distributions of the bulk plasma and magnetic field parameters. Our identification of CIRs and their features yields the following results: (1) The different pressures within CIRs are strongly correlated. (2) There is no statistical difference between planar and non‐planar CIRs in the distributions and correlations between bulk plasma and magnetic field parameters. (3) The mean observed CIR azimuthal tilt is within 1 σ of the predicted Parker spiral at 1 AU, while the mean meridional tilt is about 20°. (4) The meridional tilt of CIRs changes from one solar rotation to the next, with no relationship between successive reoccurrences. (5) The meridional tilt of CIRs in the ecliptic is not ordered by the magnetic field polarity of the parent coronal hole. (6) Although solar wind deflection is a function of CIR shape and speed, the relationship is not in agreement with that predicted by Lee (2000). We conclude the following: (1) PMSs in CIRs are not caused by a unique characteristic in the local plasma or magnetic field. (2) The lack of relationship between CIR tilt and its parent coronal hole suggests that coronal hole boundaries may be more complex than currently observed. (3) In general, further theoretical work is necessary to explain the observations of CIR tilt.

Fabrication of magnetic micro- and nanostructures by scanning probe lithography

Planar magnetic structures based on cobalt nanofilms have been obtained by scanning probe lithography. It has been shown that ferromagnetic nanoparticles with different domain structures can be formed by local oxidation of a cobalt film on a graphite substrate with the use of a conductive probe of an atomic force microscope (AFM). Using AFM nanoengraving of polymethylmethacrylate, masks were formed to obtain microcontact pads connected by cobalt nanowires with a width of 250–1400 nm and a thickness of 10–30 nm on the silicon dioxide surface. The topography and magnetization structure of the obtained samples were controlled by atomic and magnetic force microscopy.

Fitting a toroidal force‐free field to multispacecraft observations of a magnetic cloud

A torus‐type flux rope model with an arbitrary aspect ratio was applied to an interplanetary magnetic cloud observed by ACE and Nozomi on 16–18 April 1999, when Nozomi was 0.2 AU downstream of ACE in the solar wind within 3° of heliocentric longitude. The large and small radii of the torus, the direction of the symmetric axis, and the crossing points of the spacecraft were determined so that they would minimize the sum of the square of the difference between the model field and the hourly averages of the observed field. Self‐similar expansion of the flux rope was assumed in proportion with the heliocentric distance. The best fit model had large and small radii of 0.16 and 0.09 AU, respectively. Both spacecraft passed through the northern part of the torus. Difference in the magnetic field observed by the two spacecraft was explained by the difference in their paths through the magnetic cloud. The model fit was consistent with the direction of the vector normal to the preceding planar magnetic structures. The chirality of the flux rope was positive (left handed), suggesting that the solar source was on the Northern Hemisphere. Assuming a probable association with the filament disappearance observed on 13 April 1999 at N16 E00, it is inferred that the filament had traveled in interplanetary space across the ecliptic plane. It was also found that nearly the same fitting result was obtained using a single‐spacecraft observation in the case of a torus‐shaped magnetic cloud with a small aspect ratio.

Dispersion characteristics of spin waves in planar periodic structures based on ferromagnetic films

A technique for calculating the dispersion characteristics of planar periodic magnetic structures is suggested. It is based on joint application of the spin-wave mode analytical approach and the transfer matrix formalism. The dispersion characteristics of a planar periodic waveguiding medium representing a thin ferromagnetic film with an array of metallic strips (metallic grating) on its surface are calculated. It is shown that the dispersion characteristics of planar periodic structures based on ferromagnetic films depend, not only on the geometry of the waveguiding system, but also on the surface anisotropy of the initial film.

Multipoint, high time resolution galactic cosmic ray observations associated with two interplanetary coronal mass ejections

Galactic cosmic rays (GCRs) play an important role in our understanding of the interplanetary medium (IPM). The causes of their short timescale variations, however, remain largely unexplored. In this paper, we compare high time resolution, multipoint space‐based GCR data to explore structures in the IPM that cause these variations. To ensure that features we see in these data actually relate to conditions in the IPM, we look for correlations between the GCR time series from two instruments onboard the Polar and INTEGRAL (International Gamma Ray Astrophysical Laboratory) satellites, respectively inside and outside Earth's magnetosphere. We analyze the period of 18–24 August 2006 during which two interplanetary coronal mass ejections (ICMEs) passed Earth and produced a Forbush decrease (Fd) in the GCR flux. We find two periods, for a total of 10 h, of clear correlation between small‐scale variations in the two GCR time series during these 7 days, thus demonstrating that such variations are observable using space‐based instruments. The first period of correlation lasted 6 h and began 2 h before the shock of the first ICME passed the two spacecraft. The second period occurred during the initial decrease of the Fd, an event that did not conform to the typical one‐ or two‐step classification of Fds. We propose that two planar magnetic structures preceding the first ICME played a role in both periods: one structure in driving the first correlation and the other in initiating the Fd.

First analyses of planar magnetic structures associated with the Halloween 2003 events from the Earth to Voyager 1 at 93 AU

We perform the first analyses of planar magnetic structures beyond 5 AU. We also perform the first analyses of planar magnetic structures associated with the Halloween 2003 solar events. Our analyses show that planar magnetic structures (PMS) are associated with the interplanetary manifestation of the Halloween 2003 solar events over a wide range of heliocentric distances, heliolongitudes, and heliolatitudes. PMS are present at all five spacecraft (ACE, Ulysses, Cassini, Voyager 2, and Voyager 1); we investigate at distances ranging from the Earth to 93 AU. Our analyses indicate that generally the planes forming the PMS become better defined as they propagate farther out in the heliosphere. Also generally the cone angle of the PMS normal with respect to the radial direction decreases as the PMS propagate farther out in the heliosphere. This presence of PMS throughout the heliosphere is consistent with large‐scale compressions associated with the interplanetary propagation of shocks and interaction regions and with possible consequences for cosmic ray and energetic particle modulation. In the case of Voyager 2, the PMS occur in the region after the main shock and are associated with enhanced interplanetary magnetic field magnitudes. This post shock region is associated with the modulation of galactic cosmic rays (&gt;70 MeV/nuc). At Voyager 1 there is a “textbook example” of a PMS which also is associated with the modulation of galactic cosmic rays. It is tempting to associate these PMS with the “propagating diffusive barriers” of Wibberenz et al. (2002).