Local Magnetic Configuration Research Articles

Inductive electric fields in the inner magnetosphere during geomagnetically active periods

The present study examines the characteristics of electric fields in the nightside inner magnetosphere during geomagnetically active periods. Electric field and magnetic field measurements made by the Cluster spacecraft on their perigee passes are used. The results are summarized as follows: (1) The duskward electric field component EY tends to be larger in the premidnight sector and off the equator, presumably corresponding to the more frequent occurrence of substorms and boundary layer crossings, respectively. (2) The occurrence distribution of EY is biased positively with an average of 0.6–0.8 mV/m, which reflects enhanced convection at active time. (3) The occurrence distribution of EY is also characterized by extending tails with a standard deviation larger than the twice the average. Although the occurrence ratio decreases sharply with increasing magnitude of EY, ∣EY∣ occasionally exceeds 5 mV/m. (4) The sign of EY is well organized by the change of magnetic field. When the local magnetic configuration becomes more dipolar, EY tends to be positive (duskward), whereas it tends to be negative (dawnward) when the configuration becomes more stretched. (5) As for strong electric fields, EY tends to be proportional to the change of the H magnetic component, and from the induction equation, the typical spatial scale of EY is estimated at 4.2 RE. Results 4 and 5 strongly suggests that those strong electric fields are inductive. However, the corresponding process/phenomenon can be different from event to event. It is also suggested that substorm(‐like) processes inside the ring current effectively intensify the ring current.

The YOHKOH survey of partially occulted flares in hard X-rays

Flares being partially occulted by the solar limb, are the best reservoir of our knowledge about hard X-ray loop-top sources. Recently, the survey of partially occulted flares observed by the RHESSI has been published (Krucker & Lin 2008). The extensive YOHKOH database still awaits such activities. This work is an attempt to fill this gap. Among from 1286 flares in the YOHKOH Hard X-ray Telescope Flare Catalogue, for which the hard X-ray images had been enclosed, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. We found that in most cases the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are co-spatial. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. Such a difference we explain as a consequence of intrinsically dissimilar conditions ruling in coronal parts of flares, in comparison with the footpoints which usually dominate the hard X-ray emission of disk flares. At least two reasons of the difference should be taken into consideration: (1) stronger contamination of hard X-rays with emission of thermal plasma, (2) different mechanism in which non-thermal electrons radiate their energy. A schematic picture, in which thin-target mechanism is responsible for hard X-ray emission of loop-top sources and thick-target mechanism -- for emission of footpoint sources, can be modified by the presence of some coronal thick-target sources. At least a part of them suggests a magnetic trapping. Investigated flares do not respond the global magnetic configuration of the solar corona. For their characteristics conclusive is rather the local magnetic configuration in which they were developed.

Tailward flows with positive BZ in the near‐Earth plasma sheet

The present study examines tailward flows in the central plasma sheet using Geotail measurements at X ≥ −31 RE. A focus is placed on the midnight near‐Earth 8 > X ≥ −15 RE and ∣Y∣ < 5 RE) region. It is found that the BZ component is predominantly positive when the X component of the perpendicular flow velocity is negative and that a substantial portion (37%) of earthward magnetic flux transport is canceled by tailward flows. This ratio is larger than farther down the tail (−15 > X ≥ −31 RE), 25%, suggesting that there is a cause of tailward flows that works favorably in the near‐Earth region. The tailward flow velocity occasionally exceeds 200 km/s. The results of a superposed epoch analysis and case studies of such fast tailward flows are summarized as follows: (1) the typical duration of a fast tailward flow is 1 min; (2) EY is negative, suggesting that the fast tailward flow is an electric drift; (3) the local magnetic configuration tends to become more dipolar; (4) the ion temperature and density increases and decreases, respectively, and there is no significant change in ion pressure; (5) a fast tailward flow is often preceded by a fast earthward flow; and (6) the Y component of the flow velocity is generally smaller than . It is also found that the geosynchronous magnetic field on the night side rarely changes during these fast tailward flows. The rebound of fast earthward flows is suggested as the most plausible cause of fast tailward flows.

Probing the Magnetic Exchange Forces of Iron on the Atomic Scale

Applying magnetic exchange force microscopy with an Fe-coated tip, we experimentally resolve the atomic-scale antiferromagnetic structure of the Fe monolayer on W(001). On the basis of first-principles calculations, using an Fe nanocluster as a tip, we determine the distance dependence of the magnetic exchange forces. Significant relaxation of tip and sample atoms occurs, which depend sensitively on the local magnetic configuration. This shifts the onset of magnetic interactions toward larger separations and facilitates their observation. Implementing a multiatom tip in the calculations and accounting for relaxation effects are crucial to obtain the correct sign and distance dependence of the magnetic exchange interaction. By comparison with our calculations, we show that the experimentally observed contrast is due to a competition between chemical and magnetic forces.

Ferromagnetic and antiferromagnetic arrangements of uranium magnetic moments in non-stoichiometric UNi 1− xGa 1+ x compounds

The local magnetic configurations in the non-stoichiometric UNi 1− x Ga 1+ x ( x = ± 0.03 ) compounds have been investigated by Mössbauer spectroscopy technique. The compounds are characterized by coexistence of ferro- and antiferromagnetic alignments of uranium magnetic moments in the adjacent U–Ni layers. At 5 K, a rather small deviation from the exact 1:1:1 stoichiometry (both excess, as well as deficit, of Ni) leads to the drastic increase in the fraction of the ferromagnetic local configuration (up to 73% at x = + 0.03 and 66% at x = − 0.03 ). The fraction of ferromagnetic domains decreases gradually with temperature. At the vicinity of the Curie temperature, the fraction of the ferromagnetic configuration approaches 33% which corresponds to the complete restoration of the “ideal” antiferromagnetic structure, which is peculiar to the stoichiometric UNiGa compound.

Magnetic interactions and spin structure in composite Fe/Nd2Fe14B systems

Abstract Local magnetic interactions and spin configurations in α -Fe/Nd 2 Fe 14 B composite exchange-spring magnets have been investigated by Mossbauer spectroscopy and the magnetic behaviour was analysed by magnetometry. A new method for the evaluation of the coupling strength between soft and hard magnetic phases is proposed.

From stripe to speckle—the magnetic attraction of nanostructures

We discuss the principles and advantages of soft X-ray resonant magnetic scattering using polarized synchrotron radiation for the study of magnetic domain structures on a length scale from 1 to 1000 nm . This new technique is ideally suited to study magnetic superlattices and magnetic stripe domains. Speckle patterns, obtained using coherent radiation, give information on the local magnetic configuration.

Magnetic Induction Mapping in TEM of Micro- and Nano-Patterned Co/Ni Arrays

Understanding magnetic structures and properties of patterned magnetic films at nanometer length-scale is the area of immense technological and fundamental scientific importance. The patterned magnetic films can be used for magnetic sensing . applications, magnetic recording, magnetoelectronics, microactuators and hybrid magneto-superconducting devices. The optimization of film properties is crucially dependent on the understanding of their magnetic properties, which in turn, become sensitive to the specific geometry and, hence, magnetic configurations of a given system when the elements diminish in size. Recent progress in the field of noninterferometric phase retrieval brings the ordinary Fresnel microscopy to a new quantitative level, capable of recovering both the amplitude and phase of the object from the experimental images [1,2], and thus induction mapping of small magnetic elements with known geometry ranging from micro- to few nanometers in size. The key concept behind this approach is the improvement of phase recovery algorithm derived from the transport-of-intensity (TIE) equation with a fast-solution via Fourier transform. A number of quantitative in-situ TEM magnetization experiments can be realized now with the help of magnetic-field calibrated microscope (see, for example [3]). To demonstrate the practical use of the new approach in TEM magnetic imaging with nanoscale resolution we havemore » prepared several films directly on 3mm TEM-grids: (a) square-patterned magnetic films of Co islands with size of 6 pm (Fig. 1), and (b) nano-patterned arrays of Ni-nanodots (Fig.2) with lateral size about 40nm. The Co-films were prepared in UHV system by electron-beam evaporation of Co through an appropriate mask onto 30-nm-thick Si{sub 3}N{sub 4} membrane. The thickness of magnetic elements was approximately 40 nm as determined by EELS. The array of Ni-nanodots on a carbon membrane was prepared by electron-beam TEM-nanolithography followed by oblique angle deposition. Both types of the patterned arrays for Co and Ni films have been characterized by the TEM/ED methods. They were found to have a polycrystalline microstructure with the average crystallite size {approx}10 and 7 nm respectively. The Co films consisted of mixture of cubic and hcp phases. To experimentally check the sensitivity of TIE-recovered phase information to local magnetic configurations a set of in-focus and out-of-focus images was recorded on CCD (llarlk) during the in-situ magnetizing experiments in JEOL3000F microscope at different magnifications using the Gatan Imaging Filter (GIF) as function of applied field (H) and/or specimen tilt angle ({var_phi}) under constant external field. The results of the image processing (Figs.1-2), strongly suggest that TIE-phase retrieval method is a powerful tool suitable for local induction mapping B(x,y) of in-plane magnetization of magnetic elements down to few nanometers scale. The method is fast, robust, insensitive to noise, does not require the holographic equipment, and can be applied to a wide class of objects. The quantitative results can be obtained for films of known or uniform thickness.« less

Influence of vacancies on the magnetic structure of UCuGe

The local magnetic moment configurations in the UCu x Ge ( x=0.90, 0.95 and 0.98) and UCuGe y ( y=0.90 and 0.95) non-stoichiometric intermetallic compounds were studied by means of Mössbauer spectroscopy in the temperature range 10–100 K. The 119Sn nuclei in Sn atoms doped on the Ge sites were exploited as a local probe. The vacancies on the Ge sites in UCuGe y seem to have no considerable influence on the original magnetic structure of UCuGe. On the other hand, two ‘new’ magnetic moment configurations, which coexist at low temperatures with the original UCuGe magnetic structure, were observed in the Cu-deficient UCu x Ge compounds. The changes of the mutual orientation of the U magnetic moments in the ‘new’ (perturbed) magnetic structure is due to the vacancy-induced variations of the effective exchange fields, which determines also the temperature evolution of magnetization. The observed anomalous temperature dependence of the hyperfine magnetic field on the 119Sn nuclei is qualitatively interpreted within the model of ‘melting’ of frustrated spins.

Resistance jumps and hysteresis in ferromagnetic wires

A phenomenological theory leading to the principle of minimum heat generation is presented for the recently discovered exotic electric transport in ferromagnetic wires with a domain wall. In our theory, the lifetime (steadiness) of a local magnetic configuration of localized spins trapped by an impurity is assumed to be a decreasing function of the corresponding local temperature. The local temperature is an increasing function of the heat generation rate due to the conduction electron's inelastic scattering induced by the local combination potential of the impurity and the magnetic configuration. As a result, the whole magnetic structure of the system as well as the current distribution is realized as the nonequilibrium steady state to minimize the total heat generation rate of the system. Relying on this phenomenological principle of minimum heat generation, we provide a unified explanation of the negative and positive jumps and of the hysteresis loops observed in the magnetoresistance experiments. Our argument is based on a microscopic calculation as well as the principle. The result shows that the presence of a domain wall sometimes enhances the transmission probability of the conduction electrons through an impurity potential.

High-n ballooning instabilities in toroidally rotating tokamaks

High-n ballooning instabilities are studied with an initial-value code for toroidally rotating tokamaks, where n is a toroidal mode number. The effects of toroidal rotation are classified into two parts: (i) increase of effective pressure gradient due to the centrifugal force of the toroidal flow, and (ii) averaging of local magnetic equilibrium configuration over a period of poloidal angle in the case of finite flow-velocity shear. With the increase of effective pressure gradient in the rigid-rotation case, the growth rate of ballooning mode increases in the low-pressure regime as the toroidal flow velocity is increased, whereas it decreases in the high-pressure regime. The flow-velocity shear generally reduces the growth rate of the high-n ballooning mode by the averaging of the local equilibrium magnetic configuration. However, it is found that the ballooning mode becomes unstable by increasing the flow-velocity shear in a low-aspect-ratio tokamak. This is understood by the change of the local magnetic configuration, and by the changes of both the mode structure and the potential function in the ballooning space.

Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low (≤10 keV) energies

Intense (≤106 cm−2 sr−1 s−1) fluxes of upflowing energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range ∼0.1–10 keV (hydrogen assumed) by the ENA imager PIPPI (Prelude In Planetary Particle Imaging) onboard the Astrid satellite at 1000 km altitude. If a source altitude of 400 km is assumed, the ENA emissions maps to magnetic latitudes 70°–85°extending from dusk over nightside. There were no measurements in the midnight sector below approximately 80°. It is shown that the emissions cannot originate from an isotropically emitting source, which rules out any optical emissions. Local charged particle detection is ruled out on the basis of no correlation with local electron measurements and local magnetic field configuration. The ENA emissions can be explained by a conically pitch angle distributed ion source emitting ENAs in the 110°–140°pitch angle interval (source altitude 400 km). It is suggested, with support from low altitude rocket observations that the ENAs are produced by low altitude (400–600 km) ion conies of several hundreds eV charge exchanging with the upper atmosphere.

Magnetic configuration effects on the TJ-IU torsatron plasma edge turbulence

A study of plasma edge turbulence carried out in the ECRH heated TJ-IU torsatron is presented. Radial profiles of ion saturation current and floating potential, together with the fluctuation levels of these have been evaluated in the plasma edge by means of Langmuir probe arrays. The existence of two different propagation modes in the proximity of the velocity shear layer has been observed. In the plasma bulk side of the limiter radius, high-frequency fluctuations are negligible and only one propagation mode stands. A detailed examination of the data shows the existence of a quasi-coherent mode probably related to the local magnetic configuration. A radial probe scan reveals an increase in the turbulent particle flux for the location of a rational surface as calculated by the VMEC free-boundary 3D equilibrium code.

Magnetic field and plasma properties associated with pressure pulses and magnetic reconnection at the dayside magnetopause

Most of the models for the formation of magnetic flux transfer events (FTEs) at the dayside magnetopause (MP) are based on magnetic reconnection. However, it has been suggested that a wavy magnetopause motion caused by solar wind pressure variations might provide an alternative explanation. Using two‐dimensional MHD simulations, we study the dynamics and the local field and plasma signatures which are caused by the interaction of enhanced pressure regions with a current layer. The results are compared with two‐dimensional magnetic reconnection processes. The magnetopause motion due to the enhanced pressure agrees qualitatively with the suggested model. The earthward or sunward displacement of the magnetopause is driven by the interaction of a fast mode wave with the current sheet. However, the variation of the magnetic field component normal to the current layer BN is largely monopolar and for the considered cases typically smaller than for the reconnection cases. In addition, the magnetic field signatures depend strongly on the propagation direction of the boundary wave relative to the local magnetic field configuration. The normal magnetic field signature reverses sign across the magnetopause for large magnetic shear and propagation along the direction of antiparallel field components. In the reconnection case, the bipolar BN signature has the same polarity on both sides of the current sheet. Differences between magnetic reconnection and pressure driven boundary motions are manifold and systematic differences are summarized which can be used to test statistical ensembles of such events.

Giant Magnetoresistance in Magnetic Layered and Granular Materials

Publisher Summary Magnetoresistance (MR) is the change in electrical resistance of a material in response to a magnetic field. All metals have an inherent, albeit small, MR owing to the Lorentz force that a magnetic field exerts on moving electrons. Although earlier studies reported unusual magnetoresistive effects in layered structures, it was discovered that the application of magnetic fields to atomically engineered materials known as magnetic superlattices greatly reduced their electrical resistance, that is, superlattices had a giant magnetoresistance. Superlattices are a special form of multilayered structures, artificially grown under ultrahigh-vacuum conditions by alternately depositing on a substrate several atomic layers of one element, say, iron, followed by layers of another, such as chromium. The original observation of giant magnetoresistance was made on iron–chromium superlattices with nearly perfect crystallinity, which was grown by molecular beam epitaxy (MBE). Giant magnetoresistance observed in layered and granular structures arises from the dependence of the resistivity on their internal (local) magnetic configuration.

Inversion Lines of Photospheric Magnetic Fields and Solar Corona

(Solar Phys.). Two results were obtained recently at Meudon concerning the metric type III production by flares which both emphasize the importance of the local magnetic configuration involved.