Changes In Magnetic Field Direction Research Articles

Magnetic field caused enhanced absorption and circular dichroism of an achiral plasmonic nanostructure.

In this paper, modulation of light-matter interactions by a magnetic field is used to generate circular dichroism (CD) from an achiral plasmonic nanostructure. Theoretical investigations show an increase in light absorption by the nanostructure in the presence of a magnetic field. The achiral nanostructure exhibits CD in external magnetic field parallel to circularly polarized light (CPL) incidence. The CD emergence is caused by modulation of electron motion to reduced/enhanced frequencies under CPL incidence. Compared to previous studies, in this paper the mechanism of CD emergence, and the physical reasoning behind the change in CD due to change in magnetic field direction and intensity, are explained. CD intensity increases with increasing magnetic field intensity, while CD sign changes on magnetic field direction reversal. Varying structural parameters significantly influences CD intensity. This study can be helpful in magneto-optics and in magneto-chiral applications.

NUMERICAL study of MAGNETO convective Buongiorno nanofluid flow in a rectangular enclosure under oblique magnetic field with heat generation/absorption and complex wall conditions

A mathematical model is presented to analyze the magnetohydrodynamic (MHD) convective flow in a rectangular enclosure. Earlier studies on the Tiwai-Das volume fraction nanofluid model did not consider the Buongiorno nanofluid model. This is the focus of the present analysis which examines the laminar mixed convection magnetohydrodynamic flow of a nanofluid in a differentially heated rectangular enclosure with complex boundary conditions under an inclined magnetic field. Buongiorno's two-component nanofluid model is employed, which incorporates the effects of Brownian motion and thermophoretic diffusion of nanoparticles. Magnetic nanofluids have considerable potential for enhancing transport processes in energy systems such as hybrid fuel cells. The study is essential with heat generation/absorption effects. Additionally, the work is highlighted by the general case of an oblique (inclined) magnetic field. The conservation equations for mass, primary and secondary momentum, energy, and nanoparticle concentration with wall boundary conditions are dimensionless using appropriate scaling transformations. A finite-difference computational scheme known as the Harlow-Welch Marker and Cell (MAC) method is employed to solve the dimensionless nonlinear coupled boundary value problem. A mesh independence study is included. Graphical plots are presented for the impact of key control parameters on streamline contours, isotherm contours, iso-concentration (nanoparticle mass) contours, and local Nusselt number. With heat sink (absorption), the Nusselt number is enhanced in magnitude whereas it is suppressed with heat generation since there is a heat reduction transmitted to the boundary. The configurations of streamlines, isotherms, and iso-concentrations are mostly invariant to magnetic field direction changes. The obtained results show interesting behaviors of the flow and thermal fields, which mainly involve the effect of Brownian motion and thermophoresis parameters, as well as unsteady regimes, depending on specific values of the Schmidt number, Richardson number, and Prandtl numbers. Increasing the Schmidt number induces a contraction in the central cooler zone in the enclosure and also reduces the iso-concentration magnitudes in the central region across the enclosure. The core region of the enclosure heats up as the thermophoresis and Brownian motion parameters rise, pushing the previously cooler top and bottom wall zones further away from the center. There is also a decrease in iso-concentration magnitudes in particular at the upper and lower boundaries at higher values of Brownian motion and thermophoresis parameters. At decreasing buoyancy ratios, the left vortex cell first decelerates while the right vortex cell accelerates. However, when the buoyancy ratio increases, the left vortex cell streamlines magnitudes increase with a contraction in vortex size, while the right cell develops. A Very minor alteration is observed in the isotherm and iso-concentration contours with an increasing buoyancy ratio. When the Richardson number increases, the vortex cell structures shift from a strong circulation cell on the left to a weaker cell on the right, resulting in reverse distribution. With the rising Richardson number, significant cooling is also caused in the core zone, as well as a drop in iso-concentrations, with the original dual low-concentration upper and lower zones merging into a single center zone. The original symmetric left and right vortex cells are gradually twisted diagonally towards the right wall as the magnetic field increases, yet the stronger right cell and the weaker left cell are maintained.

Magnetoreceptory Function of European Robin Retina: Electrophysiological and Morphological Non-Homogeneity.

The avian magnetic compass allows orientation during migration and is shown to function properly under short-wavelength but not long-wavelength visible light. Therefore, the magnetoreceptive system is assumed to be light- and wavelength-dependent and localized in the retina of the eye. Putative candidates for the role of primary magnetosensory molecules are the cryptochromes that are known to be expressed in the avian retina and must be able to interact with phototransduction proteins. Previously, we reported that in migratory birds change in magnetic field direction induces significant effects on electroretinogram amplitude in response to blue flashes, and such an effect was observed only in the nasal quadrant of the retina. Here, we report new electroretinographic, microscopic and microspectrophotometric data on European robins, confirming the magnetosensitivity of the retinal nasal quadrant after applying the background illumination. We hypothesized that magnetoreceptive distinction of this region may be related to its morphology and analyzed the retinal distribution and optical properties of oil droplets, the filtering structures within cones. We found that the nasal quadrant contains double cones with the most intensely colorized oil droplets compared to the rest of the retina, which may be related to its magnetosensory function.

Coronal-jet-producing Minifilament Eruptions as a Possible Source of Parker Solar Probe Switchbacks

Abstract The Parker Solar Probe (PSP) has observed copious rapid magnetic field direction changes in the near-Sun solar wind. These features have been called “switchbacks,” and their origin is a mystery. But their widespread nature suggests that they may be generated by a frequently occurring process in the Sun’s atmosphere. We examine the possibility that the switchbacks originate from coronal jets. Recent work suggests that many coronal jets result when photospheric magnetic flux cancels, and forms a small-scale “minifilament” flux rope that erupts and reconnects with coronal field. We argue that the reconnected erupting-minifilament flux rope can manifest as an outward propagating Alfvénic fluctuation that steepens into an increasingly compact disturbance as it moves through the solar wind. Using previous observed properties of coronal jets that connect to coronagraph-observed white-light jets (a.k.a. “narrow CMEs”), along with typical solar wind speed values, we expect the coronal-jet-produced disturbances to traverse near-perihelion PSP in ≲25 minutes, with a velocity of ∼400 km s−1. To consider further the plausibility of this idea, we show that a previously studied series of equatorial latitude coronal jets, originating from the periphery of an active region, generate white-light jets in the outer corona (seen in STEREO/COR2 coronagraph images; 2.5–15 R ⊙), and into the inner heliosphere (seen in Solar-Terrestrial Relations Observatory (STEREO)/Hi1 heliospheric imager images; 15–84 R ⊙). Thus it is tenable that disturbances put onto open coronal magnetic field lines by coronal-jet-producing erupting-minifilament flux ropes can propagate out to PSP space and appear as switchbacks.

What magnetospheric and ionospheric researchers should know about the solar wind

Multiple properties of the solar wind that affect the earth are summarized in this overview. 13 specific solar-wind parameters that influence geomagnetic activity, the plasma sheet, the ionosphere, and the electron radiation belt are catalogued: these include the wind speed, number density, magnetic-field strength, field orientation, Mach number, Alfven speed, electron-strahl intensity, and magnetic-field fluctuation amplitude. The solar wind at Earth is categorized into four types of plasma emanating from different morphological features on the solar surface and the systematic differences between the four plasma types is assessed. The solar-cycle behavior of the solar wind is examined in terms of the changing occurrence rates of the four plasma types emanating from the changing Sun. A 27-day periodicity in the properties of the solar wind and in the reaction of the Earth are investigated: the periodicity is owed to features on the rotating Sun. The strong intercorrelations between solar-wind variables are considered and the origins of the intercorrelations are explained. Large-scale structures in the solar wind are reviewed and the different types of geomagnetic storms that they drive are summarized. The mesoscale magnetic structure of the solar wind is examined, the impact on Earth of its current sheets and cellular structure is explored, and a non-randomness in variations of the magnetic-field direction changes at Earth is demonstrated. Sudden strong velocity shears in the solar wind are discussed and their impact on the magnetosphere is shown via a global MHD simulation. The magnetic structure moving outward from the Sun faster than the plasma outflow is discussed, and it is pointed out that the velocity of the magnetic structure should be used when calculating the motional electric field of the Sun. Finally, drawbacks to monitoring the solar wind from L1 are identified.

Dynamic field line draping at comet 67P/Churyumov-Gerasimenko during the Rosetta dayside excursion

Context. The Rosetta dayside excursion took place in September–October 2015 when comet 67P/Churyumov-Gerasimenko (67P/CG) was located at ~1.36 AU from the Sun after it had passed perihelion on 13 August 2015 at ~1.25 AU. At this time, the comet was near its most active period, and its interaction with the solar wind was expected to be at its most intense, with ion pickup and magnetic field line draping. The dayside excursion was planned to move through different regions that were expected upstream of the cometary nucleus, and to possibly detect the location of the bow shock. Aims. The goal of this study is to describe the dynamic field line draping that takes place around the comet and the plasma processes that are connected to this. Methods. The data from the full Rosetta Plasma Consortium (RPC) were used to investigate the interaction of solar wind and comet, starting from boxcar-averaged magnetic field data in order to suppress high-frequency noise in the data. Through calculating the cone and clock angle of the magnetic field, we determined the draping pattern of the magnetic field around the nucleus of the comet. Then we studied the particle data in relation to the variations that are observed in the magnetic field. Results. During the dayside excursion, the magnetic field cone angle changed several times, which means that the magnetic field direction changes from pointing sunward to anti-sunward. This is caused by the changing directions of the interplanetary magnetic field that is transported toward the comet. The cone-angle direction shows that mass-loading of the interplanetary magnetic field of the solar wind leads to dynamic draping. The ion velocity and the magnetic field strength are correlated because the unmagnetized ions are accelerated more (less) strongly by the increasing (decreasing) magnetic field strength. There is an indication of an anticorrelation between the electron density and the magnetic field strength, which might be caused by the magnetized electrons being mirrored out of the strong field regions. The Rosetta RPC has shown that (dynamic) draping also occurs as mildly active comets, as was found at highly active comets such as 1P/Halley and 21P/Giacobini-Zinner, but also that determining both dynamic and nested draping will require a combination of fast flybys and slow excursions for future missions.

TWISTED MAGNETIC FLUX TUBES IN THE SOLAR WIND

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70$^0$, but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.

Observational evidence of Alfvén wings at the Earth

The solar wind at the orbit of the Earth is usually strongly super‐Alfvénic and super‐fast, causing a bow‐shock to be formed upstream of the Earth's magnetosphere. We here present observational evidence that during 24 and 25 May 2002, the solar wind at the Earth was sub‐Alfvénic (with an Alfvén Mach number as low as 0.4 in the rest frame of the Earth) and was therefore sub‐fast for time periods of up to four hours. The low Alfvén Mach number implies that the Earth's bow‐shock disappeared and two Alfvén wings formed. These Alfvén wings are two structures on both the East and West side of the Earth's magnetosphere, where the solar wind plasma is decelerated and the magnetic field direction changes. We present observations of the Geotail spacecraft, which are consistent with Geotail entering the foot of one of these Alfvén wings. We estimate that these wings reached an extension of 600 RE. Even though Alfvén wings are present at several moons in the solar system (e.g., Io, Europa, Enceladus) and are likely to occur at some extrasolar planets, this is the first time that they are observed at the Earth. We also study how the Earth is affected by this transition from a super‐fast to a sub‐Alfvénic environment and how the Alfvén wings are affected by the constantly varying solar wind. The sub‐Alfvénic solar wind is due to very low density in the solar wind. While the solar wind Alfvén Mach number was very low, the magnetosphere was geomagnetically extremely quiet. Whereas the SYM‐H index indicates a recovery phase from a small to moderate magnetic storm; the AL and AU indices show no substorm activity. In addition, there was almost no auroral activity.

Can disordered radical pair systems provide a basis for a magnetic compass in animals?

A proposed mechanism for magnetic compasses in animals is that systems of radical pairs transduce magnetic field information to the nervous system. One can show that perfectly ordered arrays of radical pairs are sensitive to the direction of the external magnetic field and can thus operate, in principle, as a magnetic compass. Here, we investigate how disorder, inherent in biological cells, affects the ability of radical pair systems to provide directional information. We consider biologically inspired geometrical arrangements of ensembles of radical pairs with increasing amounts of disorder and calculate the effect of changing the direction of the external magnetic field on the rate of chemical signal production by radical pair systems. Using a previously established signal transduction model, we estimate the minimum number of receptors necessary to allow for detection of the change in chemical signal owing to changes in magnetic field direction. We quantify the required increase in the number of receptors to compensate for the signal attenuation through increased disorder. We find radical-pair-based compass systems to be relatively robust against disorder, suggesting several scenarios as to how a compass structure can be realized in a biological cell.

Magnetic signatures of Jupiter's bow shock during the Cassini flyby

The bow shock crossings detected during the Cassini flyby of Jupiter occur at positions which imply that Jupiter's bow shock was relatively inflated during the early (before 2001, Day 3) and late (after 2001, Day 40) periods of the encounter. A variety of magnetic shock signatures are seen, corresponding to a wide range (15–75 degrees) in the angle θBN between the shock normal and the solar wind field. The time variability of this angle for the “forward” part of the shock (near the nose) is generally consistent with a shock surface similar to that derived by Slavin et al. [1985] from Voyager/Pioneer data. This implies a long‐term stability for the global shape of the Jovian bow shock. Fits of Slavin‐type models to the shock crossing locations indicate that the solar wind pressure varied by an order of magnitude during the early phase of the Cassini flyby (up to 2001, Day 18) and that the bow shock itself was consequently in a highly dynamic state. A preliminary comparison of Cassini magnetometer (MAG) data with solar wind measurements by the Cassini Plasma Spectrometer (CAPS) reveals that the relation between the shock stand‐off distance and the solar wind pressure during the Cassini flyby was similar to that during the Voyager/Pioneer encounters. This indicates that the intrinsic “compressibility” of the Jovian magnetosphere is also probably stable over long timescales. The change in magnetic field direction between solar wind and magnetosheath indicates that the Jovian bow shock is fast magnetosonic, as expected for the high solar wind Mach numbers (>10) at the location of Jupiter.

Multiple flux rope magnetic ejecta in the solar wind

Four magnetic flux rope events observed by Advanced Composition Explorer (ACE) in the solar wind at 1 AU are examined by the Grad‐Shafranov reconstruction technique. Their 2D cross sections obtained from magnetic field and plasma measurements show configurations of multiple, nonsymmetric, cylindrical flux ropes embedded within their boundaries. Each boundary is defined in consistency with two‐dimensional magnetohydrostatic theory. Among the four events, three have double flux rope configurations, and the fourth is a triplet. Parameters characterizing the events are derived. The orientation of cylindrical axis is obtained, and associated uncertainty is estimated. All events have unidirectional axial magnetic field and axial currents. The total axial magnetic flux is of the order ∼1012 Wb, and the total axial current is of the order ∼108 A. Each flux rope in the doubled flux rope pair and the triplet has the same chirality as its partner(s). Magnetic X points are formed between adjacent flux ropes on the plane perpendicular to the cylindrical axis. One event with abrupt change in magnetic field direction and another related to energetic particles are discussed.

Sub-nanosecond non-Arrhenius magnetic switching in perpendicular multilayers

Measurements are presented of sub-nanosecond magnetization reversal processes of perpendicularly magnetized (CoCr/Pt) multilayers using real-time Kerr microscopy. The multilayers are placed on a ring-type recording head which is used as a sub-ns risetime field source. In the gap of the write head the magnetic field direction changes rapidly from in-plane to out-of-plane. By scanning the laser beam across the gap the influence of the field angle on the reversal process is studied. The reversal time depends critically on the field angle, and faster switching occurs close to the gap (pole edge) due to the increase of the in-plane field. On the time scale of the study, the rotational (non-Arrhenius) switching seems to dominate.

Detachment and instability of cometotail

The two types of comet tails (ion and dust) present different morphological structures whose origin has been reviewed in this paper. The change of magnetic field direction during sector boundary crossings is known to be the source of blob formation in cometotail regions. In this paper it is shown that in the absence of sector boundary changes, the predominant velocity shear on the lower and upper side of the cometotail may result into the instability and blob formation. The growth rate of the instability for different conditions of plasma tail has been studied. It is shown that the onset of instability depends on the shape and thickness of the ion cometotail. In the extended thick ion cometotails, the onset of Kelvin-Helmholtz (K-H) instability is less likely whereas in the thin and extended tail, the K-H instability mechanism is more effective in structuring the ion cometotails into blobs.

Molecular hydrogen polarization images of OMC-1

An image of the polarization of the shocked H2 v = 1-0 S(1) line emission in the core of OMC-1 has been obtained. Along the molecular outflow of the source, the line is dichroically polarized by a medium of aligned grains located between the earth and the shock fronts. The polarization pattern traces the magnetic field direction, which is parallel to the outflow axis and to the large-scale field direction determined from far-IR continuum measurements. Close to the IR source IRc2, the likely source of the outflow, the aligned vectors twist, indicating that the magnetic field direction changes. Modeling the line ratios of scattered H2 lines in the reflection nebula, it is concluded that the size distribution of grains there is typical of the small grains in the diffuse interstellar medium. By contrast, the scattered continuum radiation from the core region suggests that the grains there are larger than this.

Field‐aligned current signatures in the near‐tail region: 1. ISEE observations in the plasma sheet boundary layer

Signatures of field‐aligned currents in the near‐tail region are examined by using ISEE magnetometer data. We focus our attention on steplike changes of magnetic field direction, which are interpreted as the spacecraft crossing of field‐aligned current sheets. Individual case studies of two substorms on March 22, 1979 (CDAW 6 events), indicate that the field‐aligned current signatures in the near‐tail region are consistent with observations at lower altitudes and that the currents in the plasma sheet boundary layer can be identified as parts of the large‐scale current system, the region 1 system. Statistical properties of the field‐aligned currents are also studied. Earthward and tailward currents tend to be observed in postmidnight and in premidnight, respectively, as expected from the distribution of current direction in the region 1 system. The present result seems to be inconsistent with that of Elphic et al. (1985). This apparent discrepancy is due to the difference in identification of field‐aligned current signatures. They have analyzed field‐aligned currents at the edge of the plasma sheet boundary layer, while we examine large‐scale currents in the present analysis. It is thus suggested that small‐scale field‐aligned currents with opposite polarities are imbedded poleward of the region 1 system. The results of these two studies are considered to be complementary to each other. The dependence of the current intensity on the geomagnetic activity and the radial distance from the Earth indicates that there are two field‐aligned current sources in the magnetotail. One is in the region RXY = (X²GSM +X²GSM)1/2 ≥ 15 Re, and the other is nearer to the Earth, RXY < 15 Re. The distant current source persistently exists and the current intensity is almost independent of the activity of the magnetosphere. On the other hand, the near‐earth current source plays an active role during substorms.

Shocks as the origin of radio-source variability

We describe a model for recent, highly polarized, centimetre-wavelength outbursts of the source BL Lacertae in which "pistons" drive shocks in a collimated, relativistic flow. The compression of a random magnetic field by the shocks produces the polarization, and very weak shocks (M of order 1) are capable of explaining the observations. Short-lived steep spectrum components and short-lived localized changes in magnetic-field direction, expected in this scenario, are exhibited by other sources, suggesting that the model for BL Lacertae is of general applicability.

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

Interplanetary magnetic clouds At 1 AU

Magnetic clouds are defined as regions with a radial dimension ≈ 0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component of nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty‐five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface‐associated clouds may have been swept up by corotating streams. Shock‐associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure, and the sum is higher than the pressure of the material outside of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alfvén speed.

Plasma electron observations in the vicinity of magnetic holes

A search of Mariner 10 electron plasma and magnetic field measurements has revealed 14 examples of narrow, well‐defined interplanetary field depressions. Eleven of these events are associated with electron enhancements, five with significant magnetic field direction changes, and six without. Thus even though such magnetic depressions usually coincide with electron enhancements at energies of ∽100 eV, these enhancements cannot always be the result of magnetic reconnection. It seems likely that some events involve local acceleration, possibly the result of reconnection, while others may involve electrons injected onto the field line at a remote point.