Spiral Magnetic Field Research Articles

Reduction of boundary effects in the spiralMRI experiment PROMISE

AbstractMagnetorotational instability (MRI) is one of the most important and most common instabilities in astrophysics. It is widely accepted that it serves as a source of turbulent viscosity in accretion disks – the most energy efficient objects in the Universe. However it is very difficult to bring this process down on earth and model it in a laboratory experiment. Several different approaches have been proposed, one of the most recent is PROMISE (Potsdam‐ROssendorf Magnetorotational InStability Experiment). It consists of a flow of a liquid metal between two rotating cylinders under applied current‐free spiral magnetic field. The cylinders must be covered with plates which introduce additional end‐effects which alter the flow and make it more difficult to clearly distinguish between MRI stable and unstable state. In this paper we propose simple and inexpensive improvement to the PROMISE experiment which would reduce those undesirable effects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

East‐west asymmetry in coronal mass ejection geoeffectiveness

This paper extends the domain of applicability of the Gosling‐McComas space weather forecast rule that applies to the postshock sheaths of fast coronal mass ejections at Earth (ICMEs). The rule is based on the draping of the sheath magnetic field around the ICME body. The original treatment considered only the radial‐from‐the‐Sun component of the preshock interplanetary magnetic field (IMF), which implied that the domain of applicability of the rule was the entire sheath region ahead of the leading face of the ICME. We show here that because of the generally prevailing Parker spiral orientation of the IMF, the domain of applicability of the rule is instead generally strongly shifted to the east side of the ICME sheath. We suggest that the eastward shift of the domain of applicability of the rule accounts for an observed greater geoeffectiveness of west hemisphere CMEs compared with east hemisphere CMEs. The approach used here to demonstrate the eastward shift of the region of potential ICME sheath geoeffectiveness, and thus to increase the accuracy of the forecast rule, is to present intensity contours of the geoeffective draping component of the IMF as computed by global MHD simulations. Since the shift depends only on a spiral magnetic field and a blunt object to drape it around, we demonstrate the generality of the principle on which the rule is based by treating both the case of the ICME and the case of Earth's magnetosphere.

Torsional Magnetic Oscillations in Type I X‐Ray Bursts

Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer of the star, ΔR(t). The layer rotates slower than the star due to angular momentum conservation. The shear between the star and the layer acts to twist the star's dipole magnetic field, giving at first a trailing spiral field. The twist of the field acts in turn to torque up the layer, increasing its specific angular momentum. As the layer cools and contracts, its excess specific angular momentum causes it to rotate faster than the star, which gives a leading spiral magnetic field. The process repeats, giving rise to torsional oscillations. We derive equations for the angular velocity and magnetic field of the layer, taking into account the diffusivity and viscosity that are probably due to turbulence. The magnetic field causes a nonuniformity of the star's photosphere (at the top of the heated layer), and this gives rise to the observed X-ray oscillations. The fact that the layer periodically rotates faster than the star means that the X-ray oscillation frequency may overshoot the star's rotation frequency. Comparison of the theory is made with observations of Chakrabarty et al. of an X-ray burst of SAX J1808.4-3658.

Modeling of the three-dimensional motion of toroidal magnetic clouds in the inner heliosphere

Context. The motion of a magnetic cloud through the heliosphere is governed by three main forces, viz. the diamagnetic force, the drag force, and gravity. Some recently derived formulas enabling the calculation of the ambient magnetic field around a toroidal magnetic cloud are applied to calculate the diamagnetic force acting on the cloud and to determine the cloud dynamics. Aims. The aim is to determine the three dimensional velocity profiles and the trajectory of the magnetic cloud, as well as the evolution of the orientation of the cloud axis from the calculated moment of the force. Methods. The method applied in this study consists of three steps. First, the r-component of the magnetic field at r = 2.5 R s is derived from a spherical harmonic analysis. Next, the field distribution in the entire heliosphere, including the spiral structure, is reconstructed in a way that is consistent with this boundary condition at r = 2.5 R s as well as with actual measurements at 1 AU. Then, a toroid is launched at a point obtained from solar observations of a specific event and the initial size, orientation, and velocity of this toroid is estimated from these observational data as well. Results. The three dimensional velocity profiles and the trajectory of the magnetic cloud, as well as the evolution of the orientation of the cloud axis have been determined for a toroidally shaped cloud moving in the interplanetary medium taking into account a spiral magnetic field.

Fast 3D spatial EPR imaging using spiral magnetic field gradient

Electron paramagnetic resonance imaging (EPRI) provides direct detection and mapping of free radicals. The continuous wave (CW) EPRI technique, in particular, has been widely used in a variety of applications in the fields of biology and medicine due to its high sensitivity and applicability to a wide range of free radicals and paramagnetic species. However, the technique requires long image acquisition periods, and this limits its use for many in vivo applications where relatively rapid changes occur in the magnitude and distribution of spins. Therefore, there has been a great need to develop fast EPRI techniques. We report the development of a fast 3D CW EPRI technique using spiral magnetic field gradient. By spiraling the magnetic field gradient and stepping the main magnetic field, this approach acquires a 3D image in one sweep of the main magnetic field, enabling significant reduction of the imaging time. A direct one-stage 3D image reconstruction algorithm, modified for reconstruction of the EPR images from the projections acquired with the spiral magnetic field gradient, was used. We demonstrated using a home-built L-band EPR system that the spiral magnetic field gradient technique enabled a 4–7-fold accelerated acquisition of projections. This technique has great potential for in vivo studies of free radicals and their metabolism.

Radial dependence of proton peak intensities and fluences in SEP events: Influence of the energetic particle transport parameters

We study the radial dependence of peak intensities and fluences of solar energetic particle (SEP) events in the framework of the focused transport theory. We solve the focused-diffusion transport equation that includes the effects of solar wind convection, adiabatic deceleration and pitch-angle scattering. We assume a Reid–Axford time profile for the particle injection at the base of a flux tube described by an Archimedean spiral magnetic field whose cross section A( r) expands as r 2 cos( ψ), where r is the radial distance and ψ( r) is the angle between the magnetic field line and the radial direction. We assume that energetic particles propagate along the field line. We locate several observers along the flux tube at radial distances ranging from 0.3 to 1.6 AU. Both peak intensities and event fluences decrease with increasing radial distance. We deduce functional forms to extrapolate peak fluxes and fluences with radial distance that depend on the energy of the particles, the pitch-angle scattering conditions, and the duration of the particle injection. The smaller the mean free path of the particles, the larger the decrease of both peak intensities and fluences with radial distance. The smaller the energy of the particles, the larger the decrease of both peak intensities and fluences with radial distance. Extended particle injections contribute to soften the decrease of the peak intensities with the radial distance but have no influence on the event fluence. We note that mobile particle sources (i.e. traveling interplanetary shocks) may vary the radial dependences deduced in this work.

Do cosmic rays drive jets?

A sudden release of high energy cosmic rays at the centre of a wind sustaining a spiral magnetic field produces cavities of low density and low magnetic field along the axis. The trajectories of high energy cosmic rays are focussed onto the axis, and lower energy cosmic rays and thermal plasma can escape through the cavities. This may explain the jets often seen in accretion systems and elsewhere. The formation of astrophysical jets and the origin of ultra-high energy cosmic rays are two outstanding problems in astrophysics. Here, they are brought together in a model in which cosmic rays generated at the centre of an accretion system initiate jet formation through their interaction with a magnetized wind above the accretion disk. Supersonic motion and shocks accompanying accretion are a natural source of cosmic rays. Low energy cosmic rays are initially confined near the shocks, but ultra-high energy cosmic rays, escaping into the magnetized wind, push aside the plasma to form cavities of low density and low magnetic field on the rotation axis. The trajectories of the highest energy cosmic rays are focussed onto the axis, and lower energy cosmic rays and thermal plasma may then escape through the cavities to form relativistic jets.

Venus–solar wind interaction: Asymmetries and the escape of [formula omitted] ions

We study the interaction between Venus and the solar wind using a global three-dimensional self-consistent quasi-neutral hybrid (QNH) model. The model treats ions ( H + , O + ) as particles and electrons as a massless charge neutralizing fluid. In the analysed Parker spiral interplanetary magnetic field (IMF) case ( IMF = [ 8.09 , 5.88 , 0 ] nT ) , a notable north–south asymmetry of the magnetic field and plasma exists, especially in the properties of escaping planetary O + ions. The asymmetry is associated with ion finite gyroradius effects. Furthermore, the IMF x-component results in a dawn–dusk asymmetry. Overall, the QNH model is found to reproduce the main observed plasma and magnetic field regions (the bow shock, the magnetosheath, the magnetic barrier and the magnetotail), implying the potential of the developed model to study the Venusian plasma environment and especially the non-thermal ion escape.

Localized “Jets” of Jovian electrons observed during Ulysses’ distant Jupiter flyby in 2003–2004

We report observations of strongly anisotropic flows of electron events originating from Jupiter that were observed during Ulysses distant Jupiter flyby. A scan of the period January 1, 2003 through February 8, 2005 (extending roughly 13 months before and after the closest approach of 0.8 AU in early 2004) identified a total of 15 events with significant and short-lived (hours) intensity increases accompanied by strongly anisotropic flows away from Jupiter. We describe the properties of these events, most of which were observed within 1.4 AU of Jupiter. We also discuss their implications for large-scale direct magnetic connections across the mean magnetic field and suggest that they may provide evidence for a previously unconsidered way of enhancing propagation of energetic charged particles across the average Parker spiral heliospheric magnetic field, thus increasing the apparent rate of latitudinal and radial cross-field diffusion.

Relativistic Solar Protons on 1989 October 22: Injection and Transport along Both Legs of a Closed Interplanetary Magnetic Loop

Worldwide neutron monitor observations of relativistic solar protons on 1989 October 22 have proven puzzling, with an initial spike at some stations followed by a second peak, which is difficult to understand in terms of transport along a standard Archimedean spiral magnetic field or a second injection near the Sun. Here we analyze data from polar monitors, which measure the directional distribution of solar energetic particles (mainly protons) at rigidities of � 1‐3 GV. This event has the unusual properties that the particle density dips after the initial spike, followed by a hump with bidirectional flows and then a very slow decay. The spectral index, determined using bare neutron counters, varies dramatically, with energy dispersion features. The density and anisotropy data are simultaneously fit by simulating the particle transport for various magnetic field configurations and determining the best-fit injection functionneartheSun.ThedataarenotwellfitforanArchimedeanspiralfield,amagneticbottleneckbeyondEarth,or particle injection along one leg of a closed magnetic loop. A model with simultaneous injection along both legs of a closed loop provides a better explanation: particles moving along the near leg make up the spike, those coming from thefarlegmakeupthehump,bothlegscontributetothebidirectional streaming,andtrappingintheloopaccountsfor the slow decay of the particle density. Refined fits indicate a very low spectral index of turbulence, q < 1, a parallel mean free path of 1.2‐2.0 AU, a loop length of 4:7 � 0:3 AU, and escape of relativistic protons from the loop on a timescale of 3 hr. The weak scattering is consistent with reports of weak fluctuations in magnetic loops, while the low q-value may indicate a smaller correlation length as well.

The Effects of a Local Interstellar Magnetic Field onVoyager 1and2Observations

We show that that an interstellar magnetic field can produce a north/south asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements, we suggest that the angle $\alpha$ between the interstellar wind velocity and magnetic field is $30^{\circ} < \alpha < 60^{\circ}$. The distortion of the shock is such that termination shock particles could stream outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within $\sim 2~AU$ of the shock. The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within $\sim 5~AU$ of the shock, but with particles from the shock streaming inward along the field. Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years.

Insulation recovery characteristics after current interruption by various vacuum interrupter electrodes

Insulation recovery characteristics after current interruption for a vacuum interrupter were investigated. We used spiral and axial magnetic field electrodes for comparison of the electrode structure, while CuBi and CuCr were also used for comparison of the contact material. This paper describes how the axial magnetic field electrode and CuCr (50%) performed best in our experimental data. In addition, we discussed reasons for the variation in characteristics between electrodes.

The stability of MHD Taylor‐Couette flow with current‐free spiral magnetic fields between conducting cylinders

AbstractWe study the magnetorotational instability in cylindrical Taylor‐Couette flow, with the (vertically unbounded) cylinders taken to be perfect conductors, and with externally imposed spiral magnetic fields. The azimuthal component of this field is generated by an axial current inside the inner cylinder, and may be slightly stronger than the axial field. We obtain an instability beyond the Rayleigh line, for Reynolds numbers of order 1000 and Hartmann numbers of order 10, and independent of the (small) magnetic Prandtl number. For experiments with Rout = 2Rin = 10 cm and Ωout = 0.27 Ωin, the instability appears for liquid sodium for axial fields of ∼20 Gauss and axial currents of ∼1200 A. For gallium the numbers are ∼50 Gauss and ∼3200 A. The vertical cell size is about twice the cell size known for nonmagnetic experiments. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Blazars with spine-sheath structures

Multi-frequency (5, 8, 15 GHz) total intensity and polarization VLBA images of three sources from a complete sample of BL Lac objects (0745+241, 1418+546, and 1652+398) are presented together with global VLBI images of the quasar 1055+018 at 1.6 and 5 GHz. These images show signs of a spiral magnetic field in the central channel of the jet, as well as a region of longitudinal magnetic field in “sheaths” at the jet edges, which may indicate an interaction between the jet and the surrounding medium on spatial scales of several parsecs.

Low‐density anomalies and sub‐Alfvénic solar wind

During 10–12 May 1999, the solar wind density dropped to an anomalously low value of ∼0.1 cm−3. The density depletion occurred in the midst of relatively slow wind flow, in between faster flows, and was apparently associated with neither a coronal mass ejection nor a fast corotating stream. While the magnetic field intensity did not show any notable variation across the density depletion, plasma analyzers on the ACE and Wind spacecraft revealed an abnormally strong nonradial flow component with an azimuthal speed that peaked at ∼100 km s−1. Usmanov et al. [2000b] suggested that the density anomaly was, in fact, a rarefaction at the trailing edge of relatively fast flow that formed as a result of suppression of coronal outflow from a region that earlier provided fast wind flow. The suppression in turn may have resulted from a rapid restructuring of solar magnetic fields during the polar field reversal. Here we show results from a two‐dimensional time‐dependent MHD simulation applied to the helioequatorial plane. The initially longitude‐independent Parker solar wind and Archimedean spiral magnetic field are disturbed by a low‐velocity/high‐density jump on an inner computational boundary at 20 R⊙. We follow the development and propagation of the rarefaction to Earth orbit and compare pseudo‐time series with near‐Earth spacecraft measurements. We show that a strong rarefaction can develop behind the fast flow and that simulation results and spacecraft observations are generally in agreement. The simulated radial magnetic field shows a relatively small variation across the density anomaly compared with that of the density. The stream interaction generates strong azimuthal velocities in the slow flow region, as observed. The simulation shows a sub‐Alfvénic flow region embedded within the low‐density region that does not extend all the way back to the Sun but which has become disconnected as the depletion propagates to Earth orbit. We discuss also the correlation between low‐density and sub‐Alfvénic events in the solar wind as inferred from spacecraft observations using the OMNI 2 data set from 1963 to 2003.

Reconnection at the heliopause

In this MHD-model of the heliosphere, we assume a Parker-type flow, and a Parker-type spiral magnetic field, which is extrapolated further downstream from the termination shock to the heliopause. We raise the question whether the heliopause nose region may be leaky with respect to fields and plasmas due to non-ideal plasma dynamics, implying a breakdown of the magnetic barrier. We analyse some simple scenarios to find reconnection rates and circumstances, under which the heliosphere can be an “open” or a “closed” magnetosphere. We do not pretend to offer a complete solution for the heliosphere, on the basis of non-ideal MHD theory, but present a prescription to find such a solution on the basis of potential fields including the knowledge of neutral points. As an example, we imitate the Parker-spiral as a monopole with a superposition of homogeneous asymptotical boundary conditions. We use this toy model for x < − R , where R = 100 AU is the distance of the termination shock to describe the situation in the nose region of the heliopause.

Modeling the effects of pitch-angle scattering processes on the transport of solar energetic particles along the interplanetary magnetic field

We present a Monte-Carlo technique to study the time-dependent transport of energetic particles in the interplanetary medium. We use the guiding center approximation between discrete finite pitch-angle scatterings to quantify the competing effects of focusing and pitch-angle scattering on energetic particles propagating along a Parker spiral magnetic field. We consider that the pitch-angle scattering process is produced by small-scale magnetic field irregularities frozen in the expanding solar wind. We also include the effects of both solar wind convection and adiabatic deceleration. We use a joint probability distribution P( h, μ′) = p( h; μ′) q( μ′; μ) to describe both the distance traveled by the particle between two scattering processes and the change in the particle pitch-angle after a scattering process. Here, p( h; μ′) is the conditional probability that the particle travels a distance h along the field line before the next scattering if it had a pitch-angle cosine μ′ after the previous scattering, and q( μ′; μ) is the conditional probability for the pitch-angle cosine μ′ if the pitch-angle cosine was μ before the scattering. We consider several functional forms to describe the processes of pitch-angle scattering, such as an isotropic scattering without any memory of the initial particle’s pitch-angle or processes in which the scattering result depends upon the initial particle’s pitch-angle. The results of our simulations are pitch-angle distributions and time-intensity profiles that can be directly compared to spacecraft observations. Comparison of our simulations with near-relativistic (45–290 keV) electron events observed by the Electron, Proton and Alpha Monitor on board the Advanced Composition Explorer allows us to estimate both the time dependence of the injection of near-relativistic electrons into the interplanetary medium and the conditions for electron propagation along the interplanetary magnetic field.

Interplanetary magnetic field control of dayside transient event occurrence and motion in the ionosphere and magnetosphere

Abstract. The pressure pulse model for dayside transient ionospheric events predicts dawnward moving events at and prior to local noon during periods of spiral interplanetary magnetic field (IMF) orientation, but duskward moving events at and after local noon during rarer periods of orthospiral IMF orientation. We use this model to interpret ground and geosynchronous magnetometer observations of a duskward-moving transient event that occurred on 10 August 1995 during a period of orthospiral IMF orientation. We then survey geosynchronous GOES-8, 9, and 10 magnetometer observations to determine the directions of motion for 67 isolated magnetic impulse events seen in South Pole magnetograms from 1995-1999. The occurrence patterns and directions of motion inferred from both case and statistical studies are consistent with pressure pulse model predictions. Key words. Interplanetary physics (Interplanetary magnetic fields; discontinuities) – Magnetospheric physics (Magnetosphere-ionosphere interactions)

Deflection of coronal mass ejection in the interplanetary medium

A solar coronal mass ejection (CME) is a large-scale eruption of plasma and magnetic fields from the Sun. It is believed to be the main source of strong interplanetary disturbances that may cause intense geomagnetic storms. However, not all front-side halo CMEs can encounter the Earth and produce geomagnetic storms. The longitude distribution of the Earth-encountered front-side halo CMEs (EFHCMEs) has not only an east–west (E–W) asymmetry (Wang et al., 2002), but also depends on the EFHCMEs' transit speeds from the Sun to 1 AU. The faster the EFHCMEs are, the more westward does their distribution shift, and as a whole, the distribution shifts to the west. Combining the observational results and a simple kinetic analysis, we believe that such E–W asymmetry appearing in the source longitude distribution is due to the deflection of CMEs' propagation in the interplanetary medium. Under the effect of the Parker spiral magnetic field, a fast CME will be blocked by the background solar wind ahead and deflected to the east, whereas a slow CME will be pushed by the following background solar wind and deflected to the west. The deflection angle may be estimated according to the CMEs' transit speed by using a kinetic model. It is shown that slow CMEs can be deflected more easily than fast ones. This is consistent with the observational results obtained by Zhang et al. (2003), that all four Earth-encountered limb CMEs originated from the east. On the other hand, since the most of the EFHCMEs are fast events, the range of the longitude distribution given by the theoretical model is E40°,W70°, which is well consistent with the observational results (E40°,W75°).

Numerical Study on the Propagation of Ultra–High‐Energy Cosmic Rays in the Galactic Magnetic Field

We study the propagation of ultra-high-energy cosmic rays (UHECRs) with E > 1018 eV in the Galactic magnetic field (GMF). We present numerical simulations for the propagation of antiprotons from the Earth toward the outside of the Galaxy in the GMF and calculate the sky map of the position of antiprotons that have reached the boundary. This sky map is interpreted as the relative probability distribution for the ability of protons to reach the Earth for the case of isotropic source distribution, considering Liouville's theorem. Basically, we adopt a GMF model that is composed of the spiral and dipole magnetic fields. We also consider the propagation of UHECRs in both of these two components of the magnetic field. This enables us to see the effect of each component on the arrival distribution of UHECRs. The main effect of the dipole magnetic field is to deflect antiprotons toward high Galactic longitudes, because this field is directed toward the north Galactic pole in the Galactic plane. In particular, antiprotons injected in the direction ~300° are strongly deflected and then rotated by the strong magnetic field in the vicinity of the Galactic center. The ideal form of the dipole magnetic field causes the clear pattern of the sky map of the positions of antiprotons that have reached the boundary when they are plotted by color according to the rotation number of the orbits. On the other hand, the spiral field mainly deflects antiprotons injected at the Earth toward the north Galactic pole, i.e., toward the Virgo cluster. This is because the spiral field in the solar system is in the direction of l ~ 90°. Even if the dipole magnetic filed is also included, we find that almost all UHECRs come from the direction of the Virgo cluster, because the spiral magnetic field in the solar system is stronger than the dipole field. This result suggests the nuclei component of UHECRs from the Virgo cluster as an attractive possibility for the origin of UHECRs.