Showing papers on "Dipole model of the Earth's magnetic field published in 2003"

Martian magnetic morphology: Contributions from the solar wind and crust

Abstract: [1] Mars Global Surveyor (MGS) Magnetometer (MAG) data provide constraints on magnetic morphology at Mars, including the relative importance of the solar wind and of crustal magnetic sources. We analyze MAG data to characterize the upstream interplanetary magnetic field (IMF) and confirm trends in the magnetic field expected from the solar wind interaction with a planetary atmosphere, including increases at the shock and magnetic pile-up boundary (MPB), postshock turbulence, and field line draping around the Martian obstacle. Crustal magnetic sources locally modify the solar wind interaction, adding variability to the Martian magnetic environment that depends on planetary rotation. We identify trends in the vector magnetic field with respect to altitude, solar zenith angle, and planetary location. Crustal sources influence the magnetic field to different altitudes above different regions, and the influence of the strongest source extends to 1300–1400 km. The draped IMF partially controls the field topology above crustal sources, and crustal magnetic field lines reconnect to this field in a systematic fashion that depends upon Mars' geography, IMF strength, and IMF orientation.

Stellar evolution with rotation and magnetic fields. I. The relative importance of rotational and magnetic effects

Abstract: We compare the current effects of rotation in stellar evolution to those of the magnetic field created by the Tayler instability. In stellar regions, where a magnetic field can be generated by the dynamo due to differential rotation (Spruit 2002), we find that the growth rate of the magnetic instability is much faster than for the thermal instability. Thus, meridional circulation is small with respect to the magnetic fields, both for the transport of angular momentum and of chemical elements. Also, the horizontal coupling by the magnetic field, which reaches values of a few 10 5 G, is much more important than the effects of the horizontal turbulence. The field, however, is not sufficient to distort the shape of the equipotentials. We impose the condition that the energy of the magnetic field created by the Taylcr-Spruit dynamo cannot be larger than the energy excess present in the differential rotation. This leads to a criterion for the existence of the magnetic field in stellar interiors. Numerical tests are made in a rotating star model of 15 M⊙ rotating with an initial velocity of 300 km s - 1 . We find that the coefficients of diffusion for the transport of angular momentum by the magnetic field are several orders of magnitude larger than the transport coefficients for meridional circulation and shear mixing. The same applies to the diffusion coefficients for the chemical elements; however, very close to the core, the strong μ-gradient reduces the mixing by the magnetic instability to values not too different from the case without magnetic field. We also find that magnetic instability is present throughout the radiative envelope, with the exception of the very outer layers, where differential rotation is insufficient to build the field, a fact consistent with the lack of evidence of strong fields at the surface of massive stars. However, the equilibrium situation reached by a rotating star with magnetic field and rotation is still to be ascertained.

Systematic bias in interstellar magnetic field estimates

Abstract: Faraday rotation of the polarization plane in magnetized thermal plasma provides one of the most efficient methods to deduce regular magnetic fields from radio astronomical observations. Since the Faraday rotation measure RM is proportional to an integral, along the line of sight, of magnetic field weighted with thermal electron density, RM is believed to yield the regular magnetic field averaged over large volume. Here we show that this is not the case in a turbulent medium where fluctuations in magnetic field and electron density are not statistically independent, and so contribute to RM. For example, in the case of pressure equilibrium, magnetic field can be anticorrelated with plasma density to produce a negative contribution. As a result, the strength of the regular magnetic field obtained from RM can be underestimated if the fluctuations in electron density and magnetic field are neglected. The anticorrelation also reduces the standard deviation of RM. We further discuss the effect of the positive correlations where the standard treatment of RM leads to an overestimated magnetic field. Because of the anisotropy of the turbulent magnetic field, the regular magnetic fields strength, obtained from synchrotron emission using standard formulae, can be overestimated. A positive correlation between cosmic-ray number density and magnetic field leads to an overestimate of the strengths of the regular and total fields. These effects can explain the difference between the strengths of the regular Galactic magnetic field as indicated by RM and synchrotron emissivity data and reconcile the magnetic field strength in the Solar vicinity with typical strength of regular magnetic fields in external galaxies.

Electric fields that "arrive" before the time derivative of the magnetic field prior to major earthquakes.

Abstract: The low frequency electric signals (emitted from the focal area when the stress reaches a critical value) that precede major earthquakes, are recorded at distances approximately 100 km being accompanied by magnetic field variations. The electric field "arrives" 1 to 2 s before the time derivative of the horizontal magnetic field. An explanation, which is still awaiting, should consider, beyond criticality, the large spatial scale as well as that the transmission of the electromagnetic fields (through an inhomogeneous weakly conductive medium like the Earth) obeys diffusion type equations.

Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands

Abstract: Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the "G band," a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic structures, thus confirming the model of radiatively heated magnetic flux concentrations. These results provide a firm basis for observational studies of the evolution and dynamics of the small-scale solar magnetic field derived through "proxy magnetometry" with G-band images.

Lepton Acceleration by Relativistic Collisionless Magnetic Reconnection

Abstract: We have calculated self-consistent equilibria of a collisionless relativistic electron-positron gas in the vicinity of a magnetic X-point. For the considered conditions, pertinent to extragalactic jets, we find that leptons are accelerated up to Lorentz factors Γ0 = κeB0L2/mc2 1, where B0 is the typical magnetic field strength, ≡ E0/B0, with E0 the reconnection electric field, L is the length scale of the magnetic field, and κ ≈ 12. The acceleration is due to the dominance of the electric field over the magnetic field in a region around the X-point. The distribution function of the accelerated leptons is found to be approximately dn/dγ ∝ γ-1 for γ Γ0. The apparent distribution function may be steeper than γ-1 due to the distribution of Γ0 values and/or the radiative losses. Self-consistent equilibria are found only for plasma inflow rates to the X-point less than a critical value.

Solar wind coupling to and predictability of ground magnetic fields and their time derivatives

Abstract: [1] A spatial and dynamic study of solar wind driving of high-latitude ground magnetic fields and their time derivatives is given. Nonlinear, data-derived basis functions are used to determine the optimal solar wind driven dynamic coupling function at high-latitude locations. The method does not assume a solar wind driving function a priori so that an unbiased determination of the relative influence of different driving processes can be made. Using this method, the locations where high-latitude models capture signatures of different driving processes, such as reconnection and the Kelvin-Helmholtz instability, are revealed for both the amplitude of the field and its time derivative. The time rate of change of the field is a disturbance measure related to geomagnetically induced currents. For the amplitude of the ground magnetic field the primary driver is reconnection; the time rate of change of the field has signatures of both reconnection driving in the nightside sector and Kelvin-Helmholtz driving in the prenoon sector. The independent influence of the solar wind ion density and azimuthal component of the magnetic field in driving magnetic field fluctuations is found to be small at auroral-zone latitudes. The data-derived coupling functions are also used to estimate the expected prediction error of a general class of models that specify the ground magnetic field or its time derivative given solar wind plasma measurements. Prediction efficiencies as small as zero and as large as 0.7 for both the amplitude of the field and its time derivative are possible. The prediction efficiency is highly dependent on spatial location and the direction of field being predicted.

Profile of an Average Magnetic Cloud at 1 au for the Quiet Solar Phase: Wind Observations

Abstract: Using WIND magnetic field (MFI) and plasma (SWE) data, an `average' profile of an interplanetary magnetic cloud was developed in terms of five physical (scalar) quantities based on appropriately selected individual clouds. The period of study was from early 1995 to late in 1998, primarily during the quiet part of a solar cycle. The physical quantities are: magnetic field magnitude, proton density, solar wind bulk speed, proton thermal speed, and proton plasma beta. Selection of the clouds was based on two considerations: (1) their `quality', determined objectively from the application of a static magnetic field model of cloud field structure, had to be good, and (2) distant spacecraft approaches from the cloud axes were not accepted. Nineteen clouds resulted out of 35 original cases. A superposed epoch analysis was performed on the 5 parameters generating summary profiles of a generic magnetic cloud at 1 AU. The density within the generic magnetic cloud reached a distinct minimum near the center and peaked in the trailing part (closest to Sun) after a slow rise. The individual clouds fall into two classes, those that have such an enhanced density feature (about \(\frac{1}{2}\) of them) and those that have an overall nearly flat density profile. For the first 85% of the generic magnetic cloud the bulk speed decreased almost uniformly by 45 km s−1 indicating marked expansion over 1 AU. The field intensity peaked very near the cloud's center but was noticeably asymmetric. Proton thermal speed was quite symmetric with local maxima at the front, center, and rear. Proton plasma beta was low throughout the cloud (0.12 on average), but had a broad minimum at its center. The relative degree of fluctuation level for the parameters ranged from the most quiet for both speed and field magnitude, to the most `noisy' for proton plasma beta, with fluctuations in density and thermal speed at intermediate levels, all being below 0.2, based on a sample-scale of frac1100 of the cloud duration. These profiles may be useful in constraining future structural and thermodynamic models of clouds with regard to their solar birth conditions and interplanetary evolution.

The magnetic field in the pile-up region at Mars, and its variation with the solar wind

Abstract: [1] The magnetic measurements from the Mars Global Surveyor satellite are used to study the magnetic field on the Martian dayside, and its variation with the solar wind. Because of the lack of solar wind measurements near Mars, solar wind measurements near Earth during a period centered on a Mars-Earth conjunction are used. Concurrent variations atMarsandEarthrelatedtotheinterplanetarysector-structure and dynamic pressure variations are demonstrated. The study is confined to the northern hemisphere of Mars in regions where the crustal anomalies are weak. Here we find a close association between the solar wind dynamic pressure and the magnetic pressure in the pile-up region, and also a strong asymmetry with the Interplanetary magnetic field (IMF) By-component, probably related to solar wind pick-up of planetary ions. INDEX TERMS: 2780 Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 5443 Planetology: Solid Surface Planets: Magnetospheres (2756); 2134 Interplanetary Physics: Interplanetary magnetic fields; 6225 Planetology: Solar SystemObjects: Mars.Citation: Vennerstrom,

The rotation of planar-2 to planar-12 dust clusters in an axial magnetic field

Abstract: Dust clusters containing from one up to 12 particles arranged in a horizontal plane were formed in an inductively coupled rf plasma. When an axial magnetic field was applied to the dust cluster, the cluster rotated as a rigid body in the left-handed direction with respect to the field. The cluster rotation occurred at a magnetic field of tens of gauss in our experiment, which was two to three orders less than that used in previous experiments and that predicted by the models in the literature. In particular, the angular velocity dependence on magnetic field strength varied with number of particles in the cluster and with the structural configuration of the cluster. Other rotational properties such as cluster radius, angular momentum and threshold magnetic field were measured. The dependence of the radial confinement electric field on the magnetic field was also obtained. Finally, comparisons were made between our experimental results and various existing theoretical models. Possible explanations for angular velocity saturation and periodic pauses for the planar-2 configuration, which were observed in our experiments, are given.

Reanalysis of Saturn's magnetospheric field data view of spin‐periodic perturbations

Abstract: [1] Periodic perturbations with their period close to that of planetary rotation are observed in most of the magnetic field data from Saturn's magnetosphere. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). The long-held view that no planetary spin-periodic modulation was present in the magnetic field observations is thus not true. Here we present several new pieces of information obtained from a careful analysis of the magnetic field data in view of this peculiar periodic feature. First, by simple considerations of the magnetic field morphology, we argue that these perturbations cannot be directly due to the planetary intrinsic field. Also, we analyze by means of two-dimensional (2-D) hodographs the rotation of the magnetic field vector, expressed in an inertial planetocentric spherical polar coordinate system, and obtain a definitive argument against the possibility of a dipole tilt signature. In addition, we find that the inbound and outbound Pioneer 11 observations of the perturbations are nearly in phase (once the spatial distance is accounted for), indicating that this periodic feature is of a global nature. Finally, we discuss the fact that in the same data set, the magnetopause position in the dawn sector seems to be modulated in phase with the radial component of the perturbation magnetic field.

Magnetic modelling and tomography: First steps towards a consistent reconstruction of the solar corona

Abstract: We undertake a first attempt towards a consistent reconstruction of the coronal magnetic field and the coronal density structure. We consider a stationary solar corona which has to obey the equations of magnetohydrostatics. We solve these equations with help of a newly developed optimization scheme. As a first step we illustrate how tomographic information can be included into the reconstruction of coronal magnetic fields. In a second step we use coronal magnetic field infor- mation to improve the tomographic inversion process. As input the scheme requires magnetic field measurements on the photosphere from vector-magnetographs and the line-of-sight integrated den- sity distribution from coronagraphs. We test our codes with well-known analytic magnetohydrostatic equilibria and models. The program is planned for use within the STEREO mission. The solar magnetic field is an important quantity which couples the solar interior, the photosphere and the atmosphere. The quasi stationary coronal magnetic field configuration is an interesting and challenging topic on its own right. But even to understand basic processes like coronal mass ejections and flares it is important to understand the quiescent magnetic configuration out of which these dynamic phenomena arise. Unfortunately the coronal magnetic field cannot be measured di- rectly, but it has to be reconstructed from photospheric measurements. A magnetic field reconstruction of the solar corona has to be consistent with the observed spa- tial variation of the coronal plasma (density, pressure, temperature) often elongated along the magnetic field. Here we are mainly interested in long-living structures which are time- independent in first order. We also concentrate on closed magnetic configurations where a stationary plasma flow (solar wind) does not significantly contribute to the force balance. Such configurations are static equilibria and have to obey the magnetohydrostatic equations (MHS). As the magnetic field B and the density distribution N are physically closely related their model reconstruction should also be linked as much as possible. In this paper we attempt to show how this can be achieved. We propose variational principles which if they can be solved should give a consistent model for an isother-

2.5‐D fluid simulations of the solar wind interacting with multiple dipoles on the surface of the Moon

Abstract: [1] Initial two-dimensional (2-D) MHD simulations indicated that mini-magnetospheres can form around magnetic anomalies on the surface of the Moon but required magnetic field strengths at 100 km above the surface an order of magnitude larger than in situ measurements. Modeling the lunar magnetic anomalies with multiple dipoles in 2.5-D MPD simulations inflates the size of the mini-magnetospheres for only small increases in the magnitude of the total magnetic field. Multiple dipoles increase the lateral distance over which solar wind plasma is held off the surface. This extended magnetic field geometry inflates the mini-magnetosphere by inhibiting fluid flow within the shock region. With multiple dipoles, a mini-magnetosphere will form with magnetic field magnitudes smaller than the lower limit for a single dipole. These results indicate that the higher order moments of the anomalous magnetic fields play a significant role in deflecting the solar wind and determining the size and shape of the mini-magnetosphere.

Transformation of VLF anomaly maps into apparent resistivity and phase

Abstract: We investigate a transformation of magnetic transfer functions into the tangential‐electric mode part of the impedance tensor in the scope of the plane‐wave electromagnetic tensor–VLF method. The transformation, which is applicable to any 2D data representing the response of arbitrary 3D geoelectric structures, overcomes the difficulties of quantitative interpretation of magnetic transfer functions, which predominantly provide a measure of the lateral changes of the electrical conductivity in the earth. We require densely sampled magnetic transfer functions of one frequency as input data. These may be decomposed into their normal and anomalous parts (deviation from the response of a layered earth) for a unit external plane‐wave source field using the Hilbert transform relationship between the magnetic field components. Faraday's law then directly provides the anomalous toroidal electric field. Unfortunately, there is no chance to estimate the normal electric field from magnetic data, since the magnetic fi...

Measurement of internal magnetic field fluctuations in a reversed-field pinch by Faraday rotation.

Abstract: Magnetic field fluctuations (and the associated current perturbation) have been measured in the core of a high-temperature reversed-field pinch using a newly developed fast-polarimetry system. Radial magnetic field fluctuation levels of � 1% are measured in standard-reversed-field pinch discharges which increase to � 4% during the sawtooth crash (enhanced dynamo). The fluctuation level is reduced fourfold for high-confinement plasmas where the core-resonant tearing modes are suppressed. In many laboratory and natural plasmas, the magnetic field fluctuates spontaneously in space and time. Magnetic fluctuations can have a large effect on the macroscopic behavior of plasmas. Two effects are of particular importance. First, magnetic fluctuations can cause magnetic field lines to wander stochastically. Energy and particle transport can then arise from particles streaming along the stochastic magnetic field lines. Second, fluctuations can generate a large-scale magnetic field through current driven by a dynamo effect. Energy transport and dynamo effects from magnetic fluctuations are especially crucial to the reversed-field pinch (RFP) laboratory plasma configuration. It has long been considered that the transport arises from stochasticity induced by overlapping magnetic islands, and that the current density profile is partly determined by the dynamo [1]. In addition, improved confinement in the RFP, induced by control of the current density profile, has been conjectured to be related to the reduction of magnetic fluctuations within the plasma [2 ‐ 4]. Understanding transport and dynamo physics requires measurement of the fluctuating magnetic field inside the high-temperature plasma. Magnetic probes have been used to measure the edge magnetic field fluctuations, as well as the internal fluctuation radial profiles in smaller, colder RFP plasmas [5,6]. Until now, however, direct nonperturbing measurement of the magnetic fluctuations in the hot plasma core has been lacking in the RFP, as well as in other toroidal configurations. In this Letter, we report the first measurement of magnetic field fluctuations (and the related current density fluctuations) in the core of a high-temperature RFP. Their relation to transport and dynamo in the RFP is investigated. This is accomplished through measurement of the Faraday rotation of an injected far-infrared laser beam using a new high-speed polarimeter system. We report three new results. First, we find that broadband internal magnetic fluctuations grow rapidly at the sawtooth crash, indicating their relation to the dynamo. The change in fluctuation amplitude correlates with the relaxation of the core current density profile and is consistent with the standard model of MHD relaxation and dynamo in the RFP. Second, fluctuating current associated with the dominant magnetic islands is measured and found to have a spatial extent � 8c mabout the rational surface. The localization of the current, while the magnetic fluctuation is radially global, is consistent with the theoretical expectation for tearing modes. Third, we find that in plasmas with improved confinement the long-wavelength core magnetic fluctuation amplitude decreases fourfold. The decrease in core magnetic fluctuations is larger than has been previously inferred from measurements at the plasma surface.

Nonlinear magnetic diffusion and magnetic helicity transport in galactic dynamos

Abstract: We have extended our previous mean-field galactic dynamo model which included algebraic and dynamic alpha nonlinearities (Kleeorin et al. 2002), to include also a quenching of the turbulent diffusivity. We readily obtain equilibrium states for the large-scale magnetic field in the local disc dynamo model, and these fields have strengths that are comparable to the equipartition field strength. We find that the algebraic nonlinearity alone (i.e. quenching of both the α effect and turbulent magnetic diffusion) cannot saturate the growth of the mean magnetic field; only the combined effect of algebraic and dynamic nonlinearities can limit the growth of the mean magnetic field. However, in contrast to our earlier work without quenching of the turbulent diffusivity, we cannot now find satisfactory solutions in the no-z approximation to the axisymmetric galactic dynamo problem.

Modelling of the Electromagnetic Field in the Interplanetary Space and in the Earth’s Magnetosphere

Abstract: A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented. In this approach the magnetopause and the Earth’s bow shock are approximated by the paraboloids of revolution. Superposition of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the observational data for September 24–26, 1998 magnetic storm (Dst min = -205 nT) and substorm occurred at 02:30 UT on January 10, 1997.

A Quantitative Method to Optimise Magnetic Field Line Fitting of Observed Coronal Loops

Abstract: Many authors use magnetic-field models to extrapolate the field in the solar corona from magnetic data in the photosphere. The accuracy of such extrapolations is usually judged qualitatively by eye, where a less judgemental quantitative approach would be more desirable. In this paper, a robust method for obtaining the best fit between a theoretical magnetic field and intensity observations of coronal loops on the solar disk will be presented. The method will be applied to Yohkoh data using a linear force-free field as an illustration. Any other theoretical model for the magnetic field can be used, provided there is enough freedom in the model to optimize the fit.

On the distribution of magnetic null points above the solar photosphere

Abstract: Many theories predict magnetic energy dissipation at locations, called null points, where the magnetic field vanishes. In several astrophysical contexts, most notably the solar corona, energy is released within a low-β magnetic field anchored to a lower boundary, the photosphere. A general expression is derived for the distribution of magnetic null points in potential magnetic fields anchored to a random, homogeneous distribution of field on the lower boundary. For all such fields the null point density decreases with height and scales with the inverse cube of the field’s characteristic length. For photospheric fields which appear unipolar at the largest scales the nulls are confined to a narrow layer. The results are applied to models of the quiet Sun whose photospheric field consists of discrete sources of mixed polarity. The number of coronal nulls depends on the degree of imbalance between positive and negative sources. Numerical experiments reveal that the greatest column density of null points occur...

Solar magnetic field reversal as seen at Ulysses

Abstract: The rapid motion of the Ulysses spacecraft from high southern to high northern latitudes in 2000–2001 provided an excellent opportunity to make inferences regarding the solar magnetic dipole's behaviour around solar maximum. A simple dipole model is fitted to Ulysses measurements of the polarity of the heliospheric magnetic field mapped back to the solar wind source surface. Although higher order components of the field are ignored, the gradual reversal in orientation of the dipole field component can be followed during solar maximum, with the dipole axis crossing the solar equator during early 2000–early 2001. The dipole appears to exhibit a rotation at a slower rate than the Carrington frame of reference, similar to previous measurements made around solar maximum in the solar equatorial regions.

Nonaxisymmetric Instabilities of a Toroidal Magnetic Field in a Rotating Sphere

Abstract: It has been suggested that nonaxisymmetric instabilities of an axisymmetric toroidal magnetic field, independent of the existence of convective turbulence, provide an important dynamo effect for stars when the convective turbulence is largely suppressed by the magnetic field. We investigate analytically such three-dimensional magnetic instabilities arising from a purely toroidal magnetic field whose strength is proportional to distance from the rotation axis of a star. A three-dimensional perturbation approach is used in our stability analysis. The leading-order problem neglects dissipative effects and describes two- or three-dimensional hydromagnetic waves in spherical geometry. For the first time we are able to obtain general explicit solutions for the problem. Interesting properties of the hydromagnetic waves are revealed by the explicit solutions. We investigate the effect of ohmic dissipation on the stability of the toroidal magnetic field by first deriving the magnetic boundary layer solution and then matching it onto the interior leading-order solution. Furthermore, for the first time we are able to derive an explicit analytic expression for the growth rate of the magnetic instabilities. We find that the most unstable mode of the instabilities is always three-dimensional and characterized by small radial and latitudinal scales. Implications of these magnetic instabilities for the solar dynamo are discussed.

Problems and Progress in Astrophysical Dynamos

Abstract: Astrophysical objects with negligible resistivity are often threaded by large scale magnetic fields. The generation of these fields is somewhat mysterious, since a magnetic field in a perfectly conducting fluid cannot change the flux threading a fluid element, or the field topology. Classical dynamo theory evades this limit by assuming that magnetic reconnection is fast, even for vanishing resistivity, and that the largescale field can be generated by the action of kinetic helicity. Both these claims have been severely criticized, and the latter appears to conflict with strong theoretical arguments based on magnetic helicity conservation and a series of numerical simulations. Here we discuss recent efforts to explain fast magnetic reconnection through the topological effects of a weak stochastic magnetic field component. We also show how mean-field dynamo theory can be recast in a form which respects magnetic helicity conservation, and how this changes our understanding of astrophysical dynamos. Finally, we comment briefly on why an asymmetry between small-scale magnetic and velocity fields is necessary for dynamo action, and how it can arise naturally.

Relaxation of writhe and twist of a bi-helical magnetic field

Abstract: In the past few years suggestions have emerged that the solar magnetic field might have a bi-helical contribution with oppositely polarized magnetic fields at large and small scales, and that the shedding of such fields may be crucial for the operation of the dynamo. It is shown that, if a bi-helical field is shed into the solar wind, positive and negative contributions of the magnetic helicity spectrum tend to mix and decay. Even in the absence of turbulence, mixing and decay can occur on a time scale faster than the resistive one provided the two signs of magnetic helicity originate from a single tube. In the presence of turbulence, positively and negatively polarized contributions mix rapidly in such a way that the ratio of magnetic helicity to magnetic energy is largest both at the largest scale and in the dissipation range. In absolute units the small scale excess of helical fields is however negligible.

A radiation belt‐ring current forecasting model

Abstract: [1] A radiation belt-ring current (RB-RC) forecasting model is presented. This model solves the convection-diffusion equation of plasma distribution in the 10 keV to a few MeV range. There are four major auxiliary components to the RB-RC model: a global magnetic field model, an electric field model, a plasma sheet model (plasma source), and a radial diffusion model. All four components are driven by solar wind and interplanetary magnetic field conditions. In this paper, a brief description of the model and input parameters is given. This model has been used to simulate several geomagnetic storms. In particular, the effects of the inductive electric field on the evolution of the radiation belt electron fluxes are investigated in detail via a case study of the 12 August 2000 storm. It is found that, in general, the inductive electric field arising from the time-varying magnetic field can enhance the flux level around geosynchronous orbit during the recovery phase of the storms. The model is validated through comparing the simulation results with the Los Alamos National Laboratory satellite measurements. Further refinement and improvement of the model is also discussed.

The Geometry of Turbulent Magnetic Fluctuations at High Heliograpahic Latitudes

Abstract: Although the orientation of wave vectors for interplanetary magnetic fluctuations is often discussed or assumed when modelling the solar wind turbulence, there were few if any direct measurements of the wave vector orientation until very recently. Indirect inferences abound. For instance, the transverse nature of magnetic fluctuations has often been used to infer a wave vector parallel to the mean magnetic field, but transverse fluctuations do not necessarily lead to parallel wave vectors — the fluctuations of two‐dimensional turbulence are fully tranverse, but their wave vectors are also transverse to the mean field. Bieber et al. [3] offer the first single‐point algorithm for separating field‐aligned wave vectors from perpendicular wave vectors using the component spectra. This method has since been used by others [5, 6, 8, 9] and is here applied to Ulysses measurements in a comparison of high latitude turbulence with the results of previous studies of near‐ecliptic observations.

Characteristics of Scatter-free Behavior of Heliospheric Pickup Ions

Abstract: Theoretical analyses of the global heliospheric pickup ion population generally involve the solution of forms of the Boltzmann or transport equation. Like their cosmic-ray counterparts, these approaches often presume a simplified geometry for the interplanetary magnetic field and that diffusive processes dominate the already picked up particles' behavior. At the opposite extreme lies the test particle picture of pickup ions, wherein their motion is tracked from the birth of the ions through their outward travel in the Parker spiral magnetic field. This approach has the advantage that it can include details of the source distribution, the initial pickup by the solar wind convection electric field at the site of ion production, and the combined effects of the radially evolving electric and magnetic fields experienced by the ions. Here test particles representing O+ pickup ions are used to examine the influences of the source characteristics, including the heliocentric radius of their birth and the velocities of their parent neutrals. The consequences for ion populations from interstellar, interplanetary dust, and Jovian sources are considered. The results suggest some potentially observable features of general pickup ion distributions that could aid in the interpretation of their different sources.