Showing papers on "Dipole model of the Earth's magnetic field published in 2007"

Numerical Dynamo Simulations

Abstract: Numerical simulations of the geodynamo that solve the fundamental equations of convection-driven flow and magnetic induction in a rotating spherical shell successfully reproduce many observed features of the geomagnetic field. Most models employ a spectral transform method based on spherical harmonic functions. Numerical dynamos can elucidate the basic mechanism that generates the magnetic field in Earth's core. This chapter also compares in detail the morphology of the magnetic field, its power spectrum and characteristic properties of secular variations in models with that of the Earth's field. Models show that persistent non-axisymmetric features of the geomagnetic field can have their origin in the influence of Earth's mantle on the core.

A semiempirical magnetohydrodynamical model of the solar wind

Abstract: We present a new MHD model for simulating the large-scale structure of the solar corona and solar wind under "steady state" conditions stemming from the Wang-Sheeley-Arge empirical model. The processes of turbulent heating in the solar wind are parameterized using a phenomenological, thermodynamical model with a varied polytropic index. We employ the Bernoulli integral to bridge the asymptotic solar wind speed with the assumed distribution of the polytropic index on the solar surface. We successfully reproduce the mass flux from Sun to Earth, the temperature structure, and the large-scale structure of the magnetic field. We reproduce the solar wind speed bimodal structure in the inner heliosphere. However, the solar wind speed is in a quantitative agreement with observations at 1 AU for solar maximum conditions only. The magnetic field comparison demonstrates that the input magnetogram needs to be multiplied by a scaling factor in order to obtain the correct magnitude at 1 AU.

Magnetospheric configurations from a high-resolution data-based magnetic field model

Abstract: [1] We present first results of the magnetospheric magnetic field modeling, based on large sets of spacecraft data and a high-resolution expansion for the field of equatorial currents. In this approach, the field is expanded into a sum of orthogonal basis functions of different scales, capable to reproduce arbitrary radial and azimuthal variations of the geomagnetic field, including its noon-midnight and dawn-dusk asymmetries. Combined with the existing method to model the global field of Birkeland currents, the new approach offers a natural way to consistently represent the field of both the tail and symmetrical/partial ring currents. The proposed technique is particularly effective in the modeling of the inner magnetosphere, a stumbling block for the first-principle approaches. The new model has been fitted to various subsets of data from Geotail, Polar, Cluster, IMP-8, and GOES-8, GOES-9, GOES-10, and GOES-12 spacecraft, corresponding to different activity levels, solar wind IMF conditions, and storm phases. The obtained maps of the magnetic field reproduce most basic features of the magnetospheric structure, their dependence on the geomagnetic activity and interplanetary conditions, as well as characteristic changes associated with the main and recovery phases of magnetic storms.

Torsional oscillations of relativistic stars with dipole magnetic fields

Abstract: We present the formalism and numerical results for torsional oscillations of relativistic stars endowed with a strong dipole magnetic field, assumed to be confined to the crust. In our approach, we focus on axisymmetric modes and neglect higher order couplings induced by the magnetic field. We do a systematic search of parameter space by computing torsional mode frequencies for various values of the harmonic index l and for various overtones, using an extended sample of models of compact stars, varying in mass, high-density equation of state (EOS) and crust model. We show that torsional mode frequencies are sensitive to the crust model if the high-density EOS is very stiff. In addition, torsional mode frequencies are drastically affected by a dipole magnetic field, if the latter has a strength exceeding roughly 10 15 G and we find that the magnetic field effects are sensitive to the adopted crust model. Using our extended numerical results we derive empirical relations for the effect of the magnetic field on torsional modes as well as for the crust thickness. We compare our numerical results to observed frequencies in soft gamma repeaters and find that certain high-density EOS and mass values are favoured over others in the non-magnetized limit. On the other hand, if the magnetic field is strong, then its effect has to be taken into account in attempts to formulate a theory of asteroseismology for magnetars.

Modelling the Global Solar Corona: Filament Chirality Observations and Surface Simulations

Abstract: The hemispheric pattern of solar filaments is considered in the context of the global magnetic field of the solar corona. In recent work Mackay and van Ballegooijen have shown how, for a pair of interacting magnetic bipoles, the observed chirality pattern could be explained by the dominant range of bipole tilt angles and helicity in each hemisphere. This study aims to test this earlier result through a direct comparison between theory and observations, using newly developed simulations of the actual surface and 3D coronal magnetic fields over a 6-month period, on a global scale. We consider two key components: (1) observations of filament chirality for the sample of 255 filaments and (2) our new simulations of the large-scale surface magnetic field. Based on a flux-transport model, these will be used as the lower boundary condition for the future 3D coronal simulations. Our technique differs significantly from those of other authors, where the coronal field is either assumed to be purely potential or has to be reset back to potential every 27 days for the photospheric field to remain accurate. In our case we ensure accuracy by the insertion of newly emerging bipolar active regions, based on observed photospheric synoptic magnetograms. The large-scale surface field is shown to remain accurate over the 6-month period, without any resetting. This new technique will enable future simulations to consider the long-term buildup and transport of helicity and shear in the coronal magnetic field over many months or years.

Fifth-generation lithospheric magnetic field model from CHAMP satellite measurements

Abstract: [1] Six years of low-orbit CHAMP satellite magnetic measurements have provided an exceptionally high-quality data resource for lithospheric magnetic field modeling and interpretation. Here we describe the fifth-generation satellite-only magnetic field model MF5. The model extends to spherical harmonic degree 100. As a result of careful data selection, extensive corrections, filtering, and line leveling, the model has low noise levels, even if evaluated at the Earth's surface. The model is particularly suited for inferring large-scale structure and composition of the lithosphere. It is also meant to serve as the long-wavelength part of continental- and global-scale marine and aeromagnetic anomaly maps.

Free Magnetic Energy in Solar Active Regions above the Minimum-Energy Relaxed State

Abstract: To understand the physics of solar flares, including the local reorganization of the magnetic field and the acceleration of energetic particles, we have first to estimate the free magnetic energy available for such phenomena, which can be converted into kinetic and thermal energy. The free magnetic energy is the excess energy of a magnetic configuration compared to the minimum-energy state, which is a linear force-free field if the magnetic helicity of the configuration is conserved. We investigate the values of the free magnetic energy estimated from either the excess energy in extrapolated fields or the magnetic virial theorem. For four different active regions, we have reconstructed the nonlinear force-free field and the linear force-free field corresponding to the minimum-energy state. The free magnetic energies are then computed. From the energy budget and the observed magnetic activity in the active region, we conclude that the free energy above the minimum-energy state gives a better estimate and more insights into the flare process than the free energy above the potential field state.

A scale-free analysis of magnetic holes at 1 AU

Abstract: [1] We describe a scale-free search for magnetic holes in the solar wind using Wind magnetometer observations between 1994 and 2004. Using magnetic field and ion measurements on the Wind spacecraft, we present the first statistical study of magnetic hole plasma signatures on the kinetic scale and we evaluate magnetic holes as kinetic and fluid phenomena. Magnetic holes are shown to be pressure-balanced structures with similar properties on all scales. Temperature anisotropy measurements are combined with magnetic field measurements to give direct evidence that magnetic holes observed at 1 AU are stable remnants of magnetic pressure depletions generated in a source region closer to the Sun, likely through the mirror-mode instability.

On the Stokes V Amplitude Ratio as an Indicator of the Field Strength in the Solar Internetwork

Abstract: The results of the determination of magnetic field strength from weak polarimetric signals in solar internetwork regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes V amplitude ratio of the Fe I λλ15652-15648 lines is a good indicator of kG magnetic field concentrations, even for magnetic fields with a complex internal structure like those in MHD simulations. The Stokes V amplitude ratio of the Fe I λλ5247-5250 lines also shows a good correlation with magnetic field strength. However, in simulations with a flux level appropriate for the internetwork, it gives values corresponding to sub-kG fields. The reason is the rapid decrease of the field strength with height in kG magnetic field concentrations. These lines sample high regions of the atmosphere, where the field is already below kG levels. We also find that the Stokes V amplitude ratio of the Fe I λλ6301-6302 lines shows no correlation with the magnetic field strength. The reason lies in the large difference in the heights of formation of these two lines. The value of the magnetic field strength obtained from the Fe I λλ6301 and 6302 lines depends crucially on the treatment of gradients of the magnetic field, line-of-sight velocity, and temperature, even at a numerical spatial resolution of 20 km.

Nonlinear force-free models for the solar corona I. Two active regions with very different structure

Abstract: Context. With the development of new instrumentation providing measurements of solar photospheric vector magnetic fields, we need to develop our understanding of the effects of current density on coronal magnetic field configurations. Aims. The object is to understand the diverse and complex nature of coronal magnetic fields in active regions using a nonlinear forcefree model. Methods. From the observed photospheric magnetic field we derive the photospheric current density for two active regions: one is a decaying active region with strong currents (AR8151), and the other is a newly emerged active region with weak currents (AR8210). We compare the three-dimensional structure of the magnetic fields for both active region when they are assumed to be either potential or nonlinear force-free. The latter is computed using a Grad-Rubin vector-potential-like numerical scheme. A quantitative comparison is performed in terms of the geometry, the connectivity of field lines, the magnetic energy and the magnetic helicity content. Results. For the old decaying active region the connectivity and geometry of the nonlinear force-free model include strong twist and strong shear and are very different from the potential model. The twisted flux bundles store magnetic energy and magnetic helicity high in the corona (about 50 Mm). The newly emerged active region has a complex topology and the departure from a potential field is small, but the excess magnetic energy is stored in the low corona and is enough to trigger powerful flares.

Quantitative estimates of magnetic field reconnection properties from electric and magnetic field measurements

Abstract: Reconnection occurs in a reconnection magnetic field geometry when there are positive electric field components tangential to the magnetopause and a magnetic field component normal to it. Because these three components are the smallest of the six electric and magnetic fields, their magnitudes are difficult to determine because of errors in, or oscillations of, the assumed constant direction normal to the current sheet. A method is described for minimizing these errors by appropriate selection of the normal direction and by analyzing the correlations between the large normal electric field and the large tangential magnetic field. The correlation coefficients are equal to ratios of the small fields, which are combined with the less accurate measurements of the averages of the small fields to produce best estimates of the small fields. For more than 120 magnetopause crossings, about 40% had such correlations that signify static conditions during those crossings. This method is applied to 22 polar subsolar magnetopause crossings to show that most were located in the ion diffusion region, as defined by the change of the total magnetic field, and that 14 had a large and steady reconnection rate with a zero parallel electric field. In these events the reconnection rate decreased with increasing guide magnetic field.

Solar Wind Parameters for Magnetospheric Magnetic Field Modeling

Abstract: [1] Magnetospheric magnetic field models are crucial for many space weather applications However, the latest empirical models require solar wind and IMF data, which are not always available Data gaps are especially common for times before the launch of the WIND spacecraft at the end of 1994, but even after then there are data gaps We present a method to interpolate the solar wind characteristics across data gaps and to evaluate the W parameters needed for the TS05 model (Tsyganenko and Sitnov, 2005) Within some distance from the edge of a data gap, the solar wind parameters from our method yield a better estimate of the observed magnetic field than that which could be found using average values of the parameters Deep within data gaps (far from measured values), the interpolated parameters are reasonable, or typical values, no better or worse than average values We have created a database of hourly data with solar wind characteristics, G, and W parameters from 1963 to 31 May 2007, which is sufficient for use in all the Tsyganenko models, including the latest TS05 model Our comparisons of the model and observed magnetic field at geosynchronous orbit give an estimate of the error in the model field as a function of status parameters defined by the interpolation scheme We also show that the model field is on average just as accurate using the hourly data as that based on 5 min data (at least at geosynchronous orbit)

Gravitational dynamos and the low-frequency geomagnetic secular variation

Abstract: Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions.

Experimental evidence of a zonal magnetic field in a toroidal plasma.

Abstract: A zonal magnetic field is found in a toroidal plasma. The magnetic field has a symmetric bandlike structure, which is uniform in the toroidal and poloidal directions and varies radially with a finite wavelength of mesoscale, which is analogous to zonal flows. A time-dependent bicoherence analysis reveals that the magnetic field should be generated by the background plasma turbulence. The discovery is classified as a new kind of phenomenon of structured magnetic field generation, giving insight into phenomena such as dipole field generation in rotational planets.

Force-free magnetic field extrapolation for MHD boundary conditions in simulations of the solar atmosphere

Abstract: Context. In recent years the accuracy of magnetic field observations in the solar atmosphere has made considerable progress. Similar progress is being made in computer modeling of complex plasma systems and computer capabilities. Aims. To consider observed solar magnetic field structure in numerical simulations a new extrapolation method for solar magnetic fields is used to incorporate such fields into three-dimensional MHD simulations Methods. The simulation employs a new modified linear magnetic field extrapolation which is specifically designed to satisfy symmetry conditions which are generic to MHD models. The model domain includes photosphere, chromosphere, and corona. Results. The new model is applied to several solar field configurations and results are compared to three-dimensional field structure from observations and another extrapolation method. The new model provides a simple and efficient method for the simulation of observed solar magnetic field structures by constructing a three-dimensional initial field that is consistent with symmetry boundary conditions of MHD simulations.

Intermittent character of interplanetary magnetic field fluctuations

Abstract: Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than velocity fluctuations in both fast and slow wind. This behavior has been interpreted in terms of the anomalous scaling observed in passive scalars in fully developed hydrodynamic turbulence. In this paper, the strong intermittent nature of the interplanetary magnetic field is briefly discussed comparing results performed during different phases of the solar cycle. The scaling properties of the interplanetary magnetic field magnitude show solar cycle variation that can be distinguished in the scaling exponents revealed by structure functions. The scaling exponents observed around the solar maximum coincide, within the errors, to those measured for passive scalars in hydrodynamic turbulence. However, it is also found that the values are not universal in the sense that the solar cycle variation may be reflected in dependence on the structure of the velocity field.

Resolution of the 180? Ambiguity for Inverse Horizontal Magnetic Field Configurations

Abstract: A well-known problem in solar physics is that solutions for the transverse magnetic field direction are ambiguous with respect to a 180° reversal in the field direction. In this paper we focus on three methods for the removal of the 180° ambiguity applied to three MHD models. These methods are (1) the reference field method, (2) the method of magnetic pressure gradient, and (3) the magnetic field divergence-free method. All three methods are noniterative, and methods 2 and 3 are analytical and fast. We apply these methods to three MHD equilibrium model fields: (1) an analytical solution of a nonlinear force-free magnetic field equilibrium from Low, (2) a simulation of an emerging twisted flux tube from Fan & Gibson, and (3) a pre-eruptive twisted magnetic flux rope equilibrium reached by relaxation from Amari et al. We measure the success of methods within "inverse horizontal field" regions in the boundary, which are mathematically defined by B⊥ ∇⊥Bz > 0. When such regions overlap with the magnetic field neutral lines, they are known as "bald patches" (BPs) or inverse topology. Our most important conclusion is that the magnetic divergence-free method is far more successful than the other two methods within BPs. This method requires a second level of measurements of the vertical magnetic field. As high-quality multilevel magnetograms will come online in the near future, our work shows that multilayer magnetic field measurements will be highly desirable to objectively and successfully tackle the 180° ambiguity problem.

Magnetospheric Contributions to the Terrestrial Magnetic Field

Abstract: The Earth's magnetic field is created and governed by processes and material in the Earth's interior This field is not restricted to the inside, the surface, or the atmosphere of the Earth, but reaches far above the Earth into space If that space were empty or only populated with neutral gases, there would be no consequences However, that space is not a vacuum but, starting at a height of about 100 km, is filled with ionized gas

A Comparative Study of Radio Wave Propagation Over the Earth Due to a Vertical Electric Dipole

Abstract: A comparative study of the electromagnetic field excited by a vertical electric dipole on the Earth is presented. Four sets of formulas for both the planar Earth model and the spherical Earth model (of large radius) are compared to find out their valid ranges. Numerical computations are also carried out specifically for a three-layered Earth model. For the planar Earth model, when both the source and observation points are on the surface, and the planar Earth covered with a thick-enough dielectric layer, the method by Zhang is more accurate; while for the fields above the surface and the thin-enough dielectric layer, the method by King and Sandler is more accurate. However, the hybrid of the trapped surface wave and the lateral wave were exhibited in the curves in, but they were not shown in the curves. Numerical calculation also shows that the amplitude of the trapped surface wave attenuates as rho-1/2 in the rho direction as expected. However, the lateral wave given by does not exhibit rho-2 decay in the rho direction. For the layered spherical Earth model, the exact series summation, which serves as an exact solution to the classic problem, is computed and compared with the residue series. Numerical results show that the residue series gives a good approximation to the field, but the smooth curve illustrates that the hybrid effect due to the trapped surface wave and the lateral wave was ignored in literature. The field strength of the trapped surface wave decreases with the dielectric layer thickness and is affected by the curvature of the Earth. The exact series shows the oscillation of the field caused by the hybrid effects, which can be considered as the dielectric resonance between the upper and lower dielectric interfaces when it is guided to propagate, but none of the other three approximations can depict the effects.

Astrophysical dynamos and magnetic helicity conservation

Abstract: Large-scale magnetic fields of celestial bodies are thought to be generated by the joint action of differential rotation and what is generally known as the alpha-effect—associated with a violation of mirror-symmetry in rotating convection or turbulence. The Coriolis force acting on rotating vortices in a stratified media results in an excess of right-hand vortices in one hemisphere and left-hand vortices in the other. This asymmetry gives a component of the mean electromotive force parallel to the mean magnetic field (the electromotive force and the electric current are perpendicular to the magnetic field in mirror-symmetric media). This is the famous alpha-effect which plays a key role in magnetic field self-excitation (the so-called mean-field dynamo) and leads to the solar cycle and other phenomena in astrophysical plasma.A new finding in this field is that the nonlinear saturation of the dynamo instability is determined by transport properties of the magnetic helicity, which is an inviscid integral of motion and describes the mirror asymmetry of the magnetic field generated. We discuss first observational results concerning helicity transport in the solar cycle in the context of solar dynamo models.

Magnetic structures in the heliosheath

Abstract: [1] We propose a solitary wave model for small scale magnetic structures observed in the solar wind and more recently in the Voyager 1 observations of the heliosheath. The model is based on the recent, fully nonlinear theory of solitary waves by McKenzie et al. (2001, 2004). Our solutions i.e., magnetic holes, humps, trains of holes and humps, are strongly nonlinear (70 to 80% change in the magnetic field at the centre), propagate at large angles (>60°) to the mean magnetic field and are well approximated by Gaussians. The structures are almost pressure balanced with an anti-correlation between the magnetic field and the plasma density, and no change in the magnetic vector across the structure. These features are consistent with observations of magnetic structures in the heliosheath.

Dynamics of the boundary of the penetration of solar energetic particles to Earth's magnetosphere according to CORONAS-F data

Abstract: The dynamics of the boundary of the penetration of solar energetic particles (electrons and protons) to Earth’s magnetosphere during solar flares and related geomagnetic disturbances in November 2001 and October–November 2003 is analyzed using CORONAS-F data. The relationship between the penetration boundary, the geomagnetic activity indices, and the local magnetic time is investigated. The correlation coefficient between the invariant latitude of the penetration boundary and the Kp and Dst indices for electrons with energies ranging from 0.3 to 0.6 MeV in the dayside sector is demonstrated to be higher than that in the nightside sector. The correlation coefficient for protons with energies from 1 to 5 MeV is higher in the nightside sector as compared to the dayside sector. For protons with energies from 50 to 90 MeV, the correlation is high at all MLT.

Cluster Observations of the Magnetospheric Low-Latitude Boundary Layer and Cusp during Extreme Solar Wind and Interplanetary Magnetic Field Conditions: II. 7 November 2004 ICME and Statistical Survey

Abstract: We present a study of the plasma properties inside and dynamics of the low- latitude boundary layer (LLBL)/cusp during the ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predom- inantly strongly northward, up to 50 nT, with some short-duration rotations. The observed LLBL/cusp is very thick (∼6 - 7° invariant latitude (ILAT)) and migrates equatorward with rates of 0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming from multiple reconnection sites. During a period of extremely north- ward IMF, signatures of pulsed dual reconnection inside the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been suggested pre-

Exoplanet-Induced Chromospheric Activity: Realistic Light Curves from Solar-type Magnetic Fields

Abstract: There is growing observational evidence for some kind of interaction between stars and close-in extrasolar giant planets. Shkolnik et al. reported variability in the chromospheric Ca H and K lines of HD 179949 and upsilon And that seemed to be phased with the planet's orbital period, instead of the stellar rotational period. However, the observations also indicate that the chromospheric light curves do not repeat exactly, which may be expected for a planet plowing through a variable stellar magnetic field. Using the complex solar magnetic field (modeled with the Potential Field Source Surface technique) as a guide, we simulate the shapes of light curves that would arise from planet-star interactions that are channeled along magnetic field lines. We also study the orbit-to-orbit variability of these light curves and how they vary from solar minimum (i.e., a more or less axisymmetric stretched dipole) to solar maximum (a superposition of many higher multipole moments) fields. Considering more complex magnetic fields introduces new difficulties in the interpretation of observations, but it may also lead to valuable new diagnostics of exoplanet magnetospheres.

Lorentz Force Effects on the Orbit of a Charged Artificial Satellite: A New Approach

Abstract: A charged artificial satellite moving relative to a magnetic field accelerates in a direction perpendicular to its velocity and the magnetic field due to the Lorentz force. The geomagnetic field is considered as a multipole potential field and the satellite electrical charged is supposed to be constant. The study is provided to compute Lorentz force acceleration of a charged satellite in Earth’s magnetic field as a function of orbital elements of the satellite. Periodic perturbations in the orbital elements of the satellite are derived using Lagrange planetary equations. Numerical results for a chosen satellites orbit shows the most effects of Lorentz force are in semi major axis, eccentricity, and the longitude of the satellite, but there aren’t any effects of the force on the inclination and the argument of the perigee of the satellite elements.

Magnetic Flux Imbalance of the Solar and Heliospheric Magnetic Fields

Abstract: A comparative analysis of solar and heliospheric magnetic fields in terms of their cumulative sums reveals cyclic and long-term changes that appear as a magnetic flux imbalance and alternations of dominant magnetic polarities. The global magnetic flux imbalance of the Sun manifests itself in the solar mean magnetic field (SMMF) signal. The north – south asymmetry of solar activity and the quadrupole mode of the solar magnetic field contribute the most to the observed magnetic flux imbalance. The polarity asymmetry exhibits the Hale magnetic cycle in both the radial and azimuthal components of the interplanetary magnetic field (IMF). Analysis of the cumulative sums of the IMF components clearly reveals cyclic changes in the IMF geometry. The accumulated deviations in the IMF spiral angle from its nominal value also demonstrate long-term changes resulting from a slow increase of the solar wind speed over 1965 – 2006. A predominance of the positive IMF Bz with a significant linear trend in its cumulative signal is interpreted as a manifestation of the relic magnetic field of the Sun. Long-term changes in the IMF Bz are revealed. They demonstrate decadal changes owing to the 11/22-year solar cycle. Long-duration time intervals with a dominant negative Bz component were found in temporal patterns of the cumulative sum of the IMF Bz.

A conductivity model for the Brazilian equatorial e-region: initial results

Abstract: This paper presents results from a new model of field-line-integrated ionospheric conductivity for the Brazilian equatorial region. It was developed aiming to calculate zonal electric fields at E-region heights in the equatorial region. The present model is based on a constant neutral atmosphere model and on an empirical electron densities model (which also gives the ion composition) adjusted by E-region electron density measured by digisonde. It is also based on a geomagnetic field model that we approximate with a dipole which is not located at the centre of the Earth due to the large magnetic declination angle in the Brazilian sector. We have also considered the eccentric dipole having an inclination of 20o with respect to the Earth rotation axis. The conductivities are calculated for the year 2002 and the results from the present model are compared to those obtained from the conductivity model of the Kyoto University.

Dynamo action in the presence of an imposed magnetic field

Abstract: Dynamo action within the cores of Ap stars may offer intriguing possibilities for understanding the persistent magnetic fields observed on the surfaces of these stars. Deep within the cores of Ap stars, the coupling of convection with rotation likely yields magnetic dynamo action, generating strong magnetic fields. However, the surface fields of the magnetic Ap stars are generally thought to be of primordial origin. Recent numerical models suggest that a primordial field in the radiative envelope may possess a highly twisted toroidal shape. We have used detailed 3-D simulations to study the interaction of such a twisted magnetic field in the radiative envelope with the core-dynamo operating in the interior of a 2 solar mass A-type star. The resulting dynamo action is much more vigorous than in the absence of such a fossil field, yielding magnetic field strengths (of order 100 kG) much higher than their equipartition values relative to the convective velocities. We examine the generation of these fields, as well as the growth of large-scale magnetic structure that results from imposing a fossil magnetic field. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)