Showing papers on "Dipole model of the Earth's magnetic field published in 2006"
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TL;DR: In this article, the authors report on the state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic field measurements and compare them quantitatively and seek to understand where each succeeds, where it fails and why.
Abstract: We report here on the present state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic field measurements. With present observations and techniques, some assumption must be made about the solar magnetic field in order to resolve this ambiguity. Our focus is the application of numerous existing algorithms to test data for which the correct answer is known. In this context, we compare the algorithms quantitatively and seek to understand where each succeeds, where it fails, and why. We have considered five basic approaches: comparing the observed field to a reference field or direction, minimizing the vertical gradient of the magnetic pressure, minimizing the vertical current density, minimizing some approximation to the total current density, and minimizing some approximation to the field's divergence. Of the automated methods requiring no human intervention, those which minimize the square of the vertical current density in conjunction with an approximation for the vanishing divergence of the magnetic field show the most promise.
319 citations
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TL;DR: The Potsdam Magnetic Model of Earth (POMME) as discussed by the authors is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields.
Abstract: The Potsdam Magnetic Model of the Earth (POMME) is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields. The internal field is represented to spherical harmonic (SH) degree 90, while the secular variation and acceleration are given to SH degree 16. Static and time-varying magnetospheric fields are parameterized in Geocentric Solar-Magnetospheric (GSM) and Solar-Magnetic (SM) coordinates and include Disturbance Storm-Time (Dst index) and Interplanetary Magnetic Field (IMF-By) dependent contributions. The model was estimated from five years of CHAMP satellite magnetic data. All measurements were corrected for ocean tidal induction and night-side ionospheric F-region currents. The model is validated using an independent model from a combined data set of Orsted and SAC-C satellite measurements. For the core field to SH degree 13, the root mean square (RMS) vector difference between the two models at the center of the model period is smaller than 4 nT at the Earth's surface. The RMS uncertainty increases to about 100 nT for the predicted field in 2010, as inferred from the difference between the two models.
136 citations
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TL;DR: In this article, a self-consistent coupling between a kinetic ring current model and a magnetospheric force-balance model was proposed to simulate the GEM Storm Challenge event.
Abstract: [1] A geomagnetic storm model needs to take into account the coupling between magnetic field and plasma, as the storm-time field in the inner nightside magnetosphere can be very depressed compared to that of the Earth dipole, thus significantly modifying plasma transport. In this paper we extend our previous "one-way" coupling between a kinetic ring current model and a magnetospheric force-balance model to a fully magnetically self-consistent approach, in which the force-balanced fields are fed back into the kinetic model to guide its evolution. The approach is applied to simulating the 21-23 April 2001 "GEM Storm Challenge" event. We use boundary and initial conditions for the kinetic model from several spacecraft, and magnetic flux boundaries for the equilibrium code from an empirical magnetic field model. We find significant differences in the self-consistent results compared to those obtained from the kinetic model with a dipolar background field (with the same particle boundary conditions and electric fields), due mainly to changes in the particle drifts. In addition to large depressions in the nightside magnetic field values compared to a dipolar field, we also find significantly lower particle density and perpendicular plasma pressure in the inner magnetosphere in the self-consistent case, as well as local, narrow pressure peaks and strongly enhanced plasma β p in localized regions on the nightside.
131 citations
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TL;DR: In this paper, the authors derived the fourth generation lithospheric field model, which is expanded to spherical harmonic degree and order 90, and fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme.
Abstract: SUMMARY The CHAMP magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy. Using almost 5 yr of data, we derive our fourth generation lithospheric field model termed MF4, which is expanded to spherical harmonic degree and order 90. After subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15, an external field model up to degree two, and the predicted magnetic field signatures for the eight dominant ocean tidal constituents, we fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme. From a subset of least disturbed tracks, we estimate the MF4 model by least squares, damping ill-determined coefficients by regularization. The resulting MF4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes, with reduced accuracy in the polar regions. Crustal features come out significantly sharper than in previous models. In particular, bands of magnetic anomalies along subduction zones become visible by satellite for the first time.
101 citations
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TL;DR: In this article, the authors studied the magnetic field strength and filling factors of the Sun magnetic field using spectropolarimetric observations of the infrared and visible Fe I lines at 6301.5, 6302, and 15653 A. The observations cover an intranetwork region at the solar disk center.
Abstract: We study the quiet Sun magnetic fields using spectropolarimetric observations of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648, and 15653 A. Magnetic field strengths and filling factors are inferred by the simultaneous fit of the observed Stokes profiles under the MISMA hypothesis. The observations cover an intranetwork region at the solar disk center. We analyze 2280 Stokes profiles whose polarization signals are above noise in the two spectral ranges, which correspond to 40% of the field of view. Most of these profiles can be reproduced only with a model atmosphere including three magnetic components with very different field strengths, which indicates the coexistence of kG and sub-kG fields in our 15 resolution elements. We measure an unsigned magnetic flux density of 9.6 G considering the full field of view. Half of the pixels present magnetic fields with mixed polarities in the resolution element. The fraction of mixed polarities increases as the polarization weakens. We compute the probability density function of finding each magnetic field strength. It has a significant contribution of kG field strengths, which concentrates most of the observed magnetic flux and energy. This kG contribution has a preferred magnetic polarity, while the polarity of the weak fields is balanced.
83 citations
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TL;DR: Landi et al. as discussed by the authors presented 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfven wave propagating away from the sun embedded in a velocity shear structure such as a microstream and showed how reversals in the magnetic field lines are generated naturally on a time-scale consistent with their observation at Ulysses.
Abstract: Magnetic field polarity inversions embedded in the predominantly unipolar fast solar wind have been observed by the Ulysses spacecraft at high latitudes. Such reversals have the nature of folded back field lines which we suggest are generated by the interaction of standard large amplitude, low frequency, Alfvenic turbulence with velocity shears in the fast solar wind. We present 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfven wave propagating away from the sun embedded in a velocity shear structure such as a microstream and show how reversals in the magnetic field lines are generated naturally on a time-scale consistent with their observation at Ulysses. The generated magnetic field and plasma signals are similar to those observed. We discuss the role turbulence-stream shear interactions might play in limiting differential velocities in the asymptotic high speed solar wind. Citation: Landi, S., P. Hellinger, and M. Velli (2006), Heliospheric magnetic field polarity inversions driven by radial velocity field structures, Geophys. Res. Lett., 33, L14101, doi:10.1029/2006GL026308.
58 citations
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University of Michigan1, Southwest Research Institute2, Uppsala University3, University of California, Berkeley4, Maynooth University5, University College London6, Finnish Meteorological Institute7, Johns Hopkins University Applied Physics Laboratory8, University of Arizona9, Rutherford Appleton Laboratory10, University of Bern11, Max Planck Society12, Braunschweig University of Technology13
TL;DR: In this paper, the authors used the results of the Mars Electron Spectrometer (ELS) instrument of the ASPERA-3 package on the Mars Express satellite to analyze the characteristics of high-altitude photoelectrons.
58 citations
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TL;DR: Magnetic perturbations to the frequencies of low-degree, high radial order, axisymmetric pulsations in stellar models permeated by large-scale magnetic fields are presented in this paper.
Abstract: Magnetic perturbations to the frequencies of low-degree, high radial order, axisymmetric pulsations in stellar models permeated by large-scale magnetic fields are presented. Magnetic fields with dipolar, quadrupolar and a superposition of aligned dipolar and quadrupolar components are considered. The results confirm that the magnetic field may produce strong anomalies in the power spectra of roAp stars. It is shown for the first time that anomalies may result both from a sudden decrease or a sudden increase of a mode frequency. Moreover, the results indicate that the anomalies depend essentially on the geometry of the problem, that is, on the configuration of the magnetic field and on the degree of the modes. This dependence opens the possibility of using these anomalies as a tool to learn about the magnetic field configuration in the magnetic boundary layer of pulsating stars permeated by large-scale magnetic fields.
44 citations
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TL;DR: In this article, the authors present the first observational evidence for a gravity-driven current system in the Earth's low-latitude ionosphere, which is clearly visible in CHAMP satellite magnetic measurements.
Abstract: [1] A gravity field acting on a collisionless, magnetized space plasma causes electrons and ions to drift into opposite directions, in addition to gyrating around the magnetic field lines. This sets up an electric current which flows perpendicular to the gravity and magnetic fields in the eastward direction. Here we present the first observational evidence for such a gravity-driven current system in the Earth's low-latitude ionosphere. Its magnetic field signal, although 10,000 times smaller than the ambient Earth's magnetic field, is clearly visible in CHAMP satellite magnetic measurements. We find a current ribbon of more than 50 kA, about 66° wide in latitude, which moves with the sun northward in summer and southward in winter. Correcting magnetic measurements for this current's signature should lead to a better agreement between low-orbiting satellite and ground-based observations.
42 citations
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TL;DR: In this article, it was shown that when the magnetospheric convection is enhanced following a southward turning of the interplanetary magnetic field (IMF), the initial high-latitude electric field will penetrate to the equatorial latitudes.
Abstract: The ‘convection’ electric field, set up in the magnetosphere by the interaction of the solar wind plasma flowing around the Earth's magnetic field, projects along magnetic field lines to low altitudes where it drives the high-latitude ionospheric convection. During active times, ionospheric electric fields are thought to originate from two sources: a disturbed wind dynamo and electric fields that penetrate from high latitudes. In the latter, when the magnetospheric convection is enhanced following a southward turning of the interplanetary magnetic field (IMF), the initial high-latitude electric field will penetrate to the equatorial latitudes.
40 citations
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TL;DR: In this paper, the authors show that two widely used but apparently different models of the turbulent interplanetary magnetic field are closely related and that the anisotropy observed in the turbulence can be incorporated into both models by properly choosing the temporal and spatial dependence of the transverse velocity field at the solar source surface.
Abstract: We show that two widely used but apparently different models of the turbulent interplanetary magnetic field are closely related. One of these is the so-called quasi-static turbulence model, in which interplanetary magnetic field fluctuations are generated at a source surface near the Sun by random transverse plasma motions (such as supergranulation at the solar photosphere or perhaps reconnection) and are thereafter carried outward at a uniform constant speed in a radial solar wind. The other model of heliospheric magnetic turbulence is known as the two-component model, in which the random part of the field is decomposed into components along the field (slab fluctuations) and normal to it. Both models have provided us with useful simplified parameterizations of interplanetary fluctuations, and both enter into calculations that show good agreement with observations. Here we show that by properly choosing the temporal and spatial dependence of the transverse velocity field at the solar source surface in the quasi-static model, we can generate a two-component model (as well as many others). In particular, the anisotropy observed in the turbulence can be incorporated into both models. This study provides us with important insights that increase our understanding of turbulence and energetic particle transport in the heliosphere.
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TL;DR: In this article, a generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002a, 2002b) empirical magnetic field model (T01).
Abstract: [1] Ultra-low-frequency (ULF) field line resonances can be used to infer the mass density along magnetospheric magnetic field lines. By specifying how mass density is distributed along the magnetic field (usually a power law as a function of distance from the Earth) and a dipole magnetic field geometry, the MHD standing wave equation can be analytically solved and mass density inferred from observed field line eigenfrequencies. However, the geometry of the Earth's magnetic field can deviate significantly from a dipole, even at relatively low L shells and on the dayside magnetosphere. This study investigates the importance of including a realistic magnetic field geometry when computing plasma mass density from observed field line eigenfrequencies. A generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002a, 2002b) empirical magnetic field model (T01). The results are compared to those found using a dipole. We find that assuming a dipole magnetic field geometry results in an overestimation of mass density. The overestimation is larger for more disturbed levels of geomagnetic activity. Our results have important implications for the inference of heavy ions in the magnetosphere. Namely, an increase in heavy ion concentration as a result of enhanced geomagnetic activity will be exaggerated unless the proper magnetic field geometry is taken into account when calculating mass density from field line eigenfrequencies.
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TL;DR: In this paper, a theoretical study of the calculation of magnetic fields in vicinity of overhead electric power lines is presented, where exact and simplified methods of the determination of the magnetic field of a straight overhead conductor based on the Fourier transform technique and on the concept of complex ground return plane are presented.
Abstract: The paper presents in a tutorial manner a theoretical study of the calculation of magnetic fields in vicinity of overhead electric power lines. Exact and simplified methods of the determination of the magnetic field of a straight overhead conductor based on the Fourier transform technique and on the concept of complex ground return plane are presented. The decomposition of the magnetic fields in two components: magnetic field obtained in free space from the Biot-Savart law and the magnetic field produced by earth current is discussed. It is shown that in practical cases the effects from earth currents can be neglected as compared with effects from line currents. A solution for modeling magnetic fields produced by sagging conductor, described by the catenary equation, is also proposed. The effect of the catenary on the magnetic field spatial distribution is investigated. The calculation results of magnetic field produced by sagging conductor and the magnetic field of a conventional model consisting of a horizontal conductor hanging on the effective height lying between the maximum and minimum heights of catenaries is compared and discussed. As an example the magnetic field of a real-power catenary high voltage 2 × 220 kV power line is demonstrated. Copyright © 2006 John Wiley & Sons, Ltd.
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TL;DR: In this paper, a wavelet analysis of the total magnetic field of the Sun as a star is used to determine the magnetic moment and direction of the dipole field for three successive solar cycles, and the results are discussed in connection with helioseismological data indicating the existence of oscillations with a period of about 1.3 yr.
Abstract: Observations of the large-scale solar magnetic field (synoptic maps) and measurements of the magnetic field of the Sun as a star (the total magnetic field) are used to determine the dipole magnetic moment and direction of the dipole field for three successive solar cycles. Both the magnetic moment and its vertical and horizontal components vary regularly during the cycle, but never disappear completely. A wavelet analysis of the total magnetic field shows that the amplitude of the 27-day variations of this field is very closely related to the magnetic moment of the horizontal dipole. The reversal of the global dipole field corresponds to a change in the inclination of its axis and occurs in a series of steps lasting one to two years rather than continuously. Before the onset of the reversal, the dipole axis precesses relative to the solar rotational axis, then shifts in a meridianal plane, reaching very low latitudes, where a substantial shift in longitude then begins. These results are discussed in connection with helioseismological data indicating the existence of oscillations with a period of about 1.3 yr and properties of dynamo processes for the case of an inclined rotator.
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TL;DR: In this paper, a line ratio technique is developed from the intrinsic variations of the line profile to explore the probability distribution function of the magnetic field strength and the spatial scale at which the observed magnetic field is horizontally organized.
Abstract: We report on spectropolarimetric observations of a near-IR line of Mn I located at 15262.702 A whose intensity and polarization profiles are very sensitive to the presence of hyperfine structure. A theoretical investigation of the magnetic sensitivity of this line to the magnetic field uncovers several interesting properties. The most important one is that the presence of strong Paschen-Back perturbations due to the hyperfine structure produces an intensity line profile whose shape changes according to the absolute value of the magnetic field strength. A line ratio technique is developed from the intrinsic variations of the line profile. This line ratio technique is applied to spectropolarimetric observations of the quiet solar photosphere in order to explore the probability distribution function of the magnetic field strength. Particular attention is given to the quietest area of the observed field of view, which was encircled by an enhanced network region. A detailed theoretical investigation shows that the inferred distribution yields information on the average magnetic field strength and the spatial scale at which the magnetic field is organized. A first estimation gives ~250 G for the mean field strength and a tentative value of ~0.45" for the spatial scale at which the observed magnetic field is horizontally organized.
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TL;DR: In this paper, a force-free solution inside an elliptic cylinder for magnetic cloud models was proposed, which may include effects of a flux rope expansion, and a comparison of this new solution with magnetic cloud observations is done for magnetic clouds with flat magnetic field magnitude profiles.
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TL;DR: In this article, the authors generalized the quasi-linear prediction for the transport and behavior of magnetic field lines to all scales and arbitrary three-dimensional turbulence spectra, and derived analytical expressions for the field line mean square cross-field displacement Δx2.
Abstract: Charged energetic particles propagating in solar wind magnetic fields span field irregularities down to very short turbulent scales, not described by the original quasi-linear theory for weak magnetic turbulence. This theory only predicts a field line diffusion on the largest scales, well above the correlation length, inverse of the spectral flattening wavenumber. The quasi-linear prediction for the transport and behavior of magnetic field lines is generalized here to all scales and arbitrary three-dimensional turbulence spectra. New analytical expressions are derived for the field line mean square cross-field displacement Δx2, and analytical proof is presented for the anomalous transport of the field lines. We find Δx2 ∝ (Δz)β, where Δz is the elapsed distance along the average field and β, the transport exponent, can take any value between 0 and 2. A decreasing turbulence spectrum results in a field line supradiffusion (β > 1), while an inverted spectrum implies a subdiffusion (β < 1). Simple expressions are derived for the transport exponent and coefficient. A powerful new method is presented to compute magnetic field lines in the quasi-linear regime of turbulence that allows rapid computation of field lines generated from any three-dimensional turbulence spectrum, including some 1015 modes and more. Individual field lines computed with this method show how a spectral steepening results in a smoothing of the field lines and how harder spectra give increasingly more short-scale fluctuations. The field line self-similarity, characteristic of power-law spectra, is demonstrated visually, and the anomalous transport of the field lines is confirmed numerically.
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TL;DR: In this paper, a new magnetosheath numerical MHD model has been developed which calculates solar wind flow around a paraboloidal obstacle, and the magnetic barrier may almost disappear for a special interplanetary magnetic field orientation nearly aligned with the solar wind velocity.
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TL;DR: In this article, a self-consistent magnetic field model was proposed to simulate the storm-time ring current in the equatorial plane, where magnetic field lines lie in meridional planes and satisfy the generic Dungey field line equation.
Abstract: [1] The storm-time ring current generates a strong and time-dependent perturbation of the magnetospheric magnetic field , and this magnetic-field perturbation can have important feedback on the dynamics of ring current particles themselves. In particular, the modification of can significantly alter the gradient-curvature drifts of ring current particles, and the induced electric field associated with ∂/∂t can inhibit ring current particle injection and energization. Thus in order to accurately simulate the storm-time ring current, a self-consistent magnetic field model that takes these effects into account is needed. This study is our first attempt to address this issue. We assume for simplicity a model for such that magnetic field lines lie in meridional planes and satisfy the generic Dungey field line equation. With these two assumptions and given the pressure distribution in the equatorial plane, the force-balanced magnetic field in the equatorial plane is obtained by solving the force balance equation. This force balance equation solver is coupled with our ring current model to provide self-consistent magnetic fields. In this study, we simulate a hypothetical storm with this magnetically self-consistent ring current model. By comparing our simulation results with statistical studies, we find that our model reasonably reproduces the disturbed magnetic field in the equatorial plane in terms of magnitude and location. The equatorial current density shows an inner eastward ring current at ∼3 RE, and a outer westward ring current at ∼4–6.6 RE, which agrees well with observations. The effects of the self-consistent magnetic field on the dynamics of ring-current particles are discussed. We find that the self-consistent magnetic field tends to prevent ring current particles from deep injection and to mitigate the energization of ring current particles. Thus the ring current simulated in a self-consistent magnetic field model will produce less of a disturbance at the center of the Earth than that simulated in the prescribed dipole or dipole-like magnetic field models without feedback from the ring current.
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TL;DR: In this paper, the authors considered the Weber-Davis model of the solar wind with alpha particles and showed that magnetic stresses pre-dominate the angular momentum loss of the Sun.
Abstract: The classic Weber-Davis model of the solar wind is reconsidered by incorporating alpha particles and by allowing the solar wind to flow out of the equatorial plane in an axisymmetrical configuration. In the ion momentum equations of the solar wind, the ion gyro-frequency is many orders of magnitude higher than any other frequency. This requires that the difference between proton and alpha velocity vectors be aligned with the background magnetic field. With the aid of this alignment condition, the governing equations of the multi-fluid solar wind are derived from the standard transport equations. The governing equations are numerically solved along a prescribed meridional magnetic field line located at colatitude $70^\circ$ at 1AU and a steady state fast solar wind solution is found. A general analysis concludes, in agreement with the Weber-Davis model, that the magnetic field helps the coronal plasma to achieve an effective corotation out to the Alfv\'enic radius, where the poloidal Alfv\'enic Mach number $M_T$ equals unity ($M_T$ is defined by equation (\ref{eq:mach})). The model computations show that, magnetic stresses predominate the angular momentum loss of the Sun. For the fast wind considered, the proton contribution to the angular momentum loss, which can be larger than the magnetic one, is almost completely canceled by the alpha particles that develop an azimuthal speed in the direction opposite to the solar rotation. The Poynting flux associated with the azimuthal components is negligible in the energy budget. However, the solar rotation can play some role in reducing the relative speed between alpha particles and protons for low latitude fast solar wind streams in interplanetary space.
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12 Oct 2006
TL;DR: In this article, the authors developed a method that allows to derive the distribution of the surface magnetic fields of magnetic white dwarfs from a set of rotation-phase resolved flux and circular polarization spectra.
Abstract: The aim of this work has been to develop a method that allows to derive the distribution of the surface magnetic fields of magnetic white dwarfs from a set of rotation-phase resolved flux and circular polarization spectra. I have computed a three-dimensional grid of 46800 theoretical Stokes I and V profiles, with the absolute magnetic field strength, the field direction relative to the line of sight, and the effective photospheric temperature as free parameters. The emerging Zeeman spectrum for a magnetic white dwarf with a given distribution of field vectors across its surface can be synthesised from this database of model spectra. For the field parametrization, either a truncated expansion in spherical harmonics or a hybrid model consisting of tilted and off-centred zonal multipole components is used. I have developed an algorithm to determine the best-fitting field structure for a set of given input spectra by a χ2-minimisation of the magnetic field parameters. For the optimisation, an evolutionary strategy is used. The method has been tested on synthetically generated input spectra with artificial noise. It has been found that a signal-to-noise ratio of 50--100 in the individual flux spectra and the inclusion of circular polarization spectra is essential for a reliable reconstruction of the original field geometry.The method has been applied to phase-resolved flux and circular polarization spectra of two objects that have been obtained with the FORS1 spectrograph at the 8-m ESO Very Large Telescope. The results yield for the first time maps of the surface magnetic field distribution with hitherto unknown detail. For HE 1045-0908, the best fitting models are in good agreement with the observations. I find strong evidence for a magnetic field that is largely dominated by a quadrupolar component. The most frequent field strength on the surface is 16 MG. The field of PG 1015+014 is more complex, and is dominated by the octupole and probably even higher multipoles. The dominating field range is 70-80 MG. Remaining discrepancies between observations and the best fitting models suggest that the true field is more complex than can be described by our field models.In the course of this thesis, the most accurate field determinations of magnetic white dwarfs so far have been obtained. Zeeman tomography has proven its usefulness in order to gain further insight into the properties of magnetic white dwarfs and their role in stellar evolution.
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TL;DR: In this paper, the solar magnetic field at the Earth's projection onto the solar-wind source surface has been calculated for each day over a long time interval (1976-2004) and the data have been compared with the daily mean solar wind velocities and various components of the interplanetary magnetic field near the Earth.
Abstract: The solar magnetic field B
s at the Earth’s projection onto the solar-wind source surface has been calculated for each day over a long time interval (1976–2004). These data have been compared with the daily mean solar wind (SW) velocities and various components of the interplanetary magnetic field (IMF) near the Earth. The statistical analysis has revealed a rather close relationship between the solar-wind parameters near the Sun and near the Earth in the periods without significant sporadic solar and interplanetary disturbances. Empirical numerical models have been proposed for calculating the solar-wind velocity, IMF intensity, and IMF longitudinal and B
z components from the solar magnetic data. In all these models, the B
s value plays the main role. It is shown that, under quiet or weakly disturbed conditions, the variations in the geomagnetic activity index Ap can be forecasted for 3–5 days ahead on the basis of solar magnetic observations. Such a forecast proves to be more reliable than the forecasts based on the traditional methods.
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TL;DR: In this article, the authors used a potential magnetic field extrapolation of synthetic photospheric magnetograms to study the magnetic network topology and the effects of a mixed-polarity background field on the network expansion with height through the solar atmosphere.
Abstract: The solar magnetic field has its footpoints in the photosphere, extends through the chromosphere, and is thought to expand through the transition region and into the corona. It is organized by fluid motions to form strong flux concentrations within the boundaries of the supergranular convection cells. These boundaries are the network lanes observed in line emission, and they display increasing width with height through the solar atmosphere. The network field concentrations are surrounded by a mixed-polarity internetwork magnetic field on the scale of granulation. We use a potential magnetic field extrapolation of synthetic photospheric magnetograms to study the magnetic network topology and the effects of a mixed-polarity background field on the network expansion with height through the solar atmosphere. We find that the expansion of the network boundary with height deviates significantly from the funnel expansion model. Moreover, we find that the background magnetic field has a considerable effect on the filling factor of the network area with height, even though the background flux is strictly equal to zero.
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TL;DR: In this paper, a new state with two spatially distinct separators connecting the same two magnetic null points, called the "dual intersecting" state, has been discovered.
Abstract: The evolution of the solar corona is dominated to a large extent by the hugely complicated magnetic field which threads it. Magnetic topology provides a tool to decipher the structure of this field and thus help to understand its behaviour. Usually, the magnetic topology of a potential field is calculated due to flux sources on a locally planar photospheric surface. We use a Green's function method to extend this theory to sources on a global spherical surface. The case of two bipolar flux-balanced source regions is studied in detail, with an emphasis on how the distribution and relative strengths of the source regions affect the resulting topological states. A new state with two spatially distinct separators connecting the same two magnetic null points, called the “dual intersecting“ state, is discovered.
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TL;DR: In this paper, closed-form expressions for the far field of a vertical magnetic dipole embedded below the sea surface are obtained by evaluating the Sommerfeld-type integrals with the aid of complex image theory.
Abstract: The propagation of radio waves in the sea is of great importance in many practical applications. Considerable effort and speculation have thus been devoted to establish the theoretical foundation for such problem. The method used for solution is essentially based on calculating the electromagnetic field due to vertical magnetic dipole buried in stratified media. In this paper closed-form expressions for the far field of a vertical magnetic dipole embedded below the sea surface are obtained by evaluating the Sommerfeld-type integrals with the aid of complex image theory.
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TL;DR: In this paper, the effects of magnetic field, varying its strength and orientation, on the model atmosphere structure, the energy distribution, photometric colors and the hydrogen Balmer line profiles were analyzed.
Abstract: Context See abstract in the paper
Aims In the last paper of this series we study the effects of the magnetic field, varying its strength and orientation, on the model atmosphere structure, the energy distribution, photometric colors and the hydrogen Balmer line profiles We compare with the previous results for an isotropic case in order to understand whether there is a clear relation between the value of the magnetic field angle and model changes, and to study how important the additional orientational information is Also, we examine the probable explanation of the visual flux depressions of the magnetic chemically peculiar stars in the context of this work
Methods We calculated one more grid of the model atmospheres of magnetic A and B stars for different effective temperatures (Teff=8000K, 11000K, 15000K), magnetic field strengths (B=0, 5, 10, 40 kG) and various angles of the magnetic field (Omega=0-90 degr) with respect to the atmosphere plane We used the LLmodels code which implements a direct method for line opacity calculation, anomalous Zeeman splitting of spectral lines, and polarized radiation transfer
Results We have not found significant changes in model atmosphere structure, photometric and spectroscopic observables or profiles of hydrogen Balmer lines as we vary the magnetic field inclination angle Omega The strength of the magnetic field plays the main role in magnetic line blanketing We show that the magnetic field has a clear relation to the visual flux depressions of the magnetic CP stars
Conclusions See abstract in the paper
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TL;DR: In this paper, the BATS-R-US global magnetohydrodynamic (MHD) simulation code was used to model the paleomagnetosphere, in particular, the magnetosphere-ionosphere coupling during such polarity transition epochs.
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TL;DR: In this paper, the difficulties associated with calculating the parameters of the interplanetary magnetic field (IMF) from solar magnetic data have been considered, and all conventional calculation patterns and available input databases have been analyzed from a unified standpoint.
Abstract: The difficulties associated with calculating the parameters of the interplanetary magnetic field (IMF) from solar magnetic data have been considered. All conventional calculation patterns and available input databases have been analyzed from a unified standpoint. It has been shown that these assumptions and limitations cannot affect the general structure and dependence on cycle of solar and interplanetary data. At the same time, the measured solar field values are underestimated as a result of the magnetograph signal saturation effect. It has been shown that the correction should depend on the heliocentric observation latitude and cycle phase. The correction method responsible for good agreement between the calculated and measured values has been proposed. The created database makes it possible to quantitatively calculate the magnetic fields in the solar wind near the Earth.
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TL;DR: In this paper, an exact analytic solution for a uniformly expanding, neutral, infinitely conducting plasma sphere in an external dipole magnetic field was obtained for the case of an infinitely conducting sphere.
Abstract: An exact analytic solution is obtained for a uniformly expanding, neutral, infinitely conducting plasma sphere in an external dipole magnetic field. The electrodynamical aspects related to the radiation and transformation of energy were considered as well. The results obtained can be used in analysing the recent experimental and simulation data.
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TL;DR: In this paper, a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetosphere geometry, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context.
Abstract: Solar wind generated magnetic disturbances are currently one of the major obstacles for improving the accuracy in the determination of the magnetic field due to sources internal to the Earth. In the present study a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetospheric geometry. The magnetic field near the Earth due to these currents is analyzed by estimating and comparing the contributions from the various parts of the system, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context. The contribution from the distant magnetospheric currents is found to consist of two, mainly opposing, contributions from respectively the dayside magnetopause currents and the cross-tail current. At high latitudes the field-aligned component is of partidular interest in connection with internal field-modelling. In the altitude regime of 400–800 km (typical for low Earth orbit satellites) the ionospheric currents are found to contribute significantly to the disturbancance, and account for more than 90% of the field-aligned disturbance. The magnetic disturbance field from field-aligned currents (FACs) is basically transverse to the main field, and they therefore contribute with less than 2% to the disturbance in total field intensity. Inhomogeneity in ionospheric conductance is identified as the main cause of main-field disturbance in the field-aligned direction. These disturbances are associated with the ionospheric Pedersen currents, and may introduce systematic errors in internal field models.