Showing papers on "Dipole model of the Earth's magnetic field published in 2002"

Dynamical Simulations of Magnetically Channeled Line-driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow

Abstract: We present numerical magnetohydrodynamic (MHD) simulations of the effect of stellar dipole magnetic fields on line-driven wind outflows from hot, luminous stars. Unlike previous fixed-field analyses, the simulations here take full account of the dynamical competition between field and flow and thus apply to a full range of magnetic field strength and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless wind magnetic confinement parameter η* (= BR/v∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement, η* ≤ 1, the field is fully opened by the wind outflow, but nonetheless, for confinements as small as η* = 1/10 it can have a significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement, η* > 1, the magnetic field remains closed over a limited range of latitude and height about the equatorial surface, but eventually is opened into a nearly radial configuration at large radii. Within closed loops, the flow is channeled toward loop tops into shock collisions that are strong enough to produce hard X-rays, with the stagnated material then pulled by gravity back onto the star in quite complex and variable inflow patterns. Within open field flow, the equatorial channeling leads to oblique shocks that are again strong enough to produce X-rays and also lead to a thin, dense, slowly outflowing disk at the magnetic equator. The polar flow is characterized by a faster-than-radial expansion that is more gradual than anticipated in previous one-dimensional flow tube analyses and leads to a much more modest increase in terminal speed (less than 30%), consistent with observational constraints. Overall, the results here provide a dynamical groundwork for interpreting many types of observations—e.g., UV line profile variability, redshifted absorption or emission features, enhanced density-squared emission, and X-ray emission—that might be associated with perturbation of hot-star winds by surface magnetic fields.

Reconstruction of magnetic clouds in the solar wind: Orientations and configurations

Abstract: [1] We develop a new approach for determining the orientation of the axis (z) of finite-β magnetic flux ropes from single-spacecraft data. It consists of an optimization procedure based on two-dimensional magnetohydrostatic theory. From the requirement that the transverse pressure, Pt = p + Bz2/2μ0, be a function of the calculated magnetic potential, A, alone, an optimal z axis is found. Benchmark studies using analytical flux rope solutions show the feasibility of this axis-determination approach. Two magnetic cloud events, on 18 October 1995 and 9 January 1997, observed by the Wind spacecraft at 1 AU, are examined. The time series of plasma and magnetic field data, collected when the magnetic structures move past the spacecraft, are utilized as spatial initial values for numerical integration of the nonlinear, plane Grad-Shafranov equation. The recovered magnetic cloud structures show helical magnetic field configurations, representative of magnetic flux ropes, with invariance along their axes. Their cross sections consist of nested irregular loops of transverse field lines rather than the concentric circles of an axially symmetric model. In addition to their axis orientations, we obtain other quantitative features of the two magnetic clouds from their recovered cross sections, including their sizes (diameters ∼ 0.2 AU), their impact parameters (|y0| ≲ 0.01 AU), their maximum axial field strengths (20 and 14 nT, respectively), and the twist of field lines (∼ 1.30–2.68 turns AU−1) around the flux rope's axis. We find that errors in the orientation of the flux rope axis may result in significant variations in the main parameters, i.e., the impact parameter, as well as the size and shape of the cross section.

Hill model of transpolar potential saturation: Comparisons with MHD simulations

Abstract: [1] We present a comparison between a simple but general model of solar wind-magnetosphere-ionosphere coupling (the Hill model) and the output of a global magnetospheric MHD code, the Integrated Space Weather Prediction Model (ISM). The Hill model predicts transpolar potential and region 1 currents from environmental conditions specified at both boundaries of the magnetosphere: at the solar wind boundary, electric field strength, ram pressure, and interplanetary magnetic field direction; at the ionospheric boundary, conductance and dipole strength. As its defining feature, the Hill model predicts saturation of the transpolar potential for high electric field intensities in the solar wind, which accords with observations. The model predicts how saturation depends on boundary conditions. We compare the output from ISM runs against these predictions. The agreement is quite good for non-storm conditions (differences less than 10%) and still good for storm conditions (differences up to 20%). The comparison demonstrates that global MHD codes (like ISM) can also exhibit saturation of transpolar potential for high electric field intensities in the solar wind. We use both models to explore how the strength of solar wind-magnetosphere-ionosphere coupling depends on the strength of Earth's magnetic dipole, which varies on short geological timescales. As measured by power into the ionosphere, these models suggest that magnetic storms might be considerably more active for high dipole strengths.

A comprehensive model of the quiet‐time, near‐Earth magnetic field: phase 3

Abstract: SUMMARY The near-Earth magnetic field is caused by sources in the Earth’s core, ionosphere, magnetosphere, lithosphere and from coupling currents between the ionosphere and the magnetosphere, and between hemispheres. Traditionally, the main field (low degree internal field) and magnetospheric field have been modelled simultaneously, with fields from other sources being modelled separately. Such a scheme, however, can introduce spurious features, especially when the spatial and temporal scales of the fields overlap. A new model, designated CM3 (Comprehensive Model: phase 3), is the third in a series of efforts to coestimate fields from all of these sources. This model has been derived from quiet-time Magsat and POGO satellite and observatory hourly means measurements for the period 1960‐1985. It represents a significant advance in the treatment of the aforementioned field sources over previous attempts, and includes an accounting for main field influences on the magnetosphere, main field and solar activity influences on the ionosphere, seasonal influences on the coupling currents, a priori characterization of the influence of the ionosphere and the magnetosphere on Earth-induced fields, and an explicit parametrization and estimation of the lithospheric field. The result is a model that describes well the 591 432 data with 16 594 parameters, implying a data-to-parameter ratio of 36, which is larger than several popular field models.

A non‐force‐free approach to the topology of magnetic clouds in the solar wind

Abstract: [1] We present a new model for the topology of magnetic clouds at 1 AU that relates the magnetic field vector with the current density of the cloud without assuming the force-free condition. In addition, the model is formulated in such a way that it can be fitted directly to the data without the need of initially determining the flux rope axis by minimum variance. The model provides both the magnetic field strength and the direction in just one fitting procedure. In addition to the orientation of its axis and the relative closest-approach distance between the spacecraft and the cloud axis, the fit of the model to the experimental data allows an estimation of the current density of plasma inside the magnetic cloud. From our study on several clouds we conclude that this has values of the order of 10−12 C m−2 s−1. Compared with the force-free model, although the improvement gained in the χ2 values is not very great, the fitting procedure is shorter and easier, and the number of parameters involved has been reduced from seven to five. Our model also lets us check the validity of force-free approximation by the calculation of the value of the current density perpendicular to the flux rope, j⟂ = j × B/|B|. This component is assumed to be zero in the force-free model, but for all analyzed cases we obtain a nonzero value. This finite value implies the existence of pressure gradients inside the cloud that cannot be explained with a force-free approximation. In this paper, four cases are presented that have already been analyzed in the literature. We show that our model can explain the profiles of the magnetic field as a magnetic cloud passes a spacecraft.

Mount Wilson Synoptic Magnetic Fields: Improved Instrumentation, Calibration, and Analysis Applied to the 2000 July 14 Flare and to the Evolution of the Dipole Field

Abstract: This paper describes the current status of the 150 foot solar tower telescope program of synoptic observations with an emphasis on the magnetic field data. A newly installed 24-channel system permits routine intercomparison of magnetic fields measured by the ?676.8 nm line used by the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory and the ?525.0 nm line used by the 150 foot tower. Two important calibration procedures for treatment of saturation and zero-point offset are described. It is demonstrated that solar rotation can be used to extract the east-west component of the slowly evolving, large-scale magnetic field in a stable fashion. This same analysis produces maps of the neutral line configurations that are well correlated with the positions of quiescent prominences. The analysis is applied to the 2000 July 14 flare and shown to demonstrate that the field was sheared due to the westward-moving intrusion of a region just north of the neutral line along which the flaring occurred. A new method for preparing synoptic charts by averaging without differential rotation smearing is presented. These synoptic charts are combined into a new format termed a supersynoptic chart, which makes possible the identification of systematic long-term trends in the magnetic field evolution. Based on these charts, distinct large-scale events of magnetic field bias opposing the old-cycle dipole field are seen. A statistical method using the skewness in the distribution of the polarity bias as a function of longitude is developed. The coincidence between pulses in this skewness and times of rapid change in the Sun's dipole moment is consistent with the idea that a tilt in the orientation of bipolar magnetic regions is responsible for the dipole field reversal. The pulses in skewness are large and limited in number, suggesting the operation of a large-scale instability such as the kink instability.

Multiple magnetic clouds in interplanetary space

Abstract: An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work.

Plasma and magnetic field inside magnetic clouds: a global study

Abstract: Data observed during spacecraft encounters with magnetic clouds have been extensively analyzed in the literature Moreover, several models have been proposed for the magnetic topology of these events, and fitted to the observations Although these interplanetary events present well-defined plasma features, none of those models have included a simultaneous analysis of magnetic field and plasma data Using as a starting point a non-force-free model that we have developed previously, we present a global study of MCs that include both the magnetic field topology and the plasma pressure In this paper we obtain the governing equations for both magnitudes inside a MC The expressions deduced are fitted simultaneously to the measurements of plasma pressure and magnetic field vector We perform an analysis of magnetic field and plasma WIND observations within several MCs from 1995 to 1998 The analysis is confined to four of these events that have high-quality data Only in one fitting procedure we obtain the orientation of the magnetic cloud relative to the ecliptic plane and the current density of the plasma inside the cloud We find that the equations proposed reproduce the experimental data quite well

Towards a rapidly rotating liquid sodium dynamo experiment

Abstract: The main characteristics of the Earth's dynamo are reviewed. The combined actions of Coriolis and Lorentz forces lead to the so-called \magnetostrophic" regime. We derive an estimate of the power needed to sustain the magnetic eld in this regime. We show that an experiment with liquid sodium can be designed to operate in the magnetostrophic regime. Such an experiment would bring most valuable information on the mechanisms of planetary dynamos. In order to prepare this large-scale experiment and explore the magnetostrophic balance, a smaller scale liquid sodium set-up has been designed and is being built. It consists of a rapidly rotating spherical shell lled with liquid sodium, in which motions are set by spinning at a dierent rotation rate an inner core permeated by a strong magnetic eld. We discuss the processes that can be explored with this new device. 1. The magnetic elds in the solar system. Most planets of the solar system have or have had an internal magnetic eld. Recent satellite missions have revealed that Mars probably had a magnetic eld in its early history (1), while two moons of Jupiter, Io and Ganymede (2), show evidence for present magnetic activity. Explaining the origin and behaviour of planetary magnetic elds is a fascinating challenge. As a starting point, one may attempt to build a model of the geodynamo since the Earth's magnetic eld is, by far, the best documented. 2. The Earth's magnetic eld. The characteristics of the magnetic eld of the Earth are known over a wide range of time scales. In historical times, observa- tory records permit to monitor the secular variation of magnetic structures, with typical drift velocities of 0.1 mm/s. Sudden variations of the internal magnetic eld, aecting the entire core surface, have also been discovered in these records, the so-called \jerks", while the total intensity of the eld displays large fluctua- tions and has dropped by more than 30% in the past 2000 years, as recorded in archaeological artefacts.The most striking property of the Earth's magnetic eld, as seen at present, but also in the paleomagnetic records over several hundred of million years, is that it is dominated by a dipole component, whose axis is aligned with the axis of rotation of the Earth. Just as fascinating is the evidence that this dipole has changed polarity many times over the Earth's history, the last inversion having taken place only 780,000 years ago (3). Models of the geodynamo aim at explaining this rich set of behaviours, starting with the dominant dipole character of the eld. 3. The mechanism. Since the pioneer work of Larmor, Bullard and Elsasser, it is widely accepted that the Earth's magnetic eld is created by self-induction in its core. In an electrically conducting liquid, fluid motions in a magnetic eld produce electrical currents, which in turn feed the magnetic eld. When the induc- tion of the magnetic eld overcomes its diusion (i.e., for a large enough magnetic

Solar Wind Magnetic Field Energy Density As Seen By Wind

Abstract: Statistical properties of the interplanetary magnetic field fluctuations can provide an important insight into the solar wind turbulent cascade. Recently, analysis of the Probability Density Functions (PDF) of the velocity and magnetic field fluctuations has shown that these exhibit non-Gaussian properties on small time scales while large scale features appear to be uncorrelated. Here we apply the finite size scaling technique to explore the scaling of the magnetic field energy density fluctuations as seen by WIND. We find a single scaling sufficient to collapse the curves over the entire investigated range. The rescaled PDF follow a non Gaussian distribution with asymptotic behavior well described by the Gamma distribution arising from a finite range Levy walk. Such mono scaling suggests that a Fokker-Planck approach can be applied to study the PDF dynamics. These results strongly suggest the existence of a common, nonlinear process on the time scale up to 26 hours.

Empirical model for μ scattering caused by field line curvature in a realistic magnetosphere

Abstract: [1] The combination of magnetic field line curvature (FLC) and weak magnetic field strength affects charged particle motion in a magnetic field by causing changes of the particle's first adiabatic invariant, μ. In this paper we refer to these changes as FLC induced μ scattering or simply μ scattering. Both large single scatterings and the cumulative effects of many small scatterings (Δμ/μ ≪ 1) influence particle populations in the Earth's magnetosphere. Because FLC-induced μ scattering is strongly dependent on the magnetic field geometry and strength, realistic magnetospheric models must be employed in order to gain a quantitative understanding of how μ scattering affects the evolution of these populations. This requires a μ-scattering model that is accurate for many different magnetic field geometries. Though several μ-scattering models have been developed previously, we demonstrate that their accuracy is limited to the simple field geometries for which they were derived. In realistic magnetic fields, there are regions where these μ-scattering models depart from Lorentz integration results by a factor of 2 or greater. The source of these discrepancies is attributed to the fact that the only magnetic field information used is the field line curvature RC and magnetic field strength B both evaluated at a single point, the magnetic equator. To generalize the analytical representation of μ scattering to a range of magnetic field geometries, we have developed a new μ-scattering model that uses two additional parameters. These are proportional to the second derivatives of the field line curvature, ∂2RC/∂S2, and the field intensity, ∂2B /∂S2, where S is the distance along the field line and the derivatives are evaluated at the magnetic equator. We have defined an error parameter that measures the fit between the model and Lorentz integration results. Using this parameter, we show that the difference between our model Δμ values and those based on Lorentz integration is, on average, only 4%; this is at least a factor of 5 less than the difference using previous analytical models. Our new model will greatly facilitate quantitative analysis of μ scattering for realistic magnetospheric magnetic field geometries including active as well as quiet conditions.

Two dimensional particle-in-cell simulations of the lunar wake

Abstract: The solar wind plasma from the Sun interacts with the Moon, generating a wake structure behind it, since the Moon is to a good approximation an insulator, has no intrinsic magnetic field, and a very thin atmosphere. Following earlier one and one-half dimensional (112D) simulations, 212D simulations have been studied for the first time. 212D simulations in the solar wind rest frame with an initial circular void reveal structures similar to those found in the earlier simulations, indicating that the 112D simulations are a good approximation to the physics. The infilling of the wake was found to be anisotropic, with the infilling occurring predominantly parallel to the magnetic field. The 212D simulations that are stationary with respect to the Moon, allow the effect of solar wind magnetic fields oblique to the solar wind flow to be studied, and these reveal additional assymetries.

Generation of remote homogeneous magnetic fields

Abstract: This paper presents a magnetic efficiency model for comparing efficiencies of various magnets for magnetic resonance imaging. It demonstrates that monohedral magnets, magnets with sources on one side, can generate remote saddle points in the field profile relatively efficiently. These magnets may be modeled by a minimum of two magnetic dipoles. The paper examines the field profile and magnetic dipole efficiency for the two-dipole model in detail, and develops some fundamental properties of homogeneous magnetic fields.

High-latitude thermospheric vertical wind activity from Dynamics Explorer 2 Wind and Temperature Spectrometer Observations: Indications of a source region for polar cap gravity waves

Abstract: [1] The large-scale distribution of thermospheric vertical wind activity, from ∼250 to 650 km altitude, was studied using observations from the Wind and Temperature Spectrometer on the Dynamics Explorer 2 satellite. We calculated the vertical velocity standard deviation, σ(Vz), within a sliding window of width 120 s, corresponding to an along-track distance of ∼900 km. Maps of σ(Vz) in local magnetic time and invariant latitude reveal a high-latitude region of enhanced activity that largely fills the polar cap, maximizing in the midnight-dawn sector. Separating the data by altitude suggests that most of the vertical wind energy present at 250–450 km is dissipated within a few hundred vertical kilometers. Northern and Southern Hemisphere high-latitude σ(Vz) fields were found to be very similar, indicating no significant hemispheric differences. No strong correlation was found between solar illumination and σ(Vz) at high latitudes, although the dayside vertical wind activity may be slightly reduced compared to night and twilight intervals. However, a clear relationship to geomagnetic activity, as measured by the AE index, was found; vertical wind activity increases with increasing AE. We interpret these results as a possible signature of polar cap gravity waves with sources in or near the midnight-dawn auroral oval. We use the probability density functions of σ(Vz), separated by AE, to infer the temporal characteristics of the wave source and consequently to provide preliminary estimates of the production rates of polar cap gravity waves.

Magnetic fields and radio polarization of barred galaxies - 3D dynamo simulations

Abstract: A three-dimensional (3D) MHD model is applied to simulate the evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo extending to about 2.8 kpc above the galactic plane. As the model input we use a time-dependent velocity field of molecular gas resulting from self-consistent 3D N -body simulations of a galactic disk. We assume that the gaseous halo rotates differentially co-rotating with the disk or decreasing its velocity in the Z direction. The dynamo process included in the model yields the amplification of the magnetic field as well as the formation of field structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed structures of polarization B -vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms, is demonstrated. The inclusion of the galactic halo constitutes a step towards a realistic model of galactic magnetic fields that includes as many dynamical components as needed for a realistic description.

Ionospheric currents and field‐aligned currents generated by dynamo action in an asymmetric Earth magnetic field

Abstract: [1] To investigate the influence of the magnetic field configuration on large-scale ionospheric electrodynamics, a geomagnetic field coordinate system based on Euler potentials is built for three magnetic field configurations: dipole, tilted dipole, and a revision of the International Geomagnetic Reference Field (IGRF). The two-dimensional ionospheric dynamo equation is expressed in this framework under the assumptions of equipotential field lines and conservation of current, including horizontal ionospheric conduction current and interhemispheric magnetic-field-aligned current. Equinoctial symmetric conductivity and neutral wind distributions are used to isolate the effects of the magnetic asymmetry about the geographic equator. In the case of the IGRF only, the coupling along realistic field lines permits the reproduction of an equinox local time shift between horizontal-current foci at all universal times. This is likely to explain the shift between the focus local times of Sq vortices that has been observed at equinox and at all universal times. The asymmetry due to nondipolar geomagnetic field distortions is found to be as efficient as conductivity and neutral wind asymmetries, which have been previously modeled, in driving Birkeland currents with an order of magnitude of 10−8 A m−2. The magnetic field asymmetry also turns out to be as important as wind and conductance asymmetries to specify the Birkeland current pattern.

Inference of Solar Magnetic Field Parameters from Data with Limited Wavelength Sampling

Abstract: We investigate the diagnostic potential of polarimetric measurements with filtergraph instruments. Numerical simulations are used to explore the possibility of inferring the magnetic field vector, its filling factor, and the thermodynamics of model atmospheres when only a few wavelength measurements are available. These simulations assume the magnetic Sun to be represented by Milne–Eddington atmospheres. The results indicate that two wavelength measurements are insufficient to reliably determine the magnetic parameters, regardless of whether magnetograph techniques or least-squares fitting inversions are used. However, as few as four measurements analyzed with the inversion technique provide enough information to retrieve the intrinsic magnetic field with an accuracy better than 10% in most cases.

Energetics of numerical geodynamo models

Abstract: SUMMARY Global energy balances provide a useful framework for assessing the operation of numerical geodynamo models. We apply a spectral decomposition to the magnetic and kinetic energy equations to assess how the magnetic field is regenerated by convection in these models. Specific analysis of the Kuang and Bloxham model indicates that dynamo action relies on the combined effects of buoyant upwelling and shear in the zonal flow. The part of the flow that contributes most to the generation of the dipole field is associated with a narrow range of local magnetic Reynolds number around Rm ≈ O(1). Shear in the zonal flow converts the dipole field into a strong toroidal field. The equilibration of field generation is revealed in the time-dependent exchanges of kinetic and magnetic energies. We also assess the turbulent cascade of energy to small scales. Transfer of kinetic energy to small scales is represented by a turbulent viscosity, which varies substantially with the length scale of the motion. This result implies that models for turbulent viscosity should depend on the wavenumber of the motion.

Transient Modulation of Cosmic Ray Intensity: Role of Magnetic Clouds and Turbulent Interaction Regions

Abstract: Two distinct regions of shock-associated magnetic clouds, (i) magnetically turbulent regions formed due to interaction between magnetic cloud and ambient magnetic field i.e. turbulent interaction region (TIR), and magnetically quiet region called magnetic cloud have been considered separately and correlation of interplanetary plasma and field parameters, magnetic field strength (B) and solar wind speed (V), with cosmic ray intensity (I) have been studied during the passage of these two regions. A good correlation between B and I and between V and I has been obtained during the passage of sheath when the magnetic field is high and turbulent, while these correlation have been found to be poor during the passage of magnetic clouds when the field is strong and smooth. Further, there is a positive correlation between enhancement in field strength and its variance in the sheath region. These results strongly support the hypothesis that most Forbush decreases are due to scattering of particles by region of enhanced magnetic turbulence. These results also suggest that it will provide a better insight if not the magnetic field enhancement alone but in addition, the nature of magnetic field enhancement is also considered while correlating the field enhancements with depressions in cosmic rays.

Constraints on the Solar Internal Magnetic Field from a Buoyancy Driven Solar Dynamo

Abstract: We report here results from a dynamo model developed on the lines of the Babcock-Leighton idea that the poloidal field is generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic recipe – wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field B c (105 G). The erupted toroidal field is then acted upon by the α-effect near the surface to give rise to the poloidal field. In this paper we study the effect of buoyancy on the dynamo generated magnetic fields. Specifically, we show that the mechanism of buoyant eruption and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining the magnitude and distribution of the magnetic field there. We also believe that a critical study of this mechanism may give us new information regarding the solar interior and end with an example, where we propose a method for estimating an upper limit of the difusivity within the overshoot layer.

Refinement of the Oblique Dipole Model in the Evolution of Rotary Motion of a Charged Body in the Geomagnetic Field

Abstract: When solving problems related to the induction of the Earth's magnetic field, the potential of which is expressed in the form of a series of spherical harmonic functions, it is necessary to use an approximate model of the geomagnetic field that satisfies the two conflicting requirements of simplicity and accuracy. As is noted in [3, p. 10], at the stage of design of satellites, especially at the stage of preliminary analysis of their dynamics, simple models of the geomagnetic field are usually employed. This offers additional possibilities for theoretical analysis of the problem. The averaged model and the model of a right dipole are just such simple models. The quadrupole model of the geomagnetic field developed in [4] is more accurate, but also more complex. The model of an oblique or skewed dipole is intermediate. The quadrupole model generalizes the simpler models mentioned above, and its analysis allows estimation of the accuracy of each model. It turns out that the oblique dipole model, which differs from the model of a right dipole by small correcting terms, does not take into account other correcting terms caused by the quadrupole part of the geomagnetic field, which are greater in magnitude. The evolution of the rotary motion of a charged rigid body in the geomagnetic field is considered, and the incorrectness of the oblique dipole model is demonstrated. The effect of the quadrupole component of the geomagnetic field on the body dynamics is revealed.

Zonal structure and meridional drift of large-scale solar magnetic fields

Abstract: Digitized synoptic charts of photospheric magnetic fields were analyzed for the past 4 incomplete solar activity cycles (1969–2000). The zonal structure and cyclic evolution of large-scale solar magnetic fields were investigated using the calculated values of the radial B r, |B r|, meridional B θ, |B θ|, and azimuthal B φ, |B φ| components of the solar magnetic field averaged over a Carrington rotation (CR). The time–latitude diagrams of all 6 parameters and their correlation analysis clearly reveal a zonal structure and two types of the meridional poleward drift of magnetic fields with the characteristic times of travel from the equator to the poles equal to ∼16–18 and ∼2–3 years. A conclusion is made that we observe two different processes of reorganization of magnetic fields in the Sun that are related to generation of magnetic fields and their subsequent redistribution in the process of emergence from the field generation region to the solar surface. Redistribution is supposed to be caused by some external forces (presumably, by sub-surface plasma flows in the convection zone).

On the interaction of the solar wind with the interstellar medium: Field aligned MHD flow

Abstract: [1] We discuss the role of the local interstellar magnetic field on the interaction of the solar wind with the local interstellar medium (LISM). In particular, we consider the case in which the LISM flow is aligned with the magnetic field. The solar wind inside the termination shock is spherically symmetric, and the LISM magnetic field varies between 0.0 and 3.5 μG. Our simulations show that for the case of field-aligned flow, increasing the interstellar magnetic field intensity Bis moves the bow shock (when it exists) and heliopause further outward from the Sun. This effect is explained in terms of the force on the plasma. We compare our results with comparable simulations carried out by Aleksashov et al. [2000]. The comparison leads to the conclusion that the mutual dependence between the neutral hydrogen number density and the strength of the magnetic field characterizes the solution for the interaction between the solar wind and interstellar medium. For a given interstellar neutral hydrogen number density (assumed to be available with present observations, or more exactly obtained from future observations), and for a given LISM magnetic field direction and intensity, the model gives distances to the heliopause and the termination shock, and the flow structure ahead of the heliopause. Thus the model can be used to estimate the LISM magnetic field intensity consistent with the observations.

The spinning Astrid-2 satellite used for modeling the Earth's main magnetic field

Abstract: The Swedish micro-satellite Astrid-2 was successfully launched into a near polar orbit in December 1998. Despite the fact that the primary science mission was auroral research, the magnetic instrument was designed to accomplish high-resolution and high-precision vector field magnetic measurements, and therefore mapping of the Earth's magnetic field was possible. The spacecraft spins about a highly stable axis in space. This fact and the globally distributed data make the magnetic measurements well suited for the estimate of a magnetic field model at the spacecraft altitude (about 1000 km). This paper describes the initial analysis of the Astrid-2 magnetic data. As a result of the study of a single day (February 7, 1999), magnetically fairly quiet, it was possible to in-flight adjust the calibration of the magnetometer and find a magnetic field model fitting the scalar component of the measurements to better than 5 nT/sub rms/ for latitudes equatorward of 50/spl deg/. Several methods for field modeling are discussed in this paper under the assumption that the direction of the spin axis in inertial space is nearly constant, and this assumption is corroborated by the observations. The approximate inertial orientation of the magnetometer could then be determined simultaneously with the instrument intrinsic calibration and the estimate of main field model coefficients.

On the motion of charged particles in a sheared force-free magnetic field

Abstract: This paper considers single-particle trajectories in a planar sheared force-free magnetic field. A specific feature of this magnetic configuration is the absence of both gradient and curvature magnetic drifts, as well as a diamagnetic force along field lines. Therefore, in the framework of the drift approximation, the motion of the particle guiding centre does not feel the magnetic field's non-uniformity. Here we discuss how the latter affects actual particle trajectories, making them quite different from simple circular gyromotion even when the Larmor radius is small. It is also shown how magnetic confinement ceases to work when the Larmor radius becomes comparable to the spatial scale of the field variation.

Stokes parameters of resonance lines scattered by a moving, magnetic medium - Theory of the two-level atom

Abstract: The aim of the present work is to present theoretical results on the Stokes parameters of a resonance spectral line, scattered by moving atoms (or ions) in the presence of a local magnetic field. We assume that the scattered line is sensitive to the Hanle effect due to the magnetic field and also to Doppler redistribution due to the atomic motions. The present theory is developed for a two-level atom, in the framework of the density matrix formalism Blum (1981). Analogous results given in Sahal-Brechot et al. (1986) for the magnetic-field effect alone, and in Sahal-Brechot et al. (1998) for the velocity-field effect alone, can be obtained from our theory by cancelling in the equations, respectively, the velocity field or the magnetic field. The results of our theory are general and can be used for astrophysical studies concerning the Hanle effect and the Doppler redistribution effect on the linear polarization parameters of the scattered radiation. They can be used particularly to interpret linear polarization of coronal spectral lines to get a complete determination of veetorial quantities such as the coronal magnetic field and the solar wind velocity field vectors. As an application, the atomic velocity field distribution is supposed to be Maxwellian with a drift velocity field vector. This latter describes the macroscopic motion of the scattering atoms. In the solar corona, it can be assimilated into the solar wind velocity field vector.