Showing papers on "Dipole model of the Earth's magnetic field published in 2005"

Modeling the dynamics of the inner magnetosphere during strong geomagnetic storms

Abstract: [1] This work builds on and extends our previous effort (Tsyganenko et al, 2003) to develop a dynamical model of the storm-time geomagnetic field in the inner magnetosphere, using space magnetometer data taken during 37 major events in 1996–2000 and concurrent observations of the solar wind and interplanetary magnetic field (IMF) The essence of the approach is to derive from the data the temporal variation of all major current systems contributing to the distant geomagnetic field during the entire storm cycle, using a simple model of their growth and decay Each principal source of the external magnetic field (magnetopause, cross-tail current sheet, axisymmetric and partial ring currents, and Birkeland current systems) is driven by a separate variable, calculated as a time integral of a combination of geoeffective parameters NλVβBsγ, where N, V, and Bs are the solar wind density, speed, and the magnitude of the southward component of the IMF, respectively In this approach we assume that each source has its individual relaxation timescale and residual quiet-time strength, and its partial contribution to the total field depends on the entire history of the external driving of the magnetosphere during a storm In addition, the magnitudes of the principal field sources were assumed to saturate during extremely large storms with abnormally strong external driving All the parameters of the model field sources, including their magnitudes, geometrical characteristics, solar wind/IMF driving functions, decay timescales, and saturation thresholds, were treated as free variables, and their values were derived from the data As an independent consistency test, we calculated the expected Dst variation on the basis of the model output at Earth's surface and compared it with the actual observed Dst A good agreement (cumulative correlation coefficient R = 092) was found, in spite of the fact that ∼90% of the spacecraft data used in the fitting were taken at synchronous orbit and beyond, while only 37% of those data came from distances 25 ≤ R ≤ 4 RE The obtained results demonstrate the possibility to develop a truly dynamical model of the magnetic field, based on magnetospheric and interplanetary data and allowing one to reproduce and forecast the entire process of a geomagnetic storm, as it unfolds in time and space

Magnetic field generation from cosmological perturbations

Abstract: In this Letter, we discuss the generation of magnetic field from cosmological perturbations. We consider the evolution of three component plasma (electron, proton, and photon) evaluating the collision term between electrons and photons up to the second order. The collision term is shown to induce electric current, which then generates magnetic field. There are three contributions, two of which can be evaluated from the first-order quantities, while the other one is fluid vorticity, which is purely second order. We estimate the magnitudes of the former contributions and show that the amplitude of the produced magnetic field is about approximately 10(-19) G at 10 Mpc comoving scale at the decoupling. Compared to astrophysical and inflationary mechanisms for seed-field generation, our study suffers from much less ambiguities concerning unknown physics and/or processes.

A non-adiabatic analysis for axisymmetric pulsations of magnetic stars

Abstract: This paper presents the results of a non-adiabatic analysis for axisymmetric non-radial pulsations including the effect of a dipole magnetic field. Convection is assumed to be suppressed in the stellar envelope, and the diffusion approximation is used to radiative transport. As in a previous adiabatic analysis, the eigenfunctions are expanded in a series of spherical harmonics. The analysis is applied to a 1.9-M ○. , main-sequence model (log T eff = 3.913). The presence of a magnetic field always stabilizes low-order acoustic modes. All the low-order modes of the model that are excited by the κ-mechanism in the Hen ionization zone in the absence of a magnetic field are found to be stabilized if the polar strength of the dipole magnetic field is larger than about 1 kG. For high-order p modes, on the other hand, distorted dipole and quadrupole modes excited by the κ-mechanism in the H ionization zone remain overstable, even in the presence of a strong magnetic field. It is found, however, that all the distorted radial high-order modes are stabilized by the effect of the magnetic field. Thus, our non-adiabatic analysis suggests that distorted dipole modes and distorted quadrupole modes are most likely excited in rapidly oscillating Ap stars. The latitudinal amplitude dependence is found to be in reasonable agreement with the observationally determined one for HR 3831. Finally, the expected amplitude of magnetic perturbations at the surface is found to be very small.

The Solar Surface Toroidal Magnetic Field

Abstract: The solar cycle of magnetic activity is thought to be a consequence of a dynamo process in which a dipole field produces a toroidal field from differential rotation (called the Ω-effect) and a twisting process produces a dipole field from the toroidal field (called the α-mechanism). These two magnetic field components are alternately destroyed and recreated in a cycle that lasts in total 22 years. Although the dipole field of the Sun has long been observed and studied, the toroidal field has never before been detected or measured. Our analysis uses solar rotation to yield meridional and east-west components of velocity and magnetic field vectors from the observed line-of-sight projection of the field. Our analysis of 18.5 yr of data from the 150 foot solar tower telescope on Mount Wilson using this method reveals for the first time a clear signal of a reversing toroidal magnetic field on the solar surface with strength comparable to that of the well-observed dipole component of the global magnetic field. The meridional velocities show a zone of convergence near latitudes of 60° during much of the observed period. Such flow convergence implies the subsidence of the toroidally magnetized fluid in this zone. If the toroidal field occupies the bulk of the polar regions of the Sun's convective envelope, then there is enough magnetic flux to reverse and rebuild the toroidal field at the convective-radiative interface known as the tachocline that is at the inner boundary of the Sun's convective envelope. These two steps—the creation of a toroidal field at high latitudes and a mechanism to reverse the tachocline toroidal field—are parts of the dynamo process that are prominent in models but have not previously had direct observational support.

On the Relation between Reconnected Magnetic Flux and Parallel Electric Fields in the Solar Corona

Abstract: Analytical theory and kinematic models are applied to magnetic reconnection in the solar corona. The emphasis of the present investigation is on the relation between the reconnection electric field, the reconnection rate, and the change in magnetic connectivity among photospheric magnetic footpoints. The results are not tied to the presence or absence of specific topological features of the magnetic field, such as a separatrix layer or separators. It is shown that the critical element in determining the location of ribbon-like bright features on the solar surface may be the parallel electric field, integrated along magnetic field lines. A general relation between the change of the reconnected magnetic flux and the integrated parallel electric field is derived. The results of this analysis are applied to the solar coronal case by means of two kinematic models, one of which affords a fully analytical treatment. The results show that reconnection-induced changes of magnetic connectivity on the corona-photosphere interface are directly related to the maximum value of the field line-integrated parallel electric field.

Comparing magnetic field extrapolations with measurements of magnetic loops

Abstract: We compare magnetic field extrapolations from a photospheric magnetogram with the observationally inferred structure of magnetic loops in a newly developed active region. This is the first time that the reconstructed 3D-topology of the magnetic field is available to test the extrapolations. We compare the observations with potential fields, linear force-free fields and non-linear force-free fields. This comparison reveals that a potential field extrapolation is not suitable for a reconstruction of the magnetic field in this young, developing active region. The inclusion of field-line-parallel electric currents, the so called force-free approach, gives much better results. Furthermore, a non-linear force-free computation reproduces the observations better than the linear force-free approximation, although no free parameters are available in the former case.

Signature of the quiet-time magnetospheric magnetic field and its electromagnetic induction in the rotating Earth

Abstract: SUMMARY Accurate models of the magnetospheric field during magnetically quiet times are essential for high-resolution mapping of core field dynamics, mantle and ocean induction, crustal fields and ionospheric currents. Satellite data sampled at low-Earth orbit allow for a separate determination of the external contributions from currents in the magnetosphere. We have used Orsted and CHAMP data from the years 1999‐2004 to investigate this field component. In contrast to earlier studies, the field is decomposed here into contributions from sources in the solar-magnetic (SM) frame and those in the geocentric-solar-magnetospheric (GSM) frame. Fo ra nobserver on the Earth, stable fields in those frames generate different diurnal and annual variations which, in response, induce currents in the subsurface. All of these effects have been modelled here. Our primary findings are: in the GSM frame, there is a dominant constant magnetic field of about 13 nT, pointing due southward. This field component is attributed to the quiet-time tail current system. The interplanetary magnetic field (IMF) contributes to the near-Earth field with 10 per cent of its Bx and about 25 per cent of its By component. For the SM frame, we obtain a constant field of 7.6 nT and a variable part which can be parametrized by the DST index. The field in SM is attributed to the combined effect of the magnetopause and ring current. A comparison of the external field variations, predicted by our satellite-derived model, with the measurements of five latitudinally distributed ground observatories shows a remarkable agreement.

Observing, Modeling, and Interpreting Magnetic Fields of the Solid Earth

Abstract: Many Earth system processes generate magnetic fields, either primary magnetic fields or in response to other magnetic fields. The largest of these magnetic fields is due to the dynamo in the Earth's core, and can be approximated by a geocentric axial dipole that has decayed by nearly 10% during the last 150 years. This is an order of magnitude faster than its natural decay time, a reflection of the growth of patches of reverse flux at the core-mantle boundary. The velocity of the North magnetic pole reached some 40 km/yr in 2001. This velocity is the highest recorded so far in the last two centuries. The second largest magnetic field in the solid Earth is caused by induced and remanent magnetization within the crust. Controlled in part by the thermo-mechanical properties of the crust, these fields contain signatures of tectonic processes currently active, and those active in the distant past. Recent work has included an estimate of the surface heat flux under the Antarctic ice cap. In order to understand the recent changes in the Earth's magnetic field, new high-quality measurements are needed to continue those being made by Orsted (launched in 1999), CHAMP and the Orsted-2 experiment onboard SAC-C (both launched in 2000). The present paper is motivated by the advent of space surveys of the geomagnetic field, and illustrates how our way of observing, modeling, and interpreting the Earth's magnetic field has changed in recent years due to the new magnetic satellite measurements.

Particle motion in collapsing magnetic traps in solar flares. i. kinematic theory of collapsing magnetic traps

Abstract: We present a model of collapsing magnetic traps in magnetic field configurations associated with solar flares. The model is based on a kinematic description of the magnetic field obeying the ideal Ohm's law. The dynamic evolution of the models is given in terms of a time-dependent transformation from Eulerian to Lagrangian coordinates. The transformation can be used to determine the corresponding flow field, the magnetic field, and the electric field from given initial conditions. The theory is formulated for translationally invariant situations, but a fully three-dimensional version is also given. The effect of various transformations and initial conditions is discussed with a view to calculating charged particle orbits in the given electromagnetic fields.

External fields on the nightside of Mars at Mars Global Surveyor mapping altitudes

Abstract: [1] More than four years of data taken from Mars Global Surveyor during its Mapping Phase Orbits (360–420 km altitude) over low field regions were examined. The nightside magnetic field data were binned according to a proxy solar wind pressure calculated from the dayside measurements. When the crustal field contribution calculated from the internal field model (FSU90) is removed, the distribution of residuals is bi-valued in the sunward component. Pass-by-pass inspections of the data sometimes show a sudden reversal of field, which occur on successive passes. Analysis indicates that for these orbits MGS traverses a current sheet that separates the two lobes of Mars' magnetotail. These results indicate that on the nightside the major contributor to the external field is the draping of the Interplanetary Magnetic Field about the planet and that care must be taken when utilizing such data for modeling Mars' internal field.

Turbulent dynamo near tachocline and reconstruction of azimuthal magnetic field in the solar convection zone

Abstract: In order to extend the abilities of the αΩ dynamo model to explain the observed regularities and anomalies of the solar magnetic activity, the negative buoyancy phenomenon and the magnetic quenching of the α effect were included in the model, as well as newest helioseismically determined inner rotation of the Sun were used. Magnetic buoyancy constrains the magnitude of toroidal field produced by the Ω effect near the bottom of the solar convection zone (SCZ). Therefore, we examined two “antibuoyancy” effects: i) macroscopic turbulent diamagnetism and ii) magnetic advection caused by vertical inhomogeneity of fluid density in the SCZ, which we call the ∇ρ effect. The Sun's rotation substantially modifies the ∇ρ effect. The reconstruction of the toroidal field was examined assuming the balance between mean-field magnetic buoyancy, turbulent diamagnetism and the rotationally modified ∇ρ effect. It is shown that at high latitudes antibuoyancy effects block the magnetic fields in the deep layers of the SCZ, and so the most likely these deep-rooted fields could not become apparent at the surface as sunspots. In the near-equatorial region, however, the upward ∇ρ effect can facilitate magnetic fields of about 3000 – 4000 G to emerge through the surface at the sunspot belt. Allowance for the radial inhomogeneity of turbulent velocity in derivations of the helicity parameter resulted in a change of sign of the α effect from positive to negative in the northern hemisphere near the bottom of the SCZ. The change of sign is very important for direction of the Parker's dynamo-waves propagation and for parity of excited magnetic fields. The period of the dynamo-wave calculated with allowance for the magnetic quenching is about seven years, that agrees by order of magnitude with the observed mean duration of the sunspot cycles. Using the modern helioseismology data to define dynamo-parameters, we conclude that north-south asymmetry should exist in the meridional field. At low latitudes in deep layers of the SCZ, the αΩ dynamo excites most efficiency the dipolar mode of the meridional field. Meanwhile, in high-latitude regions a quadrupolar mode dominates in the meridional field. The obtained configuration of the net meridional field is likely to explain the magnetic anomaly of polar fields (the apparent magnetic “monopole”) observed near the maxima of solar cycles. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the magnetic topology of October/November 2003 events

Abstract: [1] We examine the topology of several magnetic ejecta events during October–November 2003. The Grad-Shafranov reconstruction results from ACE magnetic and plasma data show magnetic flux rope configurations. Some are large scale, with sizes of tenths of an AU, typical of magnetic clouds. Some are of sizes hundredths of an AU. Their characteristic parameters are reported. Magnetic field data from Wind spacecraft are utilized to compare the prediction from numerical modeling with the actual measurement. Poor to good agreement is achieved. The deviations are discussed, particularly in the context of extremely high-speed solar wind flows.

Effect of storm‐time plasma pressure on the magnetic field in the inner magnetosphere

Abstract: [1] We investigate the effect of plasma pressure on the magnetic field in the near-Earth magnetosphere (2 to 6.5 RE) during the major magnetic storm of October 21–25, 2001. For this we obtain a time series of “snapshots”, in each of which the magnetic forces are equilibrated by plasma pressure gradient forces. Each snapshot is computed using our 3-D equilibrium code, which is fed anisotropic pressure in the equatorial plane from a kinetic ring current model. As computational boundaries we use magnetic flux surfaces obtained from the T89 empirical model [Tsyganenko, 1989], parameterized by the appropriate Kp. We analyze the computed magnetic fields and electric currents at each stage of the storm. Our findings include significant (∼10) plasma β and large field depressions near Earth at the storm peak. The results clearly show the necessity of a magnetically self-consistent treatment of plasma transport in storm modeling.

Plasma expansion in the presence of a dipole magnetic field

Abstract: Simulations of the initial expansion of a plasma injected into a stationary magnetized background plasma in the presence of a dipole magnetic field are carried out in two dimensions with a kinetic ion, massless fluid electron (hybrid) electromagnetic code. For small values of the magnetic dipole, the injected ions have large gyroradii compared to the scale length of the dipole field and are essentially unmagnetized. As a result, these ions expand, excluding the ambient magnetic field and plasma to form a diamagnetic cavity. However, for stronger magnetic dipoles, the ratio of the gyroradii of the injected ions to the dipole field scale length is small so that they remain magnetized, and hence trapped in the dipole field, as they expand. The trapping and expansion then lead to additional plasma currents and resulting magnetic fields that not only exclude the background field but also interact with the dipole field in a more complex manner that stretches the closed dipole field lines. A criterion to distinguish between the two regimes is derived and is then briefly discussed in the context of applying the results to the plasma sail scheme for the propulsion of small spacecraft in the solar wind.

Revised time‐of‐flight calculations for high‐latitude geomagnetic pulsations using a realistic magnetospheric magnetic field model

Abstract: [1] We present a simple time-of-flight analysis of Alfven pulsations standing on closed terrestrial magnetic field lines. The technique employed in this study in order to calculate the characteristic period of such oscillations builds upon earlier time-of-flight estimates via the implementation of a more recent magnetospheric magnetic field model. In this case the model employed is the Tsyganenko (1996) field model, which includes realistic magnetospheric currents and the consequences of the partial penetration of the interplanetary magnetic field into the dayside magnetopause. By employing a simple description of magnetospheric plasma density, we are therefore able to estimate the period of standing Alfven waves on geomagnetic field lines over a significantly wider range of latitudes and magnetic local times than in previous studies. Furthermore, we investigate the influence of changing season and upstream interplanetary conditions upon the period of such pulsations. Finally, the eigenfrequencies of magnetic field lines computed by the time-of-flight technique are compared with corresponding numerical solutions to the wave equation and experimentally observed pulsations on geomagnetic field lines.

Motion of a Charged Parand Plasma Equilibrium in a Dipole Magnetic Field – Can a Magnetic Field Trap a Charged Particle? Can a Magnetic Field Having Bad Curvature Trap a Plasma Stably?

Abstract: Since there is no magnetic monopole, the dipole magnetic field is the most elementary form of magnetic field. By studying carefully the motion of a single charged particle and the plasma equilibrium that results, one can obtain insight in the basic behavior of particle motion and equilibrium in a magnetic field. What will be the exact motion of a charged particle? What will be the guiding center motion? Are the equations of motion integrable? What happens if not? Is the plasma equilibrium in a magnetic field having bad curvature everywhere stable? This paper will cover these elementary questions using Hamiltonian dynamics.

Variations in cutoff latitude during selected solar energetic proton events

Abstract: Proton count rate measurements were obtained from the polar orbiting environmental satellites POES15 and POES16 during three solar proton events in 2001. From these data the invariant latitude of the cutoff in proton flux was determined at four local times. The satellites sample the edge of the northern polar cap at approximately 0300, 0830, 1230, and 1800 hours local time, covering the early morning, late morning, noon, and evening. During the event of 2 April the cutoff latitude for 35–70 MeV protons was almost constant, showing no more than 1.5° variation with local time. CME impacts occurred during the 24 September and 4 November events, the effects of which were to reduce the cutoff latitudes by between 5° and 8°. The equatorward displacement of the cutoff latitude was found to be strongly correlated with the magnetic storm indices (D st and SYM-H). Cutoff latitude prediction formulae are defined. Simulations using the Tsyganenko (2002) model of the magnetosphere are consistent with observations to within 2σ during quiet conditions and weak disturbances.

On the Origin of the Strong Intermittent Nature of Interplanetary Magnetic Field

Abstract: We briefly discuss the strong intermittent nature of the interplanetary magnetic field, observing that similarity between its statistical properties and the passive scalar ones exists, and may arise from different dynamical mechanisms.

On small scale magnetic structures in the solar photosphere

Abstract: The distribution of the magnetic field on the solar surface is as yet unknown in detail, but of considerable importance for solar physics in general. We have observed two different solar regions, one containing a small pore, the other region comprising a network in the light of the Fe I λ6301.5 A and Fe I λ6302.5 A lines at the German Vacuum Tower Telescope of the Observatorio del Teide (Tenerife, Spain) with the Gottingen Fabry-Perot Interferometer. By applying image reconstruction techniques to broad- and narrowband filtergrams we obtained continuum images, line core images, as well as line-of-sight velocity and magnetic field maps. We present scatter plots of the line core intensity vs. the Doppler velocity and the vertical component of the magnetic field, which reveal that the line core brightness is not a clear indicator for magnetic fields in the solar atmosphere. Furthermore, we estimate the swaying motion of flux tubes to be mostly smaller than 0'.'3 and thus in good agreement with the predictions of theoretical dynamic models. Finally, we show how the choice of the observed solar target influences the results and their interpretation. We claim that generalizations can mislead, and strongly depend on the presence or absence of solar (magnetic) features in the analyzed data.

Using magnetic fluids to simulate convection in a central force field in the laboratory

Abstract: Large-scale convection in planetary or stellar interiors plays a significant role but it is difficult to reproduce the central force field of those systems in experimental studies. A technique to approximate a central force field through the magnetic field from magnets acting on a magnetic liquid is presented. The thermomagnetic convection in a spherical shell filled with a magnetic liquid is analyzed in the context of a terrestrial laboratory using a 2D Finite Element model. Two configurations of magnetic fields were investigated, one resulting in a radially decreasing force field, and the other in a radially increasing force field. The results suggest that, while the actual force field does not reproduce the central gravity in planetary interiors accurately, it captures the essential qualitative character of the flow unlike other terrestrial experiments, which were either dominated by gravity or by a cylindrical radial force field. It is therefore suggested that such an experiment would provide a useful tool to investigate thermal convection in planetary interiors.

Induction effects on ionospheric electric and magnetic fields

Abstract: . Rapid changes in the ionospheric current system give rise to induction currents in the conducting ground that can significantly contribute to magnetic and especially electric fields at the Earth's surface. Previous studies have concentrated on the surface fields, as they are important in, for example, interpreting magnetometer measurements or in the studies of the Earth's conductivity structure. In this paper we investigate the effects of induction fields at the ionospheric altitudes for several realistic ionospheric current models (Westward Travelling Surge, Ω-band, Giant Pulsation). Our main conclusions are: 1) The secondary electric field caused by the Earth's induction is relatively small at the ionospheric altitude, at most 0.4 mV/m or a few percent of the total electric field; 2) The primary induced field due to ionospheric self-induction is locally important, ~ a few mV/m, in some "hot spots", where the ionospheric conductivity is high and the total electric field is low. However, our approximate calculation only gives an upper estimate for the primary induced electric field; 3) The secondary magnetic field caused by the Earth's induction may significantly affect the magnetic measurements of low orbiting satellites. The secondary contribution from the Earth's currents is largest in the vertical component of the magnetic field, where it may be around 50% of the field caused by ionospheric currents. Keywords. Geomagnetism and paleomagnetism (geomagnetic induction) – Ionosphere (electric fields and currents)

Kinetic Analysis on Plasma Flow of Solar Wind Around Magnetic Sail

Abstract: The magnetic sail utilizes the momentum of the solar wind (high-speed plasma flow) to produce the thrust by using an artificial magnetic field. Since the momentum flux from the sun is transformed into the thrust like a solar sail, no propellant is required and infinite Isp performance is available. Hence, it has a potential to reduce drastically the duration for a deep space mission, since it can accelerate the spacecraft up to its theoretical limit, that is, the speed of solar wind. In the present study, a small magnetic sail of 10m-size, which seems to be reasonable size as a test vehicle realizable within the present technology level on the space structure, is considered. Its acceleration performance and scales of the phenomena in relation to the solar wind flow around the magnetic sail are roughly estimated. Based on the estimation, the interaction of the solar wind with the applied magnetic field is numerically simulated by the full particle (PIC) method. Fundamental features of the flow field and the induced electromagnetic field around the small magnetic sail are clarified. Force acting upon the magnetic sail is also estimated by considering the Lorentz force generated by the induced electromagnetic field and the momentum change of the solar wind around the magnetic sail.

Variability of the ocean‐induced magnetic field predicted at sea surface and at satellite altitudes

Abstract: Spatial and temporal variability of the magnetic field component induced by ocean circulation is investigated on the basis of a standard thin-shell approximation of electro- and magneto-static equations. Well-known difficulties of numerical solution of the governing equations are resolved by reducing the problem to an equation for the electric field potential,(ef) as opposed to a more conventional approach focused on the vertical jump, (psi), of the magnetic field potential across a combined ocean/ marine-sediment-layer spherical shell. The present formulation permits using more realistic input data on ocean currents and ultimately yields much greater (by at least an order of magnitude) values of the magnetic field at sea surface than predicted in earlier studies. Such large values are comparable to, and in some cases exceed, magnetic field variations caused by lithospheric and ionospheric sources on monthly to interannual timescales. At the 400-km altitude (of CHAMP satellite), the field attains 6 nT. The model predictions show favorable comparisons with some in situ measurements as well as with Challenging Minisatellite Payload (CHAMP) satellite magnetometer data.

Forced magnetic reconnection at a neutral x-point

Abstract: The response of a two-dimensional X-type neutral point magnetic field to a quasi-static perturbation is considered. The latter is specified by a redistribution of the magnetic flux at a circular boundary centered on the neutral point. A special role of the dipole (m = 1) azimuthal perturbation component is revealed as the only one that results in the formation of a current sheet and subsequent magnetic reconnection. The excess magnetic energy released by such reconnective relaxation is also derived.