Dipole model of the Earth's magnetic field

About: Dipole model of the Earth's magnetic field is a research topic. Over the lifetime, 2756 publications have been published within this topic receiving 83021 citations.

Comparison of observed and model magnetic fields at high altitudes above the polar cap: POLAR initial results

Abstract: Data obtained from the high altitude, polar orbiting spacecraft, POLAR, are compared with the latest version of the data-based magnetospheric magnetic field model. The data generally agree well with the model. The major directional discrepancies at high altitudes are near the dayside cusp and on the “open” field lines over the polar cap, especially close to the boundary of the polar cap. Near the cusp, the agreement is improved if a stronger solar wind dynamic pressure and more negative IMF By and Bz are used as the model input parameters, than was actually observed. The field measured in the vicinity of the polar cusps is generally weaker than predicted by the model. Close to noon the spacecraft enters a region of additional structured field depression that appears to be the polar cusp proper. Within the limited statistics presented here, the invariant latitude of the cusp appears to be controlled by the north-south component of the IMF and the broad depression appears to be controlled by the tilt of the dipole.

How strong is the invisible component of the magnetic field in the Earth's core

Abstract: The maximum strength of toroidal magnetic field in the Earth's core is estimated on the basis of the stability of the magnetic field. In our model we take our basic magnetic field BO to be composed of both toroidal and poloidal axisymmetric decay modes of lowest order. While the strength of the poloidal component, BP is taken consistent with observation, the maximum strength of the toroidal field, |BT|max, is regarded as a parameter of the model. By demonstrating that viscous dissipation is of secondary importance and therefore that the results are independent of the parameter associated with viscosity, our model is eventually dependent on only one parameter: the ratio A of the maximum strength of the invisible toroidal field to the strength of the poloidal field at the pole of the core-mantle boundary. It is shown that |BT|max < 10|BP(θ = 0, r = ro)| in order that the basic magnetic field BO = BP + BT is stable, giving an estimated upper bound on strength of the invisible toroidal field of order 50 gauss.

Convective motions in the Earth's fluid core

Abstract: Convection in the Earth's core is affected by Lorentz and Coriolis forces. The relative importance of these is measured by the Elsasser number Λ. Previous work suggested that the optimum condition for convection occurs when the Elsasser number Λ is O(1); in particular, the critical modified Rayleigh number R* had a minimum value at some O(1) value of Λ. This gave rise to the view that the size of the magnetic field generated by the dynamo would adjust to this Λ-value because it optimised the convection. We have investigated convection in a rotating spherical shell with magnetic field distributions satisfying appropriate boundary conditions in the form of the toroidal decay modes which we believe are more realistic for the Earth's core. Our results are rather different from the classical picture. We find no optimum Elsasser number that minimises R*, but instead a monotonic decay of R* as Λ increases. At Λ ∼ 10, R* goes negative, so that rolls are driven by magnetic instability rather than convection. Naturally, as this regime is entered the rolls must be destroying magnetic field rather than creating it by dynamo action. This suggests that in the Earth's fluid outer core, the field strength is such that 1 < Λ < 10, so that the magnetic field is stable and the corresponding convection is dominated by large scales and is efficient. Another important feature of these solutions is that the azimuthal flow is primarily two-dimensional. This is surprising, as it has generally been believed that a magnetic field of this strength would break the Taylor-Proudman constraint. However, it appears that the magnetic field adjusts to preserve two-dimensionality even in the range 1 < Λ < 20.

Magnetic Component Model for Planar Structures Based on Transmission Lines

Abstract: Magnetic component models are quite complex if they take into consideration the variation of the field distribution in a three-dimensional (3-D) space. However, if the field distribution can be assumed to be one-dimensional (1-D), the magnetic component models can be drastically simplified because it is feasible to obtain accurate analytical expressions based on the solution of the Maxwell equations for a 1-D field distribution. The field distribution in magnetic components can be assumed to be 1-D when the field depends on one of its coordinates and the dependence on the other coordinates is negligible. Therefore, classical 1-D models have to be modified in order to be applied to planar transformers because their magnetic field vector has a constant direction, but its magnitude is not constant along that direction. This paper presents a 1-D model for planar magnetic transformers. Some comparisons between 2-D approaches and the proposed 1-D model have been carried out in order to show the accuracy of the proposed method.

Auroral effects on the Earth's electric field

Abstract: On three occasions of auroral appearance near the latitude of Minneapolis (October 7, November 13, and December 1, 1960), measurements of the earth's electric field and time constants of the atmosphere show abnormal conduction currents at the surface of the earth corresponding to 104 electrons cm−2 sec−1. A theoretical formulation of fair-weather electric field phenomena is given, and the measurements are interpreted in terms of this theory to show how measurements on the ground are related in general to current generators in the atmosphere.