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.

A quantitative model of the magnetospheric magnetic field

Abstract: Quantitative representations of the magnetic fields associated with the magnetopause currents and the distributed currents (tail and quiet time ring currents) have been developed. These fields are used together with a dipole representation of the main field of the earth to model the total vector magnetospheric magnetic field. The model is based on quiet time data averaged over all ‘tilt angle’ values. The weak field in the equatorial region of the inner magnetosphere and the tail field structure are included in the model. The depressed field region in the inner magnetosphere is especially important for the accurate modeling of several observed particle and magnetic field phenomena. The field representation is analytic and is given in Cartesian coordinates with power series and exponential terms. It is valid from the subsolar magnetosphere to beyond lunar orbit. This series expansion allows the magnetic field to be modeled accurately over an extended region of the magnetosphere and permits the representation of the field in the same region as that of its source currents. The model has been tested by using it to calculate several observed magnetospheric particle and field properties. The latitude cutoffs for solar cosmic rays and the trapping boundary of low-energy particles computed from the model agree well with observations. Model calculations also yield field line shapes in agreement with barium cloud observations. A simplified version of the model that can be used out past geosynchronous orbit is also presented.

The magnetic field of Mercury, 1

Abstract: An updated analysis and interpretation are presented of the magnetic field observations obtained during the Mariner 10 encounter with the planet Mercury on March 29, 1974. The combination of data relating to position of the detached bow shock wave and magnetopause and the geometry and magnitude of the magnetic field within the magnetospherelike region surrounding Mercury lead to the conclusion that an internal planetary field exists with dipole moment approximately 5.1 × 1022 G cm³. The limited data set precludes quantitative determination of an intrinsic field more complex than a centered dipole. The dipole axis has a polarity sense similar to that of earth and is tilted 7° from the normal to Mercury's orbital plane. The magnetic field observations reveal a significant distortion of the modest Hermean field (350 γ at the equator) by the solar wind flow and the formation of a magnetic tail and neutral sheet which begins close to the planet on the night side. Presently, an active dynamo mechanism in the planetary interior appears to be favored in the interpretation of the field origin, although fossil remanent magnetization cannot be excluded. The composite data set is not consistent with a complex induction process driven by the solar wind flow.

Nonlinear Force-Free Modeling of Coronal Magnetic Fields. II. Modeling a Filament Arcade and Simulated Chromospheric and Photospheric Vector Fields

Abstract: We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional, and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model. When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though, like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution.

The sub-Alfvénic interaction of the Galilean satellites with the Jovian magnetosphere

Abstract: Recent observations by the Galileo spacecraft and Earth-based techniques have motivated us to reconsider the sub-Alfvenic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub-Alfvenic, low-beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfven wing as one extreme case and the Jovian ionosphere as the other extreme case. (2) The addition of a small magnetic field of internal origin does not modify the general Alfven wing model qualitatively but only quantitatively. All magnetic moments discussed in the literature for In are small in this sense. For an aligned internal dipole and ambient Jovian magnetic field the interaction will be enhanced by focusing of the electric field. (3) A qualitative change occurs by the additional occurrence of closed magnetic field lines for larger internal magnetic fields as in the case of Ganymede. Here the focusing is even enhanced. (4) The first discussion of nonstationary plasma flows at the satellites shows that electromagnetically induced magnetic fields may play an important role if the satellite interiors are highly conducting. From the point of view of the external excitation, induction effects may be strong for Callisto, In, Europa, and Ganymede in order of decreasing importance. The magnetic field observations at the first Callisto encounter can be explained by these effects.