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.

Spatial distribution of large-scale solar magnetic fields and their relation to the interplanetary magnetic field

Abstract: The spatial organization of the observed photospheric magnetic field, as well as its relation to the polarity of the interplanetary field, have been studied using high resolution magnetograms from Kitt Peak National Observatory. Systematic patterns in the large scale field have been found to be due to contributions from both concentrated flux and more diffuse flux. It is not necessary to assume, as has often been done in previous studies, that there is a weak background solar magnetic field causing the large-scale patterns in the photosphere, although the existence of such a field cannot be excluded. The largest scale structures in the photosphere correspond to the expected pattern at the base of a warped heliomagnetic equator. The polarity of the photospheric field, determined on various spatial scales, correlates with the polarity of the interplanetary field, with the most significant correlation due to mid-latitude fields. However, because the interplanetary field is likely to be rooted in concentrated photospheric regions, rather than across an entire polarity region, both the strength and polarity of the field are important in determining the interplanetary field. Thus studies of the interplanetary field which are based on either instrumental or numerical averaging of fields in the solar photosphere are subject to serious inherent limitations. Analyses based on several spatial scales in the photosphere suggest that new flux in the interplanetary medium is often due to relatively small photospheric features which appear in the photosphere up to one month before they are manifest at the Earth. The evolution of the over-all photospheric pattern may be due to individual sub-patterns which have slightly different rotation properties and which alternate in their relative dominance of the interplanetary medium.

Magnetic Field Data Correction in Space for Modelling the Lithospheric Magnetic Field

Abstract: The Earth’s magnetic field as it is measured by low-Earth orbit satellites such as Swarm and CHAMP results from the superposition of internal and external source fields overlapping in time and in space. The Earth’s lithospheric field is one of the weakest sources detectable from space and its accurate description requires treatments of rapidly-varying magnetic fields generated by current systems in the ionosphere and magnetosphere. In this paper, we review methods most commonly used in geomagnetism to identify and then to correct for the external perturbation fields at satellite altitudes. We document the pros and cons of Fourier Filtering, polynomial and Spherical Harmonics analyses, Singular Spectral Analysis (SSA) and Line-levelling techniques. The difficulties are illustrated with an application of the methods on a common set of real Swarm magnetic field measurements and with a discussion on the differences between lithospheric field models obtained with each treatment. We finally discuss some perspectives for improvements of external field correction techniques relying on statistical or more explicit assumptions about the geographical distribution as well as the shape and strengths of the external magnetic field structures.

Magnetospheric Convection in the Nondipolar Magnetic Field of Uranus

Abstract: A method for determining the magnetospheric convection electric field using simple analytic approximations under the assumption of uniform ionospheric conductivity is described, and applied to Uranus. Magnetic field models including quadrupole and octupole moments are used to determine the shape of the polar caps and the mapping of the electric field and parallel currents between ionosphere and magnetosphere. The asymmetry in the magnetic field models between the northern and southern hemispheres leads to the inclusion of currents between the hemispheres in order to satisfy the assumption of equipotential magnetic field lines. The results show that the quadrupole moment of the Uranian magnetic field strongly influences magnetospheric convection, and that a significant octupole moment will further alter the flow pattern. Even with these modifications the basic flow is sunward in the inner magnetosphere as inferred previously. The total current which flows along field lines between the two hemispheres due to the asymmetry of the magnetic field is comparable in magnitude to that of the region 1 current system. Time dependent calculations including a self-consistent electric field show that ring current shielding of the electric field is important and may have formed the most prominent features in the plasma observations made by Voyager 2. The effectiveness of the shielding can be influenced by the magnetic field model. Other features in the data are characteristic of substorm injection, and the model has been used to show that a combination of plasma injection and electric field shielding may be applicable to the interpretation of the Voyager 2 data.

Electric field fluctuations and possible dynamo effects in the solar wind

Abstract: Magnetohydrodynamic fluctuations in different kinds of solar wind have been investigated. Electric field fluctuation spectra have been obtained from the observed velocity and magnetic field fluctuations. The mean electromotive force e, generated by the turbulent motion of the solar wind plasma and field, turns out to depend upon the nature and Alfvenicity of the fluctuations. Dynamo theory predicts a linear relationship between e and the mean magnetic field Bo. Correlation studies carried out with the intention to establish this so-called alpha effect have given negative results.