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.

Present trends in the Earth's magnetic field

Abstract: WE outline here some of the implications of the latest in a series of models of the Earth' magnetic field derived in association with the World Magnetic Charts published by the British Hydrographic Office. The model, details of which are published elsewhere1, is defined by spherical harmonic coefficients representing the internal part of the magnetic field (to twelfth order and degree; 168 coefficients), its secular variation (to eighth order and degree; 80 coefficients), and its secular acceleration (the 26 most significant coefficients) at epoch 1975.0. Because of the greatly improved distribution, accuracy and quantity of data incorporated in the analysis, we believe that this model, particularly with regard to the secular variation, provides a significantly better representation of the Earth's magnetic field than any previous model. It is of interest to compare some of the parameters derived from this model with the trends shown by its predecessors.

Spectropolarimetric forward modelling of the lines of the Lyman-series using a self-consistent, global, solar coronal model

Abstract: Context. The presence and importance of the coronal magnetic field is illustrated by a wide range of phenomena, such as the abnormally high temperatures of the coronal plasma, the existence of a slow and fast solar wind, the triggering of explosive events such as flares and CMEs. Aims. We investigate the possibility of using the Hanle effect to diagnose the coronal magnetic field by analysing its influence on the linear polarisation, i.e. the rotation of the plane of polarisation and depolarisation. Methods. We analyse the polarisation characteristics of the first three lines of the hydrogen Lyman-series using an axisymmetric, self-consistent, minimum-corona MHD model with relatively low values of the magnetic field (a few Gauss). Results. We find that the Hanle effect in the above-mentioned lines indeed seems to be a valuable tool for analysing the coronal magnetic field. However, great care must be taken when analysing the spectropolarimetry of the Lα line, given that a non-radial solar wind and active regions on the solar disk can mimic the effects of the magnetic field, and, in some cases, even mask them. Similar drawbacks are not found for the Lβ and Lγ lines because they are more sensitive to the magnetic field. We also briefly consider the instrumental requirements needed to perform polarimetric observations for diagnosing the coronal magnetic fields. Conclusions. The combined analysis of the three aforementioned lines could provide an important step towards better constrainting the value of solar coronal magnetic fields.

Precise numerical estimation of the magnetic field generated around recombination

Abstract: We investigate the generation of magnetic fields from nonlinear effects around recombination. As tight-coupling is gradually lost when approaching $z\ensuremath{\simeq}1100$, the velocity difference between photons and baryons starts to increase, leading to an increasing Compton drag of the photons on the electrons. The protons are then forced to follow the electrons due to the electric field created by the charge displacement; the same field, following Maxwell's laws, eventually induces a magnetic field on cosmological scales. Since scalar perturbations do not generate any magnetic field as they are curl-free, one has to resort to second-order perturbation theory to compute the magnetic field generated by this effect. We reinvestigate this problem numerically using the powerful second-order Boltzmann code SONG. We show that: (i) all previous studies do not have a high enough angular resolution to reach a precise and consistent estimation of the magnetic field spectrum; (ii) the magnetic field is generated up to $z\ensuremath{\simeq}10$; (iii) it is in practice impossible to compute the magnetic field with a Boltzmann code for scales smaller than 1 Mpc. Finally we confirm that for scales of a few Mpc, this magnetic field is of order $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}29}\text{ }\text{ }\mathrm{G}$, many orders of magnitude smaller than what is currently observed on intergalactic scales.