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.

Effects of Three-Dimensional Heliospheric Structures on Cosmic-Ray Modulation

Abstract: The theory of cosmic-ray transport in the heliosphere contains four distinct physical processes — diffusion, convection, adiabatic cooling, and drifts. The last of these has only recently been evaluated. Extrapolation of present understanding of the regions near the heliospheric equator to high heliographic latitudes leads to the conclusion that particle drift in the large-scale magnetic field plays an important role in cosmic-ray modulation. The large-scale, three-dimensional structure of the interplanetary magnetic field is therefore very important in understanding cosmic rays. Several key observed modulation effects are summarized, each of which is a natural consequence of drift, but which requires special assumptions if drift plays no role. It is concluded that particle drifts play an important and possibly dominant role in transport in the heliosphere.

A Comparison Between Nonlinear Force-Free Field and Potential Field Models Using Full-Disk SDO/HMI Magnetogram

Abstract: Measurements of magnetic fields and electric currents in the pre-eruptive corona are crucial to the study of solar eruptive phenomena, like flares and coronal mass ejections (CMEs). However, spectro-polarimetric measurements of certain photospheric lines permit a determination of the vector magnetic field only at the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field above multiple active regions with the help of a potential field and a nonlinear force-free field (NLFFF) extrapolation code over the full solar disk using Helioseismic and Magnetic Imager (SDO/HMI) data as boundary conditions. We compare projections of the resulting magnetic field lines with full-disk coronal images from the Atmospheric Imaging Assembly (SDO/AIA) for both models. This study has found that the NLFFF model reconstructs the magnetic configuration closer to observation than the potential field model for full-disk magnetic field extrapolation. We conclude that many of the trans-equatorial loops connecting the two solar hemispheres are current-free.

Energetics of numerical geodynamo models

Abstract: SUMMARY Global energy balances provide a useful framework for assessing the operation of numerical geodynamo models. We apply a spectral decomposition to the magnetic and kinetic energy equations to assess how the magnetic field is regenerated by convection in these models. Specific analysis of the Kuang and Bloxham model indicates that dynamo action relies on the combined effects of buoyant upwelling and shear in the zonal flow. The part of the flow that contributes most to the generation of the dipole field is associated with a narrow range of local magnetic Reynolds number around Rm ≈ O(1). Shear in the zonal flow converts the dipole field into a strong toroidal field. The equilibration of field generation is revealed in the time-dependent exchanges of kinetic and magnetic energies. We also assess the turbulent cascade of energy to small scales. Transfer of kinetic energy to small scales is represented by a turbulent viscosity, which varies substantially with the length scale of the motion. This result implies that models for turbulent viscosity should depend on the wavenumber of the motion.

The satellite along-track analysis in planetary magnetism

Abstract: SUMMARY Many physical characteristics of planets, such as their topography, magnetic and gravitational fields, are routinely detected and measured by spacecrafts. At satellite altitudes, even if little is known about the measured signal, it is possible to separate the large-scale components from other contributions by a spectral analysis carried out along the spacecraft orbit. This procedure, which dates back to the early age of the satellite era, is routinely applied with spherical harmonics analysis for the study of large-scale planetary magnetic fields, particularly those that vary rapidly in time. In this paper, we review the approximations of this procedure for investigating internal and external magnetic fields of planets. We show that the magnetic field analyses along the orbits are limited by a finite frequency resolution that is known to introduce a spectral leakage. This leakage may lead to artificial spatio-temporal variations of the magnetic field, such as an apparent internal field secular variation, an asymmetry of the magnetospheric field and a regional distortion of the lithospheric field structures. We quantify the errors for the Earth’s lithospheric field and display its distribution in the spatial and the spectral domains. We also discuss these limits for the Moon and Mars lithospheric magnetic fields. Our analytical results illustrate the pros and cons of the orbit-by-orbit analyses and should allow us to avoid the pitfall of geophysical overinterpretation of some artificial magnetic field variations.