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.

Magnetic contour maps at the core-mantle boundary.

Abstract: Contour maps of the northward, eastward, and downward components of the main geomagnetic field and of the field intensity at the surface of earth's liquid core are presented and compared with similar maps at earth's surface. The mantle is assumed to be an insulator for purposes of the extrapolation. Maximum values of the individual field components are found to be amplified by factors of order 10-20 and the maximum field intensity by 12 over the value at earth's surface and the increase in small scale structure is evident. By comparing maps of places where Br=0 at different truncation levels, we find that even for a recent magnetic model which best fits the data quite well, contours of the field extrapolated to the core are quite sensitive to truncation level. This suggests a need to develop nonlinear data-fitting procedures if one's interest is in precise location of field contours rather than in estimating field values.

Nonlinear Force-Free Reconstruction of the Global Solar Magnetic Field: Methodology

Abstract: We present a novel numerical method that allows the calculation of nonlinear force-free magnetostatic solutions above a boundary surface on which only the distribution of the normal magnetic field component is given. The method relies on the theory of force-free electrodynamics and applies directly to the reconstruction of the solar coronal magnetic field for a given distribution of the photospheric radial field component. The method works as follows: we start with any initial magnetostatic global field configuration (e.g. zero, dipole), and along the boundary surface we create an evolving distribution of tangential (horizontal) electric fields that, via Faraday’s equation, give rise to a respective normal-field distribution approaching asymptotically the target distribution. At the same time, these electric fields are used as boundary condition to numerically evolve the resulting electromagnetic field above the boundary surface, modeled as a thin ideal plasma with non-reflecting, perfectly absorbing outer boundaries. The simulation relaxes to a nonlinear force-free configuration that satisfies the given normal-field distribution on the boundary. This is different from existing methods relying on a fixed boundary condition – the boundary evolves toward the a priori given one, at the same time evolving the three-dimensional field solution above it. Moreover, this is the first time that a nonlinear force-free solution is reached by using only the normal field component on the boundary. This solution is not unique, but it depends on the initial magnetic field configuration and on the evolutionary course along the boundary surface. To our knowledge, this is the first time that the formalism of force-free electrodynamics, used very successfully in other astrophysical contexts, is applied to the global solar magnetic field.

Three-dimensional magnetohydrodynamic simulations of processes at the earth's magnetopause

Abstract: Numerical simulation plays a key role in modeling and understanding highly nonlinear two- and three-dimensional plasma processes. A typical process of this type is represented by magnetic reconnection At the Earth's magnetopause magnetic reconnection is believed to be the cause of magnetic flux transfer events or structures which are interpreted in terms of rotational discontinuities. For both observations a clear correlation to a southward interplanetary magnetic field (i.e. large magnetic shear across the magnetopause) is a typical property. From theory and computer simulation it is known that a large magnetic shear is also a requirement for magnetic reconnection. In terms of MHD simulations two major approaches may be distinguished. One of these approaches considers the interaction of the total (dayside) magnetospheric system with the solar wind plasma. These simulations may provide qualitative insight into how the three-dimensional structure of the dayside magnetosphere controls dynamical pro...

Mean Field Theory Interpretation of Solar Polarity Reversal

Abstract: A mechanism of the polarity reversal of the solar magnetic field is explored on the basis of the mean field or turbulent dynamo theory. In the low-latitude region of the convective zone, the toroidal magnetic field, which is the origin of sunspots, is generated by the rotational motion of fluids, with the turbulent cross helicity as the intermediary. This field generates the poloidal field of dipole type through the alpha or turbulent helicity effect. The latter, in turn, contributes to the annihilation of the turbulent cross helicity, resulting in the decay of the toroidal magnetic field. This process indicates less room for the occurrence of the fully developed poloidal field in the low-latitude region and paves the way for the polarity reversal through the change of the sign of the turbulent cross helicity. A simple model mimicking the periodic polarity reversal is presented, and the relationship of the reversal period to the ratio of the poloidal to toroidal fields is given. The meridional-flow velocity at the solar surface is estimated, giving a result consistent with observations.

Magnetohydrodynamic simulation of an equatorial dipolar paleomagnetosphere

Abstract: [1] During polarity reversals of the Earth's internal geomagnetic field, the paleomagnetosphere must have undergone dramatic changes because the core field was essentially different from the present-day case, where the dipole moment is approximately aligned with the Earth's rotation axis (axial dipole) and perpendicular to the solar wind flow. According to one of the possible transition scenarios, the internal dipole moment, having much reduced in magnitude, could slowly turn around, staying close to the equatorial plane for a significant time. In this paper we present magnetohydrodynamic (MHD) simulations of a so-called equatorial dipolar paleomagnetosphere, where the internal dipole moment is perpendicular to the rotation axis, i.e., the dipole axis is in the equatorial plane. The magnitude of the dipole moment was chosen as small as one-tenth of the present value. With such a dipole field orientation, the dipole tilt in GSM coordinates changes, due to the Earth's rotation, between zero and 360 degrees in the course of the day, resulting in an extremely dynamic magnetosphere on the diurnal timescale. As a first approximation, we calculated steady-state solutions with different dipole tilts, or in other words with different angles between the dipole axis and the Sun-Earth line, to represent the paleomagnetosphere at different times of the day. We describe the regular diurnal variation of the geomagnetic field-line configuration and the topology of large-scale current systems, like the magnetopause currents and the tail current sheet. We investigate the so-called pole-on magnetosphere, where the dipole axis is aligned with the Sun-Earth line, in more details using different solar wind input parameters.