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.

Nonaxisymmetric Instabilities of a Toroidal Magnetic Field in a Rotating Sphere

Abstract: It has been suggested that nonaxisymmetric instabilities of an axisymmetric toroidal magnetic field, independent of the existence of convective turbulence, provide an important dynamo effect for stars when the convective turbulence is largely suppressed by the magnetic field. We investigate analytically such three-dimensional magnetic instabilities arising from a purely toroidal magnetic field whose strength is proportional to distance from the rotation axis of a star. A three-dimensional perturbation approach is used in our stability analysis. The leading-order problem neglects dissipative effects and describes two- or three-dimensional hydromagnetic waves in spherical geometry. For the first time we are able to obtain general explicit solutions for the problem. Interesting properties of the hydromagnetic waves are revealed by the explicit solutions. We investigate the effect of ohmic dissipation on the stability of the toroidal magnetic field by first deriving the magnetic boundary layer solution and then matching it onto the interior leading-order solution. Furthermore, for the first time we are able to derive an explicit analytic expression for the growth rate of the magnetic instabilities. We find that the most unstable mode of the instabilities is always three-dimensional and characterized by small radial and latitudinal scales. Implications of these magnetic instabilities for the solar dynamo are discussed.

Theoretical determination of the form of the hollow produced in the solar corpuscular stream by interaction with the magnetic dipole field of the earth

Abstract: The interaction between a neutral stream of ionized solar corpuscles and a three-dimensional magnetic dipole representing the geomagnetic fleld is investigated. It is assumed that the stream is confined to the exterior and the magnetic field to the interior of a hollow, the boundary of which is defined by a thin current sheath. An approximate method of solution is applied, and results are presented for the coordinates of the trace of the boundary of the hollow in the meridian plane containing the sun-earth line and the dipole axis for several relative orientations. Results are presented for the trace in the equatorial plane for the case in which the dipole axis is normal to the sun-earth line. The corresponding problem in two dimensions is considered, and it is shown that the analogous approximate results are in good agreement with the results indicated by an exact solution of the same basic equations. (auth)

Sound velocity in a magnetic fluid

Abstract: A simple linearized hydrodynamical theory for magnetic fluids in the presence of a strong external magnetic field is presented. The equations are solved for a sound wave propagating at angle phi from the external field direction. The sound velocity is shown to be anisotropic, depending on phi . The anisotropy is estimated to be about 10-5 for a field of 104 G and shows an interesting frequency dependence.

Electrostatic potential in the auroral ionosphere derived from Chatanika Radar observations

Abstract: A technique is described for determining the latitudinal variation of the electrostatic potential associated with the ionospheric convection electric field. Using the north-south electric field component derived from radar convection velocity experiments, the integral of E• dl is taken northward along the magnetic meridian, starting at low latitudes. The radar data consist of up to 40 independent measurements of plasma convection spanning 15° of invariant latitude centered on Chatanika, Alaska (65°Λ), with half-hour temporal resolution. It has been found that (1) the electric field contributions to the potential at and below 60°Λ are small under most circumstances and (2) the latitudinal variation of the potential is smooth and regular, permiting the potentials to be contoured across local time. It is found from the experiments that the potential often varies uniformly over 10° latitude at dawn and dusk. Electric fields of 50 mV/m are common. It is also noted that the latitude of the greatest negative potential in the premidnight sector coincides with the Harang discontinuity in ionospheric currents. The potentials calculated from the measured plasma drifts exhibit a regular local time variation. Equipotential contours derived from the latitude-local time potential field obtained with the long-duration radar experiments, while not providing a snapshot of the instantaneous pattern, elucidate the large-scale diurnal variation of the electrostatic potential at auroral latitudes. From such contours it is found that a two-cell convection pattern with varying degrees of asymmetry is consistently present at auroral latitudes, that a cross-polar cap potential drop of 70–120 kV is present in moderately disturbed conditions, and that substorms perturb the potential pattern at all local times.

Perturbation of the radial and non-radial oscillations of a star by a magnetic field

Abstract: A generalization of the perturbation method is applied to the problem of the radial and the non-radial oscillations of a gaseous star which is distorted by a magnetic field. An expression is derived for the perturbation of the oscillation frequencies due to the presence of a weak magnetic field when the equilibrium configuration is a spheroid. The particular application to the homogeneous model with a purely poloidal field inside, due to a current distribution proportional to the distance from the axis of symmetry, and a dipole type field outside is considered in detail. The main result is that the magnetic field has a large and almost stabilizing effect on unstableg-modes, particularly on higher order modes. With the considered magnetic field the surface layers appear to have a large weight.