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.

Simple Model for an Ion‐Exosphere in an Open Magnetic Field

Abstract: A simple model for an ion‐exosphere with an open magnetic field is set up. The ions move under the influence of (1) the gravitational field, (2) the monotonic decreasing static magnetic field, and (3) the electrostatic potential due to a small charge separation. Neglecting collisions and particle drift across the magnetic field lines the particles can be classified into four classes: ballistic, escaping, trapped, and incoming particles. For each class the number density, the particle flux, the momentum fluxes, and the energy flux are calculated as a function of the electrostatic potential. Finally, it is shown how this potential can easily be computed by considering two basic physical conditions: (1) the quasineutrality, and (2) the zero current condition.

Magnetic braiding and parallel electric fields

Abstract: The braiding of the solar coronal magnetic field via photospheric motions—with subsequent relaxation and magnetic reconnection—is one of the most widely debated ideas of solar physics. We readdress the theory in light of developments in three-dimensional magnetic reconnection theory. It is known that the integrated parallel electric field along field lines is the key quantity determining the rate of reconnection, in contrast with the two-dimensional case where the electric field itself is the important quantity. We demonstrate that this difference becomes crucial for sufficiently complex magnetic field structures. A numerical method is used to relax a braided magnetic field toward an ideal force-free equilibrium; the field is found to remain smooth throughout the relaxation, with only large-scale current structures. However, a highly filamentary integrated parallel current structure with extremely short length-scales is found in the field, with the associated gradients intensifying during the relaxation process. An analytical model is developed to show that, in a coronal situation, the length scales associated with the integrated parallel current structures will rapidly decrease with increasing complexity, or degree of braiding, of the magnetic field. Analysis shows the decrease in these length scales will, for any finite resistivity, eventually become inconsistent with the stability of the coronal field. Thus the inevitable consequence of the magnetic braiding process is a loss of equilibrium of the magnetic field, probably via magnetic reconnection events.

Limits on the possible intrinsic magnetic field of Venus

Abstract: Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined for possible surface-correlated features and any possible intrinsic magnetic moment. The field variations observed in the wake do not resemble those expected for a solar wind interaction with even a weak intrinsic magnetic field. Little orbit-to-orbit persistence of features is found in the magnetic records. The magnetic field measurements in the wake are averaged in 10 deg x 10 deg bins to minimize the effects of external field sources. In these 37 bins, the average fields appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, a maximum intrinsic magnetic dipole moment is obtained of 4.3 + or - 2.0 x 10 to the 21st G cu cm, approximately an order of magnitude less than previous estimates. It is noted that a more conservative estimate of the probable error of the mean is 5.5 x 10 to the 21st G cu cm. The Pioneer Venus measurements are thus consistent with zero planetary moment. The present measurements are found to be far below estimates made on the basis of angular momentum, the so-called magnetic Bode's law, and far below the dynamo scaling law of Busse.

Signature of the quiet-time magnetospheric magnetic field and its electromagnetic induction in the rotating Earth

Abstract: SUMMARY Accurate models of the magnetospheric field during magnetically quiet times are essential for high-resolution mapping of core field dynamics, mantle and ocean induction, crustal fields and ionospheric currents. Satellite data sampled at low-Earth orbit allow for a separate determination of the external contributions from currents in the magnetosphere. We have used Orsted and CHAMP data from the years 1999‐2004 to investigate this field component. In contrast to earlier studies, the field is decomposed here into contributions from sources in the solar-magnetic (SM) frame and those in the geocentric-solar-magnetospheric (GSM) frame. Fo ra nobserver on the Earth, stable fields in those frames generate different diurnal and annual variations which, in response, induce currents in the subsurface. All of these effects have been modelled here. Our primary findings are: in the GSM frame, there is a dominant constant magnetic field of about 13 nT, pointing due southward. This field component is attributed to the quiet-time tail current system. The interplanetary magnetic field (IMF) contributes to the near-Earth field with 10 per cent of its Bx and about 25 per cent of its By component. For the SM frame, we obtain a constant field of 7.6 nT and a variable part which can be parametrized by the DST index. The field in SM is attributed to the combined effect of the magnetopause and ring current. A comparison of the external field variations, predicted by our satellite-derived model, with the measurements of five latitudinally distributed ground observatories shows a remarkable agreement.

The interplanetary modulation and transport of Jovian electrons

Abstract: Based on simultaneous measurements by Pioneer 11 of the 3-6 MeV Jovian electron flux, interplanetary magnetic field magnitude, and solar wind speed, the interplanetary transport of energetic particles is studied. It is found that corotating interaction regions (CIR's) greatly inhibit electron transport across the average field direction. Cross-field transport is also influenced by the degree of compression of the solar wind since CIR's are areas of compressed solar wind plasma. The propagation of Jovian electrons is studied by a model that includes the effects of CIR's. The model tests whether or not the three-dimensional convection-diffusion theory adequately describes the cross-field transport of electrons. The model is also valid for Jovian electron observations from earth-orbiting satellites. The model may be further applied to 1 AU from the sun where it is found that the cross-field diffusion of electrons explains why Jovian electrons are detected at the earth even during periods when the interplanetary magnetic field does not connect the earth directly to Jupiter.