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.

A comparative ray-trace study of whistler ducting processes in the Earth’s plasmasphere

Abstract: Numerical ray-trace calculations have been performed to investigate the whistler ducting characteristics of various field-aligned ionization density perturbations in the earth’s plasmasphere. The results indicate that field-line curvature has a major influence on both the ducted ray trajectories and on the probability of escape during interhemispheric propagation. A specific comparison is made of the single- and multiplehop ducting ability of “bell-shaped” versus “ledge-shaped” ducts as a function of the density perturbation and invariant latitude. Modest (20%) ledge- and bell-shaped perturbations, as well as a wide variety of “hybrid” profiles are capable of effective whistler ducting in middle latitudes, with ledge-shaped profiles having considerably better multiple-hop ducting characteristics. Furthermore, for a given duct the latitude range of starting locations from which lightning generated sferics can initially be trapped is considerably broader for ledge shaped profiles. Interhemispheric propagational losses from 20% ledge-shaped profiles in middle latitudes are between 5-12 db per hop compared to 8-20 db for similar bell-shaped profiles assuming specular reflection from the ionosphere. All transmitted downgoing rays consistently become “unducted” at altitudes near a few thousand kilometers due primarily to the ambient ionospheric topside plasma density gradients and magnetic field gradients and waves received on the ground should typically be observed at several degrees lower latitude than the ducting profile.

Problems and Progress in Astrophysical Dynamos

Abstract: Astrophysical objects with negligible resistivity are often threaded by large scale magnetic fields. The generation of these fields is somewhat mysterious, since a magnetic field in a perfectly conducting fluid cannot change the flux threading a fluid element, or the field topology. Classical dynamo theory evades this limit by assuming that magnetic reconnection is fast, even for vanishing resistivity, and that the largescale field can be generated by the action of kinetic helicity. Both these claims have been severely criticized, and the latter appears to conflict with strong theoretical arguments based on magnetic helicity conservation and a series of numerical simulations. Here we discuss recent efforts to explain fast magnetic reconnection through the topological effects of a weak stochastic magnetic field component. We also show how mean-field dynamo theory can be recast in a form which respects magnetic helicity conservation, and how this changes our understanding of astrophysical dynamos. Finally, we comment briefly on why an asymmetry between small-scale magnetic and velocity fields is necessary for dynamo action, and how it can arise naturally.

Observations of a solar‐wind‐driven modulation of the dayside ionospheric DPY current system

Abstract: The authors report here a collection of complementary measurement near local magnetic noon of a unique ionospheric convection variation observed by the Sondrestrom radar which is related to a nearly periodic variation ({approx} 25-30 min period) in the interplanetary magnetic field B{sub y} component. The authors observe also a poleward phase propagation of magnetic pulsations over a region limited in longitude and latitude near the dayside polar cusp. A series of poleward propagating radio absorption enhancements are observed in the Greenland and South Pole imaging riometer data. The pulsations and absorption enhancements are associated with latitudinally narrow and longitudinally limited intensifications of the westward convection and associated eastward Hall current, which propagates poleward over the magnetometers and radar field of view. For the cases presented the interplanetary B{sub z} is strongly negative, while the ionospheric variations are associated with the low-frequency component of variations in the interplanetary B{sub y} component. In contrast to the previously discovered traveling convection vortices, these features exhibit a poleward phase motion rather than one along lines of invariant latitude. The propagation velocity is slower ({approx} 0.5 - 1.0 km/s) and the structures cover 2 to 3 hours of local time. The authors interpret the observationsmore » as a poleward propagation of the DPY current system intensification associated with enhancements in the IMF B{sub y} component. Their observations indicate that the DPY field-aligned current system is propagating poleward and may be moving independent of the convection motion of the plasma and associated field lines. 51 refs., 14 figs., 1 tab.« less

Electromagnetic induction effects at an ocean coast

Abstract: This paper reviews recent progress in model calculations towards understanding electromagnetic induction effects at ocean coasts. Early models consisted of two adjacent quarter-spaces of different conductivity, whereas the newer models simulate the ocean with a very thin sheet of a perfect conductor placed on top of a uniform Earth medium. The inducing field is assumed to arise from a monochromatic plane wave incident vertically from above. With any of these models one succeeds at once in explaining the occurrence of large vertical magnetic fields when the inducing electric field is polarized parallel to the coast (E-polarization), thereby also confirming the highly directional character of the coast effects as discovered a few years before by Parkinson. Another important step was made when, first numerically, then analytically, the behavior of the horizontal component of the magnetic field at the surface was rigorously calculated. For H-polarization (inducing magnetic field parallel to shore) this horizontal surface field is uniform, but is not so for E-polarization. Indeed, it has now been shown that the surface field can vary appreciably close to the coast, particularly on the ocean side of shore. With E-polarization, very large currents flow in the ocean, parallel to shore, with the result that the effect of the coast is felt at very large distances over both land and sea. Under H-polarization induction the range of the coast effect is very much shorter, in fact almost an order of magnitude shorter over the land and even reducing to zero at the surface of the perfectly conducting model ocean. The magnetic fields at the ocean floor have also been calculated, which should be of interest in the rapidly expanding field of marine survey and prospecting.