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.

Small-scale magnetic structures on the Sun

Abstract: The Sun provides us with a unique astrophysics laboratory for exploring the fundamental processes of interaction between a turbulent, gravitationally stratified plasma and magnetic fields. Although the magnetic structures and their evolution can be observed in considerable detail through the use of the Zeeman effect in photospheric spectral lines, a major obstacle has been that all magnetic structures on the Sun, excluding sunspots, are smaller than what can be resolved by present-day instruments. This has led to the development of indirect, spectral techniques (combinations of two or more polarized spectral lines), which overcome the resolution obstacle and have revealed unexpected properties of the small-scale magnetic structures. Indirect empirical and theoretical estimates of the sizes of the flux elements indicate that they may be within reach of planned new telescopes, and that we are on the verge of a unified understanding of the diverse phenomena of solar and stellar activity. In the present review we describe the observational properties of the smallscale field structures (while indicating the diagnostic methods used), and relate these properties to the theoretical concepts of formation, equilibrium structure, and origin of the surface magnetic flux.

A non‐force‐free approach to the topology of magnetic clouds in the solar wind

Abstract: [1] We present a new model for the topology of magnetic clouds at 1 AU that relates the magnetic field vector with the current density of the cloud without assuming the force-free condition. In addition, the model is formulated in such a way that it can be fitted directly to the data without the need of initially determining the flux rope axis by minimum variance. The model provides both the magnetic field strength and the direction in just one fitting procedure. In addition to the orientation of its axis and the relative closest-approach distance between the spacecraft and the cloud axis, the fit of the model to the experimental data allows an estimation of the current density of plasma inside the magnetic cloud. From our study on several clouds we conclude that this has values of the order of 10−12 C m−2 s−1. Compared with the force-free model, although the improvement gained in the χ2 values is not very great, the fitting procedure is shorter and easier, and the number of parameters involved has been reduced from seven to five. Our model also lets us check the validity of force-free approximation by the calculation of the value of the current density perpendicular to the flux rope, j⟂ = j × B/|B|. This component is assumed to be zero in the force-free model, but for all analyzed cases we obtain a nonzero value. This finite value implies the existence of pressure gradients inside the cloud that cannot be explained with a force-free approximation. In this paper, four cases are presented that have already been analyzed in the literature. We show that our model can explain the profiles of the magnetic field as a magnetic cloud passes a spacecraft.

Simple mechanism for reversals of earth's magnetic field.

Abstract: We show that a model, recently used to describe all the dynamical regimes of the magnetic field generated by the dynamo effect in the von Karman sodium experiment, also provides a simple explanation of the reversals of Earth's magnetic field, despite strong differences between both systems. The validity of the model relies on the smallness of the magnetic Prandtl number.

Self‐consistent modeling of magnetic fields and plasmas in the inner magnetosphere: Application to a geomagnetic storm

Abstract: [1] A geomagnetic storm model needs to take into account the coupling between magnetic field and plasma, as the storm-time field in the inner nightside magnetosphere can be very depressed compared to that of the Earth dipole, thus significantly modifying plasma transport. In this paper we extend our previous "one-way" coupling between a kinetic ring current model and a magnetospheric force-balance model to a fully magnetically self-consistent approach, in which the force-balanced fields are fed back into the kinetic model to guide its evolution. The approach is applied to simulating the 21-23 April 2001 "GEM Storm Challenge" event. We use boundary and initial conditions for the kinetic model from several spacecraft, and magnetic flux boundaries for the equilibrium code from an empirical magnetic field model. We find significant differences in the self-consistent results compared to those obtained from the kinetic model with a dipolar background field (with the same particle boundary conditions and electric fields), due mainly to changes in the particle drifts. In addition to large depressions in the nightside magnetic field values compared to a dipolar field, we also find significantly lower particle density and perpendicular plasma pressure in the inner magnetosphere in the self-consistent case, as well as local, narrow pressure peaks and strongly enhanced plasma β p in localized regions on the nightside.

An empirical electric field model derived from Chatanika radar data

Abstract: Plasma convection velocity observations obtained with the Chatanika, Alaska incoherent scatter radar have been used to prepare a quantitative empirical model of the convection electric field at auroral latitudes between 58°Λ and 75°Λ. Vector averaging of the individual days' data and smoothing across 2.5 hours of local time were used to reduce substorm perturbations. Tables of the vector electric field components at 300-km altitude have been prepared for moderately disturbed (Kp = 3) summer conditions. Resolution of the model is 0.5° of invariant latitude and one half hour of magnetic local time. The model field is asymmetric with the afternoon convection cell dominant in magnitude and spatial extent and corresponds to conditions in which the cross polar cap potential is approximately 70 kV.