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.

Birkeland Currents: Present Understanding and Some Remaining Questions

Abstract: Kristian Birkeland determined that large-scale ionospheric currents were associated with the aurora. He deduced this from his surface magnetic field measurements in the polar regions at the beginning of the twentieth century. He suggested that these “auroral currents” originated far from the Earth and that they flowed into and away from the polar atmosphere along geomagnetic field lines. The existence of such field-aligned or Birkeland currents was widely disputed because it was not possible to unambiguously identify current systems that are field-aligned (Alfven, 1939; 1940) and those which are completely contained in the ionosphere (Vestine and Chapman, 1938) only from a study of surface magnetic field measurements. Observations acquired from a variety of rocket and satellite experiments have absolutely confirmed the presence of Birkeland currents and have demonstrated the important role that these intense currents (ranging between 106 and 107 amperes) play in the coupling of energy between interplanetary space and the lower atmosphere and ionosphere. The Birkeland currents are a critical ingredient in a variety of plasma processes on this planet (associated with aurora and radio emissions), on Jupiter (associated with Io-related radio emissions), and in our galaxy (related to comets and double radio sources, e. g., Alfven, 1981). Considerable effort has been directed, during the past number of years, in establishing the statistical characteristics of Birkeland currents. Some of this past work will be reviewed here and some major remaining questions will be posed.

Properties of the solar wind at 0.3 AU inferred from measurements at 1 AU

Abstract: A data set containing merged plasma and magnetic field measurements obtained aboard several spacecraft is presented. By using simple formulas resulting from the constant velocity approximation the field magnitude and direction and the plasma density at 0.3 AU are calculated from this data set. This is possible in about three quarters of the cases from June 1967 to July 1968 with the use of 6-hour averages. The relation between streams and sectors is examined. The cross correlation between the various parameters is calculated. It is found that the directional fluctuations of the magnetic field on this time scale are mainly due to interplanetary processes; however, the amplitude fluctuations of the magnetic field are decreased by interplanetary processes. The high-speed portions of interplanetary streams arise in regions of low plasma density. Since the data are at solar maximum, the implication is that the Skylab results for the relation between coronal holes and solar wind sources may hold for the entire solar cycle.

Limitations of force-free magnetic field extrapolations: revisiting basic assumptions

Abstract: Force-free extrapolations are widely used to study the magnetic field in the solar corona based on surface measurements. The extrapolations assume that the ratio of internal energy of the plasma to magnetic energy, the plasma-beta is negligible. Despite the widespread use of this assumption observations, models, and theoretical considerations show that beta is of the order of a few percent to more than 10%, and thus not small. We investigate what consequences this has for the reliability of extrapolation results. We use basic concepts starting with the force and the energy balance to infer relations between plasma-beta and free magnetic energy, to study the direction of currents in the corona with respect to the magnetic field, and to estimate the errors in the free magnetic energy by neglecting effects of the plasma (beta<<1). A comparison with a 3D MHD model supports our basic considerations. If plasma-beta is of the order of the relative free energy (the ratio of the free magnetic energy to the total magnetic energy) then the pressure gradient can balance the Lorentz force. This is the case in the solar corona, and therefore the currents are not properly described. In particular the error in terms of magnetic energy by neglecting the plasma is of the order of the free magnetic energy, so that the latter can not be reliably determined by an extrapolation. While a force-free extrapolation might capture the magnetic structure and connectivity of the coronal magnetic field, the derived currents and free magnetic energy are not reliable. Thus quantitative results of extrapolations on the location and amount of heating in the corona (through current dissipation) and on the energy storage of the magnetic field (e.g. for eruptive events) are limited.