Large-Scale Characteristics of Birkeland Currents

Abstract

During his polar expeditions of 1902–1903, Kristian Birkeland determined that large-scale ionospheric currents were associated with the aurora. Birkeland suggested that these currents originated far from the earth and that they flowed into and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned or Birkeland currents was disputed because it was not possible to unambiguously identify current systems that are field-aligned and those which are completely contained in the ionosphere only from a study of surface magnetic field measurements. The presence of Birkeland currents has been confirmed during the last decade with particle and magnetic field observations acquired from a variety of rocket and satellite experiments. The large-scale characteristics of Birkeland currents determined from a wide variety of observations, but principally from experiments on the near earth TRIAD and ISIS-2 satellites, are reviewed here. These include the relationship of the locations, flow directions, and intensities of Birkeland currents with the geomagnetic field, interplanetary magnetic field, visual and radar aurora, particle measurements, and auroral kilometric radiation. The Birkeland currents play an important role in the coupling of energy between the magnetosphere and lower ionosphere and atmosphere and must form the basis of any three-dimensional current system over a wide range of magnetospheric conditions. These same Birkeland currents are also the critical ingredient in a variety of complicated plasma phenomena that have important applications to this planet (associated with substorms and the aurora) to Jupiter (associated with Io-related radio emissions, Cloutier et al., 1978; Dessler and Hill, 1978), and to the galaxy (related to comets and double radio sources, Alfvén, 1977).