Satellite observations of currents and waves in space plasmas

Abstract

When viewed from outer space, the earth's magnetic field does not resemble a simple dipole, but is severely distorted into a comet-shaped configuration by the continuous flow of solar wind plasma. A complicated system of currents flows within this distorted magnetic field configuration called the ‘magnetosphere’ (See figure 1). For example, the compression of the geomagnetic field by the solar wind on the dayside of the earth is associated with a large-scale current flowing across the geomagnetic field lines, called the ‘Chapman-Ferraro’ or magnetopause current. The magnetospheric system includes large-scale currents that flow in the ‘tail’, the ring current that flows at high altitudes around the equator of the earth, field-aligned ‘Birkeland’ currents that flow along geomagnetic field lines into and away from the two auroral regions, and a complex system of currents that flows completely within the layers of the ionosphere, the earth's ionized atmosphere. The intensities of these various currents reach millions of amperes and are closely related to solar activity. The geomagnetic field lines can also oscillate, like giant vibrating strings, at specified resonant frequencies. The effects of these vibrations, sometimes described as ‘standing Alfvén waves’, have been observed on the ground in magnetic field recordings dating back to the beginning of the century. Observations of currents and waves with satellite-borne magnetic field experiments have provided a new perspective on the complicated plasma processes that occur in the magnetosphere. Some of the new observations are described here.