Dusk [dusk]


The Earth's external magnetic field is caused by a complex interaction of several systems of moving charged particles. One such system consists of currents in the ionosphere (approximately 100 km above the Earth), in the magnetosphere (several Earth radii apart), and between the two regions. The latter are known as 'field-aligned currents' because they mostly flow in the direction of the Earth's main field. The above plot illustrates the density of the radial portion of these currents at an altitude of 450 km. This plot is for one local time only (dusk) and is not a 'snap shot' of the Earth at any one universal time. Areas of red are where the currents are coming out of the ionosphere and areas of blue are where the currents are going down into the ionosphere (nA/m*m). As illustrated in the plot, the density is more intense in the polar regions where currents flow mostly inward in the polar caps but alternate directions at the boundary of the auroral region, corresponding to the well known Region 1 and Region 2 currents, respectively. Also prevalent in this plot is a current system flowing upward at the dip equator and downward on either side. This is the radial portion of the meridional current system of the Equatorial Electrojet. The Equatorial Electrojet is an intense ionospheric current which is caused by solar irradiation and is present in the dusk data but not in the dawn data.

These radial currents produce a toroidal magnetic field that was detected by the MAGnetic Field SATellite (Magsat, from 1979-1980) as a small part of the total field the satellite measured. The effect of these toroidal fields have been included in the latest field models produced at NASA's Goddard Space Flight Center. These models represent main field and secular variation as well as ionospheric and magnetospheric contributions and their variability with season, solar cycle, and level of magnetospheric activity.

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