Ringströmen

Ringströmen refers to circulating electrical currents arranged in ring-like structures in both terrestrial and space environments. In general electromagnetism, a ring current is a closed loop of current in a conductor or a model of current distribution that forms a torus around an axis. It produces a magnetic field and can be analyzed using Ampère’s law and the Biot–Savart law.

In space plasma physics, the term Ringströmen denotes the torus of energetic charged particles trapped around a planet in its magnetosphere. The ring current consists mainly of ions and electrons that drift westward due to gradient-curvature drift, forming a current system encircling the planet near the equator.

In Earth's magnetosphere the ring current is a major contributor to geomagnetic activity. During geomagnetic storms it intensifies as more energetic particles (typical energies from 10 keV to a few hundred keV) accumulate in the inner magnetosphere. The enhanced ring current produces a depression in the planet’s surface magnetic field, recorded as a negative Dst index. The current typically spans roughly 3 to 7 Earth radii and decays after storms over about one to two days.

The term can also apply to ring currents in laboratory devices and electric machines, where a current circulates in a ring-shaped conductor, affecting magnetic fields and mechanical design.