Field-Aligned Currents

Field-aligned currents, sometimes called Birkeland currents after the Norwegian scientist who first proposed their existence in the early 1900s, are electric currents that flow along Earth's magnetic field lines connecting the magnetosphere to the ionosphere. Unlike currents that flow through a simple wire, these currents follow the curving shape of Earth's magnetic field, carrying electric charge between the outer magnetosphere and the upper atmosphere at high latitudes. They form large, organized systems that roughly encircle each magnetic pole and act as a crucial link in the chain of events connecting solar wind activity to visible effects on Earth: when the solar wind and interplanetary magnetic field interact with the magnetosphere, especially during geomagnetic storms and substorms, the strength of these field-aligned currents increases dramatically, driving more energy and charged particles down into the atmosphere. This directly powers the aurora, since the electrons carried by these currents collide with atmospheric gases and cause them to glow, and it is also a major contributor to geomagnetically induced currents that can affect power grids on the ground. Satellites orbiting through the auroral zone can directly measure field-aligned currents, giving scientists real-time insight into how much energy is currently flowing from space into Earth's upper atmosphere.