Field-Aligned Currents: The Electrical System That Powers the Northern Lights

Aurora is produced when energetic electrons collide with atmospheric gases in the ionosphere. But how do those electrons get there? The answer involves one of the more elegant structures in space physics: field-aligned currents, also called Birkeland currents — sheets and filaments of electrical current that flow along Earth's magnetic field lines between the magnetosphere and the ionosphere, carrying the energy that drives aurora generation.

What Field-Aligned Currents Are

Field-aligned currents are large-scale electrical currents that flow parallel to Earth's magnetic field lines — hence the name. They form a circuit connecting the magnetosphere to the ionosphere, transporting energy that has been input by the solar wind through magnetic reconnection down into the upper atmosphere where aurora is produced.

They are named after Kristian Birkeland, a Norwegian physicist who proposed their existence in the early 20th century based on his observations of aurora and laboratory experiments with magnetized spheres in plasma — an idea that was controversial for decades before satellite measurements confirmed it in the 1970s.

What helped me picture how they work: think of Earth's magnetic field lines as wires running between the magnetosphere and the ionosphere. Field-aligned currents are the electricity flowing through those wires — carrying energy from the large-scale magnetospheric system down to the thin layer of the upper atmosphere where aurora is produced. The ionosphere acts as the resistive load in this circuit, converting the electrical energy into light through particle collisions.

Why Field-Aligned Currents Matter for Aurora Travelers

Field-aligned currents are the direct mechanism connecting space weather activity to visible aurora. When magnetic reconnection in the magnetotail releases energy during a substorm, that energy travels to the ionosphere via field-aligned currents — accelerating electrons along field lines toward Earth. The location where those currents close in the ionosphere determines where aurora appears overhead, which is why aurora tends to form in structured arcs and curtains aligned with the auroral oval rather than appearing as a random glow.

The intensity of field-aligned currents scales with geomagnetic activity. During quiet periods, they carry modest energy and produce faint, stable auroral arcs. During substorm onset, current intensities surge — producing the sudden brightening and rapid structural development that defines the most active aurora displays. The Kp index and NOAA G-scale classifications are, in an indirect sense, measuring the overall strength of this current system.

For travelers, field-aligned currents are the reason aurora forms where it does — beneath the auroral oval, in the geographic belt where the current system closes most efficiently. Our Northern Lights Tour in Fairbanks places guests directly beneath this zone, where the current system delivers energy to the ionosphere overhead on the majority of active nights.

What Field-Aligned Currents Mean for Photographers

For photographers, field-aligned currents explain the structured geometry of aurora forms. Auroral arcs, curtains, and rays are aligned with the current sheets flowing along magnetic field lines — which is why aurora has the organized, linear structure it does rather than appearing as a diffuse, shapeless glow. The folding and waving of curtains reflects variations in the current intensity and the local magnetic field geometry.

During intense substorm activity, field-aligned current surges produce the most dramatic structural changes — the rapid brightening, the development of complex folded curtains, and the formation of the auroral corona when currents are flowing strongly along field lines converging overhead. Understanding that these forms are expressions of an organized electrical system — not random — helps photographers anticipate where structure is likely to develop and how it will move.

For more on the solar events that drive the energy input into this system, see our overview of solar cycles and the northern lights.

Return to the full Northern Lights Glossary to continue through the Earth's Magnetosphere and Auroral Structure section.