Polar cap arcs: Sun-aligned or cusp-aligned?

•A new concept is developed: polar cap arcs are cusp aligned.•Improvement to the existing understanding: polar cap arcs are sun-aligned.•The new concept consistently explains the different orientations of polar cap arcs observed by ground and satellite based imagers.

Polar cap arcs are often called sun-aligned arcs. Satellite observations reveal that polar cap arcs join together at the cusp and are actually cusp aligned. Strong ionospheric plasma velocity shears, thus field aligned currents, were associated with polar arcs and they were likely caused by Kelvin–Helmholtz waves around the low-latitude magnetopause under a northward IMF Bz. The magnetic field lines around the magnetopause join together in the cusp region so are the field aligned currents and particle precipitation. This explains why polar arcs are cusp aligned.