Polar cap aurorae tell the time

North of ∼75 degrees magnetic latitude, deep within the polar cap, auroral forms oriented in the Sun-Earth direction have long been observed. However, it was not until the 1980s that the improved sensitivity of our optical instruments showed that such auroral forms are present about half the time, when the interplanetary magnetic field is close to zero or pointed northward. These auroral forms are always aligned towards the Sun. Within the central polar cap, when the Sun is well below the horizon, one can virtually tell the time by noting their direction as the Earth rotates under these special auroral forms; in effect, we have a gigantic polar sundial. Sun-aligned arcs throw new light on the nature of the Earth's upper polar atmosphere and its coupling to the solar wind.