An optical manifestation of a large-scale electrical discharge process which surrounds the Earth. The discharge is powered by the so-called solar wind–magnetosphere generator. The Sun continuously blows out its upper atmosphere, the corona, with a supersonic speed. This fully ionized and magnetized gas flow interacts with the Earth's magnetic field, resulting in a comet-shaped cavity (the magnetosphere) carved around the Earth, while the lines of force of the Earth's magnetic field and of the solar wind magnetic field interconnect. Electric power of as much as 10¹² W is generated as the solar wind blows across the interconnected field lines near the comet-shaped boundary. A part of the electric current (carried mainly by electrons) thus generated flows between the magnetospheric boundary and an annular, ring-shaped region of the polar upper atmosphere along the lines of force of the Earth's magnetic field.