The relatively good reception of distant radio signals at night results from the reflection of radio waves by the upper ionosphere. They are visible at night in the Northern Hemisphere at altitudes between 100 and 400 km. They are visible at night in the Southern Hemisphere at altitudes between 100 and 400 km. An aurora results from the interaction between the solar wind and the Earth's magnetic field. In the Northern Hemisphere, they are centered on the northwest tip of Greenland.
Auroral activity varies with the sun's activity.
When the sun is quiet, the auroral oval shrinks, but when the sun is active, the auroral oval expands equatorward, and the aurora may be visible across southern Canada and northern United States, or rarely, even further south.