When solar particles reach Earth, most are deflected into space by the magnetosphere, a magnetic field that protects the planet against harmful radiation. This invisible shield originates deep inside Earth, where convection currents in the molten outer core generate a strong magnetic dipole. Like a bar magnet, Earth has North and South Magnetic Poles connected by magnetic field lines.
The shape of the magnetic field is always changing. When conditions are calm, solar particles flow around the magnetosphere just as water flows around a large rock in a shallow stream. As the solar wind speed increases, the magnetic field compresses on the sunward side while a long magnetotail points away from the Sun on the night side.
Some particles penetrate into the magnetosphere and travel along magnetic field lines towards the Earth’s Magnetic Poles. Soon the particle will reach the upper atmosphere in the auroral oval, a circumpolar zone of concentrated geomagnetic activity that contracts and expands as it absorbs particles from space. During periods of intense solar activity, the oval normally positioned in the skies above northern Canada will stretch as far south as the southern US and Mexico. When activity decreases, the oval returns to northern latitudes.