For over 400 years, we’ve understood that the Earth’s magnetic field is like that of a giant bar magnet. Compasses align with Earth’s field, pointing north to the magnetic pole in northern Canada. With the dawn of the space age, spacecraft observations and mathematical models confirmed speculations that the Earth’s magnetic field forms a bubble in space known as the magnetosphere, which shields us from the supersonic solar wind streaming out from the Sun. The solar wind compresses the magnetosphere on the side of the Earth facing the Sun, and stretches it out into a long magnetic tail—just like an airport windsock—on the side opposite the Sun. Scientists have also known for over 100 years that auroras are caused by electric currents. About fifty years ago it was discovered that the currents were carried by particles within this “magnetic windsock.” The particles move along magnetic lines of force towards Earth’s magnetic poles, gaining speed in a region about 2,000 km above Earth’s surface. They then collide with Earth’s upper atmosphere, causing it to glow, at an altitude between 100-200 km. The glowing lights are the Northern (and Southern) Lights, or auroras. They appear to us as luminous bands of green and white stretching across the night sky—but viewed from space, you see that they actually form oval bands around each of Earth’s two magnetic poles. The auroral ovals sometimes radically increase in brightness, width and size, up to several times in one day. This change is related to processes in Earth’s magnetosphere, collectively known as a substorm.