 Magnetometers can give an indication of auroral activity before the light from the aurora becomes visible.A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is then published on the K-index.While magnetometers can be used to help map basin shape at a regional scale, they are more commonly used to map hazards to coal mining, such as basaltic intrusions (dykes, sills, and volcanic plug) that destroy resources and are dangerous to longwall mining equipment.