While space-based monitoring systems now allow scientists to detect CMEs heading towards Earth shortly after they occur, we cannot reliably predict whether a given CME will cause a major storm when it arrives. One of the first manifestations of an incipient storm is disruption of the ionospheric structure at high latitudes. This disruption then propagates to lower latitudes over a period of 4-8 hours. Thus the onset of a high-latitude disruption can give an early warning of an imminent storm in time to invoke protective measures to safeguard sensitive systems. This photo shows a reconstruction of Earth's ionosphere before and after the onset of a major storm in October 2003.