The dynasonde at Halley remotely senses the structure and motion of plasma 60-500 km up in the ionosphere and within a horizontal radius of about 500 km. The structure of the ionosphere is affected by ultra-violet and X-ray radiation from the Sun and by charged particle precipitation from geospace which also causes the aurora. The picture shows a real ionogram measured at Halley. It is a plot of the virtual height of a radio reflection as a function of the transmitted frequency. Each measurement is colour-coded according to another one of the measured parameters.