Riometer (Relative Ionospheric Opacity Meter)

The riometer at Halley measures the radiowave noise at 30MHz coming from the Galaxy. Typically there is a gentle variation of the noise during each day as the Earth turns underneath the stars. That noise signal is partially absorbed by free electrons in the ionosphere (i.e. at about 100km altitude). When an event, such as an aurora, occurs, extra free electrons are produced and the absorption of the Galactic noise increases, causing a decrease in the measured signal. Hence we are able to use this instrument to identify what is happening at 100km or so during events like the aurora, solar storms or polar black-out conditions. The instrument measures changes in the ionosphere directly above Halley, and so it is best combined with data from the other instruments at Halley (i.e. the VLF receiver, MF and SuperDarn radars, and the magnetometers), or other riometer instruments around the world, or with overhead satellite observations.