Riometers are used to measure variation in the cosmic radio noise absorption taking place in the D-region ionosphere (50-90 km). The variations are caused by changing ionization levels of the D-region, either because of auroral disturbances or solar particle events (especially over the polar caps, see PCA events). Because of the low altitude of the absorption region, the auroral events are created by higher energy particles than the visible aurora.
Previously the cosmic radio noise was typically measured at around 20-50 MHz with wide-beam antennas. Lately, however, a new generation of imaging riometers has appeared. They consist of an array (8 x 8 or so) of wire dipoles forming narrow antenna beams which cover, in total, an area of 100 km x 100 km or more at 90 km altitude. See Detrick and Rosenber (1990) for more discussion on riometer techniques.