Recent advances in radar studies of the polar ionosphere

Early studies of the auroral zone ionosphere were undertaken at Tromso in Northern Norway by Sir Edward Appleton during the Second Polar Year (1932-33). The sounding radars employed were based on the principle of radiowave reflection by the ionospheric layers. During the last 30 years a new generation of radars has been developed, based on the principle of `coherent´ and `incoherent´ scatter by irregularities in ionospheric plasma. Two of these new systems are described. The EISCAT Svalbard Radar (ESR) is a UHF incoherent scatter radar, which extends the present EISCAT viewing area from the auroral zone well into the polar cap. The second system is the CUTLASS HF coherent radar, which provides a large viewing area including both the auroral zone and polar cap regions. Some of the exciting new results obtained with these radars are discussed in detail