EISCAT as a tristatic auroral radar

We have used the European Incoherent Scatter radar (EISCAT) in a mode which allows us to use it as a tristatic auroral radar. Observing at an elevation of less than 10° with the Tromsø beam, we achieved magnetic aspect angles between 4° and 6° at 105 km altitude and recorded coherent echoes simultaneously from all three sites. The backscattered power for these echoes is up to 3 orders of magnitude higher than typical incoherent scatter echoes. Contrary to most existing auroral radars, we can calibrate the coherent echo strength and thus determine absolute values of the coherent backscatter cross section. Moreover, switching the common volume in short time intervals from E to F region heights, permits the determination of the E × B drift vector almost simultaneously with the E region coherent scatter measurements. This opens unique possibilities to study the E region plasma instabilities and their driving force. The main aim of this paper is to describe the capabilities of EISCAT as an auroral radar and to present and discuss results in terms of coherent backscatter cross sections, coherent spectra shape, irregularity phase velocities, and aspect angle dependence. In forthcoming papers several of these topics will be explored in more detail.