Incoherent scatter radars are powerful ground based instruments for ionospheric measurements. By analysis of the Doppler shifted backscatter spectrum, containing the signature of electrostatic plasma waves, plasma bulk properties are estimated. Occasionally, the backscattered radar power is enhanced several orders of magnitude above the thermal backscatter level. These enhancements occur during naturally disturbed ionospheric conditions and in ionospheric modi_cation experiments, where a powerful radio wave is incident on the ionospheric plasma. In both of the cases the non-linearity is thought to be turbulence of electrostatic Langmuir waves. The Langmuir turbulence theory and models account for many features of enhanced ionospheric radar backscatter reported on in the literature. During disturbed conditions, with precipitation of auroral electrons, Langmuir turbulence is thought to be driven by a low energy electron beam. Optical and radar observations of naturally enhanced radar backscatter indicate Alfvénic type of aurora during events reported on in the literature. However, contrasting conclusions have been drawn from optical observations. While some reports suggest that enhanced radar backscatter is observed at the edge of auroral structures others suggest that the enhanced backscatter region and auroral precipitation are co-located. Optical imagers with a narrow field of view resolve auroral structures with tens of meters scale size. The cross beam resolution of radars, however, is limited by the width of the radar beam, typically several kilometers wide at auroral altitudes. By using several radar receivers for observations - radar interferometry - the cross beam resolution is increased. Simultaneous observations of enhanced radar backscatter with radar interferometry and narrow field of view optical observations will increase the understanding of the physical processes involved and will make it possible to associate auroral structures with the enhanced radar backscatter. An interferometric radar receiver system has been built and a calibration technique for the system developed. In ionospheric modification experiments, the Langmuir turbulence is driven by a powerful electromagnetic wave incident on the ionosphere and electrons are significantly accelerated. The acceleration of electrons is not yet fully understood. Ionospheric modification experiments and ground based measurements, as reported on herein, contribute to the understa

Incoherent scatter radars are powerful ground based instruments for ionospheric measurements. By analysis of the Doppler shifted backscatter spectrum, containing the signature of electrostatic plasma waves, plasma bulk properties are estimated. At high latitudes the backscattered radar power is occasionally enhanced several orders of magnitude above the thermal backscatter level. These enhancements occur during geomagnetic disturbed conditions and are referred to as naturally enhanced ion acoustic echoes (NEIALs). NEIALs are linked to auroral activity with optical auroral emission observed in the vicinity of the radar measurement volume simultaneously to NEIALs. The backscatter enhancements are thought to be caused by wave activity above thermal level due to instability. A number of theories have been put forward including streaming instabilities and Langmuir turbulence to explain NEIAL observations. NEIALs occur in two classes distinct by their Doppler features. Observations of the first type, which has been studied more extensively, are generally modelled well by the Langmuir turbulence model. The difficulty in trying to understand the driving mechanism of the instability is the limited spatial resolution of the radar measurements. Observations of the second type, reported on more recently, have been interpreted as evidence for naturally occurring strong Langmuir turbulence by means of their Doppler features.

Aperture synthesis is a technique to increase the spatial resolution of the radar measurements to below beam width of the single receiver antennas. The technique is employed to investigate the structure of NEIALs in the plane perpendicular to the magnetic field at sub-degree scale corresponding to hundreds of meters to a few kilometres at ionospheric altitudes. Calibration of the radar interferometer is necessary and a calibration technique is presented in paper I. Interferometry observations of a NEIAL event with receivers deployed at the EISCAT incoherent scatter radar on Svalbard are presented in paper II. The size of the enhanced backscatter region is found to be limited to 900 x 500m in the plane perpendicular to the geomagnetic field. These observations constitute the first unambiguous measurements giving evidence for the limited size of the enhanced backscatter region.

In paper III observations of strong Langmuir turbulence signatures are presented. The apparent turbulent region in these observations is limited to two narrow altitude regions, 2km extent, and electron density irregularities caused by the turbulence are thought to reach down to decimeter scale length. The turbulence observations were obtained during energetic electron precipitation thereby differing from other observations during which a low energy component in the electron precipitation is reported. In paper IV a statistical study of strong Langmuir turbulence radar signatures is presented. The study reveals differing local time distributions for these signatures from type I NEIALs indicating di_ering driving conditions for the two types of NEIALs. It is found that strong Langmuir turbulence signatures are predominantly observed in the pre-midnight sector where auroral break-up aurora prevails.