2 DIFFERENT TYPES OF ENHANCED ION-ACOUSTIC FLUCTUATIONS OBSERVED IN THE UPPER IONOSPHERE

We present European incoherent scatter observations of several events that show enhanced ion acoustic fluctuations in the altitude region 40 0-1500 km. The incoherently scattered spectra have one or both the ion acoustic Lines enhanced, and sometimes a peak close to the zero frequ ency is observed. It is shown that these observations can be separated into two types depending on the surrounding physical conditions. The first type is related to strongly enhanced electron temperatures, ion outflows with large vertical velocities, an altitudinal extend of 300- 700 km, and auroral arcs and precipitating particles of 100 eV to 10 k eV. The spectra are in that case strongly asymmetric. The second type corresponds to slightly enhanced electron temperatures, no ion outflow s, spatial extends of 100-200 km, and an apparent lack of precipitatin g particles of less than 1 keV. The corresponding spectra show mostly both the ion lines enhanced. These observations are compared with the existing interpretations.