TRISTATIC EISCAT-VHF MEASUREMENTS OF THE HF MODIFIED IONOSPHERE FOR LOW-BACKGROUND ELECTRON TEMPERATURES

The effects of a powerful high-frequency electromagnetic wave transmit ted in the ordinary mode from the European Incoherent-Scatter (EISCAT) ionospheric modification facility up to the daytime ionospheric F reg ion have been studied with the EISCAT-UHF radar system. This is one of the first tristatic EISCAT-UHF incoherent scatter experiments, in whi ch the excited Langmuir and ion, acoustic fluctuations are measured si multaneously at scattering angles near parallel (Tromso site), similar to 22 degrees (Kiruna site), and similar to 31 degrees (Sodankyla sit e) to the geomagnetic field. We present measurements of the spectral s tructure and temporal evolution with a maximum resolution of 13 ms at the different scattering angles. For the first time, up to five cascad e lines in addition to the decay line in the Tromso HF-enhanced plasma line and well-developed cascade type plasma lines in Sodankyla were o bserved. In spite of low background electron temperature, and hence ne gligible electron Landau damping parallel to the geomagnetic field, th e excited plasma turbulence varied considerably.