ALTITUDE CHARACTERISTICS OF PLASMA TURBULENCE EXCITED WITH THE TROMSOSUPERHEATER

Langmuir/ion turbulence excited with the upgraded high-power (1.2-GW e ffective radiated power) HF heating facility at Tromso, Norway, has be en recently studied with the European Incoherent Scatter VHF and UHF i ncoherent scatter radars. In this report we focus on the altitudinal d evelopment of the turbulence observed at the highest HF power levels a vailable. Quite remarkably, the observed plasma turbulence plunges dow nward in altitude over timescales of tens of seconds following HF beam turn-on; the bottom altitude is generally reached after similar to 30 s. This phenomenon has a well-defined HF power threshold. It is most likely caused by changes in the electron density profile brought about by HF heating of the electron gas. If this is the case, then the heat source must be nonlinearly dependent on HF power. Overall, the charac teristics of the Tromso turbulence are quite distinctive when compared to similar high-resolution measurements made at Arecibo Observatory, Puerto Rico. After HF transmissions have been made for tens of seconds at Tromso, billowing altitude structures are often seen, in sharp con trast to layers of turbulence observed at Arecibo.