Altitude characteristics of plasma turbulence excited with the Tromsø Superheater

Langmuir/ion turbulence excited with the upgraded high‐power (1.2‐GW effective radiated power) HF heating facility at Tromsø, Norway, has been recently studied with the European Incoherent Scatter VHF and UHF incoherent scatter radars. In this report we focus on the altitudinal development of the turbulence observed at the highest HF power levels available. Quite remarkably, the observed plasma turbulence plunges downward in altitude over timescales of tens of seconds following HF beam turn‐on; the bottom altitude is generally reached after ∼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 characteristics of the Tromsø 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 Tromsø, billowing altitude structures are often seen, in sharp contrast to layers of turbulence observed at Arecibo.