Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo

Abstract

Two radar systems with beams intersecting in the HF‐modified F region were used to simultaneously measure HF‐enhanced plasma lines (HFPLs) and artificial geomagnetic field‐aligned irregularities (AFAIs). These measurements were made with the high‐power HF facility located near the Arecibo Observatory, Arecibo, Puerto Rico. The Arecibo 430‐MHz radar was used for the HFPL observations, and a portable 49.92‐MHz backscatter radar was deployed on the island of Guadeloupe to monitor the AFAIs. The experiment was designed to examine the degree to which HF‐induced plasma turbulence influences the development of AFAIs. When the HF beam is stepped up in power, sustained HFPLs and AFAIs are first observed at the same HF power level, indicating that ponderomotively driven instabilities may be involved in the early time development of AFAIs. As the HF power is increased, the HFPL backscatter power begins to saturate at ∼70 MW effective radiated power (ERP). However, the backscatter from AFAIs is linearly dependent on HF power, even at the highest (120 MW ERP) HF power levels available at Arecibo. This suggests that additional processes may contribute to the development of AFAIs. For example, ponderomotively driven instabilities may give rise to weak geomagnetic field‐aligned irregularities that are subsequently driven unstable by processes excited near the upper hybrid resonance. It is also likely that AFAIs greatly impact the development of HF‐induced plasma turbulence at late times (>1 s) following HF turn‐on. Once the ionosphere is preconditioned by high‐power HF modifications, AFAIs and HFPLs can be simultaneously sustained at a much lower HF power level than that needed to originally excite them. The nature of the preconditioning process is currently not well understood. New theoretical initiatives are clearly needed to guide future experimental activity in this area.