HF‐enhanced plasma lines in the lower ionosphere

Abstract

The ionospheric modification facility at Arecibo, Puerto Rico, has been used to study Langmuir waves excited in the lower ionosphere by a high‐power HF radio wave. Measurements of HF‐enhanced plasma lines have been made in the lower F region (< 160 km altitude) and in the E region using the 430‐MHz radar at Arecibo Observatory. These measurements complement observations made in the past at higher altitudes. Enhanced plasma line spectra observed in the lower F region peak at the so‐called “decay line” and contain additional spectral structure similar to that found in the upper F region. In the E region the spectra exhibit narrow peaks located at 430 MHz ±ƒHF, where ƒHF is the frequency of the modifying HF wave. While strong plasma line enhancements are commonly observed in sporadic E, only weak enhancements have been detected in the normal daytime E layer. Candidate mechanisms for the E region enhancements include the oscillating two‐stream instability and direct conversion of the HF wave into Langmuir waves.