The Apparent Behavior of Electron Density during an Alternating O/X-Mode Heating Experiment

Zeyun Li,Qingfeng Li,Hanxian Fang,Hongwei Gong

doi:10.3390/universe7080274

Abstract

We present the observations of the artificial ionospheric modification experiment of EISCAT on 18 October 2012 in Tromsø, Norway. When the pump of alternating O mode and X mode is switched on, the UHF radar observation shows some strong enhancements in electron density, ion lines and plasma lines. Based on some existing theories, we find the following: First, during the experiment, the frequency of plasma line (fL), ion line (fia) and pump (fh) matches fL = fh − 3fia and = fh − 5fia occasionally demonstrated that the cascade process occurred. Second, through quantitative calculation, we found that the O-mode component mixed in X-mode wave satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability, from which we infer that the HF-induced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) observed in X-mode pulse could have been caused by the O-mode component mixed in X-mode wave. Third, the UHF radar observation shows some apparent enhancements over a wide altitude range (from approximately the reflection altitude to ~670 km) in electron density during X-mode pulse, which also does not, in fact, correspond to a true increase in electron density, but due to the enhancement in ion line or the enhancement in radar backscatter induced by some unknown mechanism.

Highlights

When high-power (HF) radio waves are incident on the ionosphere, a series of complex wave–particle and wave–wave interactions will occur
HF-enhanced ion lines (HFILs) and HF-induced plasma lines (HFPLs) caused by ionospheric modification have abundant spectral structures, e.g., (1) Westman et al [6] observed up to five cascade lines in addition to the decay line in the Tromsø HF-induced plasma line (HFPL) and well-developed cascade type plasma lines for the first time; (2) Wu Jun et al [7] and Borisova et al [8] observed the plasma lines (HFPLs) excited at a frequency higher (150–250 kHz) than the pump during artificial ionospheric modification near the fifth electron gyrofrequency in EISCAT; (3) Robert et al [9] observed the frequency of HFPLs were lower than pump frequency, about 3 fia in the Arecibo artificial ionospheric modification experiment
It is obvious that the O-mode component satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability; that is to say, HFPLs and HFILs observed in the X-mode pulse could have been caused by the O-mode component mixed in X-mode wave

Summary

Introduction

When high-power (HF) radio waves are incident on the ionosphere, a series of complex wave–particle and wave–wave interactions will occur. A series of physical phenomena occur when powerful radio waves (usually termed as a pump) are injected into the ionosphere, such as Artificial Airglow [2], Anomalous Absorption [3], Stimulated Electromagnetic Emission [4] and small-scale field-aligned irregularity [5]. These phenomena, which are all caused by the O-mode pump, are closely related to parametric instability. During the ionospheric heating experiment, the most direct evidence of the excitation of Langmuir parametric instability is the HF-induced plasma lines (HFPLs), and HF-enhanced ion lines (HFILs) observed in incoherent scattering radar. HFILs and HFPLs caused by ionospheric modification have abundant spectral structures, e.g., (1) Westman et al [6] observed up to five cascade lines in addition to the decay line in the Tromsø HF-induced plasma line (HFPL) and well-developed cascade type plasma lines for the first time; (2) Wu Jun et al [7] and Borisova et al [8] observed the plasma lines (HFPLs) excited at a frequency higher (150–250 kHz) than the pump during artificial ionospheric modification near the fifth electron gyrofrequency in EISCAT; (3) Robert et al [9] observed the frequency of HFPLs were lower than pump frequency, about 3 fia in the Arecibo artificial ionospheric modification experiment

Methods

Findings

Discussion

Conclusion