Aspect Dependence of Parametric Instability Excitation by X-mode at EISCAT

Recent studies of X-mode wave heating experiments observed by EISCAT (European Incoherent Scatter Scientific Association) UHF radar indicate that parametric instability (PI) can be excited unexpectedly. In this paper, we re-investigate the ionospheric heating experiment on October 21, 2012 in details. The observed HF-enhanced ion lines (HFILs) and HF-enhanced plasma lines (HFPLs) observed during the experiment presents dependence on the heating incidence angles. The observational evidence and numerical calculation that the parallel electric field of X-mode heating wave can exceed the PI excitation threshold at certain angles indicate the decisive role of parallel electric field for the PI excitation. The excited HFPLs and HFILs during X-mode heating cycles also present the evidence for the non-Maxwellian distribution of the electrons in the heated region.