Comment on “Parametric Instability Induced by X‐Mode Wave Heating at EISCAT” by Wang et al. (2016)

Abstract

In their recent article Wang et al. (2016) analyzed observations from EISCAT (European Incoherent Scatter) Scientific Association Russian X‐mode heating experiments and claimed to explain the potential mechanisms for the parametric decay instability (PDI) and oscillating two‐stream instability (OTSI). Wang et al. (2016) claim that they cannot separate the HF‐enhanced plasma and ion lines excited by O or X mode in the EISCAT UHF radar spectra. Because of this they distinguished the parametric instability excited by O‐/X‐mode heating waves according to their different excitation heights. Their reflection heights were determined from ionosonde records, which provide a rough measure of excitation altitudes and cannot be used for the separation of the O‐ and X‐mode effects. The serious limitation in their analysis is the use of a 30 s integration time of the UHF radar data. There are also serious disagreements between their analysis and the real observational facts. The fact is that it is the radical difference in the behavior of the X‐ and O‐mode plasma and ion line spectra derived with a 5 s resolution, which provides the correct separation of the X‐ and O‐mode effects. It is not discussed and explained how the parallel component of the electric field under X‐mode heating is generated. Apart from the leakage to the O mode, results by Wang et al. (2016) do not explain the potential mechanisms for PDI and OTSI and add nothing to understanding the physical factors accounting for the parametric instability generated by an X‐mode HF pump wave.

Reply to Comment on the Article “Parametric Instability Induced by X‐Mode Wave Heating at EISCAT” by Wang et al. (2016)

“…The temporal behavior of the ion lines and plasma lines spectra presents the similar feature in the X‐mode and O‐mode heating cycles. Then we reply to the comment by Blagoveshchenskaya et al (), henceforth Comment2017, point to point in section 3. A summary is shown in section 4.…”

“…As seen in Figures and , HFILs present two maxima in the two different altitudes, which are about 20 km apart. The special feature that the ion lines present two maxima in two different altitudes is often observed in a large number of X‐mode heating experiments (Blagoveshchenskaya et al, , , ). Here the special feature is also observed in the O‐mode heating cycle.…”

“…The Russian team performed the ionosphere heating experiments by the extraordinary (X‐mode) radio wave with the EISCAT heating facility at Tromsø, Norway, to investigate the modification of the high latitudinal ionospheric F layer with various instruments, including EISCAT ultra high frequency (UHF) radar, CUTLASS radar, and optical observation system. There are a large body of nonlinear phenomena observed in the series of the ionospheric modification experiments by the X‐mode radio wave, including the electron temperature enhancement (Blagoveshchenskaya et al, , , , ), the electron density enhancement (Blagoveshchenskaya et al, , , , ), the small‐scale field‐aligned irregularities (Blagoveshchenskaya et al, , , , ), the enhanced airglow (Blagoveshchenskaya et al, ), the high‐frequency (HF)‐enhanced ion lines (HFILs) and HF‐enhanced plasma lines (HFPLs; Blagoveshchenskaya et al, , , ), and the narrow stimulated electromagnetic emissions (SEEs) spectra observed 1,000 km far away Tromsø (Blagoveshchenskaya et al, ).…”

Stimulated electromagnetic emissions spectrum observed during an X-mode heating experiment at the European Incoherent Scatter Scientific Association

“…Compared with the ponderomotive NC feature in O-mode heating experiments, the timescale of NC development was much longer in the X-mode heating experiment reported here. In X-mode heating experiments, the PDI was excited for several seconds and even minutes (Blagoveshchenskaya et al, 2017b;Wang X et al, 2018). This conclusion was reached by observing the temporal evolution of the HF-enhanced plasma lines and ion lines, both of which are signatures of PDI excitation in ionospheric heating experiments.…”

Systematic variation in observing altitude of enhanced ion line by the pump near fifth gyroharmonic

“…Previously, considering the ionosphere as a doubly refracting medium with the presence of the magnetic field, PDI and OTSI was expected to be excited only by O mode pump [21,29]. However, Blagoveshchenskaya et al [35] found that the X mode pump could also excite the enhanced down-shifted and up-shifted ion lines and down-shifted plasma line.…”