Optical and ionospheric phenomena at EISCAT under continuous X‐mode HF pumping

Abstract

We present experimental results from multiinstrument observations in the high-latitude ionospheric F 2 layer at the EISCAT (European Incoherent Scatter Scientific Association) heating facility. The results come from a set of experiments, when an X-polarized HF pump wave at high heater frequencies (f H > 6.0 MHz) was injected into the F region of the ionosphere toward the magnetic zenith. Experiments were carried out under quiet magnetic conditions with an effective radiated power of 458-548 MW. HF pumping was produced at different heater frequencies, away from electron gyroharmonic frequencies, and different durations of heater pulses. We show the first experimental evidence of the excitation of artificial optical emissions at red (630 nm) and green (557.7 nm) lines in the high-latitude ionospheric F 2 layer induced by an X-polarized HF pump wave. Intensities at red and green lines varied in the range 110-950 R and 50-350 R, respectively, with a ratio of green to red line of 0.35-0.5. The results of optical observations are compared with behaviors of the HF-enhanced ion and plasma lines from EISCAT UHF incoherent scatter radar data and small-scale field-aligned artificial irregularities from Cooperative UK Twin Located Auroral Sounding System observations. It was found that the X-mode radio-induced optical emissions coexisted with HF-enhanced ion and plasma lines and strong artificial field-aligned irregularities throughout the whole heater pulse. It is indicative that parametric decay or oscillating two-stream instabilities were not quenched by fully established small-scale field-aligned artificial irregularities excited by an X-mode HF pump wave.

A theoretical investigation on the parametric instability excited by X‐mode polarized electromagnetic wave at Tromsø

“…The issue with the excitation of Langmuir parametric decay instability (PDI) and Langmuir oscillating two-stream instability (OTSI) by X-mode heating is due to the fact that the plasma frequency near X-mode pump wave reflection height is less than the heater frequency, thus not satisfying the frequency matching condition. The comparison between the temporal behaviors of HFPLs excited by O/X-mode pump wave indicates that for the case of X-mode heating, the HFPLs are observed throughout the whole heating cycle, in contrast to a short duration (on the order of few milliseconds) just after the heater is turned on under O-mode heating, as reported by Blagoveshchenskaya et al [2014] (i.e., Figure 3, top plot). However, recent experimental observations performed by the European Incoherent Scatter (EISCAT) heating facility at Tromsø, Norway, demonstrate that an X-mode pump wave can produce HF-enhanced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) [Blagoveshchenskaya et al, 2014[Blagoveshchenskaya et al, , 2015, which indicate the excitation of parametric instability near the X-mode reflection height.…”

“…Equations (22) and (25) [Senior et al, 2010]. As shown in Figures 3 and 5 in Blagoveshchenskaya et al [2014], the HFPLs are observed during the whole heater-on period, associated with electron density and temperature enhancement, which is different from the typical O-mode heating experimental observations when HFPLs are observed only in the first few seconds. As shown in Figures 3 and 5 in Blagoveshchenskaya et al [2014], the HFPLs are observed during the whole heater-on period, associated with electron density and temperature enhancement, which is different from the typical O-mode heating experimental observations when HFPLs are observed only in the first few seconds.…”

“…In addition, it has been known that the X-mode pump wave cannot parametrically excite the Langmuir wave propagating along the geomagnetic field because of the lack of parallel electric field component. Blagoveshchenskaya et al [2014] employed the reflection heights of ordinary/extraordinary-polarized EM pump waves to distinguish the HFPLs and HFILs excited by O-mode or X-mode pumping. The comparison between the temporal behaviors of HFPLs excited by O/X-mode pump wave indicates that for the case of X-mode heating, the HFPLs are observed throughout the whole heating cycle, in contrast to a short duration (on the order of few milliseconds) just after the heater is turned on under O-mode heating, as reported by Blagoveshchenskaya et al [2014] (i.e., Figure 3, top plot).…”

“…However, recent experimental observations performed by the European Incoherent Scatter (EISCAT) heating facility at Tromsø, Norway, demonstrate that an X-mode pump wave can produce HF-enhanced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) [Blagoveshchenskaya et al, 2014[Blagoveshchenskaya et al, , 2015, which indicate the excitation of parametric instability near the X-mode reflection height. As seen in Figure 4 by Blagoveshchenskaya et al [2014], from 17:00 to 17:31 UT on 19 October 2012, the reflection heights of O/X-mode pump waves are close to 260 km and 250 km, respectively. Blagoveshchenskaya et al [2014] employed the reflection heights of ordinary/extraordinary-polarized EM pump waves to distinguish the HFPLs and HFILs excited by O-mode or X-mode pumping.…”

Survey of conditions for artificial aurora experiments by the second electron gyro-harmonic at EISCAT Tromsø using dynasonde data

“…The weather condition is another important factor affecting the optical observations, but we do not treat the weather condition in the present study for simplicity. Due to such restrictions, some of heating experiments were successful (e.g., Gustavsson et al 2005;Bryers et al 2013;Kosch et al 2005Kosch et al , 2007bKosch et al , 2009Kosch et al , 2014aBlagoveshchenskaya et al 2015), but the others were unsuccessful. Thus, it is vitally important to understand when such successful conditions are likely satisfied for planning heating experiments in campaigns.…”