First Observations of Narrowband Stimulated Electromagnetic Emissions at the Pump Frequency Second Harmonic During Ionosphere Interaction Experiments

Abstract

Reported herein are first observations of narrowband stimulated electromagnetic emissions (NSEEs) within ±30 Hz of the second harmonic of the pump wave frequency during ionospheric heating experiments at the High Frequency Active Auroral Research Program facility. The observations are juxtaposed with contemporaneous NSEEs observed within ±30 Hz of the pump wave frequency. The transmit power of the pump wave is linearly increased, and the pump wave frequency is stepped through frequencies near the third electron gyroharmonic frequency during the experiment. The first observation is that there is a transmit power threshold for the occurrence of NSEEs downshifted from the second harmonic of the pump wave frequency. Secondly, when downshifted NSEEs are present at both the pump frequency and its second harmonic, the transmit power threshold for the onset of both are comparable. Third, the frequency offset of the downshifted NSEEs spectral line from the second harmonic frequency is approximately twice the frequency offset of the downshifted NSEEs spectral line from the pump wave frequency when the downshifted NSEEs are present near both the pump and second harmonic frequency. Finally, the NSEEs at the second harmonic of the pump wave frequency are suppressed at pump frequencies very close to the third electron gyroharmonic frequency.

Pump Power Effects on Second Harmonic Stimulated Electromagnetic Emissions During Ionosphere Heating

“…Although Figure a which shows the time evolution of SEEs for the 2nd gyrofrequency power ramp experiment shows an inexplicable decrease in the power level of the SBS line during the first 3.15 s, the SBS line power generally increases with transmit power while the 2nd harmonic line grows to saturation and then decreases. Thus the evolution of the SBS and the 2nd harmonic decay lines for the 2nd gyrofrequency power ramp experiment follow the same general trend as for the 3rd gyrofrequency power ramp experiment as shown in Figure a and as reported in Yellu et al (). These are the first reported observations that show the connection between PW‐NSEEs and SHG‐NSEEs for a 2nd gyrofrequency heating experiment.…”

“…For PW-WSEEs the spectrograms and PSDs show SEEs within ±20 kHz of 0 and 2 0 , respectively. The variation of SHG-NSEEs with pump power, frequency, and correlation with PW-NSEEs affirmed that the observed SEEs were of ionospheric origin thus precluding the possibility of the observations emanating from spurious transmitter harmonics Yellu et al (2018). In this section, results for the third gyrofrequency power ramp and third gyrofrequency maximum available power experiments are presented and compared first.…”

“…The spectrograms in Figure show the time evolution of SEEs for the third gyrofrequency power ramp experiment for ω 0 =4.28 MHz. Figures 1a and 1c which show the time evolution of PW‐NSEEs ( ω 0 ±100 Hz) and SHG‐NSEEs (2 ω 0 ±100 Hz) respectively were presented and discussed in Yellu et al (). In Figure a the line downshifted from ω 0 by approximately 8 Hz and labeled “SBS” is due to SBS at the reflection altitude Yellu et al ().…”

“…At the threshold of the excitation of the decay products, the PW and the decay products must satisfy the Manly-Rowe relations 0 = 1 + 2 and ⃗ k 0 = ⃗ k 1 + ⃗ k 2 which respectively stipulate energy and momentum conservation Kruer (1988), Eliezer (2002). Parametric instabilities manifest among other things as enhanced airglow caused by impact ionization of oxygen due to high-energy tail velocity distribution Perkins et al (1974) and also in generation of secondary radiation known as stimulated electromagnetic emissions (SEEs) Leyser (2001), Scales (2016), Yellu et al (2018) which is the main focus of this manuscript.…”

NSEE Yielding Electron Temperature Measurements at the Arecibo Observatory

“…These emissions, now known as narrowband SEE (NSEE), are shifted by only up to a few tens of hertz from the pump frequency. Recent discoveries using NSEE spectral line observations have resulted in the assessment of active geomagnetic conditions, the determination of minor ion species and their densities, and measurements of the electron temperatures in the heated ionosphere, as well as a deeper understanding of the physical processes associated with electron acceleration and the formation of field-aligned irregularities (Bernhardt et al, 2009(Bernhardt et al, , 2010(Bernhardt et al, , 2011Bordikar et al, 2013;Bordikar et al, 2014;Fu et al, 2015;Fu et al, 2013;Mahmoudian et al, 2013aMahmoudian et al, , 2013bMahmoudian et al, , 2014aMahmoudian et al, , 2014bMahmoudian et al, , 2016Samimi et al, 2012Samimi et al, , 2013Scales, 2016;Yellu et al, 2018aYellu et al, , 2018b.…”