Ion gyro‐harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro‐harmonic heating

Abstract

[1] Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ground-based radio transmitters. Recently, there has been significant interest in ion gyro-harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro-harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro-harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro-harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro-frequency were observed for third electron gyro-harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro-harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro-harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro-harmonic features for pump heating near the second electron gyro-harmonic during the campaign. The observations affirm strong correlation between the gyro-structures and the pump-induced optical emissions.

Stimulated Brillouin scattering during electron gyro-harmonic heating at EISCAT

“…Stubbe and Kopka (1990) stated that the DP has been found to be a strong feature for f 0 = 3f ce and weak sign for f 0 = 4f ce , 5f ce . It is worth mentioning that Mahmoudian et al (2013a) recently noted a similar spectral feature for f 0 ≈ 2f ce pumping with frequency offset f ≈ 500 − 1000 Hz. Figure 3 shows the dependence of wideband SEE features on the pump frequency when pumping near 3f ce for the same time period and experimental conditions as the narrowband SEE in Fig.…”

“…The DP serves as a good indicator for the pump frequency close to the third electron gyro-harmonic frequency (Stubbe and Kopka, 1990;Stubbe et al, 1994;Honary et al, 1995). The DP frequency offset for the pump frequency near 3f ce can be approximately estimated based on existing theoretical models (Huang and Kuo, 1995;Hussein and Scales, 1997;Mahmoudian et al, 2013a). A detailed description of these DP models is beyond the scope of the current paper and will be pursued in future works.…”

“…This differs from SBS, which does not involve fieldaligned density irregularities in its generation process. The threshold for SBS is usually higher than DP emissions, and DP appears in the spectra almost immediately after the heater is turned on (Mahmoudian et al, 2013a).…”

“…SEE spectral lines in the scattered wave can be utilized to remotely probe the properties of the ionosphere as well as actively study radio pump-induced phenomena such as artificial airglow during modification of the ionosphere (e.g. Bernhardt et al, 2009Bernhardt et al, , 2010Pedersen et al, 2010;Mahmoudian et al, 2013a).…”

First observations of minority ion (H+) structuring in stimulated radiation during second electron gyroharmonic heating experiments

“…The dotted grid lines are at f cO harmonics, and the dashed grid lines are at f cH . The broader SEE line is due to the oblique ion acoustic decay instability with embedded oxygen gyroharmonic structuring [Samimi et al, 2012Mahmoudian et al, 2013]. The narrow emission line at n = 1 is about 2f cO below f cH and has bandwidth less than f cO .…”