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

Authors


Abstract

[1] This work presents the first observations of unique narrowband emissions ordered near the hydrogen ion (H+) gyrofrequency (fcH) in the stimulated electromagnetic emission spectrum when the transmitter is tuned near the second electron gyroharmonic frequency (2fce) during ionospheric modification experiments. The frequency structuring of these newly discovered emission lines is quite unexpected since H+ is known to be a minor constituent in the interaction region which is near 160 km altitude. The spectral lines are typically shifted from the pump wave frequency by harmonics of a frequency about 10% less than fcH (≈ 800 Hz) and have a bandwidth of less than 50 Hz which is near the O+ gyrofrequency fcO. A theory is proposed to explain these emissions in terms of a parametric decay instability in a multi-ion species plasma due to possible proton precipitation associated with the disturbed conditions during the heating experiment. The observations can be explained by including several percent H+ ions into the background plasma. The implications are new possibilities for characterizing proton precipitation events during ionospheric heating experiments.

1 Introduction

[2] For the past three decades since the discovery of stimulated electromagnetic emissions (SEE) [Thide et al., 1982] during ionospheric high-frequency (HF) high power radio wave heating experiments, interest has continued to increase. This is due to the wide range of ionospheric diagnostic information that has become available from the SEE spectrum. It can be used to determine the local magnetic field magnitude [Leyser et al., 1992], to determine the conditions for acceleration of electrons [Leyser et al., 2001], and to measure electron temperatures [Bernhardt et al., 2009] and ion composition [Bernhardt et al., 2010]. Parametric decay of the high-frequency pump wave, transmitted from the ground into other plasma wave modes, is considered to be the underlying theory for generation of the features in the SEE spectrum [Leyser et al., 2001].

[3] Recently, there has been considerable interest in SEE with structures near the ion-gyrofrequency observed in the spectrum. O+gyroharmonic structuring has been observed for second electron gyroharmonic heating [Bernhardt et al., 2011], and various aspects have been explained in terms of parametric instabilities in a single component plasma [Scales et al., 2011; Samimi et al., 2012, 2013; Mahmoudian et al., 2013]. The first observations of structuring near the H+ (proton) gyrofrequency in the SEE spectrum during second electron gyroharmonic heating are described here. This is quite unusual since H+is a minority species and is much less than a percent of the plasma ion density under quiet conditions at heating altitudes of 160 km. However, the observations were made during a period of a relatively disturbed ionosphere as indicated by disturbed magnetogram and elevated absorption thought to be associated with proton precipitation [Galand and Chakrabarti, 2006; Ebihara et al., 2010]. This observation of H+structuring implies a multi-ion component plasma with more complex wave mode behavior that must be considered. However, it provides possibilities for further diagnostics for determining the density of minority plasma species in ionospheric plasma that have not been considered before with such SEE measurements.

[4] This work describes the first observations, proposes a possible theory, and compares it to the observations. The next section provides the experimental setup and describes the observations made. Wave modes in a multicomponent plasma of minority H+ and majority NO+, O+, and O2+ ions are considered for possibilities of parametric decay of the pump field into high-frequency upper hybrid/electron Bernstein (UH/EB) waves and the appropriate low-frequency multi-ion species plasma modes. A comparison is made between the observations and the theory results, and a possible direction of future work is provided.

2 SEE Observations

[5] The experiment was conducted between 22 and 25 July 2011 at the High Frequency Active Auroral Research Program (HAARP) site at Gakona, AK. The SEE were recorded using a HF receiver setup approximately 12 km from HAARP. The receiver system consisted of a 30 m folded dipole antenna and a receiver with estimated dynamic range of 90 dB. The receiver shifted the frequency of the acquired signal by the heater frequency by mixing and sampling it at 250 kHz. The received data were windowed (Blackman) and Fourier transformed to obtain the spectrograms of the obtained signal. The frequency resolution of the spectrogram is 5 Hz. The 30 min experiment was organized into two 15 min cycles of frequency stepping from 2.9 MHz to 3.0 MHz with 0.02 MHz step size. In the first cycle, the O-Mode beam was pointing vertical, while in the second cycle it was pointing at magnetic zenith. The experiment was repeated with the same frequency and beam orientation settings to make it 1 h long. For the whole experiment, the transmitter was operated with 1 min on and 1.5 min off duty cycle at full power of 3.6 MW in magnetic zenith (ERP = 300 MW) and vertical (ERP = 443 MW) directions.

[6] During the hour-long (3:56–4:56 UT) experiment on 22 July 2011, the ionosphere was relatively disturbed with enhanced absorption recorded by digisonde and riometer from 04:36 UT to 04:42 UT. When the transmitter was turned on at 4:41 UT, both downshifted and upshifted emission lines at harmonics of frequency less than the hydrogen gyrofrequency (fcH≈789.76 Hz) by about twice the oxygen gyrofrequency (fcO≈49.36 Hz) appeared on the spectrogram and is shown in Figure 1a. HAARP was tuned at 2.9 MHz (near the second electron gyrofrequency fce) with beam pointing to magnetic zenith (azimuth 202° and zenith angle 14°). The SEE spectrum shows the pump at 2.9 MHz with the emission lines offset from the pump. The narrow lines shifted at harmonics of 689 Hz from the pump frequency appeared within a second after the pump was turned on, and their intensity lasted for approximately 15 s. These lines only exist during periods the pump is turned on, are not related to spurious harmonics of the transmitter line frequency of 60 Hz, and therefore are taken to be of geophysical origin. The fundamental downshifted (Stokes) emission line first appeared within a few tenths of a second (0.1s). The other lines developed in the following few tenths of seconds with decreasing rate with increasing harmonic number. The downshifted (Stokes) emission lines were accompanied by strong upshifted (anti-Stokes) counterparts. Although the anti-Stokes lines had higher saturation amplitude, they developed at a slower rate compared to the Stokes lines. The narrow SEE harmonic lines disappear abruptly within 0.1 s except the fundamental at 689 Hz which decays more slowly within 2 s. A broader line closer to the pump (centered near 296.16 Hz) appeared above the noise level after the pump was turned on for about 5 s and was present throughout the heating cycle. It was accompanied by a weaker upshifted anti-Stokes line. All these emission lines are seen clearly in the cross-section of the spectrogram in Figure 1b. The cross-section of the spectrogram was computed by averaging over a period of 20 s from the time the heater was turned on. The processed SEE shows that the narrow lines are at harmonics of the n  =1 line. This emission line is of maximum intensity, and its harmonics (n  =2,3,4) have monotonically decreasing intensity.

Figure 1.

(a) SEE spectrogram showing emission lines at harmonics of a frequency slightly less than the H+ gyrofrequency that disappear after 15 s. (b) SEE spectrogram averaged over 20 s from the heater turn-on time.

[7] A narrowband cross-section of the spectrogram from Figure 1a is shown in Figure 2. Similar to Figure 1b, it is averaged over a period of 20 s from the time the heater was turned on. The dotted grid lines are at fcO harmonics, and the dashed grid lines are at fcH. The broader SEE line is due to the oblique ion acoustic decay instability with embedded oxygen gyroharmonic structuring [Samimi et al., 2012, 2013; Mahmoudian et al., 2013]. The narrow emission line at n  =1 is about 2fcO below fcH and has bandwidth less than fcO.

Figure 2.

Narrowband version of Figure 1b with dotted and dashed grid lines at harmonics of fcO and fcH, respectively, offset from the pump frequency. Note narrowband emissions near fcH=789.76 Hz.

[8] These observations were obtained under disturbed ionospheric conditions. Figure 3a shows the fluxgate magnetogram, and Figure 3b riometer readings around the heating period with the dashed line indicating heater turn-on time. The magnetogram showed increased magnetic field fluctuations after 03:00 UT. The peak-to-peak amplitude is ≈ 150 nT in the H component. Absorption peaked at 0.25 dB at 04:05 UT and fell below 0.125 dB during the heating cycle as seen in the riometer readings. The temporal behavior of the ion acoustic (IA) line is most likely due to the variation in absorption during the heating cycle as can be seen from Figure 3. Figure 3 shows the beginning of relatively rapid reduction in absorption during the heating experiment which would allow development of the IA line. The IA lines are only observed during periods of negligible absorption [Samimi et al., 2012, 2013; Mahmoudian et al., 2013]. It is postulated that this disturbed event is associated with proton precipitation that produced significant quantities of H+ for short periods [Ebihara et al., 2010]. It is known that precipitating keV protons deposit much of their energy in the altitude range of 130–200 km [Basu et al., 1987; Galand and Chakrabarti, 2006]. This brackets the heating interaction altitude determined from the ionograms obtained during the same time period. Protons with higher energy and smaller pitch angle can penetrate deeper into the atmosphere with less energy degradation, while protons with lower energy are deposited into higher altitude approximately 150 km and higher [Basu et al., 1987]. The SEE emission lines described here were observed near the end of the disturbed period and lasted for the order of 10 s before disappearing. Due to the proximity of these SEE emissions to fcH (a minority species) and the narrow bandwidth, it is postulated that they involve parametric decay into low-frequency wave modes in a multi-ion component plasma as will now be described.

Figure 3.

Two hour plot of the readings of the (a) fluxgate magnetometer and (b) riometer located at Gakona, AK on 22 July 2011. The dashed line indicates heater turn-on time in Figures 1 and 2.

3 Theory

[9] At 160 km altitude, the ionospheric plasma consists primarily of nitric oxide NO+, atomic O+, and diatomic O 2+ oxygen ions. The wave mode of interest for fcO<<f<fcHand propagation near perpendicular to the magnetic field inline image is the H+-O+hybrid (or Buchsbaum) mode [Buchsbaum, 1960] which has physical analogies to the lower hybrid mode in a single-ion plasma. The approximate plasma susceptibilities (inline image) are given as inline image, inline image, and inline image, respectively, where subscripts e, i, and H denote electrons, (NO+, O+, O 2+) ions, and H+ ions, respectively; k is wave number; k⟂(∥) is the perpendicular (parallel) component to inline image; vtH is H+ thermal velocity; ωj and Ωj are the species(j) (radian) plasma and gyrofrequency; and inline imagewith ρHthe H+gyroradius. Γ1(b)=I1(b)eb, and I1 is the modified Bessel function of first order. The linear dispersion relation is given by inline image. The (radian) frequency ω is obtained by assuming ωΩH(1+Δ) for low H+ density where the frequency shift Δ<<1. The solution is

display math(1)

[10] For long wavelengths kρH<<1, inline image and the H+-O+hybrid (Buchsbaum) resonance frequency ωHOis attained.

[11] The simplified expression for Δcan be used with the shift of the SEE line below the fcH in Figure 2 to provide a rough estimate of the hydrogen density ratio. Several percent hydrogen density is consistent with the observation assuming the SEE line is roughly at ωHOestimating the hydrogen gyrofrequency to be 789.76 Hz, and the NO+, O 2+, and O+ densities to be 105, 4×104, and 2×104 cm−3as obtained from the SAMI II model [Huba et al., 2000].

[12] A three-wave parametric decay instability (PDI) of the pump wave into high-frequency upper hybrid UH/electron Bernstein and low-frequency H+-O+hybrid waves in a multi-ion plasma with minority H+ ions is proposed as a possible process for the generation of the narrowband emission lines in the SEE at the upper hybrid interaction altitude. The wave number and frequency matching conditions of inline image and ω0=ω1+ωs, where subscripts 0, 1, and s describe the pump field, high-frequency decay mode, and low-frequency decay mode, respectively, are used to describe the general dispersion relation of weak coupling [Porkolab, 1974; Ono et al., 1980]

display math(2)

where inline image, and εe(ω)=1+χe(ω). The jth plasma general susceptibility is

display math(3)

where Γn(bj)=In(bj) exp(−bj) and ζjn=(ωnΩj)/k||vtj. λDjis the Debye length of the jth species. Z is the Fried Conte function. βe is the coupling coefficient proportional to the pump field E0and is expressed as

display math(4)

and inline image. The parametric decay process is assumed to occur near the upper hybrid altitude with a double resonance condition, i.e., ωuh=2Ωe, where ωuh is the upper hybrid frequency and inline image[Samimi et al., 2012, 2013; Mahmoudian et al., 2013]. Equation ((2)) has been solved numerically with different input parameters of electron and ion temperature ratio, strength of the pump field calibrated with electron oscillating velocity vosc=eE0/meω0, and pump electric field angle (θE) with respect to inline image. Simplifications of a dipole pump field (inline image) and neglect of collisional effects due to assumption of the pump field exceeding the PDI threshold are used.

[13] Figure 4 shows numerical solution of the dispersion relation and growth rate of H+-O+hybrid mode PDI for different percentages for H+ ions (colored lines). The bold lines are the solutions of the dispersion relation, and the dashed lines are of the growth rate. Figure 4a shows dispersion relation and growth rate against the wave number, and Figure 4b shows the growth rate against the real frequency. The dotted grid lines are at harmonics of ΩO. Note that from frequency matching the low-frequency mode behavior describes the shift of the SEE line from the pump. The H+-O+hybrid mode merges into various Bernstein modes in the multi-ion plasma for small wavelengths kρH>>1. θE which is the electric field angle with respect to inline image is taken to be inline image, and oscillating velocity vosc=eE0/meω0=0.95vte(|E0|∼10V/m). It is assumed Te/Ti=3 [e.g., Samimi et al., 2012, 2013] and due to energy degradation TH/Ti=1; however, the importance of variation of this parameter will be discussed shortly. The electron density is ne=3×105cm−3, and the temperature Te=2500K. The pump frequency is taken to be ω0=2Ωe+1ΩH, and nH/ni which is the percentage of H+relative to the total ion density is varied from 1% to 5%. The variation in the solution of the hybrid mode with respect to the percentage of H+ions in the plasma is also observed in Figure 4. As the percentage of H+ in the multi-ion plasma increases, the ωHOshifts further below ΩH at long wavelengths (kρH<<1). The value of maximum growth rate and its shift below ΩH increases with increase in percentage of H+. The growth rate increases roughly linearly with increasing ratio of H+ density nH/ni. Percentages of H+near 4–5% appear reasonably consistent with the frequency shift in the experimental observations. It is noted that the growth time is approximately 0.5 s in this case. The bandwidth is typically of order of ΩO, which has reasonable consistencies with the observations as well. It is noted that increasing TH/Timay significantly increase the emission bandwidth and reduce growth rate. TH should be no more than several times Ti for bandwidths of order ΩOor less.

Figure 4.

(a) PDI growth rate and dispersion relation of H+-O+ hybrid mode for different percentage of H+ ions against the wave number with grid lines at harmonics of ΩO. (b) PDI growth rate of H+-O+ hybrid mode for different percentage of H+ ions against the real frequency normalized to ΩH with grid lines at harmonics of ΩO.

4 Discussion

[14] Evidence of minority (H+) species structuring in the SEE spectrum during second electron gyroharmonic heating under disturbed ionospheric conditions has been presented. Similar SEE observations are all correlated with magnetogram fluctuations in the 50–150 nT range and typically associated with the leading and trailing edge of the magnetic disturbance when absorption is sufficiently low. They are postulated to be the ultimate result of proton precipitation effects at the heating interaction altitude near 160 km. They are relatively short lived and are observed to last for as long as tens of seconds during the heating cycle, however, on occasion only for a few seconds. These newly discovered spectral features have the characteristics that they are narrowband, exhibit harmonic structure, and often have frequency shifts slightly less (of order of 10%) than fcHfrom the pump wave frequency. The Stokes and anti-Stokes lines also show monotonic decrease away from the pump frequency. The theory presented to produce the said emissions involves the PDI in a multi-ion component plasma with several percent of H+density. The low-frequency decay mode in this case is the H+-O+hybrid (Buchsbaum) mode. The results show reasonable agreement with experimental observations of downshifted sidebands observed in experimental observations. As an aside, it is interesting to note the possible role of the Buschbaum hybrid mode in producing these emissions and the physical analogies to the lower hybrid mode which has long been considered to be responsible for producing the prominent downshifted maximum emission in the SEE spectrum in single-ion component ionospheric plasma [Leyser, 1991]. These new observations may indicate new possibilities for investigating characteristics of protons using heating experiments during active conditions. On rare occasions, the frequency shift of the emissions has been observed to be slightly larger than fcH. Equation ((2)) can be shown to also exhibit growth of electrostatic hydrogen cyclotron waves in the multi-ion plasma that show consistencies with the data. This requires larger θE in which the electron behavior is Boltzmann [Samimi et al., 2012, 2013]. Ongoing work is considering the physics of these emissions and diagnostic possibilities in more detail. It should be noted that the weak coupling theory provided here does not describe the complex harmonic structure observed that is most likely produced by nonlinear cascading processes [e.g., Zhou et al., 1994]. Further details and more sophisticated modeling using plasma simulations, which are beyond the scope of the current discussion, will be considered in future work. Other important aspects to be investigated include threshold of the pump power for the PDI generation, the critical proton density for PDI generation (which would imply a minimum observable frequency shift in equation ((1))), and the impact of the proximity of the pump wave frequency to the second and higher electron gyroharmonic frequencies. This study indicates that more careful experiments are needed to further access diagnostic capabilities for interrogation of possible proton precipitation effects using the SEE spectrum during ionospheric heating experiments.

Acknowledgment

[15] This work was supported by National Science Foundation.

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