Unprecedentedly Strong and Narrow Electromagnetic Emissions Stimulated by High-Frequency Radio Waves in the Ionosphere

Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

Figure 1

Frequency spectra of radio emissions revealing narrow peaks (NPs) for different $f_0$. The pump beam was transmitted vertically (a) and toward geomagnetic zenith (b). A weak interference due to the power net can be seen at $f_0\pm 60\mathrm{Hz}$ and $-95\mathrm{dBm}$ in (a). Note the second set of peaks (2NPs) at approximately $f_0\pm 25\mathrm{Hz}$ in (b).

Figure 2

Pump beam transmitted towards geomagnetic zenith at $f_0=2.85\mathrm{MHz}$ (03:15–04:22 UT, 24 February 2008). The pump was cycled 2 min on/2 min off. Top: Spectrogram of received radio emissions, centered on $f_0$. Bottom: Blue (dark gray) markers show $f_{\mathrm{crit}}$, measured by the HAARP ionosonde. Red (medium gray) markers correspond to $f_{\mathrm{UH}}=\sqrt{f_{\mathrm{crit}}^2+f_e^2}$, and the green (light gray) horizontal line indicates $f_0$. The black line shows the intensity of the reflected signal, with the ordinate to the right.

Figure 3

Transient stage of wave interaction, vertical pump beam at $f_0=4.5\mathrm{MHz}$ ($20:26:00–20:38:30$ UT, 26 February 2008). Panel (a) shows a spectrogram of the received signal. A colorbar displaying the intensity in dBm is located to the far right. In panel (b) the stationary frequency spectrum is shown and in panel (c) the smoothed, relative intensities of the pump wave (black), the downshifted NP [red (medium gray)] and the upshifted NP [blue (dark gray)] are shown. In panel (d) the smoothed, absolute intensities of the downshifted NP [red (medium gray)] and the upshifted NP [blue (dark gray)] are shown for time $-0.5<t<2.5\mathrm{s}$. Original intensities are depicted by thin light gray lines.

Figure 4

Ray tracings from $x=0$ for a vertical beam with $f_0=4.5\mathrm{MHz}$ (a) and a MZ beam with $f_0=2.7\mathrm{MHz}$ (b),(c). In (a) the colored line shows $c_a$ for which the observed frequency shifts $\Delta f=\pm 28\mathrm{Hz}$ of the NPs are consistent with SBS. For the MZ beam there are two sets of NPs. In (b) the shown $c_a$ corresponds to $\Delta f=\pm 24\mathrm{Hz}$ and in (c) the $c_a$ corresponds to $\Delta f=\pm 9\mathrm{Hz}$. The beam width (10°) is shown by shaded areas. The horizontal, solid lines indicate ${\omega }_p={\omega }_0$ and the dashed lines ${\omega }_p=\sqrt{{\omega }_0^2-{\omega }_e^2}$.