The Temporal development of HF-exited Langmuir and Ion Turbulence.
Arecibo Observatory, Puerto Rico, is in the process of recovering its ability to modify the ionospheric plasma with the aid of high-power HF (5 MHz – 8.2 MHz) radio waves transmitted from the ground. One of the motivations behind the HF wave-plasma studies is to verify laser fusion theory. Because of Arecibo's unique midlatitude location and its powerful 430 MHz radar, detailed studies of Langmuir tubulence/ion turbulence excited in the plasma are possible over long time periods. During the initial experiments conducted with the new Arecibo HF facility, special emphasis will be placed on investigating electron acceleration processes in the plasma [e.g., Carlson, H. C., F. T. Djuth, and L. D. Zhang (2017), Creating space plasma from the ground, J. Geophys. Res. Space Physics, 122, 978–999, doi:10.1002/2016JA023380], The result showed that there were significant amounts of HF-induced suprathermal electrons, but the detailed link to the plasma turbulence was not examined because of the logistic difficulties during the initial experiments.
The figures below show previous ultra high resolution measurements of Langmuir tubulence/ion turbulence from Arecibo. They illustrate the temporal evolution of the intensity of Langmuir and ion turbulence versus altitude for a "cold turn-on" of a high-frequency (5.1 MHz) radio beam in a smooth background plasma. These measurements were made on May 3, 1990 at 21:10:00 local time and are totally reproducible in a cold background plasma. The upper panel shows the intensity of induced Langmuir waves traveling toward the radar (upshifted plasma line), the middle panel depicts the ion turbulence, and the bottom panel shows the Langmuir waves traveling away from the radar (downshifted plasma line). The downshifted plasma line data is expanded in the plot below. During the first few seconds after the HF beam is turned on turbulence follows the profile of the standing HF wave in the plasma. Arrows point to the standing wave maxima (SWM). During the first 1.2 s seven SWM are evident, but eventually there are only 2 then 1, then a smear created by geomagnetic field-aligned filamentation of the plasma. The upper boundary of the echo is close to the point of radio wave reflection in the plasma. With increasing time the ionosphere slowly moves upward, and the plasma turbulence follows this trend. A review of the temporal development of HF-excited Langmuir and Ion Turbulence at Arecibo is provided by Djuth, F. T., and D. F. DuBois (2015), Temporal Development of HF-Excited Langmuir and Ion Turbulence at Arecibo, Earth Moon Planets, 116, 19-53, doi: 10.1007/s11038-015-9458-x.
The temporal development of the turbulence shown above is currently not well-understood. However, recently we acquired a new data set in November 2018 that includes spectra versus altitude, which hopefully will aid in the interpretation of the results. The above research was funded by NSF grant AGS-1656898.
When the electrons in the ionosphere are heated to high temperatures (e.g., 4,500 K compared to a background of 800 K), the ion-acoustic waves in the plasma move to the tail of the plasma distribution function and are no longer heavily Landau damped. As a result, the HF wave can interact with lightly damped long-wavelength (50 m) acoustic waves as part of a parametric instability thereby enhancing the acoustic wave amplitude and yielding a large amount of HF wave backscatter from the ion wave. This is known as stimulated Brillioun Scattering (SBS). In laser fusion, this means that part of the laser beam is scattered back, so great efforts are made to mitigate it. Theoretical explanations of SBS in the ionosphere generally rely on the presence of a smooth horizontally stratified plasma. However, we have recently analyzed a large data set that indicates that theory must accommodate a heavily striated plasma, and in fact the striated plasma enhances the instability. Below we present an example of the SBS initially generated in a stratified ionosphere, but later (after 15 min) the plasma becomes heavily striated and the SBS instability is greatly excited. IAW and EICW refer to two modes in the plasma: the ion acoustic mode and the electrostatic ion cyclotron mode. 2 X IAW is a cascade line. A detailed publication on this subject is in preparation. The above research was funded by NSF grant AGS-1656898.