ANTARCTICA (above) / Nov.25, 2017. Contrast & saturation enhanced to show structural properties. https://go.nasa.gov/2hSelxX
“Nonlinear beam generated plasma waves as a source for enhanced plasma
and ion acoustic lines”
EISCAT (European Incoherent Scatter Scientific Association) operates three incoherent scatter radar systems, at 224 MHz, 931 MHz in Northern Scandinavia and one at 500 MHz on Svalbard, used to study the interaction between the Sun and the Earth as revealed by disturbances in the ionosphere and magnetosphere.
At the Ramfjordmoen facility (near Tromsø, Norway), it also operates an ionospheric heater facility, similar to HAARP.
L. K. S. Daldorff,1,a) H. L. Pe ́cseli,2,b) J. K. Trulsen,3,c) M. I. Ulriksen,4,d) B. Eliasson,5,e) and L. Stenflo6,f) 1University of Michigan, Space Research Building, 2455 Hayward Street, Ann Arbor,
Michigan 48109-2143, USA
2Department of Physics, University of Oslo, Box 1048 Blindern, N-0316 Oslo, Norway
3Institute of Theoretical Astrophysics, University of Oslo, Box 1029 Blindern, N-0315 Oslo, Norway 4Norwegian Water Resources and Energy Directorate, Drammensveien 211, Postboks 5091 Majorstua, N-0301 Oslo, Norway 5Fakulta ̈t fu ̈r Physik und Astronomie, Ruhr-Universita ̈t Bochum, D-44780 Bochum, Germany 6Department of Physics, Linko ̈ping University, SE-58183 Linko ̈ping, Sweden
(Received 21 December 2010; accepted 1 April 2011; published online 27 May 2011)
Observations by, for instance, the EISCAT Svalbard Radar (ESR) demonstrate that the symmetry of the naturally occurring ion line in the polar ionosphere can be broken by an enhanced, nonthermal, level of fluctuations (naturally enhanced ion-acoustic lines, NEIALs). It was in many cases found that the entire ion spectrum can be distorted, also with the appearance of a third line, corresponding to a propagation velocity significantly slower than the ion acoustic sound speed.
ANTARCTICA, South Shetland Islands & Elephant Island, etc. (above) / Nov.25, 2017. Note the radio-frequency/microwave transmitter pulsated ripples. These are not natural from wind patterns! https://go.nasa.gov/2zEkMA1
It has been argued that selective decay of beam excited primary Langmuir waves can explain some phenomena similar to those observed. We consider a related model, suggesting that a primary nonlinear process can be an oscillating two-stream instability, generating a forced low frequency mode that does not obey any ion sound dispersion relation. At later times, the decay of Langmuir waves can give rise also to enhanced asymmetric ion lines.
The analysis is based on numerical results, where the initial Langmuir waves are excited by a cold dilute electron beam. By this numerical approach, we can detect fine details of the physical processes, in particular, demonstrate a strong space-time intermittency of the electron waves in agreement with observations. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson’s equation.
The analysis distinguishes the dynamics of the background and beam electrons. This distinction simplifies the analysis for the formulation of the weakly nonlinear analytical model for the oscillating two-stream instability. The results have general applications beyond their relevance for the ionospheric observations. VC 2011 American Institute of Physics. [doi:10.1063/1.3582084]
ANTARCTICA (above) / Nov.25, 2017. Contrast & saturation enhanced to show structural properties. The radiation ‘spiral-coil’ ripples get exposed when I enhance the screenshots. These highly ‘metalized’ clouds are being used as inductors and capacitors. https://go.nasa.gov/2zDqWjZ
Incoherent scatter radars are some of the most versatile and widely used tools for studying the Earth’s ionosphere. For the case where the ionospheric plasma is in thermal equilibrium, the backscattered signal can be analyzed in terms of the fluctuations-dissipation theorem from basic thermodynamics and statistical mechanics,1,2 giving both the ion-acoustic and the electron plasma wave spectra. In many cases it is found, however, that the ionospheric plasma is out of equilibrium, and that particularly the ion-line signal is significantly distorted,3–5 giving rise to so-called naturally enhanced ion-acoustic lines (NEIALs). In the NEIALs, the two ion-lines will often have different amplitudes and correspond to velocities, which do not match the expected ion acoustic sound speed. In many cases also an unshifted ion line can be observed1,5 between the up- and down-shifted lines.
The Pacific Ocean (above) / Nov.25, 2017 https://go.nasa.gov/2hQWxmH
These weakly shifted lines are often sporadic and can be difficult to identify, and can appear more like a “filling- in” between the two natural ion lines. Several models were suggested,6,7 and they can account for some of these features. For instance, the symmetry of the natural ion-line is broken if a current is flowing through the plasma.1 It has also been pointed out that an external “pump wave” could give effects similar to those observed.8,9
An electron beam can enhance electron plasma waves (Langmuir waves) significantly above the thermal level, and then ion acoustic waves can be excited by parametric decay of these waves. This latter model was invoked in other studies10,11 and has gained confidence by several works.1,4,12 Consistent with the basic features of these proposed models, observations of simultaneously enhanced levels of ion, and electron plasma waves have been reported.13 The nonshifted (or weakly shifted) ion line can be explained by two basically different models.
The Pacific Ocean (above – detail) / Nov.25, 2017 https://go.nasa.gov/2hSvsQ0
Since the speed of propagation is significantly lower than the sound speed, the line is not a natural fluid mode. It is then either a feature which has to be continuously maintained by some external agency, for instance, the electron beam, or, alternatively, it is a natural mode existing beyond a standard fluid model, a linear kinetic van Kampen–Case mode,14 or a nonlinear BGK-mode.15,16
Either of these modes can in principle have any velocity. In reality, the velocities will be restricted to the range of thermal velocities of the appropriate species. Propagation velocities that differ significantly from the natural sound speed require very “artificial” shapes of the velocity distributions. Some parametrized models suggested in the literature16 represent one way of imposing conditions on the distribution functions, in order to make them match physically realistic velocity distributions at large distances from the structures.
The Pacific Ocean (above – detail) / Nov.25, 2017. The blue ‘island’ is in fact an atoll. The large one may be 20 miles long. https://go.nasa.gov/2hPPFpQ
One problem concerning the model based on Langmuir wave decay seems to be that sometimes very short wavelength primary Langmuir waves are needed to account for the observations, below a few tens of Debye lengths, kDe. It might be possible to find a low velocity electron beam which generates unstable waves for ub < 4uth, but the decay Lang- muir wave (“daughter wave”) obtained from these will be strongly Landau damped, implying that the growth rate of the decay instability becomes negligible.
For the EISCAT Svalbard Radar (ESR)-radar,3 we have a transmitter frequency of 500 MHz, giving kR 2p=kR 1⁄4 0:6 m. For an altitude 400 km, with electron temperatures of Te 3000 K, and plasma densities of n0 2 1011 m3, we find the Debye length kDe 8:5 103 m, i.e., kR=kDe 70, or kRkDe 0:09. The effects connected with plasma inhomoge- neity have been largely ignored, although a consistent treat- ment of the ionospheric plasma density gradient can bring new understanding of the observational results.17,18
Australia (above) / Nov.25, 2017. Note the very straight line. Straight lines do not occur in nature, nor do straight lines occur in cloud forms! https://go.nasa.gov/2hT92yo
The ionospheric observations as such seem to be unambiguous,3 but the interpretation is made difficult by several practical problems: the observed features are often sporadic in nature, and can vary with time as well as altitude, on second and kilometer scales, respectively.1,12 We bear in mind that the radar is usually obtaining backscatter at one selected wavevector kR, which is related to the scattering wavevector kB by selection rules, which for a monostatic radar gives kB 1⁄4 2kR.
In order to observe the ion sound wave, we should have the sound wavelength ks approximately equal to ð1=2ÞkR and the primary Langmuir wavelength kL also approximately ð1=2ÞkR. This means that in a decay process, we cannot, usually, at a given altitude, expect to observe the first generation Langmuir waves simultaneously with the sound waves forming the low frequency part of the decay products. In the case where we have a “cascade” of decaying waves, we might observe one or the other of the decay products, and it might very well be the second or third generation that is observed, instead of the first one. For the parameters mentioned before, we have kLkDe 0:18.
Australia (above) / Nov.25, 2017. Note the very straight line. Straight lines do not occur in nature, nor do straight lines occur in cloud forms!
The plasma conditions are strongly variable 19 and it is not always obvious under which conditions the enhanced ion-lines (NEIALs) are observed, since the relevant parameters are rarely monitored simultaneously. The observations are not sufficiently detailed to allow only one model for their explanation. It has been argued13 that a broad band of Langmuir waves excited by an electron beam with distributed velocities (Dub ub) can generate a wide spectrum of waves that can account for the simultaneous observa- tions of enhanced ion and electron lines at the same Bragg condition, but these calculations have seemingly not been published nor tested by numerical simulations.
The simple Langmuir wave decay can seemingly account for the enhanced ion lines, but cannot directly explain the unshifted component. In our first attempt to explain this feature we considered the possibility of excitation of ion phase-space vortices, or ion holes. These structures are well known from laboratory experiments and numerical simulations.20,21 It has been found that excitation of ion holes is ineffective when the electron/ion temperature ratio is below two,20 and this is after all the most common parameter range for many ionospheric conditions.
AFRICA west coast (above) / Nov.25, 2017. Detail with sepia enhancement for structure. https://go.nasa.gov/2zCO027
Analytical and numerical studies22 have demonstrated that ion phase space holes can be maintained by an enhanced level of Langmuir waves even for moderate ratios Te=Ti. One purpose of the present study was to search for self-consistently and spontaneously generated ion holes with a trapped electron wave component. In order to make the conditions for ion hole formation demanding, we choose a temperature ratio of Te=Ti 1⁄4 1. Actually, it is possible to construct ion phase-space vortices for any temperature ratio Te=Ti > 0, but as stated before there is empirical evidence20 that Te=Ti 2 is a limiting temperature ratio for their existence in practice.
AFRICA west coast (above) / Nov.25, 2017. https://go.nasa.gov/2hSoYkj
Electron phase-space vortices, or electron holes … can be excited and they will have interest in the present context also because such structures can have, in principle, any velocity, also one below the ion sound speed. For physically realistic velocity distribution functions, electron phase-space vortices move at or below the electron thermal velocity25 (with ion vortices having velocities at or below the ion thermal velocity). Although these vortices can be slow compared to the ion sound speed, it will be unlikely to see them confined to such slow velocities. Only their ion counterparts are realistic candidates for subsonic nonlinear structures. Electron holes are well known from laboratory experiments, but their possible role in the generation of NEIALs and the intermediate slow or subsonic ion signature is unknown.
Australia (above) / Nov.25, 2017. Detail sepia enhanced. https://go.nasa.gov/2zEspWW
Numerical simulations offer a possibility for studying the relevant plasma phenomena in detail.12,27–30 The present study is based on a direct numerical solution of the coupled electron and ion Vlasov equations solved for a linearly unsta- ble beam-background electron population. The analysis distinguishes, in particular, the dynamics of background and beam electrons as well as the ions.
Australia (above) / Nov.25, 2017. Detail slightly sepia & contrast enhanced. https://go.nasa.gov/2hRXuLA