Space Plasma Physics The Study of Solar-System Plasmas (1978) / Chapter Skim
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Understanding Plasma Instabilities in Space: Ionospheric Research and Communications Applications
Understanding Plasma Instabilities in Space: Ionospheric Research and Communications Applications
Pages 402-423
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From page 402... ...
Instabilities are ubiquitous in space plasmas because they ofttimes provide the dissipation and transport which are demanded by the general large-scale features of plasma flow in the ionosphere and magnetosphere. Indeed, dissipation and transport by instabilities is a feature which space plasms share with other large, nonlinear systems; turbulent hydrodynamic flows, geophysical fluid dynamics, astrophysical hydrodynamics and controlled fusion research.
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From page 403... ...
1. A Computational Framework for Understanding Space Plasmas The first issue to be addressed is what are the goals of Space Plasma Physics?
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From page 404... ...
Experiments, of course, play a major role in this approach to understanding space plasmas. First, they provide the initial data with which to start computational programs.
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From page 405... ...
To summarize the above paragraphs: Space Plasma Physics should set a goal of understanding and the most likely form this understanding will take is a set of nonlinear dynamic computational programs. Experiments provide three essential contributions: identifying the basic physical processes, providing initial data, and normalizing computational results.
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From page 406... ...
I Communications systems which propagate through space plasmas are 61 used increasingly for civilian and military purposes. Let us take a broad view of communications to include all forms of electromagnetic transmission of information: standard messages relayed via satellite, TV, satellite telemetry and other data, navigational signals, space track Q and ABM radars, and even raw energy from a satellite power station.
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From page 407... ...
The ionosphere is perhaps the best subject for the computational approach to understanding space plasmas, principally because the magnetic field is very strong and magnetic perturbations can be safely ignored in ionospheric dynamics. Our framework for understanding the ionosphere contains four parts: (1)
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From page 408... ...
Within reasonable adjustments of these external parameters, the calculated ionospheric electron density profiles agree well with observed ones. ' But often achieving good agreement forces one into adopting ty Q en solar EUV fluxes that are twice the measured values, 'perhaps an indication of our uncertainty of their variation during the course of the solar cycle.
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From page 409... ...
Let us set forth some more specific suggestions on what types of instabilities might occur in the ionosphere and indicate whether or not any stability analyses have been performed. Turning first to the question of mechanical equilibrium of the ionosphere, electric fields and neutral winds play an important role in controlling the height of the F-layer and in producing sporadic-E layers.
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From page 410... ...
Auroral arc models, which have very fast chemistry time scales as well as important electric currents, have yet to be 20 22 examined for instabilities other than the Farley ' instability. The gradient26 drift instability has been proposed to explain diffuse radio aurora but a stability analysis which deals satisfactorily with the high-latitude geometry remains to be carried out.
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From page 411... ...
Essentially only equatorial Farley and gradient-drift instabilities have been studied. Perhaps the most successful nonlinear studies to date have been the application of the computational simulation method first developed 66 for barium clouds, to gradient-drift and Rayleigh-Taylor instabilities in the equatorial E- and F-regions.38,56 jn the E-region, the computational result shewed how daughter instabilities with vertically-oriented wave vectors were created by the nonlinear state of the primary instability which had an approximately horizontal wave vector.
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From page 412... ...
In these analogies, field-line-integrated plasma density or conductivity takes on the role of vorticity while the electric potential serves as the stream function. The ionospheric turbulence shares with two-dimensional fluid turbulence the property that all moments of the vorticity are conserved, but the elliptic equation for the stream function is sufficiently different that steady-state vortices can not occur in ionospheric turbulence.
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From page 413... ...
If enhanced joule heating resulting from plasma instabilities occurs, then our model of the overall auroral zone heating could be in error.
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From page 414... ...
Satellite measurements experiments most likely will continue to provide the best description of magnetospheric current sources in the ionosphere, but one must be careful in deciding whether the observations uniquely define high-latitude current systems, or are merely consistent with a proposed model. Indeed, both ionospheric and magnetospheric physics could greatly benefit from a measurement program which did uniquely define the spatial pattern of electric field and field-aligned current systems.
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From page 415... ...
The purpose of this paragraph is to bring this material together for special emphasis. The unique geometry of the equatorial ionosphere means that it must carry strong currents in the E-region, and have the density stratified in a direction orthogonal to the magnetic field.38,-;6 Currently developed methods of plasma stability analysis are particularly suitable to this geometrjr ' and the two-dimensional nature of the ensuing plasma tur-' bulence permits accurate computational studies of Rayleigh-Taylor type
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From page 416... ...
The plasma instabilities which ensued when the ionosphere was illuminated by high power radio waves provided one of the first experimental demonstrations of plasma heating via nonlinear processes as well as motivating important theoretical work on the anomalous absorption caused by these instabilities. The self-focusing instability of 62 43 high-power radio waves was also observed experimentally and theory suggests that ionospheric striation via self-focusing may be an important 9 environmental impact of a satellite power station.
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From page 417... ...
The reason is that a deeper and more thorough understanding "of space plasmas is required to predict the outcome of active experiments, rather than explain observations. Hence, if active ionospheric experiments are to continue to be of general intellectual significance, qualitatively new experiments must be invented.
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From page 418... ...
Even the equilibria states of the ionosphere have been modelled only by one-dimensional codes, and these require neutral winds, solar UV flux, and global electric fields as input parameters. Three-dimensional equilibria models, required for polar regions, have not yet been created.
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From page 419... ...
F Bates, Atmospheric expansion through joule heating by horizontal electric fields, Planet.
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From page 420... ...
A Greenwald, Diffuse radar aurora and the gradient drift instability, J;.
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From page 421... ...
45. A comprehensive account of radio wave heating experiments is found in the special issue of Radio Science, Nov.
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From page 422... ...
Weinstock, Theory of the nonlinear spectrum of the gradient drift instability in the equatorial electrojet.
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From page 423... ...
F Utlaut, Ionospheric scintillation: a potential limitation to satellite communications - important unknown scintillation factors, AIAA 12th Aerospace Sciences Meeting, paper 74-56 (1974)
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