Space Plasma Physics The Study of Solar-System Plasmas (1978) / Chapter Skim
Currently Skimming:
The Ionospheric Plasma
The Ionospheric Plasma
Pages 364-401
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 364... ...
The nonlinear processes by which the turbulence develops are being actively investigated via a variety of experimental, theoretical, and computer simulation programs. Once a good correspondence between the observations in space and the simulations has been achieved, the latter should guide the development of nonlinear theories, which should have applications beyond those of space physics.
|
From page 365... ...
The effect of tropospheric storms can be detected at altitudes of several hundred kilometers; the effect of magnetospheric and ionospheric processes on the lower atmosphere can apparently also be appreciable. The possible relationship between weather and the solar cycle is currently attracting considerable attention.
|
From page 366... ...
Incoherent scatter, in addition to its usefullness as a probing technique, has provided perhaps the best detailed quantitative verification to date of the conventional linear kinetic theory of a "warm" plasma. Agreement between theory and observation is invariably within the experimental errors, which are sometimes less than one percent.
|
From page 367... ...
) | >, the expected mean square amplitude of density fluctuation waves with wave vector k and frequency a)
|
From page 368... ...
The relevance of incoherent scatter research to some other aspects of the development of plasma physics theory is mentioned in a review by Bauer . The theory leads to an expression for the radar scattering cross section a of the plasma which is given below, in the notation of Farley ' .
|
From page 369... ...
In many situations the original thermal equilibrium, collisionless, single ion species, non-magnetic theory is directly applicable to the ionospheric measurements, but in some it is not. The electron and ion temperatures may be different, collisions between ions and neutral particles may be important, the Earth's magnetic field may affect the scattering, more than one ion species may be present, the plasma as a whole may be moving, or the electrons and one or more species of ions may all be drifting at different mean velocities, the electrons may have a non-Maxwellian distribution, particularly a high energy "tail" caused by the production of photoelectrons, or the ions may even by non-Maxwellian, particularly in the auroral zone where the plasma drift velocities relative to the neutral particles can be very large.
|
From page 370... ...
This often makes the data somewhat difficult to analyze, but on the other hand when the analysis is carried out it yields a great deal of information that is obtainable in almost no other way. The ionized particles in the upper atmosphere also serve as tracers to provide information about ambient electric fields and the dynamics of the neutral atmosphere.
|
From page 371... ...
The .extensive agreement between theory and ionospheric observations has provided a detailed quantitative verification of conventional kinetic plasma theory based on the linearized Vlasov equation.
|
From page 372... ...
Indian scientists are eager to establish an observatory in their country, but lack the nesessary funds. It should be emphasized that although incoherent scatter radars are certainly not cheap, they are not really expensive when compared to the cost of major satellite programs.
|
From page 373... ...
the magnetic equator where the magnetic field geometry leads to an unusually high conductivity perpendicular to the magnetic field, and (2) the auroral zone where the electric fields perpendicular to B are often very large.
|
From page 374... ...
. In the early 1960 's the work in Peru was resumed using frequency coherent VHF radars so that Doppler spectra of the electrojet echoes could be determined .
|
From page 375... ...
Radar diagnostic techniques have been greatly improved, some rocket observations have been made, the linear theory of the instability (in both fluid and kinetic theory forms) has been extended and refined, numerous theoretical studies of various nonlinear effects have been carried out (although with only mixed success)
|
From page 376... ...
The main observed characteristics of the equatorial electrojet instability are as follows: 1. There is a high correlation with the magnitude and direction of the electrojet current.
|
From page 377... ...
, k is the wave vector, C is the ion-acoustic velocity, LN is the electron density gradient length (positive for density increasing with height) , a is the recombination coefficient, and ijj is v v./R fl.
|
From page 378... ...
One cannot explain the detailed shape of the Doppler spectra of the radar echoes or predict the amplitude of the unstable waves; both of these obviously depend upon nonlinear saturation mechanisms. Nor can vertically propagating unstable waves, for which the two driving terms go to zero, be accounted for.
|
From page 379... ...
This work which was begun at the Naval Resaerch Laboratory is now being extended at Cornell University. The early results of the Cornell simulations have already given good reproductions of radar observations of 44 the type 2 irregularities.
|
From page 380... ...
now briefly to the auroral zone E region, we find that our understanding of the plasma processes there is still in quite a primitive state. Measurements of the Doppler spectra of VHP radar signals scattered from 8 auroral zone E region plasma waves were first obtained in the 1950's ,
|
From page 381... ...
is not important in the auroral case. My feeling, which is not shared by everyone, is that a reasonable understanding of the E-region auroral zone plasma instabilities will only be achieved after the equatorial zone processes are more fully understood.
|
From page 382... ...
irregularities over a very wide range of altitudes, including regions where the mean vertical electron density gradient was positive, negative, or zero. This insensitivity to the density gradient seemed at the time to rule out gravitational instabilities as a cause.
|
From page 383... ...
Smaller scale irregularities are presumed to form on the sharp edges of the bubbles. These ideas, which a^e now generally accepted, emerged at the 1975 Gordon Conference on Space Plasma Physics, as a result of talks by M.C.
|
From page 384... ...
The barium clouds often develop striations that are highly aligned with the magnetic field and frequently break up into quite small scale sizes. Analytical work and numerical simulations have shown that the primary cause of the striations is a gradient-drift instability operating at long wavelengths, with subsequent 43 decay to shorter scale sizes.
|
From page 385... ...
As in the case of the E-region instabilities, more numerical simulation studies are planned which should help to guide future analytic theories. We have not even begun to try to sort out and interpret the complex velocity distributions within the unstable region 68 revealed by the 50 MHz ra-"ar spectral measurements of Woodman and La Hoz These Doppler spectra are sometimes simple and sometimes complex, and can vary rapidly with both altitude and time.
|
From page 386... ...
and to instabilities observed following barium cloud releases in the ionosphere (see Goldman et al. for recent work on simulating
|
From page 387... ...
Velocity shear instabilities have also been observed in the 42,45,67 F-region near auroral arcs 6. SUMMARY The ionosphere, besides being an interesting part of our environment which plays an important role in worldwide communications, is a convenient place to study certain aspects of plasma phsyics.
|
From page 388... ...
This success has of course also helped to put the kinetic theory itself on very firm quantitative grounds. We have seen that there are a number of interesting plasma instabilities in the ionosphere which lead to the development of turbulent conditions which persist for minutes or eveu hours in a reasonably steady state.
|
From page 389... ...
1142 the turbulence. Other processes not considered in detail here, such as parametric instabilities and high latitude phenomena, particularly those associated with the aurora, also hold out exciting prospects for progress in plasma physics in the next decade.
|
From page 390... ...
1143 10 FEB. 12.40 1972 l§84 7T4 SBt 342 km FIGURE 1 Incoherent scatter auto-correlation function measurements made at Arecibo.
|
From page 391... ...
200 -200 -IOO 0 IOO 200 DOPPLER SHIFT lHi) FIGURE 3 A series of power spectra of signals scattered from the equatorial electrojet near noon at Jicamarca, Peru at various angles east and west of vertical.
|
From page 392... ...
The mean local time of each 2.5 min integration is at the left of each curve. The phase velocity, rather than the Doppler shift, of the wave is plotted, and the spectra are all normalized as in Figure 3, although the maximum is not always shown.
|
From page 393... ...
-120 120 -I20 I20 DOPPLER SHIFT (HZ) FIGURE 5 Power spectra from vertically propagating 3 m electrojet irregularities.
|
From page 394... ...
1147 t = .64s = l.28s t* 2.70s FIGURE 6 Computer simulation of the development of equatorial electrojet irregularities.
|
From page 395... ...
The bubble forms at the lower interface and propagates through the top layers (from Woodman and La Hoz68)
|
From page 396... ...
0 T-S.OOOMC -to o OS 10 so to 350 100 '•lO.OOOJfC -20 0 (km) to FIGURE 9 Contour plots of 6n/n0 showing the simulation of the growth of the equatorial F region Rayleigh-Taylor instability and the rising of a "bubble." The growth rate here is unrealistically small, due to the choice of initial density profile, shown by the dashed line (after Scannapieco and Ossakow62)
|
From page 398... ...
Hagen, Incoherent scattering of radio waves by whistler mode oscillations in the ionosphere, submitted to Radio Sci., 1977.
|
From page 399... ...
Woodman, Oblique VHF radar spectral studies of the equatorial electrojet, J Geophys.
|
From page 400... ...
Sudan, Numerical studies of type 2 equatorial electrojet irregularity development, Radio Sci., 10, 247-254, 1975.
|
From page 401... ...
Jamin, Two-dimensional nonlinear processes associated with "type 1" irregularities in the equatorial electrojet, J Geophys.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.