NGC 3646 in Leo, a large peculiar galaxy which has been reproduced in at least 2 different orientations, as if it had been "flipped" or rotated or something in its processing, is shown in Peratt's textbook in Figure 3.29, captioned "
Optical photograph of NGC 3646. Note the well-defined diocotron instability in the spiral's arm." The Burbidges didn't seem to be sure if the wavy, kinked area was a part of an arm or a complete elliptical form not necessarily co-centered with the classic spiral centered portion of the galaxy, in a paper 3 decades earlier earlier, at [url]http:
www.adsabs.edu/full/1961Apj...134...237B[/url]. Without going through the Hubble images and those of other agencies at other wavelengths, I can't honestly say I have observed anything that looks like diocotron instabilities in other galaxies than NGC 3646. I'll keep an eye out, and maybe browse around a bit.
Here is a later reference to this odd galaxy, [url]http:
www.adsabs.edu/full/1996Apj...469...299G[/url], titled "
Stability of Stellar Disks of Flat Galaxies II",by Griv and Peter, a better referenced and mathematically described Jeans instability criterion.
I have not drawn conclusions as to whether or not a certain vortex-like morphology represents a diocotron stability or not, and leave that up to those who know more about them than do I. Peratt describes the Diocotron Instability in 1.7.3, noting that the phenomenon "o
bserved closely resembles that associated with the Kelvin-Helmholtz fluid dynamical shear instability, in which vortices develop in a fluid when a critical velocity of the flow is exceeded, with a large increase in the resistance to flow [Chandrasekhar 1961]." This is also similar to Von Karmen vortices, but, being in plasma, the threshold trigger is determined by the beam current or distance of propagation. As a guess, only, on my part, galactic jets imaged in radio frequencies are often able to travel significant distances as a highly collimated electron beam, and often tend, at distance, to become uncollimated and unstable and distort into feathery asymmetric plumes.
To the degree that large scale knots and kinks (in galaxies, nebulae, nova remnants and the like) reflect the lab scale electron beam patterns etched onto carbon witness plates or exciting a fluorescent screen(Peratt's figure 1.20), it may not be completely erroneous to posit that those instabilities seen in telescopes do seem to imitate the much scaled down plasma beams in the lab.
Peratt states, in the same section, that "the instability leading to the filamentation of the beam is known as the "slipping steam" [sic] or "diocotron" and occurs when charge neutrality is not locally maintained, for example, when electrons and ions separate." Peratt's typo should have read "slipping
stream", the translation of the Greek "diocotron", meaning as when two different current sheets or filaments slip closely past each other with different velocities, creating a shear gradient between them.
One wonders if a plasma sheath or double layer becomes set up by this slippage, leading to charge separation across the shear boundary and possible acceleration by the ensuing electric fields of charged particles to relativistic velocities observed as, among other things, synchrotron radiation. This seems common enough on the Sun, where similar accelerations can occur in plasma filaments arcades and lead to high energy particle emission.
In his
Fundamentals of Plasma Physics, 2006,Paul Bellan at CalTech covers "streaming instabilities" and the related "Landau problem" in great depth with more math than I could learn in another lifetime, but he never once mentions "diocotron" in connection with this form of instability, nor does he indicate that it takes certain shapes which have cognates in fluid dynamics, and that there are cosmic phenomena in which plasma instabilities, which have been fairly well researched and described on Earth at lab scales,
appear to exist at various cosmic scales, too.
To his credit, at the beginning of his excellent book, Bellan writes, "Plasma physics is usually not a precise science. It is rather a web of overlapping points of view, each modeling a limited range of behavior. Understanding of plasmas is developed by studying these various points of view, all the while keeping in mind the linkages between [sic; among] these points of view." and "Plasma dynamics is determined by the
self-consistent interaction between electromagnetic fields and statistically large numbers of charged particles..."
While many modern plasma physics textbooks allow that "space plasmas" exist, they, most definitely unlike Peratt, do not make much effort to relate their lessons and endless mathematics (which is good math as far as it goes, but generally ignores important discrete behaviors at the particle scale, operating as electric currents). How they plan to discover a way to get electricity out of their tokamaks without utilizing the electric current details of plasma physics is beyond me.
Jim
