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Plasma Physics Mechanisms Relevant to Striation Structuring and Decay

20. 
Striation falling due to motion perpendicular to B. 
(This occurs 
for a nonvert iCal magnet Ic field through the setting up of electric 
fields perpendicular to B dut to components of the neutral veloc i ty 
or gravity perpendicular to B.) 
Diffusion perpendicular to B due to drift—dissipation mode tur— 
bul ence. 
Distance scales for striation structure for model situations of 
(The scales are 
striation pinching and striation tip steepening. 
calculated as functions Of initial dimensions and diffusion 
coeffic ients. 
Time scales for striation decay due to the above processes. 
The effect of ionization falling appears to be capable of removing 
ion inhomogeneities on a time scale of order I hour; diffusion 
perpendicular to the magnetic field is strongly dependent on striation 
dimensions and diffusion mechanisms, but significant turbulent diffu— 
s ion appears possible in some cases on time scales of the order Of ten 
minutes. There is some indication in the experimental data of plasma 
turbulent diffusion. These results are of the nature of estimates 
rather than detailed calculations and hence are suggestive rather than 
def init ive.

In this paper we have attempted to present a basis for the physical 
understanding of drift modes as well as background information for 
their application to striation decay problems . 
If on€ neglects the 
effect of a component of the ambient field, E, in the direc— 
t ion of the background density gradient, Vn (with both perpendicular tc 
to the ambient magnetic field, B), drift modes appear capable of pre— 
venting elongated striations from getting thinner than 0.1 km in many 
cases of interest. The inclusion of electric field effects for drift 
rw»des is an ongoing problem.

The E x B instability is analyzeé in the presence of plasma velocity 
shear, with the rnagnetic field, B, in the z—direction, the background 
density gradient in the x—direction, and electric field components 
E (x) and E with E / E non—zero. 
It is shown that the structure of 
E x B modes under circumstances can account naturally for a k 
-2 
ionospheric power density spectrum, provided one assumes that the modes 
grow until the modal density gradient in the direction of the ambient 
density gradient becomes comparable with the ambient density gradient