SP-345 Evolution of the Solar
System
FIGURE 6.7.1. Simple model of negative
diffusion. The region of collision is shown (above) for two
particles, one with guiding center at x1 =
r -
r' and
another with guiding center at x2
= r'
- r. Both particles are
oscillating radially with amplitude x0. In the
x1
x2 diagram (below)
collisions can take place if x1
+ x2 is located
inside the triangle. If a perfectly inelastic collision takes place
in the domain a the guiding center x2 is transmitted
outward through r'. If the collision takes place in domain b, a similar inward
transmission of the guiding center at x1 takes place. If
the density increases with r, the collision
frequency is larger in domain a than in domain
b, with
the result that the diffusion goes outward; i.e., toward the higher
density region. Hence the diffusion coefficient is negative.
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