J. Wang et al., 3-DIMENSIONAL ELECTROMAGNETIC MONTE-CARLO PARTICLE-IN-CELL SIMULATIONS OF CRITICAL IONIZATION VELOCITY EXPERIMENTS IN-SPACE, J GEO R-S P, 101(A1), 1996, pp. 371-382
Although the existence of the critical ionization velocity (CIV) is kn
own from laboratory experiments, no agreement has been reached as to w
hether CIV exists in the natural space environment. In this paper we m
ove towards more realistic models of CIV and present the first fully t
hree-dimensional, electromagnetic particle-in-cell Monte-Carlo collisi
on (PIC-MCC) simulations of typical space-based CIV experiments. In ou
r model, the released neutral gas is taken to be a spherical cloud tra
veling across a magnetized ambient plasma. Simulations are performed f
or neutral clouds with various sizes and densities. The effects of the
cloud parameters on ionization yield, wave energy growth, electron he
ating, momentum coupling, and the three-dimensional structure of the n
ewly ionized plasma are discussed. The simulations suggest that the qu
antitative characteristics of momentum transfers among the ion beam, n
eutral cloud, and plasma waves is the key indicator of whether CIV can
occur in space. The missing factors in space-based CN experiments may
be the conditions necessary for a continuous enhancement of the beam
ion momentum. For a typical shaped charge release experiment, favorabl
e CIV conditions may exist only in a very narrow, intermediate spatial
region same distance from the release point due to the effects of the
cloud density and size. When CIV does occur, the newly ionized plasma
from the cloud forms a very complex structure due to the combined for
ces from the geomagnetic field, the motion induced emf, and the polari
zation. Hence the detection of CIV also critically depends on the sens
or location.