An analysis is presented on the motion of contaminant ions picked up b
y the solar wind within velocity shears. An expression is obtained for
the ion density in terms of the flow speed across the velocity shear.
It is argued that enhanced densities occur in the region where the fl
ow speed is small and that local values may become significantly large
r than the ion density outside the velocity shear. A comparison is mad
e with measurements obtained across the plasma wake of comet Giacobini
-Zinner with the International Cometary Explorer (ICE) spacecraft. Wit
hin the velocity shear of that comet's ionosheath the cometary ion den
sity becomes large and reaches values that are different from those ex
pected when the ion population depends only on the ionization of a uni
form outflow of neutral particles from the cometary nucleus. The enhan
ced plasma density expected within the velocity shear is compatible wi
th the shape of the density profile of contaminant ions detected in th
at region. (C) 1996 American Institute of Physics.