  We examine the evolution of the Parker instability in galactic disks using 3D
numerical simulations. We consider a local Cartesian box section of a galactic
disk, where gas, magnetic fields and cosmic rays are all initially in a
magnetohydrostatic equilibrium. This is done for different choices of initial
cosmic ray density and magnetic field. The growth rates and characteristic
scales obtained from the models, as well as their dependences on the density of
cosmic rays and magnetic fields, are in broad agreement with previous
(linearized, ideal) analytical work. However, this non-ideal instability
develops a multi-modal 3D structure, which cannot be quantitatively predicted
from the earlier linearized studies. This 3D signature of the instability will
be of importance in interpreting observations. As a preliminary step towards
such interpretations, we calculate synthetic polarized intensity and Faraday
rotation measure maps, and the associated structure functions of the latter,
from our simulations; these suggest that the correlation scales inferred from
rotation measure maps are a possible probe for the cosmic ray content of a
given galaxy. Our calculations highlight the importance of cosmic rays in these
measures, making them an essential ingredient of realistic models of the
interstellar medium.
