  We present mid-IR imaging observations of a high Galactic latitude cirrus
obtained with the ISO camera ISOCAM at 6" angular resolution. The observations
were done with two filters LW2 (5-8.5 microns) and LW3 (12-18 microns) that
measure respectively the aromatic carbon bands and the underlying continuum
emission from small dust particles. Three 0.05 square degree images sample
atomic and molecular sections in the Ursa Major cirrus. These images are
compared with Hi, CO and IRAS observations. In such a cloud transparent to
stellar light (Av < 0.5) the mid-infrared to 100 microns and the mid-IR
emissivity per hydrogen are related to the abundance and the optical properties
of small dust particles independently of any modelling of the penetration of
the radiation. Within the atomic section of the cloud, the comparison between
ISOCAM images and 21 cm interferometric data highlights an enhancement of the
mid-IR emitters abundance by a factor ~5 in an Hi filament characterized by a
large transverse velocity gradient suggestive of rotation. Furthermore, a drop
in the abundance of the same mid-IR emitters is observed at the interface
between the atomic and molecular cirrus sections. We propose that these
abundance variations of the mid-IR emitters are related to the production of
small dust particles by grain shattering in energetic grain-grain collisions
generated by turbulent motions within the cirrus and inversely by their
disappearance due to coagulation on large grains. At the atomic-molecular
interface we also observe a decrease of the Lw2/Lw3 ratio by a factor 2 in the
molecular region. This could result from a modification of the dust size
distribution or of the intrinsic optical properties of the small dust
particles.
