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 \mu m ) and LW3 ( 12-18 \mu m ) 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 ( A _ { V } < 0.5 ) the mid-infrared to 100 \mu m 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 \sim 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 Hi -H _ { 2 } interface we also observe a change in the I _ { \nu } ( LW2 ) / I _ { \nu } ( LW3 ) ratio by a factor 2 .
This color variation indicates that the amplitude of the continuum near 15 \mu m , relative to the aromatic bands , rises inside the molecular region .
It could result from a modification of the dust size distribution or of the intrinsic optical properties of the small dust particles .