  An ultracold wave packet of bosonic atoms loaded into a travelling laser wave
may form a many-atom soliton.This is disturbed by a homogeneous force field,
for example by the inevitable gravitation. The wave packet is accelerated and
therefore the laser frequency appears to be chirped in the rest frame of the
atoms. We derive the effective nonlinear Schr\"odinger equation. It shows a
time dependent nonlinearity coefficient which amounts to a damping or
antidamping, respectively. The accelerated packet solution remains a soliton
which changes its shape adiabatically. Similarly, an active shaping can be
obtained in the force-free case by chirping the laser frequency thus
representing a way of coherent control of the soliton form. The experimental
consequences are discussed.
