  The influence of the chiral mean field on the kaon dynamics in heavy ion
reactions is investigated. Inside the nuclear medium the kaons are described as
dressed quasi-particles carrying effective masses and momenta. A momentum
dependent part of the interaction which resembles a Lorentz force originates
from spatial components of the vector field and provides an important
contribution to the in-medium kaon dynamics. This contribution is found to
counterbalance the influence of the vector potential on the $K^+$ in-plane flow
to a strong extent. Thus it appears to be difficult to restrict the in-medium
potential from the analysis of the corresponding transverse flow.
