The nonlinear growth theory of chorus emissions is used to develop a
simple model of the subpacket formation. The model assumes that the
resonant current, which is released from the source to the upstream
region, radiates a new whistler mode wave with a slightly increased
frequency, which triggers a new subpacket. Saturation of the growth in
amplitude is controlled by the optimum amplitude. Numerical solution of
advection equations for each subpacket, with the chorus equations acting
as the boundary conditions, produces a chorus element with a subpacket
structure. This element features an upstream shift of the source region
with time and an irregular growth of frequency, showing small decreases
between adjacent subpackets. The influence of input parameters on the
number of subpackets, the shift of the source, the frequency sweep rate
and the maximum amplitude is analyzed. The model well captures basic
features of instantaneous frequency measurements provided by the Van
Allen Probes spacecraft. The modeled wave field can be used in future
particle acceleration studies.