  Non conventional localized pulses, like offset bipolars have been reported in
the Earths magnetosphere. The reports were sporadic and the theoretical
interpretations were mainly event based which differ for slow or fast moving
pulses. Here it was shown that the offset bipolar pulses are manifestations of
an ideal, mathematically defined nonlinear coherent structure called a flat top
soliton. Adopting Sagdeev pseudopotential technique, and assuming a simple
plasma model, it was shown that a slow moving offset bipolar pulse in the
auroral region can be interpreted as an ion acoustic flat top solitary wave.
The proposed theory is a generic one and can easily be extended to a fast
moving one under an appropriate plasma model. One important characteristic of
such structures is that they generally occur at the boundary of two different
phases of nonlinear dynamical processes.
