  It is known that peculiar plasmons whose frequencies are purely imaginary
exist in the interior of a two-dimensional electronic system described by the
Drude model. We show that when an external magnetic field is applied to the
system, these bulk plasmons are still non-oscillating and are isolated from the
magnetoplasmons by the energy gap of the cyclotron frequency. These are mainly
in a transverse magnetic mode and can combine with a transverse electronic mode
locally at an edge of the system to form edge magnetoplasmons. With this
observation, we reveal the intrinsic long lifetime of edge magnetoplasmons for
the first time.
