  In a collisionless Vlasov-Poisson (V-P) electron plasma system, two types of
modes for electric field perturbation exist: the exponentially Landau damped
electron plasma waves and the initial-value sensitive ballistic modes. Here,
the V-P system is modified slightly to a Vlasov-Ampere (V-A) system. A new
constant residual mode is revealed. Mathematically, this mode comes from the
Laplace transform of an initial electric field perturbation, and physically
represents that an initial perturbation (e.g., external electric field
perturbation) would not be damped away. Thus, this residual mode is more
difficult to be damped than the ballistic mode. [Physics of Plasmas 20, 112108
(2013); doi: 10.1063/1.4831761]
