  We study theoretically the mode conversion and the associated resonant
absorption of p-polarized electromagnetic waves into longitudinal plasma
oscillations in cold, unmagnetized and stratified plasmas with periodic spatial
density variations. We consider sinusoidal density configurations for which the
frequency band where mode conversion occurs is well included within a
transmission band of the one-dimensional plasma photonic crystal. We calculate
the mode conversion coefficient, which measures the fraction of the
electromagnetic wave energy absorbed into the plasma, and the spatial
distribution of the magnetic field intensity for various values of the wave
frequency and the incident angle using the invariant imbedding theory of mode
conversion. We find that the absorption is greatly enhanced over a wide range
of frequency and incident angle due to the interplay between the mode
conversion and the photonic band structure. The enhancement occurs because for
frequencies within a transmission band, the wave reflection is strongly
suppressed and the waves penetrate more deeply into the inhomogeneous region,
thereby increasing the possibility for them to reach many resonance points
where the dielectric permittivity vanishes.
