  We investigated the polarization characteristics of a zebra pattern (ZP) in a
type-IV solar radio burst observed with AMATERAS on 2011 June 21 for the
purpose of evaluating the generation processes of ZP. Analyzing highly resolved
spectral and polarization data revealed the frequency dependence of the degree
of circular polarization and the delay between two polarized components for the
first time. The degree of circular polarization was 50-70 percent right-handed
and it varied little as a function of frequency. Cross-correlation analysis
determined that the left-handed circularly polarized component was delayed by
50-70 ms relative to the right-handed component over the entire frequency range
of the ZP and this delay increased with the frequency. We examined the obtained
polarization characteristics by using pre-existing ZP models and concluded that
the ZP was generated by the double plasma resonance process. Our results
suggest that the ZP emission was originally generated in a completely polarized
state in the O-mode and was partly converted into the X-mode near the source.
Subsequently, the difference between the group velocities of the O-mode and
X-mode caused the temporal delay.
