  The analysis of the spectral characteristics of the burst radio emission on
June 21, 2011 was carried out on the basis of an improved methodology for
determining harmonic numbers for the corresponding stripes of the zebra
structure. By using the parameters of the zebra structure in the time frequency
spectrum and basing on the double plasma resonance model, the magnetic field
and its dynamics, electron density, and the time variation of the distance
between the stripes with harmonics s = 55 and 56 and adjacent stripes near the
frequency 183 MHz have been determined in the burst generation region. The
relationships between the scale characteristics of the field and the density
along and across the axis of the power tube and their dependence on time have
been also determined. The field obtained (1.5 G for the first harmonic and 0.75
G for the second harmonic of the plasma frequency) turned out to be so small
that, firstly, it fails to explain the dynamic features of the spectrum based
on MHD waves, and secondly, it results in large values for plasma betta (>1).
Other possible difficulties of the generation mechanism of bursts with zebra
pattern based on the double plasma resonance are also noted. Another possible
mechanism, with whistlers explains qualitatively the main observational
characteristics of this zebra. The magnetic field required in this case is
about 4.5 G, and the plasma betta is 0.14, which fully corresponds to the
coronal conditions.
