  We investigate instabilities of classical Yang-Mills fields in a
time-dependent spatially homogeneous color magnetic background field in a
non-expanding geometry for elucidating the earliest stage dynamics of
ultra-relativistic heavy-ion collisions. The background gauge field
configuration considered in this article is spatially homogeneous and
temporally periodic, and is alluded by Berges-Scheffler-Schlichting-Sexty
(BSSS). We discuss the whole structure of instability bands of fluctuations
around the BSSS background gauge field on the basis of Floquet theory, which
enables us to discuss the stability in a systematic way. We find various
instability bands on the $(p_z, p_T)$-plane. These instability bands are caused
by parametric resonance despite the fact that the momentum dependence of the
growth rate for $|\mathbf{p}| \leq \sqrt{B}$ is similar to a Nielsen-Olesen
instability. Moreover, some of instability bands are found to emerge not only
in the low momentum but also in the high momentum region; typically of the
order of the saturation momentum as $|\mathbf{p}| \sim \sqrt{B} \sim Q_{\rm
s}$.
