We study the monopole (BΛ00β) and quadrupole (BΛ20β) moments
of the 21-cm bispectrum (BS) from EoR simulations and present results for
squeezed and stretched triangles. Both BΛ00β and BΛ20β are
positive at the early stage of EoR where the mean neutral hydrogen (HI) density
fraction xΛHIββ0.99. The subsequent evolution of
BΛ00β and BΛ20β at large and intermediate scales (k=0.29 and
0.56Mpcβ1 respectively) is punctuated by two sign changes which
mark transitions in the HI distribution. The first sign flip where
BΛ00β becomes negative occurs in the intermediate stages of EoR
(xΛHIβ>0.5), at large scale first followed by the intermediate
scale. This marks the emergence of distinct ionized bubbles in the neutral
background. BΛ20β is relatively less affected by this transition, and
it mostly remains positive even when BΛ00β becomes negative. The second
sign flip, which affects both BΛ00β and BΛ20β, occurs at the
late stage of EoR (xΛHIβ<0.5). This marks a transition in the
topology of the HI distribution, after which we have distinct HI islands in an
ionized background. This causes BΛ00β to become positive. The negative
BΛ20β is a definite indication that the HI islands survive only in
under-dense regions.Comment: Accepted for publication in MNRA