  Recent observations of core-collapse supernovae (SNe) suggest aspherical
explosions. Globally aspherical structures in SN explosions are regarded as the
key for understanding their explosion mechanism. However, the exact explosion
geometries from the inner cores to the outer envelopes are poorly understood.
Here, we present photometric, spectroscopic and polarimetric observations of
the Type IIP SN 2021yja and discuss its explosion geometry, in comparison to
those of other Type IIP SNe that show large-scale aspherical structures in
their hydrogen envelopes (SNe 2012aw, 2013ej and 2017gmr). During the plateau
phase, SNe 2012aw and 2021yja exhibit high continuum polarization characterized
by two components with perpendicular polarization angles. This behavior can be
interpreted to be due to a bipolar explosion, composed of a polar (energetic)
and an equatorial (bulk) components of the SN ejecta. In such a bipolar
explosion, an aspherical axis created by the polar ejecta would be dominating
at early phases, while the perpendicular axis along the equatorial ejecta would
emerge at late phases after the receding of the photosphere in the polar
ejecta. The interpretation of the bipolar explosions in SNe 2012aw and 2021yja
is also supported by other observational properties, including the time
evolution of the line velocities and the line shapes in the nebular spectra.
The polarization of other Type IIP SNe that show large-scale aspherical
structures in the hydrogen envelope (SNe 2013ej and 2017gmr) is also consistent
with the bipolar-explosion scenario, although this is not conclusive.
