  To date, the dayside thermal structure of ultrahot Jupiters (UHJs) is
generally considered to be inverted, but their nightside thermal structure has
been less explored. Here we explore the impact of nightside thermal emission on
high-resolution infrared transmission spectroscopy, which should not be
neglected, especially for UHJs. We present a general equation for the
high-resolution transmission spectrum that includes planetary nightside thermal
emission. This provides a new way to infer the thermal structure of the
planetary nightside with high-resolution transmission spectroscopy. Using the
cross-correlation technique, we find evidence for the presence of an H$_2$O
emission signature on the UHJ WASP-33b during the transit, indicating an
inverted temperature structure on its nightside. Such a result suggests a
stronger heat transport through the circulation than currently expected. An
alternative explanation is that the rotating visible hemisphere during transit
leads to the potential contribution of the limb and dayside atmospheres to the
detected emission signature. In the future, the combination of high-resolution
full-phase curve spectroscopic observations and general circulation models will
hopefully solve this puzzle and provide a complete picture of the
three-dimensional nature of the chemistry, circulation, and thermal structure
of UHJs.
