  The optically dark states play an important role in the electronic and
optical properties of monolayers (MLs) of semiconducting transition metal
dichalcogenides. The effect of temperature on the in-plane-field activation of
the neutral and charged dark excitons is investigated in a WSe$_2$ ML
encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark
(X$^D$) and grey (X$^G$) excitons and the negative dark trion (T$^D$) differ
substantially at a particular temperature. More importantly, they vanish
considerably by about 3 -- 4 orders of magnitude with the temperature increased
from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied
by the two-order-of-magnitude increase in the emissions of their neutral bright
counterparts, $i.e.$ neutral bright exciton (X$^B$) and spin-singlet (T$^S$)
and spin-triplet (T$^T$) negative trions, due to the thermal activations of
dark states. Furthermore, the energy splittings between the dark X$^D$ and
T$^D$ complexes and the corresponding bright X$^B$, T$^S$, and T$^T$ ones vary
with temperature rises from 4.2 K to 100 K. This can be explained in terms of
the different exciton-phonon couplings for the bright and dark excitons
stemming from their distinct symmetry properties.
