  We introduce an Uhlenbeck closure of the space of based maps from projective
line to the Kashiwara flag scheme of an untwisted affine Lie algebra. For the
algebra $\hat{sl}_n$ this space of based maps is isomorphic to the moduli space
of locally free parabolic sheaves on $P^1\times P^1$ trivialized at infinity.
The Uhlenbeck closure admits a resolution of singularities: the moduli space of
torsion free parabolic sheaves on $P^1\times P^1$ trivialized at infinity. We
compute the Intersection Cohomology sheaf of the Uhlenbeck space using this
resolution of singularities. The moduli spaces of parabolic sheaves of various
degrees are connected by certain Hecke correspondences. We prove that these
correspondences define an action of $\hat{sl}_n$ in the cohomology of the above
moduli spaces.
