For Cylindrical $Y\times\mathbb R$, where $Y$ is a closed oriented 3-manifold.
If it is necessary, we could consider the $b_1(Y)=0$ case.
Fix a Line bundle $L\to Y\times \mathbb R$ and a Hermitian connection $A_0$, we know that all other Hermtian connections, denoted by $\mathcal C$, can be regarded as the 1 forms on $Y\times\mathbb R$, i.e. $\mathcal C\overset{\sim}{=}\sqrt{-1}\Omega^1(Y\times\mathbb R)$.
And the $U(1)$-gauge group $\mathcal G:=Map(Y\times\mathbb R,U(1))$, the action is given by $$(g,A_0+a)\mapsto (A_0,a-g^{-1}dg).$$
Question: Is there a method to find a subspace $V\subset \mathcal C$ such that $$V\overset{\sim}{=}\mathcal{C/G}.$$
PS:
We know that for the temporal gauge, $\{a\in \mathcal C~|~ a \mbox{ contains no } dt \mbox{ part and } d^*_Ya=0 \}$, is surjective to the quotient space.