Characterization of $\mathcal L^1_\kappa$

The logic $\mathcal L^1_\kappa$ was introduced by Shelah in [3]. In [4], he proved that for a strongly compact cardinal $\kappa$, it admits the following algebraic characterization: two structures are $\mathcal L^1_\kappa$-equivalent if and only if they have isomorphic iterated ultrapowers via $\kappa$-complete ultrafilters. We give a presentation of the logic $\mathcal L^1_\kappa$ and a simplified and slightly modified proof of this result.