  Large-scale motions, also known as superstructures, are dynamically relevant
coherent structures in a wall-bounded turbulent flow, that span the entire
domain in wall-normal direction and significantly contribute to the global
energy and momentum transport. Recent investigations in channel and Couette
flow, suggest that these large-scale motions are self-sustained, implying they
are not driven by small-scale motions at the wall. Whether large-scale motions
are self-sustained has however not yet been answered for open boundary layers,
which are relevant for many applications. Here, using the asymptotic suction
boundary layer flow at the friction Reynolds number $Re_\tau = 1\,168$ as a
testbed, we show that large-scale motions are self-sustained also in boundary
layers. Together with the previous investigations in confined flows, this
observation provides strong evidence of the robust and general nature of
coherent self-sustained processes in turbulent wall-bounded flows. The dynamics
of the large-scale self-sustained process involving the growth, breakdown and
regeneration of quasi-streamwise coherent streaks and vortices within the
boundary layer shows temporal phase relations reminiscent of bursting as
observed in buffer layer structures. The dynamics however differs from the
quasi-periodic large-scale streak-vortex regeneration cycle observed in
confined flows. Based on the similarity of the dynamics of large-scale motions
in boundary layers and of small-scale buffer layer structures, we conjecture
that the bursting behaviour is associated with the dynamical relevance of only
one wall, while two confining walls lead to the quasi-periodic cycle.
