  We present the first large sample of scintillation arcs in millisecond
pulsars, analysing 12 sources observed with the Large European Array for
Pulsars (LEAP), and the Effelsberg 100\,m telescope. We estimate the delays
from multipath propagation, measuring significant correlated changes in
scattering timescales over a 10-year timespan. Many sources show compact
concentrations of power in the secondary spectrum, which in PSRs J0613$-$0200
and J1600$-$3053 can be tracked between observations, and are consistent with
compact scattering at fixed angular positions. Other sources such as PSRs
J1643$-$1224 and J0621+1002 show diffuse, asymmetric arcs which are likely
related to phase-gradients across the scattering screen. PSR B1937+21 shows at
least three distinct screens which dominate at different times and evidence of
varying screen axes or multi-screen interactions. We model annual and orbital
arc curvature variations in PSR J0613$-$0200, providing a measurement of the
longitude of ascending node, resolving the sense of the orbital inclination,
where our best fit model is of a screen with variable axis of anisotropy over
time, corresponding to changes in the scattering of the source. Unmodeled
variations of the screen's axis of anisotropy are likely to be a limiting
factor in determining orbital parameters with scintillation, requiring careful
consideration of variable screen properties, or independent VLBI measurements.
Long-term scintillation studies such as this serve as a complementary tool to
pulsar timing, to measure a source of correlated noise for pulsar timing
arrays, solve pulsar orbits, and to understand the astrophysical origin of
scattering screens.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics