  In this work we study variations in the parabolic scintillation arcs of the
binary millisecond pulsar PSR J1643-1224 over five years using the Large
European Array for Pulsars (LEAP). The 2D power spectrum of scintillation,
called the secondary spectrum, often shows a parabolic distribution of power,
where the arc curvature encodes the relative velocities and distances of the
pulsar, ionised interstellar medium (IISM), and Earth. We observe a clear
parabolic scintillation arc which varies in curvature throughout the year. The
distribution of power in the secondary spectra are inconsistent with a single
scattering screen which is fully 1D, or entirely isotropic. We fit the observed
arc curvature variations with two models; an isotropic scattering screen, and a
model with two independent 1D screens. We measure the distance to the
scattering screen to be in the range 114-223 pc, depending on the model,
consistent with the known distance of the foreground large-diameter HII region
Sh 2-27 (112+/-17 pc), suggesting that it is the dominant source of scattering.
We obtain only weak constraints on the pulsar's orbital inclination and angle
of periastron, since the scintillation pattern is not very sensitive to the
pulsar's motion, since the screen is much closer to the Earth than the pulsar.
More measurements of this kind - where scattering screens can be associated
with foreground objects - will help to inform the origins and distribution of
scattering screens within our galaxy.

High Energy Astrophysical Phenomena;High Energy Physics - Phenomenology