  The discovery of a fast radio burst (FRB) associated with a magnetar in the
Milky Way by the Canadian Hydrogen Intensity Mapping Experiment FRB
collaboration (CHIME/FRB) and the Survey for Transient Astronomical Radio
Emission 2 (STARE2) has provided an unprecedented opportunity to refine FRB
emission models. The burst discovered by CHIME/FRB shows two components with
different spectra. We explore interstellar scintillation as the origin for this
variation in spectral structure. Modeling a weak scattering screen in the
supernova remnant associated with the magnetar, we find that a superluminal
apparent transverse velocity of the emission region of $>9.5\,c$ is needed to
explain the spectral variation. Alternatively, the two components could have
originated from independent emission regions spaced by $>8.3\times10^4\,$km.
These scenarios may arise in "far-away" models where the emission originates
from well beyond the magnetosphere of the magnetar (for example through a
synchrotron-maser mechanism set up by an ultra-relativistic radiative shock),
but not in "close-in" models of emission from within the magnetosphere. If
further radio observations of the magnetar confirm scintillation as the source
for the observed variation in spectral structure, this scattering model thus
constrains the location of the emission region.
