  On 21 August 2017 a total solar eclipse swept across the contiguous United
States providing excellent opportunities for diagnostics of the Sun's corona.
The Sun's coronal structure is notoriously difficult to observe except during
solar eclipses; thus theoretical models must be relied upon for inferring the
underlying magnetic structure of the Sun's outer atmosphere. These models are
necessary for understanding the role of magnetic fields in the heating of the
corona to a million degrees and generation of severe space weather. Here we
present a methodology for predicting the structure of the coronal field based
on long-term surface flux transport simulations whose output is utilized to
extrapolate the coronal magnetic field structures. This prescription was
applied to the 21 August 2017 solar eclipse. Post-eclipse analysis shows good
agreement between model simulated and observed coronal structures and their
locations on the limb. We demonstrate that slow changes in the Sun's surface
magnetic field distribution driven by long-term flux emergence and evolution
govern large-scale coronal structures with a (plausibly cycle-phase dependent)
dynamical memory timescale on the order of few solar rotations -- opening up
the possibility of large-scale, global corona predictions at least a month in
advance.
