  The circular polarization of black hole accretion flows can encode properties
of the underlying magnetic field structure. Using general relativistic
magnetohydrodynamics (GRMHD) simulations, we study the imprint of magnetic
field geometry on circular polarization images potentially observable by the
Event Horizon Telescope (EHT). We decompose images into the different
mechanisms that generate circular polarization in these models, which are
sensitive to both the line of sight direction and twist of the magnetic field.
In these models, a stable sign of the circular polarization over time, as
observed for several sources, can be attributed to a stability of these
properties. We illustrate how different aspects of a generic helical magnetic
field geometry become imprinted on a circular polarization image. We also
identify novel effects of light bending that affect the circular polarization
image on event horizon scales. One consequence is the sign flipping of
successive photon rings in face-on systems, which if observable and uncorrupted
by Faraday rotation, can directly encode the handedness of the approaching
magnetic field.
