  UV photochemistry in the surface layers of protoplanetary disks dramatically
alters their composition relative to previous stages of star formation. The
abundance ratio CN/HCN has long been proposed to trace the UV field in various
astrophysical objects, however to date the relationship between CN, HCN, and
the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS
(Molecules with ALMA at Planet-forming Scales), we present observations of CN
N=1-0 transitions at 0.3'' resolution towards five disk systems. All disks show
bright CN emission within $\sim$50-150 au, along with a diffuse emission shelf
extending up to 600 au. In all sources we find that the CN/HCN column density
ratio increases with disk radius from about unity to 100, likely tracing
increased UV penetration that enhances selective HCN photodissociation in the
outer disk. Additionally, multiple millimeter dust gaps and rings coincide with
peaks and troughs, respectively, in the CN/HCN ratio, implying that some
millimeter substructures are accompanied by changes to the UV penetration in
more elevated disk layers. That the CN/HCN ratio is generally high (>1) points
to a robust photochemistry shaping disk chemical compositions, and also means
that CN is the dominant carrier of the prebiotically interesting nitrile group
at most disk radii. We also find that the local column densities of CN and HCN
are positively correlated despite emitting from vertically stratified disk
regions, indicating that different disk layers are chemically linked. This
paper is part of the MAPS special issue of the Astrophysical Journal
Supplement.
