  Protoplanetary disk evolution is strongly impacted by ionization from the
central star and local environment, which collectively have been shown to drive
chemical complexity and are expected to impact the transport of disk material.
Nonetheless, ionization remains a poorly constrained input to many detailed
modeling efforts. We use new and archival ALMA observations of N$_2$H$^+$ 3--2
and H$^{13}$CO$^+$ 3--2 to derive the first observationally-motivated
ionization model for the IM Lup protoplanetary disk. Incorporating ionization
from multiple internal and external sources, we model N$_2$H$^+$ and
H$^{13}$CO$^+$ abundances under varying ionization environments, and compare
these directly to the imaged ALMA observations by performing non-LTE radiative
transfer, visibility sampling, and imaging. We find that the observations are
best reproduced using a radially increasing cosmic ray (CR) gradient, with low
CR ionization in the inner disk, high CR ionization in the outer disk, and a
transition at $\sim 80 - 100$ au. This location is approximately coincident
with the edge of spiral structure identified in millimeter emission. We also
find that IM Lup shows evidence for enhanced UV-driven formation of HCO$^+$,
which we attribute to the disk's high flaring angle. In summary, IM Lup
represents the first protoplanetary disk with observational evidence for a CR
gradient, which may have important implications for IM Lup's on-going
evolution, especially given the disk's young age and large size.
