  Ionization drives important chemical and dynamical processes within
protoplanetary disks, including the formation of organics and water in the cold
midplane and the transportation of material via accretion and
magneto-hydrodynamic (MHD) flows. Understanding these ionization-driven
processes is crucial for understanding disk evolution and planet formation. We
use new and archival ALMA observations of HCO+, H13CO+, and N2H+ to produce the
first forward-modeled 2D ionization constraints for the DM Tau protoplanetary
disk. We include ionization from multiple sources and explore the disk
chemistry under a range of ionizing conditions. Abundances from our 2D chemical
models are post-processed using non-LTE radiative transfer, visibility
sampling, and imaging, and are compared directly to the observed radial
emission profiles. The observations are best fit by a modestly reduced CR
ionization rate ($\zeta_{CR}$ ~ 10$^{-18}$ s$^{-1}$) and a hard X-ray spectrum
(hardness ratio [HR] = 0.3), which we associate with stellar flaring
conditions. Our best-fit model under-produces emission in the inner disk,
suggesting that there may be an additional mechanism enhancing ionization in DM
Tau's inner disk. Overall, our findings highlight the complexity of ionization
in protoplanetary disks and the need for high resolution multi-line studies.
