  The chemical properties of protoplanetary disks are especially sensitive to
their ionization environment. Sources of molecular gas ionization include
cosmic rays, stellar X-rays and short-lived radionuclides, each of which varies
with location in the disk. This behavior leads to a significant amount of
chemical structure, especially in molecular ion abundances, which is imprinted
in their submillimeter rotational line emission. Using an observationally
motivated disk model, we make predictions for the dependence of chemical
abundances on the assumed properties of the ionizing field. We calculate the
emergent line intensity for abundant molecular ions and simulate sensitive
observations with the Atacama Large Millimeter/Sub-millimeter Array (ALMA) for
a disk at D=100 pc. The models readily distinguish between high ionization
rates ($\zeta\gtrsim10^{-17}$ s$^{-1}$ per H$_2$) and below, but it becomes
difficult to distinguish between low ionization models when
$\zeta\lesssim10^{-19}$ s$^{-1}$. We find that \htdp\ emission is not
detectable for sub-interstellar CR rates with ALMA (6h integration), and that
\ntdp\ emission may be a more sensitive tracer of midplane ionization. HCO$^+$
traces X-rays and high CR rates ($\zeta_{\rm{CR}}\gtrsim10^{-17}$ s$^{-1}$),
and provides a handle on the warm molecular ionization properties where CO is
present in the gas. Furthermore, species like HCO$^+$, which emits from a wide
radial region and samples a large gradient in temperature, can exhibit
ring-like emission as a consequence of low-lying rotational level de-excitation
near the star. This finding highlights a scenario where rings are not
necessarily structural or chemical in nature, but simply a result of the
underlying line excitation properties.

Solar and Stellar Astrophysics