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We observed HCO$^+$ $J=1-0$ and H$^{13}$CO$^+$ $J=1-0$ emission towards thefive protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480as part of the MAPS project. HCO$^+$ is detected and mapped at0.3\arcsec\,resolution in all five disks, while H$^{13}$CO$^+$ is detected(SNR$>6 \sigma$) towards GM Aur and HD 163296 and tentatively detected (SNR$>3\sigma$) towards the other disks by a matched filter analysis. Inside a radiusof $R\sim 100$ au, the HCO$^+$ column density is flat or shows a central dip.At outer radii ($\gtrsim 100$ au), the HCO$^+$ column density decreasesoutwards, while the column density ratio of HCO$^+$/CO is mostly in the rangeof $\sim 10^{-5}-10^{-4}$. We derived the HCO$^+$ abundance in the warm CO-richlayer, where HCO$^+$ is expected to be the dominant molecular ion. At $R\gtrsim100$ au, the HCO$^+$ abundance is $\sim 3 \times 10^{-11} - 3\times 10^{-10}$,which is consistent with a template disk model with X-ray ionization. At thesmaller radii, the abundance decreases inwards, which indicates that theionization degree is lower in denser gas, especially inside the CO snow line,where the CO-rich layer is in the midplane. Comparison of template disk modelswith the column densities of HCO$^+$, N$_2$H$^+$, and N$_2$D$^+$ indicates thatthe midplane ionization rate is $\gtrsim 10^{-18}$ s$^{-1}$ for the disksaround IM Lup, AS 209, and HD 163296. We also find hints of an increasedHCO$^+$ abundance around the location of dust continuum gaps in AS 209, HD163296, and MWC 480. This paper is part of the MAPS special issue of theAstrophysical Journal Supplement.

Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO$^+$ and disk ionization structure