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Snowlines of major volatiles regulate the gas and solid C/N/O ratios in theplanet-forming midplanes of protoplanetary disks. Snow surfaces are the 2Dextensions of snowlines in the outer disk regions, where radiative heatingresults in a decreasing temperature with disk height. CO and N$_2$ are two ofthe most abundant carriers of C, N and O. N$_2$H$^+$ can be used to probe thesnow surfaces of both molecules, because it is destroyed by CO and formed fromN$_2$. Here we present Atacama Large Millimeter/submillimeter Array (ALMA)observations of N$_2$H$^+$ at 0.2$''$-0.4$''$ resolution in the disks aroundLkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctiveemission morphologies: N$_2$H$^+$ is either present in a bright, narrow ringsurrounded by extended tenuous emission, or in a broad ring. These emissionpatterns can be explained by two different kinds of vertical temperaturestructures. Bright, narrow N$_2$H$^+$ rings are expected in disks with a thickVertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur,DM Tau) where the N$_2$H$^+$ emission peaks between the CO and N$_2$ snowlines.Broad N$_2$H$^+$ rings come from disks with a thin VIRaM layer (V4046 Sgr, AS209, IM Lup). We use a simple model to extract the first sets of CO and N$_2$snowline pairs and corresponding freeze-out temperatures towards the disks witha thick VIRaM layer. The results reveal a range of N$_2$ and CO snowline radiitowards stars of similar spectral type, demonstrating the need for empiricallydetermined snowlines in disks.

Probing CO and N$_2$ Snow Surfaces in Protoplanetary Disks with N$_2$H$^+$ Emission