  Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the
planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D
extensions of snowlines in the outer disk regions, where radiative heating
results in a decreasing temperature with disk height. CO and N$_2$ are two of
the most abundant carriers of C, N and O. N$_2$H$^+$ can be used to probe the
snow surfaces of both molecules, because it is destroyed by CO and formed from
N$_2$. Here we present Atacama Large Millimeter/submillimeter Array (ALMA)
observations of N$_2$H$^+$ at 0.2$''$-0.4$''$ resolution in the disks around
LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive
emission morphologies: N$_2$H$^+$ is either present in a bright, narrow ring
surrounded by extended tenuous emission, or in a broad ring. These emission
patterns can be explained by two different kinds of vertical temperature
structures. Bright, narrow N$_2$H$^+$ rings are expected in disks with a thick
Vertically 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, AS
209, 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 with
a thick VIRaM layer. The results reveal a range of N$_2$ and CO snowline radii
towards stars of similar spectral type, demonstrating the need for empirically
determined snowlines in disks.
