Distributions of CO gas, N₂ gas and N₂H⁺ in the simple chemical model (model CH) with CO and N₂ abundances of 3 × 10^-6. To focus on the region around the CO snow surface, the vertical scale is limited to a scale height z/r ≤ 0.2. The rightmost panel highlights the region where N₂H⁺ is present near the disk midplane. The dashed and dash-dotted contours represent the CO and N₂ snow surfaces, respectively, and the corresponding midplane snowlines are indicated by arrows below the horizontal axis of the rightmost panel. The midplane radius with the highest N₂H⁺ abundance is marked with a red arrow.