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We present a spatially resolved map of integrated-intensity and abundance ofNeptune's stratospheric hydrogen cyanide (HCN). The analyzed data were obtainedfrom the archived 2016 observation of the Atacama LargeMillimeter/submillimeter Array. A 0.42 $\times$ 0.39 arcseconds synthesizedbeam, which is equivalent to a latitudinal resolution of $\sim$20 degrees atthe disk center, was fine enough to resolve Neptune's 2.24 arcseconds diameterdisk. After correcting the effect of different optical path lengths, a spatialdistribution of HCN emissions is derived over Neptune's disk, and it clearlyshows a band-like HCN enhancement at the equator. Radiative transfer analysisindicates that the HCN volume mixing ratio measured at the equator was 1.92 ppbabove the 10$^{-3}$ bar pressure level, which is 40$\%$ higher than thatmeasured at the southern middle and high latitudes. The spatial distribution ofHCN can be interpreted as either the effect of the transportation of N$_{2}$from the troposphere by meridional atmospheric circulation, or an externalsupply such as cometary collisions (or both of these reasons). From themeridional circulation point of view, the observed HCN enhancement on both theequator and the pole can be explained by the production and accumulation of HCNat the downward branches of the previously suggested two-cell meridionalcirculation models. However, the HCN-depleted latitude of 60 S does not matchwith the location of the upward branch of the two-cell circulation models.

A belt-like distribution of gaseous hydrogen cyanide on Neptune's equatorial stratosphere detected by ALMA