用ALMA探测的海王星赤道平流层气态氰化氢的带状分布
A belt-like distribution of gaseous hydrogen cyanide on Neptune's equatorial stratosphere detected by ALMA
摘要：
我们提出了一个空间分辨的地图的综合强度和丰富的海王星的平流层氰化氢（HCN）。分析数据来自2016年阿塔卡马大型毫米/亚毫米阵列的存档观测。一束0.42 $\times$ 0.39角秒的合成光束，相当于在盘中心的纬度分辨率为$\sim$ 20度，足以分辨海王星直径为2.24角秒的盘。在校正了不同光程长度的影响后，得到了海王星盘面上HCN辐射的空间分布，并清楚地显示了赤道处HCN的带状增强。辐射转移分析表明，在10 $^{-3}$ bar气压水平上，在赤道测得的HCN体积混合比为1.92 ppb，比在南半球中高纬测得的高出40 $\%$。HCN的空间分布可以解释为对流层中N $_{2}$通过纬向大气环流输送的影响，也可以解释为彗星碰撞等外部供应的影响（或两者兼而有之）。从纬向环流的角度来看，所观察到的HCN增强赤道和极点可以解释由HCN的生产和积累在以前提出的双细胞纬向环流模型的向下分支。然而，60 S的HCN贫化纬度与双单体环流模式的上升支位置不匹配。
英文摘要：
We present a spatially resolved map of integrated-intensity and abundance of Neptune's stratospheric hydrogen cyanide (HCN). The analyzed data were obtained from the archived 2016 observation of the Atacama Large Millimeter/submillimeter Array. A 0.42 $\times$ 0.39 arcseconds synthesized beam, which is equivalent to a latitudinal resolution of $\sim$20 degrees at the disk center, was fine enough to resolve Neptune's 2.24 arcseconds diameter disk. After correcting the effect of different optical path lengths, a spatial distribution of HCN emissions is derived over Neptune's disk, and it clearly shows a band-like HCN enhancement at the equator. Radiative transfer analysis indicates that the HCN volume mixing ratio measured at the equator was 1.92 ppb above the 10$^{-3}$ bar pressure level, which is 40$\%$ higher than that measured at the southern middle and high latitudes. The spatial distribution of HCN can be interpreted as either the effect of the transportation of N$_{2}$ from the troposphere by meridional atmospheric circulation, or an external supply such as cometary collisions (or both of these reasons). From the meridional circulation point of view, the observed HCN enhancement on both the equator and the pole can be explained by the production and accumulation of HCN at the downward branches of the previously suggested two-cell meridional circulation models. However, the HCN-depleted latitude of 60 S does not match with the location of the upward branch of the two-cell circulation models.
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