Neptune’s global circulation deduced from multi-wavelength observations
Neptune’s global circulation deduced from multi-wavelength observations
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从多波长观测推导出海王星的全球环流
DOI：
10.1016/j.icarus.2014.02.030
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发表时间：
2014
期刊：
影响因子：
3.2
通讯作者：
P. Marcus
中科院分区：
文献类型：
--
作者：
I. Pater;L. Fletcher;S. Luszcz;D. DeBoer;B. Butler;H. Hammel;M. Sitko;G. Orton;P. Marcus
摘要2003年6月至10月，我们用10米长的WM Keck II和I望远镜分别在近红外和中红外波段观测了海王星，并用甚大阵列在射电波段观测了海王星。在近红外波长，利用宽带和窄带滤光器耦合到自适应光学系统的NIRC2获得了图像。在中红外，我们在8到22μm的波长对海王星成像，在8-13μm和18-22μm获得了狭缝分辨光谱。在射电波长，我们在0.7到6 cm的离散滤光片中绘制了海王星的地图。我们分别使用针对特定波长区域进行优化的辐射传输程序对每个数据集进行了分析。在中纬度南部，中红外波段的大气似乎比地球上其他任何地方都要凉爽。我们认为这是由中纬度地区从对流层上层到≲0.1mbar高度的空气上升引起的绝热冷却造成的。在近红外波段，我们在这些纬度发现了两个不同的云层：对流层中气压水平P∼300-≳600mbar的相对较深的云层(可能是甲烷)，我们认为这是由大规模上升流及其伴随的绝热冷却和甲烷凝结造成的；以及平流层中20-30mbar的较高的空间间歇性云层。这些高云的纬度围绕着纬向气流的反气旋带，这表明它们可能是由于与当地反气旋有关的强烈但局部化的垂直上升，而不是对流(即气旋)风暴中的羽流。北中纬度的云层位于大气中最高的高度，接近10mbar。海王星的南极在中红外和无线电波长上的亮度都大大增强，即从平流层的∼0.1mbar水平下降到对流层的数十巴。我们认为这是由于从平流层一直到对流层深处的下沉运动造成的。在中红外波段观测到的亮度增强是由于平流层压缩的绝热加热所致，射电波段亮度温度的增强表明极地上空的下沉空气是非常干燥的；在∼40bar的NH4 SH云上方，极地上空H2 S的相对湿度仅为5%。低湿区域从南极向下延伸到66°S的纬度，这与南极进动急流接近相同的纬度，表明极地涡旋的边界。我们认为，60-70°纬度的南极特征(SPF)是由斜压不稳定产生的对流风暴，预计在南极急流附近的纬度会产生斜压不稳定。总而言之，我们的数据表明了一种全球环流模式，其中空气从对流层上升到中纬度以上，从对流层上升到平流层，而干空气从平流层向下从极地和赤道下沉到对流层。我们建议这种模式从≲0.1mbar一直延伸到≳40bar的压力。
Abstract We observed Neptune between June and October 2003 at near-and mid-infrared wavelengths with the 10-m WM Keck II and I telescopes, respectively; and at radio wavelengths with the Very Large Array. Images were obtained at near-infrared wavelengths with NIRC2 coupled to the adaptive optics system in both broad-and narrow-band filters between 1.2 and 2.2 μ m. In the mid-infrared we imaged Neptune at wavelengths between 8 and 22 μ m, and obtained slit-resolved spectra at 8–13 μ m and 18–22 μ m. At radio wavelengths we mapped the planet in discrete filters between 0.7 and 6 cm. We analyzed each dataset separately with a radiative-transfer program that is optimized for that particular wavelength regime. At southern midlatitudes the atmosphere appears to be cooler at mid-infrared wavelengths than anywhere else on the planet. We interpret this to be caused by adiabatic cooling due to air rising at midlatitudes at all longitudes from the upper troposphere up to≲ 0.1 mbar levels. At near-infrared wavelengths we find two distinct cloud layers at these latitudes: a relatively deep layer of clouds (presumably methane) in the troposphere at pressure levels P∼ 300–≳ 600 mbar, which we suggest to be caused by the large-scale upwelling and its accompanying adiabatic cooling and condensation of methane; and a higher, spatially intermittent, layer of clouds in the stratosphere at 20–30 mbar. The latitudes of these high clouds encompass an anticyclonic band of zonal flow, which suggests that they may be due to strong, but localized, vertical upwellings associated with local anticyclones, rather than plumes in convective (ie, cyclonic) storms. Clouds at northern midlatitudes are located at the highest altitudes in the atmosphere, near 10 mbar. Neptune’s south pole is considerably enhanced in brightness at both mid-infrared and radio wavelengths, ie, from∼ 0.1 mbar levels in the stratosphere down to tens of bars in the troposphere. We interpret this to be due to subsiding motions from the stratosphere all the way down to the deep troposphere. The enhanced brightness observed at mid-infrared wavelengths is interpreted to be due to adiabatic heating by compression in the stratosphere, and the enhanced brightness temperature at radio wavelengths reveals that the subsiding air over the pole is very dry; the relative humidity of H 2 S over the pole is only 5% at altitudes above the NH 4 SH cloud at∼ 40 bar. The low humidity region extends from the south pole down to latitudes of 66° S. This is near the same latitudes as the south polar prograde jet signifying the boundary of the polar vortex. We suggest that the South Polar Features (SPFs) at latitudes of 60–70° are convective storms, produced by baroclinic instabilities expected to be produced at latitudes near the south polar prograde jet. Taken together, our data suggest a global circulation pattern where air is rising above southern and northern midlatitudes, from the troposphere up well into the stratosphere, and subsidence of dry air over the pole and equator from the stratosphere down into the troposphere. We suggest that this pattern extends all the way from≲ 0.1 mbar down to pressures of≳ 40 bar.