Saturn's seasonal variability from four decades of ground-based mid-infrared observations
Saturn's seasonal variability from four decades of ground-based mid-infrared observations
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来自四十年地面中红外观测的土星季节变化
DOI：
10.1016/j.icarus.2022.115347
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发表时间：
2023
期刊：
影响因子：
3.2
通讯作者：
Blake J
中科院分区：
文献类型：
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作者：
Blake J
利用几十年的土星地基中红外（7-25 μ m）图像记录，探索了超过一个土星年（1984-2022）的热辐射的季节性和非季节性变化。由3米和8米级天文台（特别是NASA的红外望远镜设施，斯巴鲁和ESO的超大望远镜）测量的热辐射与基于卡西尼温度记录和辐射气候模型预测的合成图像相比毫不逊色。我们发现，8米级的设施能够解决土星的带，区，极六边形和极地气旋的规模上的热反差，叠加到大规模的季节性不对称。观测到平流层亮温的季节变化，平流层亮温约为30 K，对流层上层亮温约为10 K，这是由于平流层北方极涡和平流层南北极极涡（NPSV和SPSV）在春季形成，秋季消散。辐射气候模型成功地再现了温暖的极地涡旋首次出现的时间，确认它们是辐射现象，尽管它们被夹带在受动力学影响的尖锐边界内。轴对称的热带（每半球4-5个）显示出与土星纬向风密切相关的温度梯度，表明风的强度随着从云顶到0.01毫巴的高度而衰减，并暗示土星对流层上层和平流层的纬向环流圈形成了冷带和暖带系统。土星的热结构在很大程度上每年都是可重复的（通过比较1989年和2018年的红外图像），低纬度地区除外。在这里，我们发现年际变化的证据，因为赤道带在7。9 μ m与土星赤道平流层振荡的15年周期不一致，也就是说，它不是严格的半年周期。要么振荡有一个更长的周期，接近20年，要么它的进展是自然变化的，并被对流层气象（如风暴）中断。最后，2017-2022年之间的观测扩展了卡西尼使命的遗产，揭示了NPSV在北方夏季的持续变暖与辐射气候模型的预测一致。
A multi-decade record of ground-based mid-infrared (7–25 μ m) images of Saturn is used to explore seasonal and non-seasonal variability in thermal emission over more than a Saturnian year (1984–2022). Thermal emission measured by 3-m and 8-m-class observatories (notably NASA’s Infrared Telescope Facility, Subaru, and ESO’s Very Large Telescope) compares favourably with synthetic images based on both Cassini-derived temperature records and the predictions of radiative climate models. We find that 8-m class facilities are capable of resolving thermal contrasts on the scale of Saturn’s belts, zones, polar hexagon, and polar cyclones, superimposed onto large-scale seasonal asymmetries. Seasonal changes in brightness temperatures of∼ 30 K in the stratosphere and∼ 10 K in the upper troposphere are observed, as the northern and southern polar stratospheric vortices (NPSV and SPSV) form in spring and dissipate in autumn. The timings of the first appearance of the warm polar vortices is successfully reproduced by radiative climate models, confirming them to be radiative phenomena, albeit entrained within sharp boundaries influenced by dynamics. Axisymmetric thermal bands (4–5 per hemisphere) display temperature gradients that are strongly correlated with Saturn’s zonal winds, indicating winds that decay in strength with altitude from the cloud-tops to the∼ 1-mbar level, and implying meridional circulation cells in Saturn’s upper troposphere and stratosphere forming the system of cool zones and warm belts. Saturn’s thermal structure is largely repeatable from year to year (via comparison of infrared images in 1989 and 2018), with the exception of low-latitudes. Here we find evidence of inter-annual variations because the equatorial banding at 7. 9 μ m is inconsistent with a∼ 15-year period for Saturn’s equatorial stratospheric oscillation, ie, it is not strictly semi-annual. Either the oscillation has a longer period closer to∼ 20 years, or its progression is naturally variable and interrupted by tropospheric meteorology (eg, storms). Finally, observations between 2017–2022 extend the legacy of the Cassini mission, revealing the continued warming of the NPSV during northern summer in line with predictions of radiative climate models.