突发钠层形成机制的深入研究

Sporadic sodium layer (SSL or Nas) is a mesospheric phenomenon in which the density of sodium layer increases very fast and could more than double the background value (factor is greater than 2) within a very short time (e.g. within five minute) in a narrow altitude range. Several mechanisms have been proposed to explain the formation of the SSLs, such as the direct injection of meteors, the energetic auroral particle bombardment on the dust surface to release free sodium atoms, the recombination of sodium ions from downward sporadic E layers (ES theory), and the trigging by high temperature in a local narrow altitude due to the gravity wave breaking (temperature control mechanism). Up to now, however, none of these theories could explain all the observed SSLs. In our previous studies, it was found that SSLs at different altitudes and latitudes may have distinct mechanisms. A new temperature controlled icy dust sodium reservoir model was also proposed. Based on these results, this project will focus on three points: (Ⅰ) The applicability of ES theory: SSLs accompanied by ES will be selected, and the process of forming a SSL directly from ES will be discussed through relevant calculations. (Ⅱ) The applicability of temperature controlled theory: SSLs in close relationship with temperature increase processes will be distinguished. Observations from OH imager will confirm that whether the high temperature region is caused by the gravity wave breaking. (Ⅲ) Further research into the temperature controlled theory: we will do further studies on the temperature mechanism to verify the possibility of icy dust sodium reservoir model. The icy dust model may contain two dynamic processes: first, sodium species are collected by the dust surface through extremely cold weather; second, the gathered sodium species are released under hot temperature to form a SSL. The expected results of this project will propose a classification of SSLs in different altitudes and latitudes, which could resolve some international disputes of the mechanisms for SSLs. The icy dust model discussed in content (Ⅲ) will establish a new scope of mechanism for some typical SSLs in subtropical area.

突发钠层是指极狭窄的高度范围内钠层密度在极短时间迅速增加至背景钠密度2倍以上的一种现象。之前的工作结果显示，不同纬度不同高度的突发钠层可能具有完全不同的形成机制，并且申请者等人还针对亚热带地区提出一种全新的含冰尘埃钠储库模型机制。基于这些结果，本次项目拟重点针对三方面的研究内容展开：（Ⅰ）ES理论的适用性：筛选子午工程各台站与突发E层密切相关的突发钠层事件，确定ES理论适用范围，结合相关计算探讨由ES形成突发钠层的具体过程；（Ⅱ）升温理论的适用性：利用SABER卫星和钠测风/温度激光雷达，筛选与升温过程相关的事例，并结合OH气辉成像仪结果，提取突发钠层事件发生时刻重力波信号，进一步分析局部剧烈升温是否由重力波破碎加热引起；（Ⅲ）对升温理论相关过程的深入探索：将与升温机制相关的事例进行深入分析，验证含冰尘埃钠储库模型的两个动态过程，即突发前低温相位钠储库聚集钠源以及高温相位激发释放钠源。

在地球大气中间层顶附近存在着一层金属钠原子层形成钠层，钠层最显著的变化是突发钠层的现象，即指在极狭窄的高度极短时间内钠层密度增加至同一高度整夜平均背景钠密度2倍以上的一种突发现象。突发钠层可以作为中层顶各项大气参数的示踪 (tracer)，研究突发钠层对于了解中层顶大气各项参数耦合过程具有重要意义。. 本次研究工作就对中层顶突发钠层的形成机制进行了深入研究。研究结果显示：中纬度北京地区的突发钠层可能受到升温和风剪切以及重力波的影响；亚热带合肥地区突发钠层与温度的变化联系紧密但与突发E层的关系不大，突发E层不太可能作为大多数突发钠层的直接钠源存在；低纬度海口地区突发钠层与突发E和升温均密切相关，两种机制共同控制低纬度区域突发钠层的形成，其中升温与96 km以下突发钠之间联系更紧密，而ES理论则更倾向于控制96 km以上的突发钠。此外，项目组利用子午工程合肥站中国科学技术大学钠荧光散射激光雷达的钠密度观测数据，与测风/温度激光雷达的同步温度观测结果，筛选了发生于2013年5月12日及2013年5月13日的两次事件作为典型事例进行了深入研究。同步温度观测结果显示，当夜从激光雷达开始运行直到突发钠发生前的长时间内，突发高度附近经历了极低温阶段，温度极小值已达到134 K，低于该高度范围内水汽凝结温度的理论计算值；而突发钠发生时刻则伴随了超过40 K的剧烈升温过程，突发峰值廓线与温度峰值廓线有对应关系。通过分析总结这些观测结果，项目组完善了针对亚热带典型突发钠事件的可能机制：在潮汐的低温相位，冰晶在极低温度下凝结形成，并不断吸附周围的钠原子，形成钠储库；在潮汐的高温相位，钠储库受到激发，从而释放钠原子形成突发钠层，而激发可能是重力波破碎倾注能量造成的剧烈升温过程。