PMSE dependence on aerosol charge number density and aerosol size
PMSE dependence on aerosol charge number density and aerosol size
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PMSE 对气溶胶电荷数密度和气溶胶尺寸的依赖性
DOI：
10.1029/2002jd002650
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发表时间：
2003
影响因子：
--
通讯作者：
T. Blix
中科院分区：
文献类型：
--
作者：
M. Rapp;F. Lübken;P. Hoffmann;R. Latteck;G. Baumgarten;T. Blix
[1]人们普遍认为，极地中间层夏季回波(PMSE)的存在取决于带电气溶胶的存在，因为它们相对较重，并减少了由于双极力而产生的自由电子的扩散。简单的微物理模型表明，这种扩散系数降低与Ra2(Ra=气溶胶半径)成正比，但只有当大量电荷结合在气溶胶上，使得NA|Za|/Ne>；1.2(NA=气溶胶数量，Za=气溶胶电荷，Ne=自由电子数)。背景电子分布经常显示PMSE高度的大耗尽(“Biteout”)这一事实被认为是支持这一观点的，因为在Biteout中缺少大部分自由电子，即结合在气溶胶上。本文从电子密度的现场测量以及雷达和激光雷达的观测表明，PMSE也可以存在于只有一小部分自由电子结合在气溶胶上的区域，即没有Bit Out且有NA|Za|/Ne≪1的区域。我们提供了强有力的实验证据，证明PMSE的存在是由产物NA|Za|Ra2决定的。例如，小的气溶胶电荷可以用大的气溶胶半径来补偿。我们发现，该产品复制了PMSE的主要特征，特别是PMSE的平均高度分布和存在夜光云(NLCS)时的高度。因此，我们将这款产品视为PMSE的“代理”。这一代理与PMSE的主要特征之间的一致性意味着，简单的微观物理模型不能令人满意地描述PMSE物理，需要改进。该代理可以很容易地用于高层大气模式，以更好地了解PMSE的季节和地理变化，例如，长期争论的北半球和南半球PMSE之间的差异。
[1] It is commonly accepted that the existence of polar mesosphere summer echoes (PMSEs) depends on the presence of charged aerosols since these are comparatively heavy and reduce the diffusion of free electrons due to ambipolar forces. Simple microphysical modeling suggests that this diffusivity reduction is proportional to rA2 (rA = aerosol radius) but only if a significant amount of charges is bound on the aerosols such that NA∣ZA∣/ne > 1.2 (NA = number of aerosols, ZA = aerosol charge, ne = number of free electrons). The fact that the background electron profile frequently shows large depletions (“biteouts”) at PMSE altitudes is taken as a support for this idea since within biteouts a major fraction of free electrons is missing, i.e., bound on aerosols. In this paper, we show from in situ measurements of electron densities and from radar and lidar observations that PMSEs can also exist in regions where only a minor fraction of free electrons is bound on aerosols, i.e., with no biteout and with NA∣ZA∣/ne ≪ 1. We show strong experimental evidence that it is instead the product NA∣ZA∣rA2 that is crucial for the existence of PMSEs. For example, small aerosol charge can be compensated by large aerosol radius. We show that this product replicates the main features of PMSEs, in particular the mean altitude distribution and the altitude of PMSEs in the presence of noctilucent clouds (NLCs). We therefore take this product as a “proxy” for PMSE. The agreement between this proxy and the main characteristics of PMSEs implies that simple microphysical models do not satisfactorily describe PMSE physics and need to be improved. The proxy can easily be used in models of the upper atmosphere to better understand seasonal and geographical variations of PMSEs, for example, the long debated difference between Northern and Southern hemisphere PMSEs.