Midlatitude Sporadic E‐Layer Horizontal Structuring Modulated by Neutral Instability and Mixing in the Lower Thermosphere
Midlatitude Sporadic E‐Layer Horizontal Structuring Modulated by Neutral Instability and Mixing in the Lower Thermosphere
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DOI：
10.1029/2022ja030929
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发表时间：
2023-02
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影响因子：
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通讯作者：
Michelle X. Bui;D. Hysell;M. Larsen
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作者：
Michelle X. Bui;D. Hysell;M. Larsen
利用位于纽约伊萨卡的甚高频成像雷达获得了来自零星-E电离层的30 MHz相干后向散射的观测。雷达探测到的体积位于相对较高的磁纬度，在中纬度地区的北缘，在电离层槽下面。在伊萨卡观测到的带状准周期(QP)回波与在中纬度较低地区发现的回波相似。观测到的多普勒频移较小，到目前为止，似乎还没有达到法利-邦曼不稳定性的阈值。然而，许多回声表现出细微的尺度结构，次级带或辫子斜向初级带。次生条带在中低纬度地区很少见。在以前的观测中，QP散射与不稳定的中性风切变有关。在低热层中常见的中性风切变可能在这些不规则的形成中起到关键作用，并解释了由此产生的等离子体密度不规则和雷达回波的某些形态特征。我们考虑了低热层中的中性不稳定和湍流是否是零星-E层中某些结构的可能原因。中层大气动力学的3D数值模拟结果支持这一命题。特别是，我们关注Ekman型不稳定，与更常见的Kelvin-Helmholtz不稳定一样，是拐点不稳定，尽管特别与旋转切变有关，并导致对流卷曲接近平均风向，较小尺度的次级波垂直于主要结构。
Observations of 30‐MHz coherent backscatter from sporadic‐E ionization layers were obtained with a VHF imaging radar located in Ithaca, New York. The volume probed by the radar lies at relatively high magnetic latitudes, on the northern edge of the mid‐latitude region and underneath the ionospheric trough. Banded, quasi‐periodic (QP) echoes observed from Ithaca are similar to those found in lower midlatitude regions. The Doppler shifts observed are smaller and, so far, do not appear to reach the threshold for Farley‐Buneman instability. However, many of the echoes exhibit fine‐scale structure, with secondary bands or braids oriented obliquely to the primary bands. Secondary bands have been seen only rarely at lower middle latitudes. In previous observations, the QP scattering has been linked to unstable neutral wind shears. Neutral wind shear commonly found in the lower thermosphere could play a key role in the formation of these irregularities and explain some morphological features of the resulting plasma density irregularities and the radar echoes. We consider whether neutral instability and turbulence in the lower thermosphere is the likely cause for some of the structuring in the sporadic‐ E layers. Results of 3D numerical simulations of atmospheric dynamics in the mesosphere to lower thermosphere support the proposition. In particular, we focus on Ekman‐type instabilities that, like the more common Kelvin‐Helmholtz instabilities, are inflection point instabilities, although specifically associated with turning shears, and result in convective rolls aligned close to the mean wind direction, with smaller‐scale secondary waves aligned normal to the primary structures.