Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion
Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion
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DOI：
10.1007/s00585-000-0945-6
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发表时间：
2000-08
影响因子：
1.9
通讯作者：
H. Rishbeth;I. Müller-Wodarg;L. Zou;T. Fuller‐Rowell;G. Millward;R. Moffett;D. W. Idenden;A. Ayl
中科院分区：
文献类型：
--
作者：
H. Rishbeth;I. Müller-Wodarg;L. Zou;T. Fuller‐Rowell;G. Millward;R. Moffett;D. W. Idenden;A. Ayl
Zouet等人的配套论文表明，中纬度F2层电子密度峰值（NmF 2）的年度和半年变化可以通过热层-电离层耦合计算模式（CTIP）再现，而不需要求助于太阳风或源自低层大气的波浪和潮汐等外部影响。本工作更详细地讨论了物理。结果表明，正午NmF_2与环境原子/分子浓度比密切相关，NmF_2随地理经度和磁经度的变化主要是由于极光椭圆的几何形状所致。电场在中纬度热层动力学中没有起重要作用。我们的模拟导致了以下全球三维热层环流的图像，正如邓肯所设想的那样，这是解释F2层变化的关键。夏至时，夏季高纬度地区几乎持续不断的太阳输入驱动着盛行的夏季至冬季风，在低纬度地区和整个夏季半球的大部分地区都有上升流，而在冬季半球，就在赤道方向的极光椭圆形区域有一个下降流区。这些运动比昼夜交替（上下）运动对热层组成的影响更大。因此，整个热层在至日比在春分时更分子化。结合众所周知的F2层电子密度与中性空气中原子/分子比的关系，这解释了在低纬度和中纬度盛行的NmF 2中的F2层半年效应。在较高的中纬度地区，季节性行为取决于冬季下降流区的地理纬度，虽然成分变化的影响是由大太阳天顶角在仲冬修改。天顶角效应在远离磁极的天体中尤为重要。在这里，下降流发生在高地理纬度，天顶角的影响变得压倒性的，并导致隆冬的电子密度的抑郁症，尽管增强的原子/分子比。这导致NmF 2的半年变化。在靠近磁极的南极洲，冬季的情况有所不同，那里的下沉流发生在相对较低的地理纬度，因此太阳辐射足够强，可以产生很大的NmF 2值。这种环流驱动的机制提供了一个合理的完整的解释所观察到的模式F2层的年度和半年的平静日的变化。
The companion paper by Zouet al.shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external influences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noonNmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations ofNmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric fields play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions affect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual effect inNmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the effect of the composition changes is modified by the large solar zenith angle at midwinter. The zenith angle effect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle effect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/molecular ratio. This leads to a semiannual variation ofNmF2. A different situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values ofNmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.