Latitudinal four-peak structure of the nighttime F region ionosphere: Possible contribution of the neutral wind
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摘要: 欧空局Swarm星座包含三颗飞行在不同地方时的卫星，其为研究夜侧电离层纬向四峰结构随时间的演化提供了很好的机会. 在2017年1月31日夜间，Swarm A和C两颗并排飞行的卫星在黄昏前后（17:55/18:01地方时）的美洲扇区并没有观测到赤道电离异常的两个峰，而Swarm B卫星在约4.5小时后飞行于大致相同的经度扇区，并观测到夜侧电离层呈现出明显的纬向四峰结构. 该观测证明了纬向四峰结构中靠近低纬的两个内峰不是黄昏前后赤道电离异常峰的残余. 在该事件中，位于秘鲁的Jicamarca非相干散射雷达从黄昏至午夜观测到向下的等离子体垂直漂移速度，表明向上的等离子体漂移速度并不是引起夜间纬向四峰结构的必要条件；而位于Arecibo的法布里-珀罗干涉仪观测到中性风显示出东向和南向分量的增强，表明中性风对夜侧纬向四峰结构的形成有着重要作用. SAMI2模型的模拟结果与卫星、非相干散射雷达及法布里-珀罗干涉仪的观测一致. 模拟结果显示夜侧东向与赤道向风为纬向四峰结构的形成提供了有利条件；但当赤道向风过大时，会导致背景电子密度出现显著半球不对称性，从而阻碍夜侧纬向四峰结构的发展.Abstract: In this study, we provide a detailed analysis of the latitudinal four-peak structure of the F region electron density observed by the Swarm B satellite during the night of January 31, 2017. Consisting of three satellites flying at different local times, Swarm provides an opportunity to investigate the temporal evolution of the nighttime latitudinal four-peak structure. For this event, Swarm A/C did not observe the two crests of equatorial ionization anomaly (EIA) at 17:55/18:01 LT, but Swarm B, which flew over the same longitudinal sector approximately 4.5 h later, observed a clear latitudinal four-peak structure. This provides direct evidence that the two inner peaks of the latitudinal four-peak structure are not remnants of the EIA crests from sunset. In addition, simultaneous measurements of the vertical plasma drift from the incoherent scatter radar at Jicamarca and neutral wind from the ground-based Fabry–Perot interferometer (FPI) at Arecibo were compared to reveal the possible driving mechanisms. The observed F region vertical plasma drift was generally downward from sunset throughout the local night of January 31, 2017, which reveals that an enhanced upward plasma drift is not necessary to cause the nighttime latitudinal four-peak structure. The neutral wind measurements from the FPI located at Arecibo showed enhanced eastward and southward components during the night when the latitudinal four-peak structure was observed by Swarm B, both with a difference of approximately 100 m/s compared with the other two days. This suggests that the neutral winds play an important role in the nighttime latitudinal four-peak structure. Further simulations using the SAMI2 (another model of the ionosphere) model support these observations. In general, eastward and equatorward winds from both hemispheres at night provided favorable conditions for the latitudinal four-peak structure. However, when the equatorward wind is too large, a strong hemispheric asymmetry of the background Ne can occur, which prevents the development of the latitudinal four-peak structure at night.
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Figure 1. (a) Two consecutive orbits of Swarm A (red) and C (blue) on January 31, 2017, as well as one orbit of Swarm B (green) on February 1, 2017. (b) Electron density profiles from the orbits of Swarm A/C on the west side of the Swarm B orbit. (c) Same as (b) but for the orbits of Swarm A/C on the east side of Swarm B
Figure 2. Closest orbits of Swarm A/B/C on one day (a) before and (b) after January 31, 2017 around the longitude sector of Jicamarca
Figure 4. Plasma vertical drift from the Jicamarca ISR on the three days; Vz has been averaged over the altitude range of 300–500 km to focus on the F region
Figure 5. Neutral winds from ground FPIs located at Arecibo (18.35° N, −66.75° E, Mlat: 27.1°) on three consecutive nights. The middle panel represents the night during which the four-peak structure was observed by Swarm B
Figure 6. Evolution of Ne from the SAMI2 model simulation in the altitude versus Mlat frame
Figure 7. Evolution of the zonal wind from the SAMI2 model simulation in the altitude versus Mlat frame
Figure 8. Evolution of the meridional wind from the SAMI2 model simulation in the altitude versus Mlat frame
Figure 10. Mlat profiles of Ne at 400 km from the SAMI2 model simulations with modified (a) zonal wind and (b) meridional wind
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