Abstract Post-sunset ionospheric irregularities are common features of the equatorial ionosphere that affect radio communication and navigation systems; their triggering physical mechanism is not yet fully understood. Atmospheric gravity wave is considered as a seeding mechanism for the occurrence of ionospheric irregularities (Abdu et al. 2009). To understand the effects of atmospheric waves, characteristics of wavelike oscillation from ionospheric total electron content (TEC) fluctuation that can be obtained from superposition of different oscillation modes have been investigated. Decomposing fluctuating TEC into different oscillation modes and investigating oscillation characteristics of each component is also important to get insight about the characteristics of individual atmospheric waves that may cause TEC fluctuation. In this paper we have investigated characteristics of components of fluctuating TEC obtained from SCINDA GPS receiver installed at Bahir Dar, (geographic coordinate, 11.5oN, 37.6o E, and dip latitude of 2.5oN) Ethiopia during April 2012. First Empirical Mode Decomposition (EMD) has been applied to decompose TEC fluctuation into different oscillation modes that are known as Intrinsic Mode Function (IMF). Hilbert-Huang Transform (HHT) and Continuous Wavelet Transform (CWT) have been applied to investigate the characteristics of wave-like oscillations. Applying EMD on fluctuating vTEC corresponding to a GPS satellite, five components are found. Results from HHT and CWT have shown excellent agreement. In addition, it is found out that the median periods of oscillation of those five components are 9, 17, 47, 78, and 118 minutes. Of these periods, 17 and 47 minutes respectively are oscillation periods of components of TEC fluctuation with occurrence frequency of 92% and 91% that may be interpreted as the manifestation of two frequently occurring components of atmospheric gravity waves that are likely generated by the motion of solar terminator.

中文翻译：

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通过分解波动TEC研究日落后赤道电离层不规则性波状振荡特征

摘要 日落后电离层不规则性是赤道电离层的共同特征，影响无线电通信和导航系统；它们的触发物理机制尚未完全了解。大气重力波被认为是电离层不规则性发生的种子机制（Abdu et al. 2009）。为了了解大气波的影响，研究了电离层总电子含量 (TEC) 波动的波状振荡特征，该波动可以通过不同振荡模式的叠加获得。将波动的 TEC 分解为不同的振荡模式并研究每个分量的振荡特性对于深入了解可能导致 TEC 波动的单个大气波的特性也很重要。在本文中，我们研究了 2012 年 4 月埃塞俄比亚巴希尔达尔（地理坐标，北纬 11.5 度，东经 37.6 度，倾角 2.5 度）从 SCINDA GPS 接收器获得的波动 TEC 分量的特征。 第一次经验模式分解（ EMD) 已被应用于将 TEC 波动分解为不同的振荡模式，称为本征模式函数 (IMF)。Hilbert-Huang 变换 (HHT) 和连续小波变换 (CWT) 已被应用于研究波状振荡的特性。在对应于 GPS 卫星的波动 vTEC 上应用 EMD，发现了五个分量。HHT 和 CWT 的结果显示出极好的一致性。此外，还发现这五个分量的振荡周期中位数分别为 9、17、47、78 和 118 分钟。在这些时期中，