The ionospheric response over the low and mid-latitudes along the $78^{\circ}\pm 3^{\circ}$ E longitude sector is studied for the 20th December 2015 geomagnetic storm. The global positioning system (GPS) derived total electron content (TEC) over mid & low-latitude stations and also an ionosonde observation of the F2 layer critical frequency over a mid-latitude station are analyzed. On the day of storm (20th December) the interplanetary magnetic field turned southward around 0330 UT and reached to its minimum value around −20 nT. SYM-H index started to decline at the same time and attained to its minimum value of about −180 nT at around 2330 UT after a very long main-phase duration (∼20 hours). During the storm’s main phase, a large TEC variation is observed for both the locations which have been attributed to the prompt penetration electric field (PPEF). The uniqueness of the observed TEC variation lies in the fact that while over the mid-latitude the TEC returned to its quiet time variations after the recovery phase, the enhancement in TEC over low-latitude has been observed even after two days (anomalous increase on 22nd December) of the storm which indicates the absence of disturb dynamo electric field (DDEF) for low-latitude over 78°E longitude. The thermospheric O/N2 data obtained both from, observations as well as CTIPe model simulation are in good agreement with the observed TEC variation except for the 22nd December over the mid-latitude location. This anomalous behavior on 22nd December over low-latitude could be due to some local dynamics or the transport of plasma by the background seasonal and storm time meridional winds over the studied region.

中文翻译：

---

中低纬度电离层对2015年12月20日地磁暴的响应研究

研究了 2015 年 12 月 20 日地磁风暴沿 $78^{\circ}\pm 3^{\circ}$ E 经度扇区的低纬度和中纬度的电离层响应。分析了全球定位系统 (GPS) 中低纬度站上的总电子含量 (TEC) 以及中纬度站上 F2 层临界频率的电离探空仪观测结果。在风暴日（12 月 20 日），行星际磁场在 0330 UT 附近向南转向，并在 -20 nT 附近达到最小值。SYM-H 指数同时开始下降，并在很长的主相持续时间（~20 小时）后在 2330 UT 附近达到其最小值约 -180 nT。在风暴的主要阶段，对于归因于快速穿透电场 (PPEF) 的两个位置，都观察到了较大的 TEC 变化。观测到的 TEC 变化的独特性在于，虽然在中纬度地区 TEC 在恢复阶段后恢复到其安静的时间变化，但即使在两天后也观察到了低纬度地区 TEC 的增强（在12 月 22 日），这表明在 78°E 经度以上的低纬度地区没有扰动发电机电场 (DDEF)。除了 12 月 22 日在中纬度位置外，从观测和 CTIPe 模型模拟获得的热层 O/N2 数据与观测到的 TEC 变化非常吻合。