This paper presents the performance of the latest version of the International Reference Ionosphere (IRI-2016) model for estimating the Total Electron Content (TEC) variations in the mid-latitudinal Turkish regions during low solar activity years (2015–2018). Moreover, TEC is estimated from dual-frequency Global Navigation Satellite System (GNSS) receivers operating in different regions of Turkey: Zonguldak (41.44 °N, 31.77 °E), Ankara (39.88 °N, 32.75 °E) and Antalya (36.88 °N, 30.66 °E). The diurnal, monthly and seasonal variations in the measured TEC are compared with the prediction of modelled TEC. The results reveal minimum variations in the diurnal TEC after midnight hours 21:00–24:00 Universal Time ( $\text{UT}=\text{LT}+3{:}00~\text{hrs}$ ) and maximum TEC enhancement during the daytime from 9:00–15:00 UT (12:00–18:00 LT). The monthly mean TEC from measured and modelled estimation depicts the highest and lowest values in April and December during the years 2015–2018, respectively. Moreover, the low and high seasonal variations are observed in equinoxes and winter solstice, respectively. Furthermore, it is observed that IRI-2016 model underestimates the measured values, especially during the daytime (12:00–18:00 LT) at each station. The correlation coefficient between the measured and modelled TEC is found in the range ( $R=0.81\text{--}0.85$ ) during the years 2015–2018, which exhibits a high positive correlation over the regions. The model predicted TEC is showing a good agreement with measured TEC of the ionospheric dynamics over the regions, which manifests the radio signals accuracy over the mid-latitude regions, particularly Turkey.

中文翻译：

---

土耳其低太阳活动期间 IRI-2016 和 GPS 的 TEC 比较

本文介绍了最新版本的国际参考电离层 (IRI-2016) 模型的性能，用于估计低太阳活动年 (2015-2018) 期间土耳其中纬度地区的总电子含量 (TEC) 变化。此外，TEC 是根据在土耳其不同地区运行的双频全球导航卫星系统 (GNSS) 接收器估算的：宗古尔达克 (41.44 °N, 31.77 °E)、安卡拉 (39.88 °N, 32.75 °E) 和安塔利亚 (36.88 °北，30.66 °E）。将测得的 TEC 的日变化、月变化和季节变化与模拟 TEC 的预测进行比较。结果显示在世界标准时间午夜 21:00–24:00 ( $\text{UT}=\text{LT}+3{:}00~\text{hrs}$ ) 之后的昼夜 TEC 的最小变化和最大值白天 9:00–15:00 UT（12:00–18:00 LT）期间的 TEC 增强。来自测量和模型估计的月平均 TEC 分别描绘了 2015-2018 年 4 月和 12 月的最高和最低值。此外，分别在春分和冬至中观察到低和高的季节性变化。此外，观察到 IRI-2016 模型低估了测量值，尤其是在每个站点的白天（12:00-18:00 LT）。在 2015-2018 年期间，测量的和模拟的 TEC 之间的相关系数在 ($R=0.81\text{--}0.85$) 范围内，在区域内表现出高度的正相关。模型预测的 TEC 与这些地区电离层动力学的实测 TEC 非常吻合，这表明中纬度地区，特别是土耳其的无线电信号精度。