Statistical Study of the Ionospheric Slab Thickness at Beijing Midlatitude Station.

The ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2) is known as the ionospheric equivalent slab thickness (EST, also known as t), and it is a crucial indicator of the ionosphere. Using TEC and NmF2 data from the years 2010 to 2017, this work conducts a comprehensive statistical analysis of the ionospheric slab thickness in Beijing, which is in the midlatitude of East Asia. The outcomes show that the t have different diurnal variations at different seasons for high/low-solar-activity years. On the whole, daytime t significantly greater than nighttime t in summer, and it is the opposite for the t in winter regardless of the solar cycle, whereas the t during equinox shows different morphology for high/low-solar-activity years. Specifically, daytime t is larger than nighttime t during equinox in years of high-solar activity, while the opposite situation applies for the t during equinox in years of low-solar activity. Moreover, the pre-sunrise and post-sunset peaks are most pronounced during winter for low-solar-activity years. In summer, there is a great increase in t during the morning hours when compared with other seasons. Furthermore, the t decreases with the solar activity during nighttime, whereas it seems there is no correlation between daytime t and solar activity. This paper explained the primary diurnal variations in t across different seasons during high-/low-solar-activity years by analyzing relative fluctuations of TEC and NmF2 throughout the corresponding period. In addition, based on the disturbance index (DI), which is calculated by instantaneous t and its corresponding median, this paper found that the storm-time t might increase when compared with its median value during the daytime, while it may both increase and decrease during the nighttime, especially around dawn and dusk hours. To further analyze the physical mechanism, an example on 2 October 2013 is also presented. The results indicate that the positive disturbance of t during the main phase of a geomagnetic storm might be caused by the prompt penetration electric field and neutral wind during the storm, and the t increases during the early recovery phase might be due to the disturbance dynamo electric field as well as the neutral wind during the storm. Moreover, there is a negative disturbance of t in the recovery phase during the most disturbed sunrise hours, and it might be due to the electric field reversal, neutral wind or other factors during this period. This paper notes the differences of t in midlatitude between different longitudinal sectors from the related climatology and storm-time behavior, as it would be helpful to improve the current understanding of t at midlatitudes in East Asia.