The Ionospheric Equivalent Slab Thickness: A Review Supported by a Global Climatological Study Over Two Solar Cycles

Abstract

The ionospheric equivalent slab thickness ($\tau $ τ ) is a parameter characterizing both the distribution of the plasma in the ionosphere and the shape of the corresponding vertical electron density profile. It is calculated as the ratio of the vertical total electron content (vTEC) to the ionospheric F2-layer electron density maximum (NmF2). Since its definition dated back in the 60s, a lot of information on the behavior of $\tau $ τ for different helio-geophysical conditions has been cumulated and the connection with several plasma properties has been also demonstrated. The beginning of the Global Positioning System (GPS) era in the 90s had a strong effect on the studies about $\tau $ τ because GPS signals allow to obtain the vTEC up to about 20000 km of altitude. Recently, $\tau $ τ has also found application in many data-assimilation methodologies, especially for the improvement of empirical ionospheric models based on near real-time data. All of these topics are reviewed and discussed in this paper based on the literature published in the last sixty years.Moreover, to highlight and summarize the main global climatological features of $\tau $ τ , in this work we selected thirty-two ionospheric stations globally distributed and co-located with ground-based Global Navigation Satellite System (GNSS) receivers, for the last two solar cycles. This allowed to collect a dataset of NmF2 and vTEC that represents the largest and most complete ever analyzed for studies concerning $\tau $ τ , which gave the chance to deeply investigate its spatial, diurnal, seasonal, and solar activity variations. The corresponding results are presented and discussed in the light of the existing literature.

On the global model of the ionospheric equivalent slab thickness

“…The creation of a global model of the ionospheric equivalent slab thickness τ has attracted increased interest [1][2][3][4][5]. This is due to the fact that τ allows one to determine the critical frequency foF2 using TEC.…”

“…Figure 3 Consequently, when building a global model τ it is necessary to carefully select data. Authors [4] note, that development of global model is difficult task, so they recommend to start with local and regional models.…”

“…Results showed that τ anomalies are much stronger than the corresponding anomalies for other parameters making τ a good candidate for weather monitoring and alerts. The paper [4] gives a historical review of τ studies, presents features of τ behavior in different regions of the globe under different conditions of solar activity in the form of daily and seasonal dependences of τ, which can be used as a reference material. Herewith: (1) it is pointed out that the main application of τ is the determination of the critical frequency foF2 using the total electron content TEC, (2) it is noted that the recently developed global models of τ [1][2] have not been sufficiently tested, (3) as a direction for further research, the development of a model of τ behavior during disturbed conditions is proposed.…”

Statistical Study of the Ionospheric Slab Thickness at Beijing Midlatitude Station

“…Its capacity to provide information on both TEC and NmF2 makes it a valuable tool for researching the vertical plasma distribution in the ionosphere-plasmasphere system, as well as the vertical electron density profile [1][2][3][4]. In a scientific sense, it also relates to important physical variables and ionosphere properties, such as neutral temperature and plasma scale height [3][4][5][6][7][8][9]. In terms of engineering applications, it can play a key role in fields such as ionospheric modeling and data-assimilation methodologies due to its ability to convert the TEC and NmF2 to each other [6,[9][10][11][12][13].…”

“…In a scientific sense, it also relates to important physical variables and ionosphere properties, such as neutral temperature and plasma scale height [3][4][5][6][7][8][9]. In terms of engineering applications, it can play a key role in fields such as ionospheric modeling and data-assimilation methodologies due to its ability to convert the TEC and NmF2 to each other [6,[9][10][11][12][13]. Therefore, the field has been enriched by numerous statistical and modeling studies in the past decades since TEC measurements became available [14][15][16][17][18][19][20][21][22][23][24].…”