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

The ionospheric equivalent slab thickness (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 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 τ because GPS signals allow to obtain the vTEC up to
about 20000 km of altitude. Recently, 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 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 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.