Long term trends of the equivalent slab thickness of the ionosphere over two solar cycles

Abstract

Besides the total electron content (TEC) and the peak electron density of the F2 layer (NmF2) also the equivalent slab thickness is considered as a key parameter of the ionosphere. The equivalent slab thickness is a shape parameter of the vertical electron density profile defined by the ratio TEC/NmF2. Because the equivalent slab thickness is closely related to the neutral gas scale height under steady-state conditions at daytime, the sllab thickness is correlated with thermospheric temperature changes. Based on earlier studies that have revealed a continuous decrease of the slab thickness over European ionosonde stations since 1996, the presentation analyses trends in relation to TEC and NmF2 for an extended data base up to the end of 2022. A continuous TEC data base is available from DLR and the International GNSS Service (IGS). The peak electron density NmF2 is obtained from vertical sounding stations located over Europe covering solar cycles 23 and 24. In addition to the ground-based observations of NmF2 also radio occultation measurements are used to derive the peak electron density for equivalent slab thickness computations. It is found that the slab thickness behavior continues decreasing, e.g. by 50-60 km over the Juliusruh ionosonde station since 1996. The geophysical and solar activity background will be considered to explain the significant contraction of the equivalent slab thickness associated with a long-term cooling of the thermosphere. It is concluded that the slab thickness analysis has a high potential for estimating long-term changes in the thermosphere/ionosphere systems.