Comparisons of the root mean square errors (RMSE) between the ionosonde B ₀ data (27 stations) and the Bilitza et al. (2000) model (Standard), the Gulyaeva (1987) model (Gulyaeva) and the Altadill et al. (2009) B ₀ model (SH) for the years 1998–2006. The plot at the top shows the solar activity levels corresponding to each year, the middle plot shows the RSME for B ₀ and the bottom plot shows the RSME for B ₁ (Altadill et al. 2009).