Calculated ∆f _calc versus measured ∆f _meas values of the IAR frequency scale as a result of calculations based on the modified IRI-2012 model with the inclined profile elongated along the local geomagnetic field line. Bold brown line is the regression line running through the origin of coordinates; thin lines are the 99% confidence limits; gray line is the line of perfect match Δf _calc = Δf _meas.