The additional data used to constrain the model parameters. The leftmost panel shows the altitude of peak ionospheric electron density (hmF2) measured from the Juliusruh ionosonde (54.6°N, 13.6°E). The middle panel shows the distribution of north-south vTEC (vertical TEC) gradients (positive implies increasing to the north) calculated from MADRIGAL GNSS TEC based on the difference over 4° of latitude centred on points between 58°–61°N, 5°–7°E, and 10:15–11:45UT. The third panel shows vTEC measured from the Southampton (SOTN) GNSS receiver (50.9°N, 1.5°W) for all azimuths between 0° and 60° clockwise from the north.