Formation heights (derived from the VAL F model) versus the mean longitudinal field strengths determined from inverting the different lines. A regular monotone decrease of the longitudinal magnetic field is found, corresponding to the ordering of the ions as indicated in Table 1. A vertical component gradient (absolute value) of the magnetic field of − 1.18 G/km is determined from the linear relationship. The average is taken on a region delimited by magnetic field values from 200 to 1000 G on the map of the line Fe i 6302.50 Å, as indicated in Fig. 6. Standard errors for the regression coefficients are 179.3 and 0.25 for the linear fit 1043.85 and − 1.11.