Linearised sensitivity of the auroral zones surface area change, $\partial A_i/\partial {\beta }_l^m$, with respect to the Gauss coefficients $g_l^m$ and $h_l^m$, in 2020 AD. Left panel: the reference main field is the IGRF-13, truncated at degree L = 13. Right panel: the reference field is a quasi-axial dipole field, described in the main text. The top and bottom panels refer to, respectively the northern and southern auroral zone.