Variation with time of the magnetic field derived from NLFFF and of the magnetic field recorded at 1 AU. Panel a: temporal variation of the observed magnetic field (solid black line) at 1 AU together with the magnetic field derived from the solution of the NLFFF extrapolation at a height of r = 2.5 R_s (triple dot-dashed red line), r = 2.0 R_s (dashed orange line), and r = 1.8 R_s (dot-dashed violet line) and with the magnetic field derived from the potential field at r = 1.8 R_s (dotted blue line). Panel b: ratio between the observed and derived radial unsigned magnetic field at 1 AU. The derived magnetic field was calculated from the solution of the NLFFF at coronal heights of r = 2.5 R_s (dash-dotted black line), r = 2.0 R_s (dotted red line), and r = 1.8 R_s (dashed green line).