Two-dimensional surface map of the magnetic field amplitude as saved in a coupling file. The magnetic field amplitude is computed using the three magnetic field components B_r, B_lon, B_clt saved in the 138th ASCII file used for the coupling between COCONUT and EUHFORIA for the case named ‘TDm_3’. The 138th file, marking the time t = 5.5 hours, captures when the flux rope starts crossing the surface at R_b = 21.5 R_⊙.