Magnetic field strength on the solar surface and selected magnetic field lines for Carrington rotations 2077 (December 2008 with low solar activity, left) and 2158 (December 2014 close to the solar activity maximum, right), based on GONG magnetogram data and the PFSS model. Field lines are shown between the solar surface and the source surface at 2.5 solar radii, with blue indicating outgoing (B_r >  0), brown indicating incoming (B_r <  0) polarity. Magnetic field strength on the solar surface is shown using a rainbow color scale. It can clearly be seen that the dipolar structure which typically prevails during solar minimum turns into a more irregular field structure towards solar maximum.