  We investigate how representing active regions with bipolar magnetic regions
(BMRs) affects the end-of-cycle polar field predicted by the surface flux
transport model. Our study is based on a new database of BMRs derived from the
SDO/HMI active region patch data between 2010 and 2020. An automated code is
developed for fitting each active region patch with a BMR, matching both the
magnetic flux and axial dipole moment of the region and removing repeat
observations of the same region. By comparing the predicted evolution of each
of the 1090 BMRs with the predicted evolution of their original active region
patches, we show that the bipolar approximation leads to a 24% overestimate of
the net axial dipole moment, given the same flow parameters. This is caused by
neglecting the more complex multipolar and/or asymmetric magnetic structures of
many of the real active regions, and may explain why previous flux transport
models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR
database and the Python code to extract it are freely available.
