  Small-scale internetwork (IN) magnetic fields are considered to be the main
building blocks of the quiet Sun magnetism. For this reason, it is crucial to
understand how they appear on the solar surface. Here, we employ a
high-resolution, high-sensitivity, long-duration Hinode/NFI magnetogram
sequence to analyze the appearance modes and spatio-temporal evolution of
individual IN magnetic elements inside a supergranular cell at the disk center.
From identification of flux patches and magnetofrictional simulations, we show
that there are two distinct populations of IN flux concentrations: unipolar and
bipolar features. Bipolar features tend to be bigger and stronger than unipolar
features. They also live longer and carry more flux per feature. Both types of
flux concentrations appear uniformly over the solar surface. However, we argue
that bipolar features truly represent the emergence of new flux on the solar
surface, while unipolar features seem to be formed by coalescence of background
flux. Magnetic bipoles appear at a faster rate than unipolar features (68 as
opposed to 55 Mx cm$^{-2}$ day$^{-1}$), and provide about 70% of the total
instantaneous IN flux detected in the interior of the supergranule.
