  We study the circular polarization of the Mn I resonance lines at 279.56,
279.91, and 280.19 nm (hereafter, UV multiplet) by means of radiative transfer
modeling. In 2019, the CLASP2 mission obtained unprecedented
spectropolarimetric data in a region of the solar ultraviolet including the Mg
II h and k resonance lines and two lines of a subordinate triplet, as well as
two Mn I resonance lines. The first analysis of such data, in particular those
corresponding to a plage region, allowed the inference of the longitudinal
magnetic field from the photosphere to the upper chromosphere just below the
transition region. This was achieved by applying the weak field approximation
to the circular polarization profiles of the Mg II and Mn I lines. While the
applicability of this approximation to the Mg II lines was already demonstrated
in previous works, this is not the case for the Mn I UV multiplet. These lines
are observed as absorptions between the Mg II h and k lines, a region whose
intensity is shaped by their partial frequency redistribution effects.
Moreover, the only Mn I stable isotope has nuclear spin $I=5/2$ and thus
hyperfine structure must be, a priori, taken into account. Here we study the
generation and transfer of the intensity and circular polarization of the Mn I
resonance lines accounting for these physical ingredients. We analyze their
sensitivity to the magnetic field by means of their response function, and we
demonstrate the applicability of the weak field approximation to determine the
longitudinal component of the magnetic field.
