  This work focuses on the analysis of the mean longitudinal magnetic field as
a stellar activity tracer in the context of small exoplanet detection and
characterisation in radial-velocity (RV) surveys. We use SDO/HMI filtergrams to
derive Sun-as-a-star magnetic field measurements, and show that the mean
longitudinal magnetic field is an excellent rotational period detector and a
useful tracer of the solar magnetic cycle. To put these results into context,
we compare the mean longitudinal magnetic field to three common activity
proxies derived from HARPS-N Sun-as-a-star data: the full-width at
half-maximum, the bisector span and the S-index. The mean longitudinal magnetic
field does not correlate with the RVs and therefore cannot be used as a
one-to-one proxy. However, with high cadence and a long baseline, the mean
longitudinal magnetic field outperforms all other considered proxies as a solar
rotational period detector, and can be used to inform our understanding of the
physical processes happening on the surface of the Sun. We also test the mean
longitudinal magnetic field as a "stellar proxy" on a reduced solar dataset to
simulate stellar-like observational sampling. With a Gaussian Process
regression analysis, we confirm that the solar mean longitudinal magnetic field
is the most effective of the considered indicators, and is the most efficient
rotational period indicator over different levels of stellar activity. This
work highlights the need for polarimetric time series observations of stars.
