  We use a magnetometer probe based on the Zeeman shift of the rubidium
resonant optical transition to explore the atomic magnetic response for a wide
range of field values. We record optical spectra for fields from few tesla up
to 60 tesla, the limit of the coil producing the magnetic field. The atomic
absorption is detected by the fluorescence emissions from a very small region
with a submillimiter size. We investigate a wide range of magnetic interactions
from the hyperfine Paschen-Back regime to the fine one, and the transitions
between them. The magnetic field measurement is based on the rubidium
absorption itself. The rubidium spectroscopic constants were previously
measured with high precision, except the excited state Land\'e $g$-factor that
we derive from the position of the absorption lines in the transition to the
fine Paschen-Back regime. Our spectroscopic investigation, even if limited by
the Doppler broadening of the absorption lines, measures the field with a 20
ppm uncertainty at the explored high magnetic fields. Its accuracy is limited
to 75 ppm by the excited state Land\'e $g$-factor determination.
