  The fluorescence spectra of a sub-micron atomic cesium vapor layer observable
under resonant excitation on D2 line have been studied in the presence of an
external magnetic field. Substantial changes in amplitudes and frequency
positions of the individual (resolved) hyperfine transitions have been recorded
in moderate magnetic fields (up to ~ 50 Gauss). These features are caused by
mixing of the hyperfine states of the upper level resulting from comparable
values of the hyperfine splitting of the 62^P_{3/2} manifold and Larmor
frequencies of the magnetic sublevels. The results of simulation show a good
agreement with the experimental spectra. Possible application of the results
for high spatial resolution magnetometry is discussed.
