The satellite of the 4d_3/2 – 2p_1/2 emission line (Lγ) has been traditionally interpreted as a signature of the bare (unscreened) magnetic moment of the 4f electrons in the lanthanides [see e.g. B. Maddox et al., PRL 96, 215701 (2006)]. Here, a collaboration of theorists from the University of California at Davis and Stanford along with experimenters from the University of Washington and Lawrence Livermore National Laboratory find that the intensity of the satellite structure of the early light lanthanides is actually tracking with the 4f occupancy. This offers a unique diagnostic tool to determine whether the 4f electrons become itinerant under compression or remain correlated. The experimental observations were taken at the HPCAT 16-ID-D beamline of the Advanced Photon Source and have been confirmed using state-of-the-art extended atomic structure codes.  For more, see Wei-Ting Chiu et al., Physical Review Letters 122, 066401 (2019).