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Aims. This study investigates the time-varying excitation of the Earth's retrograde free core nutation (RFCN) by atmospheric pressure variations. Methods. The amplitude of the RFCN resulting from diurnal pressure variations in the external fluid layers is derived. The case of atmospheric excitation is investigated using the National Center for Environmental Prospect/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis project data set. Results. The diurnal atmospheric excitation is about half that required to explain the observed RFCN amplitude. Moreover, the time variability of the atmospheric noise does not account for the observed variability of the RFCN amplitude over the time period 1984-2006. Possible sources of error like the pressure coefficient, the RFCN quality factor, or the time varying RFCN apparent period are investigated but cannot account for the discrepancy. Conclusions. Improvements are needed in atmospheric modeling at diurnal frequencies. Until this is achieved, the physical link between the observed RFCN and its atmospheric excitation, especially concerning its time variability, will remain unclear.

Atmospheric excitation of the Earth's free core nutation