ULF field line resonance frequencies (f R) of three different magnetic shells (L = 1.61, 1.71, and 1.83) have been monitored during a 4-year period (2001--2004) using a cross-phase analysis of magnetic measurements recorded at the South European Geomagnetic Array (SEGMA). We find that the variations of the daily averages of f R, which reflect changes in plasmaspheric mass density, follow the variations of the daily values of the 10.7-cm solar radio flux F 10.7 with an estimated time delay of 1--2 days. The analysis of selected events indicates that the sensitivity of f R to short-term (27-day) variations in the solar flux is the same as that for long-term (solar cycle related) variations. On the other hand, the results of the overall statistical analysis seem to indicate a lower sensitivity (by a factor of ~2) of f R to short-term solar flux variations with respect to long-term variations. Geomagnetic activity effects and/or different solar variabilities of EUV and 10.7-cm flux for different timescales are suggested as a possible cause for such a difference. Experimental results are also compared with those provided by a physical-numerical model of the ionosphere-plasmasphere system. Last, we find some evidence for a slight annual variation in f R with an estimated summer/winter ratio of ~1.1--1.2. The inferred corresponding annual variation in the equatorial mass density is in line with previous estimates for the European longitudinal sector as obtained from whistler measurements.