Dependence of geomagnetic field line resonant frequencies on solar irradiance

By performing a cross-phase analysis of ULF wave magnetic measurements recorded at the South European Geomagnetic Array (SEGMA) we have investigated the temporal variation of the resonance frequency (f(R)) of a low-latitude geomagnetic field line (L = 1.61) during a 3-year period from 2002 to 2004. We find that f(R) is clearly anticorrelated with the 10.7 cm solar radio flux F-10.7. This result indicates that a variation of the ionization rate in the ionosphere, induced by a variation of the solar EUV/X-ray radiation, influences the whole distribution of the plasma along low-latitude field lines.