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 similar to 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 similar to 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.

Solar activity dependence of geomagnetic field line resonance frequencies at low latitudes

Vellante M.;U. Villante;
2007

Abstract

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 similar to 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 similar to 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11697/4949
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