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Field line resonances (FLR) driven by compressional waves are an importantmechanism for the generation of ULF geomagnetic pulsations observed at alllatitudes during local daytime. References to observations of toroidalstanding Alfvén mode oscillations with clearly L-dependent frequenciesfrom spacecraft in the outer magnetosphere for L>3 are limited in theliterature. Such observations in the inner magnetosphere for L<3 havenot yet been reported in the literature. This study offers two interestingcase studies of observations of ULF waves by the low Earth orbiting CHAMPsatellite. The magnetic field measurements from CHAMP, which are ofunprecedented accuracy and resolution, are compared to Hermanus magnetometerdata for times when CHAMP crosses the ground station L-shell, namely for 13February 2002 and 18 February 2003. The data were analysed for Pc3 pulsationactivity using the Maximum Entropy Spectral Analysis (MESA) method tovisualise FLRs in the vector magnetometer data. For the first timeobservations of Pc3 toroidal oscillations with clearly L-dependentfrequencies for lower L-shell values (L<3) observed by an LEO satelliteare reported. These observations show FLR frequencies increasing as afunction of decreasing latitude down to L=1.6 and then decreasing as aresult of the larger plasma density of the upper ionosphere. The L-dependentfrequency oscillations were observed in the presence of a broadbandcompressional wave spectrum. Our observations thus confirm the well-knownmagnetohydrodynamic (MHD) wave theoretical prediction of a compressionalwave being the driver of the field line resonance

A study of L-dependent Pc3 pulsations observed by low Earth orbiting CHAMP satellite