Abstract
Abstract. Field line resonances (FLR) driven by compressional waves are an important mechanism for the generation of ULF geomagnetic pulsations observed at all latitudes during local daytime. References to observations of toroidal standing Alfvén mode oscillations with clearly L-dependent frequencies from spacecraft in the outer magnetosphere for L>3 are limited in the literature. Such observations in the inner magnetosphere for L<3 have not yet been reported in the literature. This study offers two interesting case studies of observations of ULF waves by the low Earth orbiting CHAMP satellite. The magnetic field measurements from CHAMP, which are of unprecedented accuracy and resolution, are compared to Hermanus magnetometer data for times when CHAMP crosses the ground station L-shell, namely for 13 February 2002 and 18 February 2003. The data were analysed for Pc3 pulsation activity using the Maximum Entropy Spectral Analysis (MESA) method to visualise FLRs in the vector magnetometer data. For the first time observations of Pc3 toroidal oscillations with clearly L-dependent frequencies for lower L-shell values (L<3) observed by an LEO satellite are reported. These observations show FLR frequencies increasing as a function of decreasing latitude down to L=1.6 and then decreasing as a result of the larger plasma density of the upper ionosphere. The L-dependent frequency oscillations were observed in the presence of a broadband compressional wave spectrum. Our observations thus confirm the well-known magnetohydrodynamic (MHD) wave theoretical prediction of a compressional wave being the driver of the field line resonance.
Highlights
Magnetometers on board spacecraft and on the Earth’s surface detect the magnetic signature of ultra-low frequency (ULF) geomagnetic field line resonances known as geomagnetic pulsations
Observations suggest that ULF waves generated in the region upstream of the bow shock are moved downstream by the solar wind into the magnetosheath by convection
We presented two Pc3 pulsation events characterised by a broadband compressional source spectrum and toroidal resonant oscillations with clear continuous Ldependent frequency
Summary
Magnetometers on board spacecraft and on the Earth’s surface detect the magnetic signature of ultra-low frequency (ULF) geomagnetic field line resonances known as geomagnetic pulsations. They identified toroidal standing Alfven wave oscillations with clearly L-dependent frequencies between L=3 and 6 Their ability to detect small amplitudes in CCE magnetometer data lead to the possibility of identifying the origin of various waves types of magnetic pulsation observed on the ground at low and middle latitudes using in situ measurements by satellites. They generated dynamic spectra using the FFT method computed for a 6-min window progressively shifted by 1.5 min at each step. In order to identify FLRs in ground-based and satellite data PEF orders of 15 and 20, respectively, are regarded as adequate
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