Ultra low frequency (ULF) waves observed at mid to low latitudes during daytime using low Earth orbit (LEO) satellite and ground-based data.

Abstract

The launch of German geoscience satellite CHAMP inspired the increased interest in the study of ultra low frequency waves. In this work data from low Earth orbiting (LEO) German CHAMP satellite and South African ground-based magnetometer data were used to study geomagnetic pulsations, in particular continuous pulsations, Pc3, with periods in the range 10-45 seconds. Both Fast Fourier Transform (FFT) and Maximum Entropy Spectral Analysis (MESA) were used as analysis techniques to compute and compare spectra. We simulated a Pc3 oscillation using a sinusoidal function in order to test and establish appropriate parameters to use on the application of these analysis techniques. In this study the region chosen for a low latitude geomagnetic pulsations study excludes high current regions such as polar regions where field aligned currents occur. The structure of low-latitude pulsations was studied by comparing satellite and ground magnetic field measurements. The magnetic field measurements observed in the topside ionosphere by CHAMP were compared to Hermanus data for times when CHAMP crossed the ground station L-shell. The data were analysed for Pc3 pulsation activity using the MESA method to visualise field line resonance (FLR) in the vector magnetometer data. A number of discrete frequency oscillations for the fast mode wave were observed, one of which drives FLR at characteristic latitude as detected by both ground and satellite measurements. The toroidal mode frequency on CHAMP experiences a Doppler shift due to the rapid motion across the resonance region. Polarization hodograms in the resonance region show the expected 900 rotation of the field line resonant magnetic field components. We present first time ob- servations of toroidal standing Alfv´en mode oscillation with clearly L-dependent frequencies in the inner magnetosphere for L < 3. Our observations show FLR frequency continuously increasing as a function of decreasing latitude down to L = 1.6 and then decrease as a result of the larger plasma density of the upper ionosphere. The L-dependent frequency oscillations were observed in the presence of broadband compressional wave spectra. Our observation confirms the well-known magnetohydrodynamic (MHD) wave theoretical prediction of a compressional wave being the driver of the field line resonance, and that the Pc3 pulsations do not have a source with the same frequency structure. Keywords: ULF waves, Fast Fourier Transform, Maximum Entropy Spectral Analysis, CHAMP satellite, Geomagnetic pulsations, Pc3, Alfv´en wave, Field line resonance.