Ultra low frequency (ULF) waves observed at mid to low latitudes during daytime using low Earth orbit (LEO) satellite and ground-based data.
Loading...
Date
2011
Journal Title
Journal ISSN
Volume Title
Publisher
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.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.
Keywords
Satellites., Geophysics., Geomagnetism., Theses--Physics.