2 research outputs found
Ultra low frequency (ULF) waves observed at mid to low latitudes during daytime using low Earth orbit (LEO) satellite and ground-based data.
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.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
A study of the time-dependent modulation of galactic cosmic rays in the heliosphere
Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.Time-dependent cosmic ray modulation in the heliosphere is studied by using a two-dimensional
time dependent modulation model. To compute realistic cosmic ray modulation a compound
approach is used, which combines the effect of the global changes in the heliospheric magnetic
field magnitude and the current sheet tilt angle to establish realistic time dependent diffusion
and drift coefficients. This approach is refined by scaling down drifts additionally (compared
to diffusion) towards solar maximum. The amount of drifts needed in the model to realistically
compute 2.5 GV proton and electron and 1.2GV electron and helium intensities, as measured
by Ulysses from 1990 to 2004, is established. It is shown that the model produces the correct
latitudinal gradients evident from the observations during both the Ulysses fast latitude scan
periods. Also, much can be learned on the magnitude of perpendicular diffusion in the polar
direction, K┴θ, especially for solar minimum conditions and for polarity cycles when particles
drift in from the poles. For these periods K┴θ = 0.12K║ in the polar regions (with K║ the parallel
diffusion coefficient)and K┴θ /K║ can vary between 0.01 to even 0.04 in the equatorial
regions depending on the enhancement factor toward the poles. The model is also applied to
compute radial gradients for 2.5 GV cosmic ray electrons and protons in the inner heliosphere.
It is shown that, for solar minimum, and in the equatorial regions, the protons (electrons) have
a radial gradient of 1.9 %/AU (2.9 %/AU), increasing for both species to a very fluctuating
gradient varying between 3 to 4 %/AU at solar maximum. Furthermore, the model also computes
realistic electron to proton and electron to helium ratios when compared to Ulysses observations,
and charge-sign dependent modulation is predicted up to the next solar minimum
expected in 2007. Lastly the model is also applied to model simultaneously galactic cosmic
ray modulation at Earth and along the Voyager 1 trajectory, and results are compared with> 70
MeV count rates from Voyager 1 and IMP8. To produce realistic modulation, this model gives
the magnitude of perpendicular diffusion in the radial direction as Kâ”´r/Kâ•‘= 0.035 and that
the modulation boundary seemed to be situated between at 120 AU and 140 AU.Master