Browsing by Author "Stephenson, Judy Ann Elizabeth."

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The effects of traveling ionospheric disturbances on SuperDARN near range echoes.
(2022) Hiyadutuje, Alicreance.; Kosch, Michael Jurgen.; Stephenson, Judy Ann Elizabeth.
Traveling Ionospheric Disturbances (TIDs) and Near Range Echoes (NREs) are both natural phenomena observed by SuperDARN High Frequency (HF) radars. This study presents for the first time observations of NREs in the lower E-region whose amplitudes are moderately modulated by medium-scale TIDs propagating in the F-region that have been observed by the same radar at another time in the far ranges. Two events during geomagnetic storms in winter recorded by the SANAE radar and two events during quiet times in summer recorded by the Zhongshan radar, both radars in the southern hemisphere, are described. The Gradient Drift Instability (GDI) proved to be the likely mechanism. The GDI is driven by the velocity difference between neutrals and plasma in the E-region ionosphere, due to the global convection electric field, and can be modulated by the polarization electric field of a passing TID via the near-vertical equipotential magnetic field lines.
An investigation of the nature of Pc5 pulsations using SuperDARN and magnetometer data.
Mtumela, Zolile.; Stephenson, Judy Ann Elizabeth.; Walker, Anthony David Mortimer.
Pc5 pulsations are global magnetohydrodynamic events in the magnetosphere. We employed an Automated Pulsation Finder program to identify significant Pc5 pulsation events in SuperDARN data. Those events for which a resonance of similar frequency band was observed in more than one HF radar were selected. The three events presented here are such that a similar resonance was observed in Goose Bay, Saskatoon and Pykkvibaer HF radars, located in the northern polar region. One event was isolated in which the resonance was observed at the conjugate hemisphere at Sanae, Antarctica. Those events have a good data from magnetometer chains within the field of view of HF radars are chosen for analysis. These two instruments complement each other. We combined these two instruments to investigate the nature of the pulsation, determining its qualitative polarization characteristics. Observations of a resonance that extend over a large fraction of the polar region are rarely reported. A complex demodulation technique was employed to determine the amplitude and phase relationship between field components observed by the radars and magnetometer chains, this in turn, affords resolution of other characteristics of pulsations such as wave number and phase velocity. We present results in a graphical form and discuss them in the context of MHD theory of magnetic pulsations, speculating on their generation mechanism.
Solar influences on Polar ozone.
(1994) Stephenson, Judy Ann Elizabeth.; Scourfield, Malcolm W. J.; Rash, Jonathan Paul Stuart.
Measurements by the TOMS instrument aboard the Nimbus 7 satellite, of total column ozone over polar regions have been studied to determine the effects of solar induced natural ozone modulation. Two different analysis methods were employed to ascertain short term (days to months) and long term (months to years) solar influences on polar ozone. Bursts of intense solar activity can result in solar proton events (SPE's). The high energy protons, originating in solar flares, produce secondary electrons which can generate large concentrations of odd nitrogen in the middle atmosphere. These reactive species can catalytically destroy ozone. Three case studies are presented in an attempt to quantify the effect of SPE's on ozone mass over a latitude region 90 to 70°. In order to monitor the ozone response following a SPE over both hemispheres simultaneously, the SPE must occur during the equinox period when both poles are irradiated. Fortuitously, a SPE was recorded in March 1989, the analysis of which forms a case study in this thesis. Ozone depletions of 7.4 x 10 to the power of 9 kg for the south polar cap and 8.0 x 10 to the power of 9 kg for the north polar cap indicate the degree of symmetry for this event. Longer term effects of solar variability are investigated by Fourier techniques. A Fourier transform of eleven years of total ozone mass values, over the region 90 to 70° S, was performed. Inspection of the Fourier spectrum reveals peaks associated with solar cycle, annual and semi-annual oscillations, that may be attributed directly to solar variation. Other peaks, corresponding to QBO and ENSO periodicities, may be ascribed to indirect solar influences i.e. thermally driven dynamics. Finally, a comparison between the phase of the solar cycle peak in this spectrum with that in a spectrum of daily values of solar radio flux, reveals that the austral polar ozone solar cycle periodicity lags solar forcing by 2.8 years. Portions of chapters have been reported at the 1990 South African Institute of Physics Annual Conference, University of Port Elizabeth, South Africa and as a poster at the 1992 Quadrennial Ozone Symposium, Charlottesville, United States of America, 4-13 June 1992. In addition, various parts of this work has been submitted for publication, viz: Stephenson, J. A. E. and M. W. J. Scourfield, Importance of energetic solar protons in ozone depletion, Nature, 352, 137: 1991.