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Ann. Geophys., 26, 877-892, 2008
www.ann-geophys.net/26/877/2008/
© European Geosciences Union 2008
Complexity in the high latitude HF radar spectral width boundary region
M. L. Parkinson, K. M. Hannah, and P. L. Dyson
Department of Physics, La Trobe University, Victoria 3086, Australia
Abstract. SuperDARN radars are sensitive to the collective Doppler
characteristics of decametre-scale irregularities in the high latitude
ionosphere. The radars routinely observe a distinct transition from large
spectral width (>100 m s−1) located at higher latitudes to low
spectral width (<50 m s−1) located at lower latitudes. Because of its
equatorward location, the TIGER Tasmanian radar is very sensitive to the
detection of the spectral width boundary (SWB) in the nightside auroral
ionosphere. An analysis of the line-of-sight velocities and 2-D
beam-swinging vectors suggests the meso-scale (~100 km) convection is
more erratic in the high spectral width region, but slower and more
homogeneous in the low spectral width region. The radar autocorrelation
functions are better modelled using Lorentzian Doppler spectra in the high
spectral width region, and Gaussian Doppler spectra in the low spectral
width region. However, paradoxically, Gaussian Doppler spectra are
associated with the largest spectral widths. Application of the Burg maximum
entropy method suggests the occurrence of double-peaked Doppler spectra is
greater in the high spectral width region, implying the small-scale (~10 km)
velocity fluctuations are more intense above the SWB. These
observations combined with collective wave scattering theory imply there is
a transition from a fast flowing, turbulent plasma with a correlation length
of velocity fluctuations less than the scattering wavelength, to a slower
moving plasma with a correlation length greater than the scattering
wavelength. Peak scaling and structure function analysis of fluctuations in
the SWB itself reveals approximately scale-free behaviour across temporal
scales of ~10 s to ~34 min. Preliminary scaling exponents for
these fluctuations, αGSF=0.18±0.02 and αGSF=0.09±0.01,
are even smaller than that expected for MHD
turbulence.
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