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Ann. Geophys., 24, 689-705, 2006
www.ann-geophys.net/24/689/2006/
© European Geosciences Union 2006
Dynamical critical scaling of electric field fluctuations in the greater cusp and magnetotail implied by HF radar observations of F-region Doppler velocity
M. L. Parkinson
Department of Physics, La Trobe University, Bundoora Campus, Victoria 3086, Australia
Abstract. Akasofu's solar wind ε parameter describes the coupling of
solar wind energy to the magnetosphere and ionosphere. Analysis of
fluctuations in ε using model independent scaling techniques including the
peaks of probability density functions (PDFs) and generalised structure
function (GSF) analysis show the fluctuations were self-affine
(mono-fractal, single exponent scaling) over 9 octaves of time scale from
~46 s to ~9.1 h. However, the peak scaling exponent α0 was a
function of the fluctuation bin size, so caution is required when comparing
the exponents for different data sets sampled in different ways. The same
generic scaling techniques revealed the organisation and functional form of
concurrent fluctuations in azimuthal magnetospheric electric fields implied
by SuperDARN HF radar measurements of line-of-sight Doppler velocity, vLOS,
made in the high-latitude austral ionosphere. The PDFs of vLOS
fluctuation were calculated for time scales between 1 min and 256 min, and
were sorted into noon sector results obtained with the Halley radar, and
midnight sector results obtained with the TIGER radar. The PDFs were further
sorted according to the orientation of the interplanetary magnetic field, as
well as ionospheric regions of high and low Doppler spectral width. High
spectral widths tend to occur at higher latitude, mostly on open field lines
but also on closed field lines just equatorward of the open-closed boundary,
whereas low spectral widths are concentrated on closed field lines deeper
inside the magnetosphere. The vLOS fluctuations were most self-affine
(i.e. like the solar wind ε parameter) on the high spectral width field lines
in the noon sector ionosphere (i.e. the greater cusp), but suggested
multi-fractal behaviour on closed field lines in the midnight sector (i.e. the
central plasma sheet). Long tails in the PDFs imply that "microbursts"
in ionospheric convection occur far more frequently, especially on open
field lines, than can be captured using the effective Nyquist frequency and
volume resolution of SuperDARN radars.
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