Complexity in the high latitude HF radar spectral width boundary region

Abstract

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, α GSF =0.18±0.02 and α GSF =0.09±0.01, are even smaller than that expected for MHD turbulence.

Complexity in the scaling of velocity fluctuations in the high-latitude F-region ionosphere

“…The velocities were analysed separately for Population A echoes with low spectral width (<75 m s −1 ), and Population B echoes with high spectral width (>150 m s −1 ). Population A echoes are concentrated on closed field lines and Population B echoes on open fields, but there is no consensus about the physical meaning of the SuperDARN spectral widths (Parkinson et al, 2008). The velocities were also analysed using separate 2-h bins in MLT, but too few samples were available for accurate calculations in some MLT sectors.…”

“…Chisham and Freeman, 2003). It is not known whether regions of large and small spectral width are more dynamically fundamental than regions found above and below the SWB (Parkinson et al, 2008). The overall spatial distribution of velocity counts shown in Fig.…”