On the Altitude Dependence of the Spectral Characteristics of Decametre-Wavelength E-Region Backscatter and the Relationship with Optical Auroral forms
S.E. Milan, M. Lester,
University of Leicester, U.K.
N. Sato,
NationalInstitute of Polar Research, Japan
and
H. Takizawa
Tohuku University, Japan
Observations of E region backscatter by the Iceland East SuperDARN HF radar from the 30 minute period 2330 to 2400 UT, 13 September 1999, are presented, along with simultaneous observations of auroral luminosity from two all-sky cameras. Interferometric techniques are employed to estimate the altitude of origin of each echo observed by the radar. Under investigation is a region of backscatter which is L-shell aligned and exists in a region of low auroral luminosity bounded to the north and the south by two auroral arcs. The spectral characteristics of the backscatter fall into three main populations. The first two - broad, low Doppler shift spectra and narrow, high Doppler shift spectra - are similar to type II and type I spectra observed with VHF radars, respectively. These populations scatter from near the peak of the E region. The high Doppler shift population appears to exist in a region of sub-critical electric field. The third population - exceptionally narrow, low Doppler shift spectra - originate below the E region peak, at altitudes between 80 and 100 km. We argue for a non-coherent scattering process to be responsible for this backscatter.