Abstract
Abstract. The statistical occurrence of decametre-scale ionospheric irregularities, average line-of-sight (LOS) Doppler velocity, and Doppler spectral width in the sub-auroral, auroral, and polar cap ionosphere ( - 57°L to - 88°L) has been investigated using echoes recorded with the Tasman International Geospace Environment Radar (TIGER), a SuperDARN radar located on Bruny Island, Tasmania (147.2° E, 43.4° S geographic; - 54.6 °L). Results are shown for routine soundings made on the magnetic meridian beam 4 and the near zonal beam 15 during the sunspot maximum interval December 1999 to November 2000. Most echoes were observed in the nightside ionosphere, typically via 1.5-hop propagation near dusk and then via 0.5-hop propagation during pre-midnight to dawn. Peak occurrence rates on beam 4 were often > 60% near magnetic midnight and ~ - 70 °L. They increased and shifted equatorward and toward pre-midnight with increasing Kp (i.e. Bz southward). The occurrence rates remained very high for Kp > 4, de-spite enhanced D-region absorption due to particle precipitation. Average occurrence rates on beam 4 exhibited a relatively weak seasonal variation, consistent with known longitudinal variations in auroral zone magnetic activity (Basu, 1975). The average echo power was greatest between 23 and 07 MLT. Two populations of echoes were identified on both beams, those with low spectral width and a mode value of ~ 9 ms-1 (bin size of 2 ms-1) concentrated in the auroral and sub-auroral ionosphere (population A), and those with high spectral width and a mode value of ~ 70 ms-1 concentrated in the polar cap ionosphere (population B). The occurrence of population A echoes maximised post-midnight because of TIGER’s lower latitude, but the subset of the population A echoes observed near dusk had characteristics reminiscent of "dusk scatter" (Ruohoniemi et al., 1988). There was a dusk "bite out" of large spectral widths between ~ 15 and 21 MLT and poleward of - 67 °L, and a pre-dawn enhancement of large spectral widths between ~ 03 and 07 MLT, centred on ~ - 61 °L. The average LOS Doppler velocities revealed that frequent westward jets of plasma flow occurred equatorward of, but overlapping, the diffuse auroral oval in the pre-midnight sector.Key words. Ionosphere (auroral ionosphere; electric fields and currents, ionospheric irregularities)
Highlights
It is well known that the transfer of solar-wind energy and momentum to the coupled magnetosphere-ionospherethermosphere system is constantly changing due to fluctuations in magnetic reconnection, turbulent boundary layer processes, and dynamic pressure in the solar wind (Kivelson and Russell, 1995)
Note the poleward edge of the discrete auroral oval delineates a fixed polar cap boundary, but the radar statistics encompass a broad range of geomagnetic activity
From the auroral oval boundaries (Starkov, 1994b) superimposed in Fig. 4, we infer that most of the ionospheric echoes observed at ranges > 630 km on beam 4 were from decametre-scale irregularities drifting in the nightside discrete auroral and polar cap ionosphere
Summary
It is well known that the transfer of solar-wind energy and momentum to the coupled magnetosphere-ionospherethermosphere system is constantly changing due to fluctuations in magnetic reconnection, turbulent boundary layer processes, and dynamic pressure in the solar wind (Kivelson and Russell, 1995). Some authors have found evidence suggesting these processes, including the magnetospheric boundaries formed by them, produce reasonably distinct signatures in the motion of ionospheric irregularities The Super Dual Auroral Radar Network (SuperDARN) is a ground-based network of high frequency (HF) backscatter radars providing global scale coverage of the occurrence, amplitude, and motion of decametre-scale ionospheric irregularities in response to dynamic high-latitude processes (Greenwald et al, 1985, 1995). The fluid E × B (gradient drift) instability (see Tsunoda, 1988; Kelley, 1989) is the mechanism most often
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
