Discrete and diffuse auroral belts in antarctica

The morphology of the ''discrete'' and ''diffuse'' auroral belts in the southern polar region is statistically examined as a function of geomagnetic Ksub(p)-index, using 350 examples of USAF-DAPP auroral photographs. The morphological characteristics of the two auroral belts are as follows: a) The diffuse auroral belts is always located equatorward of the discrete one. b) The discrete auroras are observed mostly in the evening-to-midnight sectors, whereas the diffuse auroras are dominant in the midnight-to-early morning sectors. c) The discrete auroras are observable in the evening sector even when Ksub(p)-index is very low; the discrete auroral belt in such a case is identified as the ''auroral oval''. d) The discrete auroral belt approaches a circular form along the geomagnetic latitudes 710-720 when the Ksub(p)-index approaches Ksub(p) >= 5. e) The diffuse auroral belt is confined to the night sector during geomagnetically quiet periods, but expands towards the evening sector along a geomagnetic latitude circle of 680-690, accompanied by a rapid expansion of the belt width, in association with an increase of the Ksub(p)-index. Comparisons of DAPP auroral photographs with all-sky camera auroral photographs and scanning auroral spectrophotometric data for Hβ 4861 A and OI 5577 A, obtained at Syowa Station, indicate that the discrete and diffuse auroral belts are identifiable with the electron and proton auroral belts, respectively. (auth.)