A statistical study of the interplanetary magnetic field control of sporadic E-layer occurrence in the southern polar cap ionosphere

The influence of the interplanetary magnetic held (IMF) on the occurrence o f sporadic E (Es)-layers in the southern polar cap ionosphere has been inve stigated. We statistically analysed ionogram and Doppler velocity observati ons made using a HF digital ionosonde located at Casey, Antarctica (66.3 de grees S, 110.5 degrees E; 81 degrees S magnetic latitude) during the two su mmer campaign intervals I January to 18 February, and 1 November to 31 Dece mber 1997. The ionogram and Doppler Velocity measurements were used to dete rmine the Es-occurrence and electric held vectors (assuming E x B/B-2 drift ), respectively. Concurrent IMF data were obtained from measurements made o n board the Wind spacecraft. First, the gross properties of the IMF depende nce of Es-formation were obtained: the occurrence rate was higher for negat ive B-y and/or positive B-z, and lower for positive B-y and/or negative B-z . To reconcile these gross properties with the electric held theory of Es-l ayer formation, the detailed diurnal variation of both Es-occurrence and th e ionospheric electric field were obtained for different orientations of th e IMF. The main statistical results are that: (1) the B-y component mainly controls the occurrence of the midnight Es-layers through its influence on the corresponding South West electric field; and (2) the B-z component main ly controls the occurrence of the evening Es-layers. However, the change in the occurrence rate for evening Es-layers was not related to the strength of the associated North West and North East electric fields. The total occu rrence of Es-layers depended more on B-y than on B-z, owing to the dominanc e of B-y-controlled midnight Es-layers in the occurrence distribution. Neve rtheless, the dependence of Es-occurrence on B-z was important. We suggest that the increase in Es-occurrence for positive B-z might be explained by t he intermittent production of lower F-region ionisation by polar showers an d squalls, which also increase in frequency and intensity for positive B-z. The importance of metallic ion transport within the ionosphere is also con sidered. (C) 2000 Elsevier Science Ltd. All rights reserved.