PROBING DISCRETE AURORAL ARCS BY IONOSPHERIC TOMOGRAPHY

Optical observations of 557.7 nm and 630.0 nm emissions from discrete auroral arcs in the post-noon sector have been related to localised fi eld-aligned enhancements in the spatial distribution of E- and F-layer electron density respectively seen in images reconstructed by ionosph eric tomography. Results from two case studies are presented in which meridian scanning photometer and all-sky camera observations on Svalba rd have been compared to electron-density structures found by tomograp hic inversion of measurements made by reception of radio signals at a chain of four stations at high latitude. The F-layer features are long -lived and show exact correspondence to the red-line emissions. Transi ent arcs in green-line intensity result in E-region structures that ar e resolved in one case, but not in another where the dynamic auroral f orms are separated by less than one degree of latitude. The signature of an inverted-V precipitation event is clearly evident in one example .