The ionosphere as a focusing lens: A case study involving simultaneous type III solar radio storm measurements from the ISIS 1 and 2 and ISEE 3 satellites

摘要

For several decades ionospheric focusing has been suggested as a means of improving antenna resolution at frequencies less than about 10 MHz for the observations of spatial features of the cosmic noise background as well as the spectra of discrete radio sources. In this paper the possibility of using terrestrial ionospheric focusing to improve the directivity of electric dipoles on space missions has been investigated by comparing simultaneous observations of a solar radio storm by the ISIS 1 and ISIS 2 spacecraft, in near Earth orbit, and the ISEE 3 spacecraft located beyond the magnetosphere. To this end, a three-dimensional ray tracing in a spherically stratified ionosphere has been carried out for conditions appropriate to the observations by the ISIS spacecraft of a solar radio storm in September 1983. This procedure allows Poynting flux spectral densities measured on ISIS to be converted to spectral densities well outside the ionosphere where they can be compared directly with simultaneous observations on ISEE 3. The results demonstrate good agreement over their common observing frequency range (1–2 MHz). Furthermore, since ISIS covers higher frequencies than ISEE 3, the spectrum of the solar radio storm was extended to 4 MHz.