The Uranian corona as a charge exchange cascade of plasma sheet protons

Abstract

Models of magnetospheric convection and interparticle interactions are used in a study of the collisional interactions between atmospheric neutral hydrogen and magnetospheric charged particles observed by Voyager to be convecting through the Uranian magnetosphere. The most important reactions are e−‐H collisional ionization and p+‐H charge exchange with the atmosphere. The e−‐H collisional ionization process, continually reenergized by compressional heating of the electrons as they drift toward Uranus, produces a cascade of new plasma. This process has been suggested by many authors as the source of the warm (10 eV at L = 5) plasma and is found by this work to continue in a cascade to even cooler and more abundant plasma. This newly created plasma consists almost entirely of electrons and protons because He and H2 are nearly absent from the very upper layers of the atmosphere. The densities computed for the warm plasma are within an order of magnitude of the values observed by Voyager. The cold plasma should occur only near or sunward from Uranus, where it is too cold to have been detected by Voyager on its inbound trajectory. Low‐altitude p+‐H charge exchange between the atmosphere and the new plasma produces a significant flux of hot H. This hot H may comprise the Uranian H corona, and its production serves as a high‐altitude thermospheric heat source, both of which are implied by Voyager UVS observations (although this heat source is apparently inadequate to produce the high thermospheric temperature that was observed). The generation of significant new plasma near Uranus leads to the following speculation: If this plasma crosses the dayside magnetopause and mixes with magnetopause boundary layers such as the plasma mantle, there to be swept back along the magnetotail, reincorporated into the magnetotail by the same processes postulated for solar wind plasma entry, and reenergized in the magnetotail current sheet, it would constitute an important source for the hot plasma (1 keV at L = 5) observed by Voyager.