Saturn's inner magnetospheric convection pattern: Further evidence

摘要

Observations of the radial locations of satellite absorption microsignatures in energetic particle data at Saturn have suggested the existence of an average convection pattern, fixed in local time, that is superimposed on the dominant near-corotation of the inner magnetosphere. Such a pattern should have additional observational consequences, and we use several different Cassini data sets to test these expectations. These include day/night asymmetries in the A-ring absorption signature of high-energy particles and total electron density, day/night asymmetries in plasma ion and electron temperatures, and day/night asymmetries in energetic-particle phase-space densities. For L > 4, the observations are found to be consistent with expectations based on the suggested convective drifts in a global noon-to-midnight electric field, such that particles drift outward on the dawn side of the magnetosphere and inward on the dusk side, resulting in drift orbits with an outward offset toward noon. The different data sets yield similar estimates of the required radial offsets, ～ 0.5-1 Rs in the region inside L = 10. The corresponding convection electric field appears to decrease with increasing radial distance, from ～ 0.3 mV/m near Tethys to ～ 0.1 mV/m beyond Dione. The source of such an electric field remains a puzzle, but whatever the source, it appears to be a dominant factor in the circulation of plasma in Saturn's inner magnetosphere. For L < 4, the observations are not fully consistent with such a global convection field, and other explanations for A-ring absorption asymmetries are needed.