Ion composition in interchange injection events in Saturn's magnetosphere

摘要

Interchange injection events are commonly observed by the Cassini spacecraft in the region between about 6 and 12 R_s (1 R_s = 60,268 km) and even frequently beyond. In this study, 13 examples of interchange injection events are identified in Cassini/Cassini Plasma Spectrometer data under special conditions such that time-of-flight (TOF) mass spectra could be obtained from entirely within the events. Using the TOF data to separate the main ion species H~+, H_2~+, and W~+, approximate densities of each species are calculated under the assumption that all distributions were isotropic. The light-ion density ratios, H_2~+/H~+, in the injection events are not discernibly different from those ratios in control intervals from the ambient plasma. However, the water-group ratio, W~+/H~+, is significantly lower than ambient. The comparison of the measured density ratios with the range of values observed throughout Saturn’s magnetosphere indicates that the values of W~+/H~+ that are as low as those observed within the injection events are found primarily beyond L~14 (where L is the equatorial crossing distance, in Saturn radius, of a dipole field line), indicating that the injection events are delivering plasma from the outer magnetosphere at times traveling at least 6 R_s.