Coincidence of composition and speed boundaries of the slow solar wind

Recent studies have raised questions about the plasma characteristics that define what is called the “slow solar wind.” At issue is whether solar wind with low speed located far from the heliospheric current sheet (HCS) lacks the composition signatures characteristic of a streamer belt origin, where the streamer belt is understood to encompass the HCS. To test for this possibility, superposed epoch analysis of composition data at 1 AU was performed at stream interfaces. Since interfaces mark the boundary between what was originally fast and slow wind when it left the Sun, the analysis sidesteps the need to identify slow wind by its speed. At those interfaces where the slow wind precedes fast wind, the resulting pressure ridge creates an east‐west flow deflection that identifies the boundary. The analysis of 258 interfaces displaying an unambiguous deflection reveals sharp drops in the ratios of O 7+ /O 6+ , C 6+ /C 5+ , and Fe/O coincident with interface passage. The coincidence indicates that essentially all slow wind, including slow wind from pseudostreamers, has ionic and elemental composition characteristic of the streamer belt, independent of distance from the HCS. This finding is not unexpected in view of the well‐known drop in density and increase in proton temperature characteristic of interface passage. These parameters control entropy, which is well‐correlated with O 7+ /O 6+ .