Comparison of Ion-Proton Differential Speed between ICMEs and Solar Wind near 1 au

Abstract

The elemental abundance of ICMEs and solar wind near 1 au is often adopted to represent the abundance in the corresponding coronal sources. However, the absolute abundance of heavy ions (relative to hydrogen) near 1 au might be different from the coronal abundance due to the ion-proton differential speed ($V_{ip}$). To illustrate the $V_{ip}$ characteristics and explore whether it influences the absolute abundance analysis for ICMEs and solar wind, we perform a statistical study on the $V_{ip}$ for He$^{2+}$, C$^{5+}$, O$^{6+}$, and Fe$^{10+}$ in both ICMEs and solar wind based on measurements of Advanced Composition Explorer. The results show that the $V_{ip}$ is negligible within ICMEs and slow solar wind ($<$ 400 km s$^{-1}$), while obvious in the intermediate (400 -- 600 km s$^{-1}$) and fast wind ($>$ 600 km s$^{-1}$). Previous studies showed that the $V_{ip}$ in ICMEs keeps negligible during propagation from 0.3 to 5 au, but in solar wind it increases with the decreasing heliocentric distance. Therefore, it might be questionable to infer the absolute abundance of coronal sources through in-situ abundance near 1 au for solar wind. Fortunately, the ion-oxygen (O$^{6+}$) differential speed ($V_{io}$) is negligible for He$^{2+}$, C$^{5+}$, and Fe$^{10+}$ within both ICMEs and solar wind, and previous studies suggested that the $V_{io}$ does not vary significantly with the heliocentric distance. This indicates that various heavy ions always flow at the same bulk speed and their relative abundance (relative to oxygen) near 1 au can represent the coronal abundance for both ICMEs and solar wind.