Comparison of Ion–Proton Differential Speed between Interplanetary Coronal Mass Ejections and Solar Wind near 1 au

Abstract

The elemental abundance of interplanetary coronal mass ejections (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 ( 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.

Keywords

• Solar coronal mass ejections
• Solar wind
• Solar abundances
• Solar cycle