Solar Wind Heavy Ions and Alpha Particles Within Earth's Magnetosphere and Their Variability With Upstream Conditions

Abstract

Working toward the goal of understanding solar wind (SW) entry to the Earth's magnetosphere, this study examines solar-origin ion composition in the magnetotail. During its trajectory, Wind spent a significant amount of time in the Earth's magnetotail, where its SupraThermal Ion Composition Spectrometer (STICS) measured the mass and mass per charge of protons, alpha particles, and heavy ions with an energy/charge ratio up to 226 keV/e. For this reason, STICS measurements within the magnetosphere from 1995 to 2002 help us identify preferential entry between the different SW ion species. This study statistically analyzes how the density ratio between SW heavy ions and alpha particles ([O⁶⁺ + C⁵⁺ + Fe¹⁰⁺]/He²⁺) varies for different upstream conditions and locations within the magnetosphere: Interplanetary Magnetic Field (IMF) orientation, low versus high SW density (N_SW), low versus high SW dynamic pressure (P_Dyn), slow versus fast SW (V_SW), and dawn versus dusk. Our results indicate that the SW heavies enter the magnetosphere more efficiently than He²⁺ during northward IMF and high N_SW. In addition, these ratios exhibit a dawn-dusk asymmetry, highly skewed toward the dawn side for most upstream cases likely due to charge-exchange processes.