Geoeffectivity of Solar Wind Heavy Ions

The influence of solar wind properties, such as the southward interplanetary magnetic field, on geomagnetic activity has been well established. However, the role of heavy ions (i.e., particles heavier than protons) in the solar wind on geomagnetic storms is not fully understood. Notably, scientific models and simulations of the Earth's magnetosphere predominately assume a pure-proton solar wind, despite heavy ions constituting over 15% of the solar wind mass density on average during solar maximum periods. By utilizing data from Solar Wind Ion Composition Spectrometer onboard the Advanced Composition Explorer (ACE), we investigate the impact of heavy ion dynamic pressure during Interplanetary Coronal Mass Ejection (ICME) events on Earth's magnetosphere. Including heavy ion dynamic pressure into magnetopause calculations leads to an average reduction of 3.25% in Earth's magnetopause standoff distance during the ICME sheath interval, which could be larger during specific events. Additionally, we observe that heavy ions yield stronger correlations between maximum ion dynamic pressures during ICMEs and the corresponding minimum Dst and maximum auroral electrojet (AE) indices. The correlations with minimum Dst and maximum AE are stronger when considering minor ions (i.e., heavy ions excluding helium), with correlation coefficients of $r=-0.57$ and $r=0.60$, respectively, compared to $r=-0.48$ and $r=0.43$ for protons alone. These findings underscore the importance of including heavy ions in solar wind models to improve our understanding of geomagnetic activity and advance space weather prediction capabilities.