Near-Earth High-Speed and Slow Solar Winds: A Statistical Study on Their Characteristics and Geomagnetic Impacts

Abstract

Near-Earth solar winds are separated into two groups: slow solar wind (SSW) with plasma speed [$V_{\mathrm{sw}}$] $< 500$ km s⁻¹ and high-speed solar wind (HSW) with $V_{\mathrm{sw}} > 700$ km s⁻¹. A comparative study is performed on the plasma and interplanetary magnetic field (IMF) properties of the near-Earth SSW and HSW, using solar wind measurements propagated to Earth’s bow shock nose from 1963 through 2022. On average, HSW is characterized by higher alpha-to-proton density ratio [$N_{\mathrm{a}}/N_{\mathrm{p}}$] (67%), ram pressure [$P_{ \mathrm{sw}}$] (95%), proton temperature [$T_{\mathrm{p}}$] (370%), reconnection electric field [$VB_{\mathrm{s}}$] (141%), Alfvén speed [$V_{\mathrm{A}}$] (76%), magnetosonic speed [$V_{\mathrm{ms}}$] (65%), and lower proton density [$N_{\mathrm{p}}$] (52%) and plasma-$\beta$ (54%) than SSW. In $VB_{\mathrm{s}}$, $V = V_{\mathrm{sw}}$, $B_{\mathrm{s}}$ is the southward component of IMF. $V_{\mathrm{A}} = B_{0}/\sqrt{\mu_{0}\rho}$, $V_{\mathrm{ms}} = \sqrt{V_{\mathrm{A}}^{2}+V_{\mathrm{S}}^{2}}$, where $B_{0}$ is the IMF magnitude, $\mu_{0}$ is the free space permeability, $\rho$ is the solar wind mass density, and $V_{\mathrm{S}}$ is the sound speed. $\beta$ is defined as the plasma pressure to the magnetic-pressure ratio. The geomagnetic activity is found to be enhanced during HSW, as reflected in higher average auroral electrojet index [AE] (213%) and stronger geomagnetic Dst index (367%) compared to those during SSW. The SSW characteristic parameters $N_{\mathrm{a}}/N_{\mathrm{p}}$, $T_{\mathrm{p}}$, $B_{0}$, $V_{\mathrm{A}}$, and $V_{\mathrm{ms}}$ exhibit medium to strong correlations (correlation coefficients $r = 0.51$ to 0.87) with the $F_{\mathrm{10.7}}$ solar flux, while $\beta$ and Mach numbers exhibit strong anti-correlations ($r = -0.82$ to −0.90) with $F_{\mathrm{10.7}}$. The associations are weaker or insignificant for HSW.