Near-Earth Sub-Alfvénic Solar Winds: Interplanetary Origins and Geomagnetic Impacts

Abstract

The near-Earth solar wind is in general super-Alfvénic and supermagnetosonic. Using all available near-Earth solar wind measurements between 1973 and 2020, we identified 30 intervals with sub-Alfvénic solar winds. The majority (83%) of the events occurred within interplanetary coronal mass ejection magnetic clouds (MCs)/driver gases. These MC sub-Alfvénic events are characterized by exceptionally low plasma densities (N _sw) of ~0.04-1.20 cm^-3, low temperatures (T _sw) of ~0.08 × 10⁵ K to 12.46 × 10⁵ K, enhanced magnetic field intensities (B ₀) of ~8.3-53.9 nT, and speeds (V _sw) of ~328-949 km s^-1. The resultant high Alfvén wave speeds (V _A) ranged from ~410 to 1471 km s^-1. This is consistent with a mechanism of the MC expansions as they propagate radially outward, causing small pockets of sub-Alfvénic wind regions within the MCs. The remainder of the sub-Alfvénic intervals (17%) occurred within the extreme trailing portions of solar wind high-speed streams (HSSs). These HSS sub-Alfvénic winds had low N _sw of ~0.04-0.97 cm^-3, low T _sw of ~0.06 × 10⁵ K to 0.46 × 10⁵ K, B ₀ of ~6.3-18.2 nT, V _sw of ~234-388 km s^-1, and a V _A range of ~364-626 km s^-1. This is consistent with a mechanism of solar wind super-radial expansions in the trailing HSS regions. During sub-Alfvénic solar wind intervals, Earth's bow shock nose exhibited rapid evanescence, and the estimated geocentric magnetopause distance increased by ~33%-86%. The inner magnetosphere was more or less unaffected by the sub-Alfvénic solar winds. No significant impact was observed in the outer radiation belt relativistic electrons, and no geomagnetic storms or substorms were triggered during the sub-Alfvénic solar wind events.