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

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.