Extremely Slow (V sw < 300 km s−1) Solar Winds (ESSWs) at 1 au: Causes of Extreme Geomagnetic Quiet at Earth

Abstract

A search for extremely slow (V sw < 300 km s−1) solar winds (ESSWs) at 1 au has been conducted using hourly average solar wind data from 1963 through 2021. 297 ESSW events were identified with an average duration of ∼2.0 ± 1.4 days. The lowest speed detected was 156 km s−1. Ten of the lowest-speed events were analyzed in detail. It was found that all 10 events were located at the ends of high-speed solar wind (HSSW) streams. The termination of the ESSWs was caused by high-density plasmas, either a corotating interaction region associated with the next HSSW stream or a heliospheric plasma sheet encounter. There was a greater occurrence of ESSW events in solar cycles 23 and 24 than in previous solar cycles. This phenomenon is associated with the much lower solar activity during these two cycles. The decrease in solar polar open magnetic fields was accompanied by an increase in low and midlatitude open magnetic fields. The ESSWs were accompanied by low interplanetary magnetic fields (4.4 ± 2.1 nT) and low negative interplanetary B z fields (−1.7 ± 1.5 nT), which led to extreme geomagnetic quiet: AE = 67 ± 78 nT and Dst = 2.2 ± 9.9 nT. We encourage magnetospheric researchers to use ESSW events to better understand the ground states of the magnetosphere and ionosphere.