A Possible Cause of the Diminished Solar Wind During the Solar Cycle 23 – 24 Minimum

Abstract

Interplanetary magnetic field and solar wind plasma density observed at 1 AU during Solar Cycle 23 – 24 (SC-23/24) minimum were significantly smaller than those during its previous solar cycle (SC-22/23) minimum. Because the Earth’s orbit is embedded in the slow wind during solar minimum, changes in the geometry and/or content of the slow wind region (SWR) can have a direct influence on the solar wind parameters near the Earth. In this study, we analyze solar wind plasma and magnetic field data of hourly values acquired by Ulysses. It is found that the solar wind, when averaging over the first (1995.6 – 1995.8) and third (2006.9 – 2008.2) Ulysses’ perihelion ( ${\sim}\,1.4~\mbox{AU}$ ) crossings, was about the same speed, but significantly less dense ( ${\sim}\,34~\%$ ) and cooler ( ${\sim}\,20~\%$ ), and the total magnetic field was ${\sim}\,30~\%$ weaker during the third compared to the first crossing. It is also found that the SWR was ${\sim}\,50~\%$ wider in the third ( ${\sim}\,68.5^{\circ}$ in heliographic latitude) than in the first ( ${\sim}\,44.8^{\circ}$ ) solar orbit. The observed latitudinal increase in the SWR is sufficient to explain the excessive decline in the near-Earth solar wind density during the recent solar minimum without speculating that the total solar output may have been decreasing. The observed SWR inflation is also consistent with a cooler solar wind in the SC-23/24 than in the SC-22/23 minimum. Furthermore, the ratio of the high-to-low latitude photospheric magnetic field (or equatorward magnetic pressure force), as observed by the Mountain Wilson Observatory, is smaller during the third than the first Ulysses’ perihelion orbit. These findings suggest that the smaller equatorward magnetic pressure at the Sun may have led to the latitudinally-wider SRW observed by Ulysses in SC-23/24 minimum.