Heliospheric plasma sheet inflation as a cause of solar wind anomaly during the solar cycle 23-24 minimum

Abstract

The unusual low solar wind density and weak magnetic field observed at the near-Earth orbit during the 2007-2009 solar minimum years has been an actively studied subject. Our previous numerical simulation suggested that an inflation of the heliosphere current/plasma sheet (HCS/HPS) could be the cause of the density and field anomaly [1]. The inflation of HCS/HPS hypothesizes that HCS/HPS is thicker if Sun’s polar magnetic field is weaker and thinner if Sun’s polar magnetic field is stronger. In this study, we provide additional evidence to support this hypothesis using photosphere magnetographs from the ground-based stations (NSO, MWO or WMO). Specifically, we compute and compare average values of the solar magnetic field for two different periods: 1995-1997 and 2007-2009. We also integrate the magnetic field over the equatorial (< 7.5°), the polar (47.5°-87.5°), and the mid-latitude (12.5°-42.5°) regions. It is found that at 2.5 solar radii the averaged radial-component of the magnetic field (Br) is smaller in the 2007-2009 period than in the 1995-1997 period by ∼33% in the polar region, ∼36% in the mid-latitude region, and ∼11% in the equatorial region. When moving outward with the solar wind, the relatively large magnetic field at the equator may inflate the HCS/HPS at a farther distance causing the unusually low solar wind density and weak magnetic field at the equator during 2007-2009.