Global Solar Magnetic Field and Interplanetary Scintillations During the Past Four Solar Cycles

Abstract

The extended minimum of Solar Cycle 23, the extremely quiet solar-wind conditions prevailing and the mini-maximum of Solar Cycle 24 drew global attention and many authors have since attempted to predict the amplitude of the upcoming Solar Cycle 25, which is predicted to be the third successive weak cycle; it is a unique opportunity to probe the Sun during such quiet periods. Earlier work has established a steady decline, over two decades, in solar photospheric fields at latitudes above $45^{\circ}$ and a similar decline in solar-wind micro-turbulence levels as measured by interplanetary scintillation (IPS) observations. However, the relation between the photospheric magnetic fields and those in the low corona/solar-wind are not straightforward. Therefore, in the present article, we have used potential-field source-surface (PFSS) extrapolations to deduce global magnetic fields using synoptic magnetograms observed with National Solar Observatory (NSO), Kitt Peak, USA (NSO/KP) and Solar Optical Long-term Investigation of the Sun (NSO/SOLIS) instruments during 1975 – 2018. Furthermore, we have measured the normalized scintillation index [$m$] using the IPS observations carried out at the Institute of Space–Earth Environment Research (ISEE), Japan during 1983 – 2017. From these observations, we have found that, since the mid-1990s, the magnetic field over different latitudes at $2.5~\mathrm{R}_{\odot}$ and $10~\mathrm{R}_{\odot}$ (extrapolated using the PFSS method) has decreased by ${\approx}\,11.3\,\mbox{--}\,22.2\%$. In phase with the declining magnetic fields, the quantity $m$ also declined by ${\approx}\, 23.6\%$. These observations emphasize the inter-relationship among the global magnetic field and various turbulence parameters in the solar corona and solar-wind.