Global Solar Magnetic-field and Interplanetary Scintillations During the Past Four Solar Cycles

TL;DR

This study analyzes the decline in solar magnetic fields and interplanetary scintillation over four solar cycles, revealing a significant correlation between magnetic field weakening and turbulence reduction in the solar corona and wind.

Contribution

It provides a comprehensive analysis of the long-term decline in solar magnetic fields and scintillation indices using PFSS extrapolations and IPS data from 1975 to 2018, highlighting their interrelationship.

Findings

Magnetic fields at 2.5 and 10 solar radii decreased by approximately 11.3-22.2%.

Normalized scintillation index declined by about 23.6%.

The decline indicates a strong link between magnetic field weakening and turbulence reduction.

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 $\rm R_{\odot}$ and 10 $\rm R_{\odot}$(extrapolated using PFSS method) has decreased by $\approx 11.3-22.2 \%$. In phase with the declining magnetic-fields, the quantity m also declined by $\approx 23.6 \%$. These observations emphasize the inter-relationship between the global magnetic-field and various turbulence parameters in the solar corona and solar wind.