A Twenty Year Decline in Solar Photospheric Magnetic Fields: Inner-Heliospheric Signatures and Possible Implications?

TL;DR

This study documents a 20-year decline in solar photospheric magnetic fields and related heliospheric signatures, predicting a continued decrease until 2020 and estimating a lower peak for solar Cycle 25.

Contribution

It provides a long-term analysis linking photospheric magnetic field decline with heliospheric magnetic field predictions and solar cycle forecasts.

Findings

20-year decline in photospheric magnetic fields since 1995

Predicted solar Cycle 25 peak sunspot number of approximately 62

Estimated heliospheric magnetic field floor value of around 3.2 nT

Abstract

We report observations of a steady 20 year decline of solar photospheric fields at latitudes $\geq$45${^{o}}$ starting from $\sim$1995. This prolonged and continuing decline, combined with the fact that Cycle 24 is already past its peak, implies that magnetic fields are likely to continue to decline until $\sim$2020, the expected minimum of the ongoing solar Cycle 24. In addition, interplanetary scintillation (IPS) observations of the inner heliosphere for the period 1983--2013 and in the distance range 0.2--0.8 AU, have also shown a similar and steady decline in solar wind micro-turbulence levels, in sync with the declining photospheric fields. Using the correlation between the polar field and heliospheric magnetic field (HMF) at solar minimum, we have estimated the value of the HMF in 2020 to be 3.9 ($\pm$0.6) and a floor value of the HMF of $\sim$3.2 ($\pm$0.4) nT. Given this floor value for the HMF, our analysis suggests that the estimated peak sunspot number for solar Cycle 25 is likely to be $\sim$ 62 ($\pm$12).