Evidence of persistence of weak magnetic cycles driven by meridional plasma flows during solar grand minima phases

TL;DR

This study uses a flux transport dynamo model to show that weak magnetic cycles driven by meridional plasma flows persist during solar grand minima, with characteristic frequency modulations and a decayed polar field.

Contribution

It demonstrates that periodic magnetic activity and amplitude modulation continue during grand minima, driven by meridional flow speed, even when active region emergence ceases.

Findings

Weak magnetic activity persists during minima.

Periodic polar field modulation continues during grand minima.

Meridional flow speed governs cycle persistence and frequency.

Abstract

Long-term sunspot observations and solar activity reconstructions reveal that the Sun occasionally slips into quiescent phases known as solar grand minima, the dynamics during which is not well understood. We use a flux transport dynamo model with stochastic fluctuations in the mean-field and Babcock-Leighton poloidal field source terms to simulate solar cycle variability. Our long-term simulations detect a gradual decay of the polar field during solar grand minima episodes. Although regular active region emergence stops, compromising the Babcock-Leighton mechanism, weak magnetic activity continues during minima phases sustained by a mean-field $\alpha$-effect; surprisingly, periodic polar field amplitude modulation persist during these phases. A spectral analysis of the simulated polar flux time series shows that the 11-year cycle becomes less prominent while high frequency periods and periods around 22 years manifest during grand minima episodes. Analysis of long-term solar open flux observations appears to be consistent with this finding. Through numerical experimentation we demonstrate that the persistence of periodic amplitude modulation in the polar field and the dominant frequencies during grand minima episodes are governed by the speed of the meridional plasma flow -- which appears to act as a clock.