Letter to the Editor

Extreme fluctuations in the Sun’s activity over the Modern Maximum: Understanding the enigmatic solar cycles 19-20

¹ Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
² Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
³ Department of Technical Education, Training and Skill Development, Government of West Bengal, Kolkata, India

^⋆ Corresponding author: dnandi@iiserkol.ac.in

Received: 14 May 2025
Accepted: 11 July 2025

Abstract

Over the past century, the Sun has gone through a phase of enhanced activity known as the Modern Maximum. Notably, the strongest sunspot cycle on record during this period–and indeed since direct sunspot observations began–was cycle 19; this was followed by a significantly weaker cycle 20. Understanding and reconstructing this extreme variability has remained elusive. Utilizing data-driven, coupled models of magnetic field evolution on the Sun’s surface and within its convection zone, here we show that random deviations in the tilt angle and polarity orientation of bipolar sunspot pairs can explain these observed, extreme fluctuations. Our results demonstrate that perturbations in the Babcock-Leighton poloidal field source of the dynamo mechanism–mediated via the emergence of anomalous solar active regions–can adequately explain extreme variations observed over cycles 19-20 and century-scale trends that manifest as supradecadal envelops in sunspot time series. This study has implications for understanding how the Sun may transition between extreme, average, and quiescent low activity phases–such as the Maunder Minimum.