Evidence for distinct modes of solar activity^⋆

¹ Sodankylä Geophysical Observatory (Oulu unit) and Physics Dept., ReSoLVE Center of Excellence, University of Oulu, Finland
e-mail: Ilya.Usoskin@oulu.fi
² Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 75005 Paris, France
³ Climate and Environmental Physics, Physics Institute, and Oeschger Centre for Climate Change Research, University of Bern, Switzerland
⁴ Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia
⁵ IEPT Russian Academy of Sciences, 117997 Moscow, Russia

Received: 9 January 2014
Accepted: 6 February 2014

Abstract

Aims. The Sun shows strong variability in its magnetic activity, from Grand minima to Grand maxima, but the nature of the variability is not fully understood, mostly because of the insufficient length of the directly observed solar activity records and of uncertainties related to long-term reconstructions. Here we present a new adjustment-free reconstruction of solar activity over three millennia and study its different modes.

Methods. We present a new adjustment-free, physical reconstruction of solar activity over the past three millennia, using the latest verified carbon cycle, ¹⁴C production, and archeomagnetic field models. This great improvement allowed us to study different modes of solar activity at an unprecedented level of details.

Results. The distribution of solar activity is clearly bi-modal, implying the existence of distinct modes of activity. The main regular activity mode corresponds to moderate activity that varies in a relatively narrow band between sunspot numbers 20 and 67. The existence of a separate Grand minimum mode with reduced solar activity, which cannot be explained by random fluctuations of the regular mode, is confirmed at a high confidence level. The possible existence of a separate Grand maximum mode is also suggested, but the statistics is too low to reach a confident conclusion.

Conclusions. The Sun is shown to operate in distinct modes – a main general mode, a Grand minimum mode corresponding to an inactive Sun, and a possible Grand maximum mode corresponding to an unusually active Sun. These results provide important constraints for both dynamo models of Sun-like stars and investigations of possible solar influence on Earth’s climate.