Direct observations over the past four centuries1 show that the number of sunspots observed on the Sun’s surface varies periodically, going through successive maxima and minima. Following sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field2,3 and an unusually large number of days without sunspots4. Sunspots are strongly magnetized regions5 generated by a dynamo mechanism6 that recreates the solar polar field mediated through plasma flows7. Here we report results from kinematic dynamo simulations which demonstrate that a fast meridional flow in the first half of a cycle, followed by a slower flow in the second half, reproduces both characteristics of the minimum of sunspot cycle 23. Our model predicts that, in general, very deep minima are associated with weak polar fields. Sunspots govern the solar radiative energy8,9 and radio flux, and, in conjunction with the polar field, modulate the solar wind, the heliospheric open flux and, consequently, the cosmic ray flux at Earth3,10,11.
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This work was supported through the Ramanujan Fellowship of the Government of India at the Indian Institute of Science Education and Research, Kolkata, and by a NASA Living With a Star grant to the Smithsonian Astrophysical Observatory and Montana State University. We are grateful to D. Hathaway for providing the observational data on days without sunspots; the analysis of these data is reported in Supplementary Fig. 1.
The authors declare no competing financial interests.
This file contains Supplementary Text and Data comprising a description of the observed characteristics of the minimum of solar cycle 23, details of the dynamo model used for the simulations, an analysis of sensitivity of results to parameter changes and a note on solar plasma flow observations, Supplementary Figures 1-4 with legends, Supplementary Tables 1-3 and additional references. (PDF 311 kb)
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Nandy, D., Muñoz-Jaramillo, A. & Martens, P. The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations. Nature 471, 80–82 (2011). https://doi.org/10.1038/nature09786
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