Nature’s Third Cycle: A Story of Sunspots, Arnab Rai Choudhuri, Oxford U. Press, 2015. $39.95 (281 pp.). ISBN 978-0-19-967475-6 Buy at Amazon
The cyclic magnetic activity of the Sun is among the most intriguing phenomena in the universe. It results from complex interactions in a magnetized, turbulent plasma that cover a wide range of spatial and temporal scales. Solar activity is usually associated with an 11-year variation in the sunspot number and in strong, highly energetic events—for example, coronal mass ejections and flares—that release energy equivalent to a billion 10-megaton hydrogen bombs. The resulting flux of high-energy particles interacts with Earth’s magnetosphere, causing auroras and geomagnetic storms that can disrupt electrical power systems, navigation systems, satellites, and more.
In Nature’s Third Cycle: A Story of Sunspots, solar physicist Arnab Rai Choudhuri presents the history of empirical and theoretical studies of global solar variations and discusses the influence of that activity on Earth’s environment. Choudhuri’s captivating narrative starts with the massive power blackout of 13 March 1989 in Quebec, an event caused by a strong solar flare from a few days earlier. Such strong solar eruptions are associated with sunspots, clusters of which form active regions with topologically complicated magnetic field structures.
The author details such historical highlights as the discovery of the 11-year solar cycle and makes a brief excursion into the solar interior’s dynamics and evolution. His simple description of a turbulent plasma in a magnetic field serves as an introduction to the deep theoretical studies of magnetic field generation and to Eugene Parker, Horace Babcock, and Robert Leighton, the scientists who developed the foundation of solar dynamo theory.
The long record of solar observations shows that the duration and strength of solar cycles have varied over time. Our understanding of the solar cycle is determined by the ability of theoretical models to make reliable forecasts, and until recently, available models failed to correctly predict either a cycle’s amplitude or the time of the next solar maximum. During the past 10 years, however, a new approach has emerged: Realizing that our knowledge of the Sun’s interior is still incomplete, solar physicists are combining theoretical modeling with observational data.
Although Choudhuri does not detail the new “data assimilation” approach in the book, he has previously suggested a weather forecasting scheme that combines observational data with a type of Babcock–Leighton mean-field dynamo model. Because of the known correlation between the strength of global magnetic fields during a solar minimum and during the next solar maximum, the magnetic field forecast is updated once per solar cycle, at the solar minimum, by means of an empirically determined dipole magnetic index. The assimilation of that data with the dynamo model enabled physicists to correctly predict the observed amplitude of the current solar cycle, which began in 2008 and is the 24th since extensive recording of sunspot activity began in 1755.
Nature’s Third Cycle includes approaches to model the solar cycles and their predictions using rigorous mathematical methods that are briefly described in the appendices. The next step is to incorporate nonlinear magnetohydrodynamic models and data assimilation; the new prediction models that result may be considered as an ensemble of possible states sequentially adjusted with observational data. In my recent work with Alexander Kosovichev, we have applied that approach to describe the observed asymmetry of solar cycles, which grow faster than they decay (the so-called Waldmeier effect). Our initial results are in good agreement with the actual observed evolution of the current solar cycle—the good agreement suggests the possibility of making reliable predictions at least seven years forward after a solar minimum.
I strongly recommend Nature’s Third Cycle to readers who are interested in learning about solar activity, its effects on Earth, and the history of the field. It includes technical details and references for delving deeper into a fascinating topic, yet it is easy and enjoyable to read.
Irina Kitiashvili is a research scientist at NASA’s Ames Research Center at Moffett Field, California. Her research interests include developing numerical simulations of solar and stellar dynamics and magnetism based on first-principle physics modeling and analysis of observational spectropolarimetric data.
