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A History of Solar Activity Over Millennia

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A History of Solar Activity Over Millennia Living Rev. Sol. Phys. (2017) 14:3 DOI 10.1007/s41116-017-0006-9 REVIEW ARTICLE A history of solar activity over millennia Ilya G. Usoskin1 Received: 6 September 2016 / Accepted: 3 February 2017 © The Author(s) 2017. This article is published with open access at Springerlink.com Abstract Presented here is a review of present knowledge of the long-term behavior of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the special indices used to quantify different aspects of variable solar activity, with special emphasis upon sunspot number. Over long timescales, quantitative information about past solar activity can only be obtained using a method based upon indirect proxies, such as the cosmogenic isotopes 14C and 10Be in natural stratified archives (e.g., tree rings or ice cores). We give an historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that this method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene. A separate section is devoted to reconstructions of strong solar energetic-particle (SEP) events in the past, that suggest that the present-day average SEP flux is broadly consistent with estimates on longer timescales, and that the occurrence of extra-strong events is unlikely. Finally, the main features of the long-term evolution of solar magnetic activity, including the This article is a revised version of http://dx.doi.org/10.12942/lrsp-2013-1. Change summary Major revision, updated and expanded. Change details Several major updates have been made, in particular, about recent updates of the sunspot number series, new multi-millennial solar activity reconstructions, including long-period cycles, Grand minima and maxima of solar activity, the extreme SEP even of 775 AD, about nitrate in ice cores as a possible index of solar variability. The number of references has increased from 401 to 474. B Ilya G. Usoskin ilya.usoskin@oulu.fi 1 Sodankylä Geophysical Observatory (Oulu Unit), University of Oulu, 90014 Oulu, Finland 123 3 Page 2 of 97 Living Rev. Sol. Phys. (2017) 14:3 statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed. Keywords Solar activity · Paleo-astrophysics · Cosmogenic isotopes · Solar- terrestrial relations · Solar physics · Long-term reconstructions · Solar dynamo Contents 1 Introduction ............................................... 2 Solar activity: concept and observations ................................. 2.1 The concept of solar activity .................................... 2.2 Indices of solar activity ....................................... 2.2.1 Direct solar indices ...................................... Wolf (WSN) and International (ISN) sunspot number series .................... Group sunspot number (GSN) series ................................ Updates of the series ........................................ Other indices ............................................ 2.2.2 Indirect indices ........................................ 2.3 Solar activity observations in the pre-telescopic epoch ....................... 2.3.1 Instrumental observations: camera obscura ......................... 2.3.2 Naked-eye observations ................................... 2.3.3 Mathematical/statistical extrapolations ............................ 2.4 The solar cycle and its variations .................................. 2.4.1 Quasi-periodicities ...................................... 11-years Schwabe cycle ....................................... Phase catastrophe? ......................................... Centennial Gleissberg cycle ..................................... 210-years Suess/de Vries cycle ................................... Millennial Eddy cycle ........................................ ≈2400-years Hallstatt cycle ..................................... 2.4.2 Randomness versus regularity ................................ 2.4.3 A note on solar activity predictions ............................. 2.5 Summary .............................................. 3 The proxy method of past solar-activity reconstruction ......................... 3.1 The physical basis of the method .................................. 3.1.1 Heliospheric modulation of cosmic rays ........................... 3.1.2 Geomagnetic shielding .................................... 3.1.3 Cosmic-ray–induced atmospheric cascade .......................... 3.1.4 Transport and deposition ................................... 3.2 Radioisotope 14C .......................................... 3.2.1 Measurements ........................................ 3.2.2 Production .......................................... 3.2.3 Transport and deposition ................................... 3.2.4 The Suess effect and nuclear bomb tests ........................... 3.2.5 The effect of the geomagnetic field .............................. 3.3 Cosmogenic isotope 10Be ...................................... 3.3.1 Measurements ........................................ 3.3.2 Production .......................................... 3.3.3 Atmospheric transport .................................... 3.3.4 Effect of the geomagnetic field ................................ 3.4 Other potential proxy ........................................ 3.5 Towards a quantitative physical model ............................... 3.5.1 Regression models ...................................... 3.5.2 Reconstruction of heliospheric parameters .......................... 3.5.3 A link to sunspot numbers .................................. 123 Living Rev. Sol. Phys. (2017) 14:3 Page 3 of 97 3 3.6 Solar activity reconstructions .................................... 3.7 Verification of reconstructions ................................... 3.7.1 Comparison with direct data ................................. 3.7.2 Meteorites and lunar rocks: A direct probe of the galactic cosmic-ray flux ......... 3.7.3 Comparison between isotopes ................................ 3.8 Composite reconstruction ...................................... 3.9 Summary .............................................. 4 Variability of solar activity over millennia ................................ 4.1 Quasi-periodicities and characteristic times ............................ 4.2 Grand minima of solar activity ................................... 4.2.1 The Maunder minimum ................................... 4.2.2 Grand minima on a multi-millennial timescale ........................ 4.3 Grand maxima of solar activity ................................... 4.3.1 The modern episode of active sun .............................. 4.3.2 Grand maxima on a multi-millennial timescale ....................... 4.4 Related implications ......................................... 4.4.1 Theoretical constrains .................................... 4.4.2 Solar-terrestrial relations ................................... 4.5 Summary .............................................. 5 Solar energetic particles in the past ................................... 5.1 Cosmogenic isotopes ........................................ 5.1.1 The event of 775 AD: the worst case scenario? ....................... 5.1.2 Occurrence rate ........................................ 5.2 Lunar and meteoritic rocks ..................................... 5.3 Nitrates in polar ice ......................................... 5.4 Summary .............................................. 6 Conclusions ............................................... References .................................................. 1 Introduction The concept of the perfectness and constancy of the sun, postulated by Aristotle, was a strong belief for centuries and an official doctrine of Christian and Muslim countries. However, as people had noticed already before the time of Aristotle, some slight tran- sient changes of the sun can be observed even with the naked eye. Although scientists knew about the existence of “imperfect” spots on the sun since the early seventeenth century, it was only in the nineteenth century that the scientific community recognized that solar activity varies in the course of an 11-year solar cycle. Solar variability was later found to have many different manifestations, including the fact that the “solar constant”, or the total solar irradiance, TSI, (the amount of total incoming solar elec- tromagnetic radiation in all wavelengths per unit area at the top of the atmosphere) is not a constant. The sun appears much more complicated and active than a static hot plasma ball, with a great variety of nonstationary active processes going beyond the adiabatic equilibrium foreseen in the basic theory of sun-as-star. Such transient nonstationary (often eruptive) processes can be broadly regarded as solar activity, in contrast to the so-called “quiet” sun. Solar activity includes active transient and long-lived phenomena on the solar surface, such as spectacular solar flares, sunspots, prominences, coronal mass ejections (CMEs), etc. The very fact of the existence of solar activity poses an enigma for solar physics, leading to the development of sophisticated models of an upper layer known as the convection zone and the solar corona.
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