Solar cycle

Solar activity, driven by both the solar cycle and transient aperiodic processes, governs the environment of interplanetary space by creating space weather and impacting space- and ground-based technologies as well as the Earth's atmosphere and also possibly climate fluctuations on scales of centuries and longer.

The idea of a cyclical solar cycle was first hypothesized by Christian Horrebow based on his regular observations of sunspots made between 1761 and 1776 from the Rundetaarn observatory in Copenhagen, Denmark.

In 1852, Rudolf Wolf designated the first numbered solar cycle to have started in February 1755 based on Schwabe's and other observations.

[8] In 1961 the father-and-son team of Harold and Horace Babcock established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.

Horace's Babcock Model described the Sun's oscillatory magnetic field as having a quasi-steady periodicity of 22 years.

[21] Several predictions have been made for solar cycle 25[22] based on different methods, ranging from very weak to strong magnitude.

[35] The best information today comes from SOHO (a cooperative project of the European Space Agency and NASA), such as the MDI magnetogram, where the solar "surface" magnetic field can be seen.

The evolution of plage areas is typically tracked from solar observations in the Ca II K line (393.37 nm).

[36] The amount of facula and plage area varies in phase with the solar cycle, and they are more abundant than sunspots by approximately an order of magnitude.

For reasons not yet understood in detail, sometimes these structures lose stability, leading to solar flares and coronal mass ejections (CME).

Flares and CME are caused by sudden localized release of magnetic energy, which drives emission of ultraviolet and X-ray radiation as well as energetic particles.

[8][48][49][50] As pioneered by Ilya G. Usoskin and Sami Solanki, associated centennial variations in magnetic fields in the corona and heliosphere have been detected using carbon-14 and beryllium-10 cosmogenic isotopes stored in terrestrial reservoirs such as ice sheets and tree rings[51] and by using historic observations of geomagnetic storm activity, which bridge the time gap between the end of the usable cosmogenic isotope data and the start of modern satellite data.

[52] These variations have been successfully reproduced using models that employ magnetic flux continuity equations and observed sunspot numbers to quantify the emergence of magnetic flux from the top of the solar atmosphere and into the heliosphere,[53] showing that sunspot observations, geomagnetic activity and cosmogenic isotopes offer a convergent understanding of solar activity variations.

CMEs (coronal mass ejections) produce a radiation flux of high-energy protons, sometimes known as solar cosmic rays.

[63] The increased irradiance during solar maximum expands the envelope of the Earth's atmosphere, causing low-orbiting space debris to re-enter more quickly.

Their concentration can be measured in tree trunks or ice cores, allowing a reconstruction of solar activity levels into the distant past.

This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the "bright" network, that are brighter (hotter) than the average photosphere.

Since the upper atmosphere is not homogeneous and contains significant magnetic structure, the solar ultraviolet (UV), EUV and X-ray flux varies markedly over the cycle.

Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.

Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions.

Sunspot activity has a major effect on long distance radio communications, particularly on the shortwave bands although medium wave and low VHF frequencies are also affected.

[83] Speculations about the effects of cosmic-ray changes over the cycle potentially include: Later papers showed that production of clouds via cosmic rays could not be explained by nucleation particles.

[96] The amount of ultraviolet UVB light at 300 nm reaching the Earth's surface varies by a few percent over the solar cycle due to variations in the protective ozone layer.

Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators and shortwave broadcasters.

[106] Eleven-year cycles have been found in tree-ring thicknesses[14] and layers at the bottom of a lake[15] hundreds of millions of years ago.

The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon described by Hale's law.

The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change in the polarity of the Sun's large-scale magnetic field.

[116] Although the tachocline has long been thought to be the key to generating the Sun's large-scale magnetic field, recent research has questioned this assumption.

[117] A 2012 paper proposed that the torque exerted by the planets on a non-spherical tachocline layer deep in the Sun may synchronize the solar dynamo.

[23] In 1974 the book The Jupiter Effect suggested that the alignment of the planets would alter the Sun's solar wind and, in turn, Earth's weather, culminating in multiple catastrophes on March 10, 1982.

Line graph showing historical sunspot number count, Maunder and Dalton minima, and the Modern Maximum
400 year sunspot history, including the Maunder Minimum
"The prediction for solar cycle 24 gave a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013 so the official maximum was at least that high. The smoothed sunspot number rose again towards this second peak over the last five months of 2016 and surpassed the level of the first peak (66.9 in February 2012). Many cycles are double peaked but this is the first in which the second peak in sunspot number was larger than the first. This was over five years into cycle 24. The predicted and observed size made this the smallest sunspot cycle since cycle 14 which had a maximum of 64.2 in February of 1906." [ 1 ]
Evolution of magnetism on the Sun
Reconstruction of solar activity over 11,400 years
Solar activity events recorded in radiocarbon. Present period is on right. Values since 1900 not shown.
A drawing of a sunspot in the Chronicles of John of Worcester , ca. 1100 [ 33 ]
This version of the sunspot butterfly diagram was constructed by the solar group at NASA Marshall Space Flight Center. The newest version can be found at [ http://solarcyclescience.com/solarcycle.html solarcyclescience.com
Time vs. solar latitude diagram of the radial component of the solar magnetic field, averaged over successive solar rotation. The "butterfly" signature of sunspots is clearly visible at low latitudes. Diagram constructed by the solar group at NASA Marshall Space Flight Center. The newest version can be found at [ http://solarcyclescience.com/solarcycle.html solarcyclescience.com
Solar plage area evolution over time
An overview of three solar cycles shows the relationship between the solar cycle, galactic cosmic rays, and the state of Earth's near-space environment. [ 44 ]
2,300 year Hallstatt solar variation cycles
Activity cycles 21, 22 and 23 seen in sunspot number index, TSI, 10.7cm radio flux, and flare index. The vertical scales for each quantity have been adjusted to permit overplotting on the same vertical axis as TSI. Temporal variations of all quantities are tightly locked in phase, but the degree of correlation in amplitudes is variable to some degree.
A solar cycle: a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a solar cycle, from after August 30, 1991, to September 6, 2001. Credit: the Yohkoh mission of ISAS (Japan) and NASA (US).