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.