The solar cycle, or the solar magnetic activity cycle, is the main source of periodic variation of all solar phenomena driving variations in space weather. Powered by a hydromagnetic dynamo process driven by the inductive action of internal solar flows, the solar cycle
The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today.
The average duration of the sunspot cycle is about 11 years (about 28 cycles in the 309 years between 1699 and 2008), but cycles as short as 9 years and as long as 14 years have been observed. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.
Following the numbering scheme established by Wolf, the 1755-1766 cycle is traditionally numbered "1". The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing data, and coincides with the Little Ice Age. This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer. In the second half of the nineteenth century it was also noted (independently) by Richard Carrington and by Spörer that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by the husband-wife team of E. Walter and Annie Maunder in the early twentieth century (see Figure 2). Images of the sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-latitude diagram.
The physical basis of the solar cycle was elucidated in the early twentieth century by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (this was the first detection of magnetic fields outside the Earth), and in 1919 went on to show that the magnetic polarity of sunspot pairs:
Hale's observations revealed that the solar cycle is a magnetic cycle with an average duration of 22 years. However, because very nearly all manifestations of the solar cycle are insensitive to magnetic polarity, it remains common usage to speak of the "11-year solar cycle".
Half a century later, the father-and-son team of Harold Babcock and Horace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order a dipole; and that this dipole also undergoes polarity reversals with the same period as the sunspot cycle (see Fig. 3 below). These various observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the sun as a whole.
The sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to a host of phenomena collectively known as solar activity. All of solar activity is strongly modulated by the solar magnetic cycle, since the latter serves as the energy source and dynamical engine for the former.
The dipolar component of the solar magnetic field is observed to reverse polarity around the time of solar maximum, and reaches peak strength at the time of solar minimum. Sunspots, on the other hand, are produced from a strong toroidal (longitudinally-directed) magnetic field within the solar interior. Physically, the solar cycle can be thought of as a regenerative loop where the toroidal component produces a poloidal field, which later produces a new toroidal component of sign such as to reverse the polarity of the original toroidal field, which then produces a new poloidal component of reversed polarity, and so on.
The total solar irradiance (TSI) is the amount of solar radiative energy impinging on the Earth's upper atmosphere. It is observed to vary in phase with the solar cycle, with yearly averages going from 1365.5 Watt per square meter at solar minimum, up to 1366.6 at solar maximum, with fluctuations about the means of about +/- 1 Watt per square meter on timescales of a few days (see Figure 4, yellow and red curves). The min-to-max variation, at the 0.1% level, is far too small to affect Earth's climate directly, but it is worth keeping in mind that continuous reliable measurements of the TSI are only available since 1978; the minimum and maximum levels of solar activity have remained roughly the same from then to now, spanning cycle 21 through 23.
Interestingly, the Sun is slightly brighter at solar maximum, even though sunspots are darker than the rest of the solar photosphere. This is because at solar maximum, a great many magnetized structures other than sunspots appear on the solar surface and many of them, such as faculae and active elements of the network, are brighter than the photosphere. They collectively end up slightly overcompensating for the overall irradiance deficit associated with the larger but less numerous sunspots. Recent observations indicate that the primary driver of TSI changes is the varying photospheric coverage of these different types of solar magnetic structures, although contributions from long-timescale variations associated with a deep-seated physical process, such as cycle-mediated small changes in the efficiency of convective energy transport, cannot be ruled out entirely as yet.
With a temperature of 5870 kelvin, the unmagnetized regions of the Sun's atmosphere emit very little short-wave radiation, such as extreme ultraviolet (EUV) and X-Rays. However, magnetized regions emit more short-wave radiation. Since surface coverage of magnetic structures varies markedly in the course of the cycle, the level of diffuse, non-flaring solar UV, EUV and X-Ray flux varies accordingly.
Figure 5 illustrates this variation for soft X-Ray, as observed by the Japanese satellite YOHKOH. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the SOHO or TRACE satellites.
Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-Ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.
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. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere.
It has been proposed that 10.7 cm solar flux can interfere with point-to-point terrestrial communications.
The occurrence frequency of coronal mass ejections and flares is strongly modulated by the solar activity cycle. Flares of any given size are some 50 times more frequent at solar maximum than at minimum. Large coronal mass ejections occur on average a few times a day at solar maximum, down to one every few days at solar minimum The size of these events themselves does not depend sensitively on the phase of the solar cycle. A good recent case in point are the three large X-class flares having occurred in December 2006, very near solar minimum; one of these (an X9.0 flare on Dec 5) stands as one of the brightest on record.
Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents to cause occasionally nuclear spallation reactions. Some of the fission products include radionuclides such as 14C and 10Be, which settle down on Earth's surface. Their concentration can be measured in ice cores, allowing a reconstruction of solar activity levels into the distant past. Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that Maunder minimum-like epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.