The solar cycle is the main source of periodic solar variation on Earth which drives variations in space weather and to some degree weather on the ground and possibly climate change.
See also Sunspots
The solar cycle, or the solar magnetic activity cycle, is the dynamical engine and energy source behind all solar phenomena driving space weather. Powered by a hydromagnetic dynamo process relying on the inductive action of internal solar flows, the solar cycle
- structures the sun's atmosphere, corona and wind;
- modulates the solar irradiance;
- modulates the flux of short-wavelength solar radiation, from ultraviolet to X-Ray;
- modulates the occurrence frequency of flares, coronal mass ejections, and other geoeffective solar eruptive phenomena;
- indirectly modulates the flux of high-energy galactic cosmic rays entering the solar system.
The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of diligent observations of the sun noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Very much impressed by Schwabe's discovery, Rudolf Wolf
compiled and studied earlier observations, and managed to reconstruct
the cycle back to 1745, eventually pushing these reconstructions to the
earliest observations of sunspots by Galileo and contemporaries in the
opening decades of the seventeenth century. Because sunspots come in
many sizes and different levels of grouping, 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 11.1 years, 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. 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 Spoerer 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. 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
- is always the same in a given solar hemisphere throughout a given sunspot cycle;
- is opposite across hemispheres throughout a cycle;
- reverses itself in both hemispheres from one sunspot cycle to the next.
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
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. These various observations
established that the solar cycle is a spatiotemporal magnetic process
unfolding over the sun as a whole.
Impacts of the solar cycle
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 leads 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.
Sunspots may exist anywhere from a few days to a few months, but
they eventually decay, and this releases magnetic flux in the solar
photosphere. This magnetic field is dispersed and churned by turbulent
convection, and solar large-scale flows. These transport mechanisms
lead to the accumulation of the magnetized decay products at high solar
latitudes, eventually reversing the polarity of the polar fields.
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 of 1366.6 at solar
maximum, with fluctuations about the means of about +/- 1 Watt per
square meter on timescales of a few days. The min-to-max variation, at the 0.1% level, is far too small
to affect Earth's climate, 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 been
remained roughly the same from then to now, spanning cycle 21 through
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 5870 kelvins,
the unmagnetized regions of the Sun's atmosphere emit very little
short-wavelength radiation, such as extreme ultraviolet (EUV) and
X-Rays. This is no longer the case where magnetic fields are present,
magnetized regions being observed to emit short-wavelength 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 is modulated accordingly Figure 5 illustrates this variations 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 the total
solar irradiance, the impact of solar UV, EUV and X-Ray radiation on
the Earth's upper atmosphere are profound. The solar UV flux is a major
driver of stratospheric chemistry, and the ionosphere responds strongly to increases in ionizing radiation, through heating and changes in electrical conductivity.
Solar radio flux
Emission from the Sun at centimetric (radio) wavelength is due
primarily to coronal plasma trapped in the magnetic fields overlying
active regions .
The F10.7 index is a measure of the solar radio flux per unit frequency
at a wavelength of 10.7cm, near the peak of the observed solar radio
emission. It represents a measure of diffuse, nonradiative heating of
the coronal plasma trapped by magnetic fields over active regions, and
is an excellent indicator of overall solar activity levels. The solar
F10.7 cm record extends back to 1947, and is the longest direct record
of solar activity available, other than sunspot-related quantities.
It has been proposed that 10.7 cm solar flux can interfere with
point-to-point terrestrial communications. "The Effect of 10.7 cm Solar
Radiation on 2.4GHz Digital Spread Spectrum Communications", NARTE
News, Volume 17 Number 3 July - October 1999.
Radio Communications Interference
Solar flares also create a wide spectrum of radio noise; at VHF (and
under unusual conditions at HF) this noise may interfere directly with
a wanted signal. The frequency with which a radio operator experiences
solar flare effects will vary with the approximately 11-year sunspot
cycle; more effects occur during solar maximum (when flare occurrence
is high) than during solar minimum (when flare occurrence is very low).
A radio operator can experience great difficulty in transmitting or
receiving signals during solar flares due to more noise and different
Geoeffective eruptive phenomena
The solar magnetic field structures the corona, giving it its
characteristic shape visible at times of solar eclipses. Complex
coronal magnetic field structures evolve in response to fluid motions
at the solar surface, and emergence of magnetic flux produced by dynamo
action in the solar interior. For reasons not yet understood in detail,
sometimes these structures lose stability, leading to coronal mass ejections into interplanetary space, or flares,
caused by sudden localized release of magnetic energy driving copious
emission of ultraviolet and X-ray radiation as well as energetic
particles. These eruptive phenomena can have a significant impact on
Earth's upper atmosphere and space environment, and are the primary
drivers of what is now called space weather.
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
Cosmic ray flux
The outward expansion of solar ejecta into interplanetary space
provides overdensities of plasma that are efficient at scattering
high-energy cosmic rays
entering the solar system from elsewhere in the galaxy. Since the
frequency of solar eruptive events is strongly modulated by the solar
cycle, the degree of cosmic ray scattering in the outer solar system
varies in step. As a consequence, the cosmic ray flux in the inner
solar system is anticorrelated with the overall level of solar
activity. This anticorrelation is clearly detected in cosmic ray flux
measurements at the Earth's surface.
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.
Impact on Biosphere and human circadian cycle
The impact of Solar cycle on living organisms is covered in part by interdisciplinary studies in the fields of science known as Chronobiology, Heliobiology, and Astrobiology. In 1924 Alexander Chizhevsky, graduate of Medical School at Moscow University,
published interdisciplinary works: "Physical factors behind the process
of history" and "Epidemiological catastrophes and periodic activity of
the Sun" studying cycles in living organisms in connections with solar cycle and cycle of lunar phases. Chizhevsky developed a new discipline, Heliobiology, a branch of Astrobiology. In 1939 Chizhevsky was elected Honorary President of International Congress in Biological Physics, for his 1936 publication The Terrestrial Echo of Solar Storms, 366 pp. 1976, Moscow, (First published in 1936 in Russian. However, soon Chizhevsky was arrested by the Soviet government and exiled to Siberia under the dictatorship of joseph Stalin. Chizhevsky's publications were censored and his 1930s research of blood and electromagnetic parameters of erythrocytes
in connection with cycles in human circadian system was banned, it was
published 40 years later, in 1973. Chizhevsky's 1928 publication
"Influence of Cosmos on human psychoses" was censored in the Soviet
Union, albeit in 2003 this work was referenced in Journal of Circadian Rhythms article.
List of Solar Cycles
Here is the list of Solar cycles (or sunspot cycles), tracked since 1755:
- March 1755 - June 1766
- June 1766 - June 1775
- June 1775 - September 1784
- September 1784 - May 1798
- May 1798 - December 1810
- December 1810 - May 1823
- May 1823 - November 1833
- November 1833 - July 1843
- July 1843 - December 1855
- December 1855 - March 1867
- March 1867 - December 1878
- December 1878 - March 1890
- March 1890 - February 1902
- February 1902 - August 1913
- August 1913 - August 1923
- August 1923 - September 1933
- September 1933 - February 1944
- February 1944 - April 1954
- April 1954 - October 1964
- October 1964 - June 1976
- June 1976 - September 1986
- September 1986 - May 1996
- May 1996 - (expected to end in 2007)
- Future cycle
Note: A new cycle starts by definition, when the first sunspots show up at high latitudes.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)