# Stars/Sun/Solar cycles

The surface of the Sun (surface of the photosphere) undergoes a cycle of sunspot activity with a period around 11 years. The exact cause, or even approximate cause, of the sunspot, or solar cycle, is unknown.

## Colors

At right is a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001.

## Spörer's law

"For the greater part of the sun-spot period there is practically but one zone of spots in each hemisphere. The departure from this condition of things near or at the time of minimum, when the spots of the dying cycle are approaching the equator, and the forerunners of the new cycle are beginning to appear in high latitudes, is the only case in which the solar spots are distinctly separated into more than a single zone in each hemisphere." "Spöerer's law ... involves that in a minimum year the zone about 15° should be entirely barren".
"First of all there were only fifteen groups seen during the entire year [1901], north and south put together. Of these, seven were in the north, and the mean latitude for the north was 8.6°, exactly the latitude of one spot of the seven, and this very naturally, seeing that it was by far the greatest group of the year, the celebrated "eclipse group."" Bold added. "Greatest group" and "eclipse group" are both relative synonyms for "dominant group".
"[T]he spot-groups have been carefully examined for cases of return, and where it appeared clear that the same group has returned a second time or more frequently, without any temporary disappearance or subsidence, such a long-continued group has been treated as an entity throughout." Bold added. "It has been forgotten that, whatever the cause which produces this variation of rotation rate with latitude, the causes producing difference of rate within any given latitude are more effective still."
"[T]here is a slight retardation of the rotation period from the first cycle to the second, shown by both northern and southern hemispheres."

## Theoretical solar cycles

"The principal objective of this study is to invent a simple, quantitative, closed kinematical model of the solar cycle which is based as far as possible either on well-accepted physical effects or on observed facts (whether fully understood or not), in the hope that such a model might single out the most important factors for detailed study, and might suggest further directions for investigation."
"[T]he model describes ... the time variation of average sunspot latitudes (Spoerer's law) and the width of the eruption zone (Maunder's butterfly diagram)".
The conjunctions of Venus and Jupiter can occur at about opposite sides of Jupiter's orbital period of ~ 11 yrs around the Sun.

## Flip-flop cycles

Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. Flip-flop phenomena are observed for both binary RS Canum Venaticorum variable RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.

## Plasma objects

Def. the luminous plasma atmosphere of the Sun or other star, extending millions of kilometres into space, most easily seen during a total solar eclipse is called a corona, or stellar corona.
"Beginning with the daguerreotype of the corona of 1851, the Reverend Lecturer had thrown on the screen pictures illustrating the form of the corona in different years. The drawings of those of 1867, 1878, and 1900 all showed long equatorial extensions with openings at the solar poles filled with beautiful rays." "The intermediate years, as, for example, 1883, 1886, and 1896 showed the four groups of synclinals which mainly constitute the corona gradually descending towards the equator of the sun, with a corresponding opening of the polar regions."
"Some of the theories of the solar corona were then illustrated and discussed."
* "The corona is not of the nature of an atmosphere round the sun, for the pressure at the sun's limb would be enormous, while the thinness of the chromospheric lines show that it is not."
* "comets, such as that of 1843, have approached the sun with enormous velocities within the region of the prominences without suffering disruption or retardation."
* "If not an atmosphere of particles of gas, still less is it an atmosphere of solid stones or meteorites."
* "Meteor streams do circle round the sun, but there is no reason why the positions of the orbits, or the intrinsic brightness of such streams should vary with the sun-spot period."
* "the appearance of the corona does not seem to be such as the projection of meteor streams upon the celestial vault would give."
* "Prof. Schaeberle has proposed a mechanical origin of the solar corona, due to the forces of ejection of particles from the solar limb, as evidenced by the prominences, and the force of gravity under the particular conditions of the solar rotation and the inclination of its axis to the earth's orbit."
* "The electrical theory of the corona does not negative the mechanical theory, but supplements it. In addition to the forces of gravity and ejection, it takes account of the repulsive force which the sun exerts on matter which has the same electrical sign as itself, and which has been ejected from it."
* "it would seem that the solar corona is of the nature of an electrical aurora round the sun."
* "the coronoidal discharges in poor vacua obtained by Prof. Pupin about an insulated metal ball are exceedingly like the rays and streamers of the solar corona."
The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.
The sun's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. Inconsistencies in this wind and larger events on the surface of the star, such as coronal mass ejections, form a system that has features analogous to conventional weather systems (such as pressure and wind) and is generally known as space weather. Coronal mass ejections have been tracked as far out in the solar system as Saturn. The activity of this system can affect planetary atmospheres and occasionally surfaces. The interaction of the solar wind with the terrestrial atmosphere can produce spectacular aurorae, and can play havoc with electrically sensitive systems such as electricity grids and radio signals.

## Maxima

"The other component [of cosmic radiation] comes from the sun and is known as solar wind; it fluctuates with solar eruptions which produce large quantities of particles, mainly protons. The atomic number and energy value of particles of solar origin are generally lower than those of the galactic and extra-galactic component. Average solar activity occurs over an 11-year cycle (1-2)."
"The particles [of solar cosmic radiation] exercise widespread pressure on the magnetic field on the side exposed to the sun and produce a magnetospheric tail at the rear. This perturbation is considerable at high altitudes. At altitudes below a few earth radii (a radius is equal to 6370 km), the dipolar structure of the magnetic field predominates; this phenomenon explains the presence of polar cones centred around the magnetic poles where the magnetic field offers less resistance to incoming charged particles. There are belts of charged particles, known as Van Allen belts, corresponding to a drop in pressure of the magnetic field in which the charged particles are trapped. Generally speaking, there are two belts beyond a few hundred kilometres, depending on the type of particles. The first belt is made up of electrons and the second, larger belt of protons. The environment fluctuates with the flow of cosmic particles, particularly during solar eruptions. These perturbations, which can be considerable, may cause magnetic storms and the injection of particles into the belts close to the magnetic poles (1-2)."
The "lowest average dose rates for long-haul flights are observed on routes close to the equator and when solar activity is at a peak (minimum amount of cosmic radiation at ground level); for example, in the period 91-92 on the Paris-Buenos Aires flight, the average rate throughout the flight was around 3 μSv.h$^{-1}$."
"Sounding rockets played an important role in the International Geophysical Year (IGY), an 18-month period (1 July 1957 to 31 December 1958) coinciding with high solar activity. The IGY was an intensive investigation of the natural environment-the earth, the oceans, and the atmosphere-by 30 000 participants representing 66 nations. More than 300 instrumented sounding rockets launched from sites around the world made significant discoveries regarding the atmosphere, the ionosphere, cosmic radiation, auroras, and geomagnetism."
A list of solar cycles including maxima is available back to 1755 (245 b2k) 24 cycles.

## Minima

The image on the right shows the sunspot minimum around January 2009 and the most recent sunspot maximum around March 2015.
Solar activity minima tend to be correlated with colder temperatures on Earth.
In the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during this period. During this era, known as the Maunder minimum or Little Ice Age, Europe experienced unusually cold temperatures.
Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.
Near "or at the time of minimum, when the spots of the dying cycle are approaching the equator, and the forerunners of the new cycle are beginning to appear in high latitudes, is the only case in which the solar spots are distinctly separated into more than a single zone in each hemisphere."
In "a minimum year the zone about 15° should be entirely barren".
At "times corresponding to minimum of sun-spottedness, the polar diameter is relatively larger; that, at times of maximum sun-spottedness, the equatorial diameter is relatively larger. The amplitude of the variation is extremely small, but its reality would seem to be established. [This] at least renders the existence of such periodic fluctuations in the shape of the sun more probable than their non-existence."
A "fundamental change in the nature of the [sun’s magnetic] dynamo may be in progress."
"It’s backed up by NASA’s Solar Dynamics Observatory’s daily snaps, which have shown a spotless sun for 44 days in a row."
"Solar minimums are known to spark lots of cosmic ray activity that can penetrate our atmosphere."
"But when it happened back in 2008-2009, scientists suggested that climate change might be adding to the cooling and contracting in the upper layer of our atmosphere."
“This is not how it used to be and the rotation rate [of the sun] has slowed a bit at latitudes around about 60 degrees. We are not quite sure what the consequences of this will be but it’s clear that we are in unusual times. However, we are beginning to detect some features belonging to the next cycle and we can suggest that the next minimum will be in about two years."
"By the standard of spotless days, the ongoing solar minimum is the deepest in a century: NASA report. In 2008, no sunspots were observed on 266 of the year's 366 days (73%). To find a year with more blank suns, you have to go all the way back to 1913, which had 311 spotless days (85%)".
During "the previous minimum (around 2008), no less than 817 spotless days were recorded, whereas the minimum period leading into solar cycle 23 (around 1996 [solar cycle 22]) counted only 309 such blemishless days."
"One stable component [of cosmic radiation] is due to galactic and extra-galactic radiation; it comprises ions whose energy value can reach 10$^{20}$ electronvolts, averaging out at a few 10$^9$ electronvolts."
"The charged particles move around and interact with the interstellar magnetic field".
"Secondary particles (neutrons, ions, electrons, gamma rays, muons etc.) are produced by the break up of cosmic ions and atoms of interstellar and atmospheric gas."
"Because of the magnetic field and the atmosphere, only the most energetic ions, mainly those contained in galactic cosmic radiation, reach low altitudes where they interact. This galactic component is modulated by the solar wind outside the magnetosphere, being more heavily influenced when there is considerable solar activity. These phenomena explain why the flow of cosmic particles at ground level is lowest when solar activity is at a peak and vice versa."
"The magnetosphere and the atmosphere together form a powerful shield protecting us from cosmic rays. Without it, the dose received on the earth's surface would exceed 1 Sv.year$^{-1}$."
During "the period 1996-98 [solar minimum, the] maximum integrated dose, 150 μSv, is for the round trip Paris - Tokyo and San Francisco; for Buenos Aires, the longest flight, the dose is 30 % lower (100 μSv). The dose received for the round trip Paris - Washington (14.6 hours) is comparable to the one for New York with Concorde (7 hours); this comparison points out the effect of altitude (up to 18,000 metres for Concorde)."
"At higher latitudes and when the solar activity is lower, the values are higher; for the Paris-Tokyo flight passing over Siberia, the average dose rate measured in 97 was 6.6 μSv.h$^{-1}$. For the cargo flight between Tokyo and Paris passing over the North Pole with a stop at Fairbanks, the average dose rate was 5 μSv.h$^{-1}$. On polar routes at a given altitude, the cosmic radiation flow can generally be compared to that of Siberian routes and indeed beyond a geomagnetic latitude of 65°, it is taken as being constant. The measured value is lower than on Siberian routes because the average altitude is lower. As far as supersonic flights are concerned, the dose levels are far higher due to altitude (up to 18,000 metres); throughout the Paris-New York flight during a period of low solar activity, the average rate was approximately 9.5 μSv.h$^{-1}$."
"The International Years of the Quiet Sun (1 January 1964 to 31 December 1965), a full-scale follow-up to the IGY, was an intensive effort of geophysical observations in a period of minimum solar activity. Instrumented sounding rockets again played a significant role in the investigation of earth-sun interactions. By the end of 1974, some 20 countries had joined NASA in cooperative projects launching more than 1700 rockets from ranges in the United States and abroad."

## Grand solar minima

Solar minimum events and approximate dates :
| Event | Start | End |
| Homeric minimum | 950BC | 800BC |
| Oort minimum | 1040 | 1080 |
| Medieval maximum | 1100 | 1250 |
| Wolf minimum | 1280 | 1350 |
| Spörer Minimum | 1450 | 1550 |
| Maunder Minimum | 1645 | 1715 |
| Dalton Minimum | 1790 | 1820 |
| Glassberg Minimum | 1880 | 1914 |
| Modern Maximum | 1914 | 2007 |
A list of historical grand solar minima includes also Grand minima ca. 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC, 9170 BC.

## Space weather

The solar cycle has a great influence on space weather, and a significant influence on the Earth's climate since the Sun's luminosity has a direct relationship with magnetic activity. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during this period. During this era, known as the Maunder minimum or Little Ice Age, Europe experienced unusually cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.
"MOST current literature on solar activity assumes that the planets do not affect it, and that conditions internal to the Sun are primarily responsible for the solar cycle. Bigg$^1$, however, has shown that the period of Mercury's orbit appears in the sunspot data, and that the influence of Mercury depends on the phases of Venus, Earth, and Jupiter."
"It is shown that starting with the alignment of Venus with Jupiter at perihelion position, these two planets will perfectly align at Jupiter's perihelion after every 23.7 years".
"The tidal forces hypothesis for solar cycles has been proposed by Wood (1972) and others. Table 2 below shows the relative tidal forces of the planets on the sun. Jupiter, Venus, Earth and Mercury are called the "tidal planets" because they are the most significant. According to Wood, the especially good alignments of J-V-E with the sun which occur about every 11 years are the cause of the sunspot cycle. He has shown that the sunspot cycle is synchronous with the alignments, and J. Schove's data for 1500 year of sunspot maxima supports the 11.07 year J-V-E period average."
"Both the 11.86 year Jupiter tropical period (time between perihelion's or closest approaches to the sun and the 9.93 year J-S alignment periods are found in sunspot spectral analysis. Unfortunately direct calculations of the tidal forces of all planets does not support the occurrence of the dominant 11.07 year cycle. Instead, the 11.86 year period of Jupiter's perihelion dominates the results. This has caused problems for several researchers in this field."
"[B]y assuming a harmonic variation having a period of 11.13 years ... the phases of such a variation [in polar diameter minus equatorial diameter of the Sun] coincide to within one-fifth of a year with the phases of the sun-spot fluctuations; that, at times corresponding to minimum of sun-spottedness, the polar diameter is relatively larger; that, at times of maximum sun-spottedness, the equatorial diameter is relatively larger. The amplitude of the variation is extremely small, but its reality would seem to be established. [This] at least renders the existence of such periodic fluctuations in the shape of the sun more probable than their non-existence."
"Solar oblateness, the difference between the equatorial and polar diameters, reflects certain fundamental properties of the Sun. ... the oblateness reflects properties of the Sun's interior, ... [There is] a time varying, excess equatorial brightness [producing] a difference between the equatorial and polar limb darkening functions ... at times when the excess brightness is reduced, the intrinsic visual oblateness can be obtained from the observations without detailed knowledge of the excess brightness. A period of reduced excess brightness occurred in 1973 September." The period of reduced excess equatorial brightness occurred between solar cycle maximum around 1970 and minimum around 1975. Considering excess equatorial brightness and seeking to perform measurements at opportunities of reduced excess equatorial brightness has the effect of reducing solar oblateness from some 86.6 ± 6.6 milli-arcsec to 18.4 ± 12.5 milli-arcsec.
The Babcock Model describes a mechanism which can explain magnetic and sunspot patterns observed on the Sun.
* The start of the 22-year cycle begins with a well-established dipole field component aligned along the solar rotational axis. The field lines tend to be held by the highly conductive solar plasma of the solar surface.
* The solar surface plasma rotation rate is different at different latitudes, and the rotation rate is 20 percent faster at the equator than at the poles (one rotation every 27 days). Consequently, the magnetic field lines are wrapped by 20 percent every 27 days.
* After many rotations, the field lines become highly twisted and bundled, increasing their intensity, and the resulting buoyancy lifts the bundle to the solar surface, forming a bipolar field that appears as two spots, being kinks in the field lines.
* The sunspots result from the strong local magnetic fields in the solar surface that exclude the light-emitting solar plasma and appear as darkened spots on the solar surface.
* The leading spot of the bipolar field has the same polarity as the solar hemisphere, and the trailing spot is of opposite polarity. The leading spot of the bipolar field tends to migrate towards the equator, while the trailing spot of opposite polarity migrates towards the solar pole of the respective hemisphere with a resultant reduction of the solar dipole moment. This process of sunspot formation and migration continues until the solar dipole field reverses (after about 11 years).
* The solar dipole field, through similar processes, reverses again at the end of the 22-year cycle.
* The magnetic field of the spot at the equator sometimes weakens, allowing an influx of coronal plasma that increases the internal pressure and forms a magnetic bubble which may burst and produce an ejection of coronal mass, leaving a coronal hole with open field lines. Such a coronal mass ejections are a source of the high-speed solar wind.
* The fluctuations in the bundled fields convert magnetic field energy into plasma heating, producing emission of electromagnetic radiation as intense ultraviolet (UV) and X-rays.

## Planetary astronomy

The image on the right shows the sunspot minimum around January 2009 and the most recent sunspot maximum around March 2015.
On the left is an image of the Moon, Venus and Jupiter, from left to right, at about the time of the solar cycle minimum for cycle 24. The second image on the left has a more specific date of December 1, 2008. The third image down on the left shows a conjunction of Mars and Jupiter on May 1, 2011, about half way between solar minimum and solar maximum for cycle 24.
The images down on the right show another conjunction of Venus and Jupiter. This one is at about the time, or just after, the solar maximum for cycle 24, 30 June 2015. The first is from Quezon City, Philippines, and the second is from Orange County, California.

## Earth polar ice caps

"The records show that prior to 2005, while sea ice extent in the Arctic varied for the month of January, it was always greater than 14.25 million sq. km. Since 2005, it has always been below that figure. There has been a 3.2 percent decline in winter Arctic ice for the decade."
"The February North American snow cover extent was 16.68 million square km (6.44 million square miles), 440,000 square km (170,000 square miles) below average. This was the 13th smallest February SCE on record, and the smallest since 2005."
"According to the National Snow and Ice Data Center (NSIDC), the Northern Hemisphere (Arctic) sea ice extent — which is measured from passive microwave instruments onboard NOAA satellites — averaged for February 2016 was 14.22 million square km (5.49 million square miles), 1.16 million square km (450,000 square km), or 7.54 percent, below the 1981-2010 average. This was the smallest February sea ice extent on record for the Arctic, 180,000 square km (70,000 square miles) smaller than the previous record in 2005."
"All 10 of the smallest sea ice extents on record have occurred since 2005. In the nearly four decades of satellite monitoring, sea ice has disappeared at a clip of 13.4 percent per decade. This year’s cracked ice also continues a troubling trend of disappearing old ice. Though some of that ice will refreeze together this winter, some has disappeared for good and new ice will be left to fill in the gaps."
For January, the "month’s average now sits at negative 3.2 percent every decade based on data collected since 1979. It also continues a trend of a less than 5.5-million-square-mile extent reported in each consecutive January since 2005."
"Despite what appears to be a consistent downward tendency in sea ice extent, a recent study published in Geophysical Research Letters suggests that the rate of Arctic ice loss may actually be slowing, at least in the Atlantic. Even though the January trend is on a negative trajectory, a tendency for more ice overall has been observed since 2005."
“There is little doubt that we will see a decline in Arctic sea ice cover in this century in response to anthropogenic warming, However, internal climate variations and other external forcings could generate considerable spread in Arctic sea ice trends on decadal timescales.”
"Edinburgh and Day (2016) used historical monitoring records to conclude that “the [Antarctic sea ice] levels in the early 1900s were in fact similar to today“."
"Arctic sea ice has also remained essentially unchanged since the 1930s and 1940s too, and is overall still quite high relative to recent centennial- and millennial-scale historical periods. Even for the last few decades, the trends are not unusual."
The "IPCC referenced NOAA satellite data that extended back to 1972, not 1979, in the first UN report (1990). It showed that there had been a slight increasing trend in sea ice for 1972-1990 due to the low extent recorded during the early 1970s, and the very high extent in the late 1970s, when the current satellite datasets begin. Now, the IPCC (and NOAA, NSIDC) discard the 1972-1978 data from the sea ice record, instead using 1979 as the starting point, or the year with the highest sea ice extent since the early 20th century. This way, the decline in sea ice extent to the present can be steepened considerably in modern graphics."
"Between 1990 and 2006, Arctic sea ice declined rapidly. Since 2006, however, the sea ice decline has undergone a pause, as shown in NSIDC data (using WoodForTrees.org interactive graphs):"
"For the early 20th century, there was a dramatic decline in Arctic sea ice between the 1920s and 1940s that was concomitant with the as-warm-as-present Arctic surface temperatures (top graph). After this abrupt warming trend ended, the Arctic cooled for several decades and a subsequent increase in sea ice occurred through the late 1970s. Hoffert and Flanney (1985) furnish a graph with recorded sea ice trends for 1920-1975."
"Solar forcing [is] an important trigger for West Greenland sea-ice variability over the last millennium … Here, we use diatom assemblages from a marine sediment core collected from the West Greenland shelf to reconstruct changes in sea-ice cover over the last millennium. The proxy-based reconstruction demonstrates a generally strong link between changes in sea-ice cover and solar variability during the last millennium. Weaker (or stronger) solar forcing may result in the increase (or decrease) in sea-ice cover west of Greenland. In addition, model simulations show that variations in solar activity not only affect local sea-ice formation, but also control the sea-ice transport from the Arctic Ocean through a sea-ice–ocean–atmosphere feedback mechanism."
"The Arctic sea ice experienced a drastic reduction that was phased with warming temperatures 1923 to 1940. This reduction was followed by a sharp cooling and sea ice recovery. This permits us to also conclude that very likely the Arctic sea ice extent also has a quasi-60 years’ oscillation. The recognition of a quasi-60 year’s oscillation in the sea ice extent of the Arctic similar to the oscillation of the temperatures and the other climate indices may permit us to separate the natural from the anthropogenic forcing of the Arctic sea ice. The heliosphere and the Earth’s magnetosphere may have much stronger influence on the climate patterns on Earth including the Arctic sea ices than has been thought."
The "approximately 80 year variability of the Koch [sea ice] index [has been compared] to the similar periodicity found in the solar cycle length, which is a measure of solar activity. A close correlation (R=0.67) of high significance (0.5 % probability of a chance occurrence) is found between the two patterns, suggesting a link from solar activity to the Arctic Ocean climate. … The 'low frequency oscillation' that dominated the ice export through the Fram Strait as well as the extension of the sea-ice in the Greenland Sea and Davis Strait in the twentieth century may therefore be regarded as part of a pattern that has existed through at least four centuries. The pattern is a natural feature, related to varying solar activity."
"Updated data from NASA satellite instruments reveal the Earth’s polar ice caps have not receded at all since the satellite instruments began measuring the ice caps in 1979. Since the end of 2012, moreover, total polar ice extent has largely remained above the post-1979 average. The updated data contradict one of the most frequently asserted global warming claims – that global warming is causing the polar ice caps to recede."
"Beginning in 2005, however, polar ice modestly receded for several years. By 2012, polar sea ice had receded by approximately 10 percent from 1979 measurements. (Total polar ice area – factoring in both sea and land ice – had receded by much less than 10 percent, but alarmists focused on the sea ice loss as “proof” of a global warming crisis.)"
"In late 2012, however, polar ice dramatically rebounded and quickly surpassed the post-1979 average. Ever since, the polar ice caps have been at a greater average extent than the post-1979 mean."
"Since the decadal variation of the [Arctic Oscillation] AO is recognized as the natural variability of the global atmosphere, it is shown that both of decadal variabilities before and after 1989 in the Arctic can be mostly explained by the natural variability of the AO not by the external response due to the human activity."
The "strong warming in Spitsbergen in the latest decades is not driven by increased frequencies of "warm" [large-scale atmospheric circulation] AC types but rather from sea ice decline, higher sea surface temperatures, and a general background warming."
"Observations show an increase in the rate of winter sea ice loss in the North Atlantic sector of the Arctic up until the late 1990s followed by a slowdown in more recent years. The observed trend over the period 2005 to 2015 is actually positive (a tendency for more ice)".
"The importance of the September minimum sea ice extent in the Arctic derives from the observation that the four lowest values of this extent in the 38-year period 1979-2016 occurred in 2007, 2012, 2015, and 2016. These statistics are cited as evidence of the dangerous effects of [anthropogenic global warming] AGW. They are also cited as harbingers of an ice free Arctic in summer with a corresponding loss in albedo that could cause AGW to feed on itself and accelerate. Our results are inconsistent with these arguments because they do not indicate that September sea ice extent in the Arctic can be explained by temperature alone and that other factors such as winds, ocean currents, clouds, and solar irradiance anomalies may also be important (Kay, 2008) (Holland, 2012) (Francis, 2005) (Gordon, 2000) (Nicol, 2000) (Vinje, 2001)."

## Starspots

Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.
The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.

## Pennsylvanian

The Pennsylvanian lasted from 318.1 ± 1.3 to 299.0 ± 0.8 Mb2k.
Studies of stratigraphic data have suggested that the solar cycles have been active for hundreds of millions of years, if not longer; measuring varves in precambrian sedimentary rock has revealed repeating peaks in layer thickness, with a pattern repeating approximately every eleven years. It is possible that the early atmosphere on Earth was more sensitive to changes in solar radiation than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity.
This would presume annual layering; however, alternate explanations (diurnal) have also been proposed.

## Prehistory

The prehistory period dates from around 7 x 10$^6$ b2k to about 7,000 b2k.

## Paleolithic

The paleolithic period dates from around 2.6 x 10$^6$ b2k to the end of the Pleistocene around 12,000 b2k.

## Mesolithic

The mesolithic period dates from around 13,000 to 8,500 b2k.
Analysis of tree rings has revealed a detailed picture of past solar cycles: Dendrochronologically dated radiocarbon concentrations have allowed for a reconstruction of sunspot activity dating back 11,400 years, far beyond the four centuries of available, reliable records from direct solar observation.

## Ancient history

The ancient history period dates from around 8,000 to 3,000 b2k.

## Early history

The early history period dates from around 3,000 to 2,000 b2k.
The earliest surviving record of sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.

## Classical history

The classical history period dates from around 2,000 to 1,000 b2k.

## Recent history

The recent history period dates from around 1,000 b2k to present.
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles. Although they are at temperatures of roughly 3,000–4,500 K (2,727–4,227 °C), the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (49,710 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere.
"For the greater part of the sun-spot period there is practically but one zone of spots in each hemisphere. The departure from this condition of things near or at the time of minimum, when the spots of the dying cycle are approaching the equator, and the forerunners of the new cycle are beginning to appear in high latitudes, is the only case in which the solar spots are distinctly separated into more than a single zone in each hemisphere." "Spöerer's law ... involves that in a minimum year the zone about 15° should be entirely barren".
"First of all there were only fifteen groups seen during the entire year [1901], north and south put together. Of these, seven were in the north, and the mean latitude for the north was 8.6°, exactly the latitude of one spot of the seven, and this very naturally, seeing that it was by far the greatest group of the year, the celebrated "eclipse group."" Bold added. "Greatest group" and "eclipse group" are both relative synonyms for "dominant group".
"[T]he spot-groups have been carefully examined for cases of return, and where it appeared clear that the same group has returned a second time or more frequently, without any temporary disappearance or subsidence, such a long-continued group has been treated as an entity throughout." Bold added. "It has been forgotten that, whatever the cause which produces this variation of rotation rate with latitude, the causes producing difference of rate within any given latitude are more effective still."
"[T]here is a slight retardation of the rotation period from the first cycle to the second, shown by both northern and southern hemispheres."
The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.

## Sunspots

Galileo di Vincenzo Bonaiuti de' Galilei used the telescope for scientific observations of sunspots.
Galileo made naked-eye and telescopic studies of sunspots.
"Drawing of the large sunspot seen by naked-eye by Galileo, and shown in the same way to everybody during the days 19, 20, and 21 August 1612".
in his Letters on Sunspots, Galileo reported that the telescope revealed the shapes of both stars and planets to be "quite round". From that point forward, he continued to report that telescopes showed the roundness of stars, and that stars seen through the telescope measured a few seconds of arc in diameter. He also devised a method for measuring the apparent size of a star without a telescope.
Galileo made studies of sunspots.
A dispute over claimed priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with Christoph Scheiner. In the middle was Mark Welser, to whom Scheiner had announced his discovery, and who asked Galileo for his opinion. Both of them were unaware of Johannes Fabricius' earlier observation and publication of sunspots.
Galileo's main written works are as follows: History and Demonstration Concerning Sunspots (1613; in Italian: Istoria e dimostrazioni intorno alle macchie solari; work based on the Three Letters on Sunspots, Tre lettere sulle macchie solari, 1612),

## Solar cycle 1

The maximum smoothed sunspot number observed during the solar cycle was 144.1 (June 1761), and the starting minimum was 14.0.
Cycle #1 was discovered by Johann Rudolph Wolf who, inspired by the discovery of the solar cycle by Heinrich Schwabe in 1843, collected all available sunspot observations going back to the first telescopic observations by Galileo and was able to improve Schwabe's estimate of the mean length of the cycle from about a decade to 11.11 years. However, he could not find enough observations before 1755 to reliably identify cycles, hence the 1755–1766 cycle is conventionally numbered as cycle #1. Wolf published his results in 1852.
* start date: February 1755.
* end date: June 1766.
* duration: 11.3 y.
* max count: 144.1.
* max count date: June 1761.
* min count: 14.0.

## Solar cycle 14

The maximum smoothed sunspot number observed during the solar cycle was 107.1, in February 1906 (the lowest since the Dalton Minimum), and the starting minimum was 4.5.
During the minimum transit from solar cycle 14 to 15, there were a total of 1023 days with no sunspots (the second highest recorded of any cycle to date).
* start date: January 1902.
* end date: July 1913.
* duration: 11.5.
* max count: 107.1.
* max count date: February 1906.
* min count: 4.5.
* spotless count: 1023 d.

## Solar cycle 15

Solar cycle 15 was the fifteenth solar cycle since 1755, when extensive recording of solar sunspot activity began.
The maximum smoothed sunspot number observed during the solar cycle was 175.7 (August 1917), and the starting minimum was 2.5.
During the minimum transit from solar cycle 15 to 16, there were a total of 534 days with no sunspots.
* start date: July 1913.
* end date: August 1923.
* duration: 10.1 y.
* max count: 175.7.
* max count date: August 1917.
* min count: 2.5.
* spotless count: 534 d.

## Solar cycle 21

* start date: March 1976.
* end date: September 1986.
* duration: 10.5 d
* max count: 232.9 maximum smoothed sunspot number.
* max count date: December 1979.
* min coun: 17.8 d
* spotless count: 273 d

## Solar cycle 22

The starting minimum was 13.5. During the minimum transit from solar cycle 22 to 23, there were a total of 309 days with no sunspots.
"Sunquakes" caused by solar flares were first observed during this cycle. These are running acoustic waves, as opposed to the standing waves that constitute the material of helioseismology. The flare impulse launches these waves from the photosphere, and they arc deeply into the interior of the Sun before refracting back to the surface to appear as faint ripples, expanding radially away from the flare some tens of minutes later. The flare in which these waves were first observed, SOL1996-07-09, occurred more than three years after the last previous major event.
* start date: September 1986.
* end date: August 1996.
* duration: 9.9 yrs.
* max count: 212.5 sunspots.
* max count date: November 1989
* min count: 13.5 sunspots.
* spotless count: 309 d.

## Solar cycle 23

The maximum smoothed sunspot number observed during the solar cycle was 180.3 (November 2001), and the starting minimum was 11.2. During the minimum transit from solar cycle 23 to 24, there were a total of 817 days with no sunspots.
One of the first major aurora displays of solar cycle 23 occurred on 6 April 2000, with bright red auroras visible as far south as Florida and South Europe. On 14 July 2000, the CME hurled by a X5.7 solar flare provoked an extreme (G5 level) geomagnetic storm the next day, known as the Bastille Day event, this storm caused damage to GPS systems and some power companies, where auroras were visible as far south as Texas.
A G5 level geomagnetic storm blasted the Earth's magnetosphere over the next two days. A few days later, the largest solar flare ever measured with instruments occurred on 4 November; initially measured at X28, it was later upgraded to an X45-class.
* start date: August 1996.
* end date: December 2008.
* duration: 12.3 y.
* max count: 180.3.
* max count date: November 2001.
* min count: 11.2.
* spotless count: 817.

## Solar cycle 24

Sunspots did not begin to appear immediately after the last minimum (in 2008) and although they started to reappear in late 2009, they were at significantly lower rates than anticipated.
Activity was minimal until early 2010.
It reached its maximum in April 2014 with a 23 months smoothed sunspot number of 81.8.
The first peak reached 99 sunspots in 2011 and the second peak came in early 2014 at 101.
* start date: December 2008.
* end date: December 2019.
* duration: 11.0.
* max count: 81.8.
* max count date: April 2014.
* min count: 2.2.
* spotless days: 489.

## Solar cycle 25

On the diagram on the right, blue/red show negative/positive polarity fields scaled between ±5 Gauss. Two black arrows mark approximate location of two latitudinal bands of cycle 25. Data are acquired by Global Oscillations Network Group (GONG) instruments operated by NISP/National Solar Observatory (NSO)/Association of Universities for Research in Astronomy (AURA)/National Science Foundation (NSF).
Solar cycle 25 is the current solar cycle that began in December 2019 with a smoothed minimum sunspot number of 1.8.
As of April 2018, the Sun showed signs of a reverse magnetic polarity sunspot appearing and beginning this solar cycle.
The polarward reversed polarity sunspots suggest that a transition to cycle 25 is in process.
The first cycle 25 sunspot may have appeared in early April 2018 or even December 2016.
In November 2019, two reversed polarity sunspots appeared, possibly signaling the onset of cycle 25.
Analysis of the polarity orientation of bipolar magnetic regions observed in December 2019: magnetic regions with the underlying orientation of solar cycle 25 toroidal field component were brewing in the solar convection zone, representing early signs of the new cycle.
Supersynoptic (time vs. solar latitude) map of the radial component of the solar magnetic field for cycles 24-25 based on observations from the Global Oscillations Network Group (GONG) shows magnetic activity of cycle 25 beginning November 2019 at about 30 degree latitudes in both solar hemispheres.
Supersynoptic map is available.
Spotless days by year (Solar cycle 25 vs 24):
* 2022 : 0 (0%) [as at May 16 2022]
* 2021 : 50 (14%)
* 2020 : 192 (52%)
* 2019 : 274 (75%)
* 2018 : 208 (57%)
* 2017 : 96 (26%)
* 2016 : 27 (7%)
* 2015 : 0 (0%)
* 2014 : 1 (0%)
* 2013 : 0 (0%)
* 2012 : 0 (0%)
* 2011 : 2 (1%)
* 2010 : 44 (12%)
* 2009 : 262 (72%)
* 2008 : 265 (72%)
* 2007 : 163 (45%)
* 2006 : 65 (18%)
* start date: December 2019
* max count: 45.0
* max count date: October 2021 (progressive)
* min count: 1.8

## Numbers

"[T]he longest data set, the Wolf sunspot number (R), is determined from visible wavelength observations of the solar surface."
The Wolf number (also known as the International sunspot number, relative sunspot number, or Zürich number) is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the sun.
This number has been collected and tabulated by researchers for over 150 years.
The relative sunspot number $R$ is computed using the formula (collected as a daily index of sunspot activity):
    $R=k(10g+s)\,$
where
* $s$ is the number of individual spots,
* $g$ is the number of sunspot groups, and
* $k$ is a factor that varies with location and instrumentation (also known as the observatory factor or the personal reduction coefficient$K$).

## Technology

"The device used to take the measurements, NAUSICAA, was developed for estimating the doses received by astronauts (3). A sample operated on board the Mir space station for three and a half years (4). Portable versions are used in other situations, particularly aboard aircraft and in radiation facilities (5)."
"The detector is a Tissue Equivalent Proportional Counter (TEPC), considered by specialists as the reference detector for measuring doses from cosmic radiation (6). It is sensitive to directly ionising particles (ions, electrons and gamma rays) as well as to neutrons via the charged secondary particles created by them in the walls of the counter. The sensitive volume is a 5x5 cm cylinder filled at low pressure (33 hPa) with a gas « equivalent » to biological tissue. This gas is based on propane: 50% C$^3$H$^8$, 40% CO$^2$ and 5% N$^2$. The detector simulates a 3 micron-long biological site located inside the organism at a depth of 1 cm."
"Each event detected is analysed using a pulse height analysis method (PHA) and stored to produce the lineal energy distribution spectrum, y; y is the energy deposited over the average chord of the detector. The system uses a logarithmic amplifier because of the dynamic range of y (104) and a 256 multi-channels analyser. There is a relation between y and the linear energy transfer (LET) which is related to the quality factor (Q). The sum of all events provides the absorbed dose (D), an assessment of the ambient dose equivalent (H*(10)) and the average quality factor (Q = H*(10)/D) of the radiation. An internal source of alpha particles ($^{244}$Cm) is used to adjust the high voltage of the system. The ambient dose equivalent calibration is done with a 60Co source for LET lower than 4 keV/μm and with a AmBe neutron source for higher LET."

## Hypotheses

* Some configuration of Venus and Jupiter precisely corresponds to the major solar cycle period.