# Solar activity

Change in the frequency of sunspots since 1610
Graph of various parameters of solar activity since 1975
Solar radiation graph since 1975 based on the full spectrum
Solar flare on August 20, 2002

The variable properties of the sun , which are related to the turbulence of its extremely hot gas and continuous changes in the magnetic field , are called solar activity . The changes can be cyclical or irregular. The solar activity is most noticeable in the changing frequency of the sunspots and their position in relation to the heliographic equator , which can be determined simply by observing the sun with small telescopes.

The sunspot cycle has an average period of 11.1 years, but can range from 9 to 14 years over the course of a century. The mean monthly number of sunspots fluctuates from 0 to 20 at the sunspot minimum and between 80 and 300 at the maximum. The highest known maximum so far was 1957/59 with a monthly mean of the sunspot relative number of 285. At the maximum in 2013/14, the mean monthly number of spots was mostly between 80 and 120. In mid-2016, daily values ​​from zero to around 60 and monthly averages below 40 were observed. This means that activity has fallen by almost half since autumn 2015.

In addition to the interplay of the sunspots, there are also irregular bursts of plasma and radiation ( flares ), changes in the solar wind , isolated geomagnetic storms and proton showers , and the huge gas jets of the prominences .

Although the sunspots have a temperature 1000–1600 ° lower than the rest of the sun's surface (5500 ° C), the sun shines with a slightly higher output during the activity maximum than at the sunspot minimum. The sun flares (hotter areas with around 7000 °) contribute to this. Solar activity is responsible for events in space weather and has a direct impact on satellites, but also on technical facilities on earth. It also influences the interplanetary magnetic field , the earth's magnetic field , the ionosphere and thus the propagation of radio waves and the polar lights .

## Determination of solar activity

Solar activity is quantified by various indices. Indices can be based on direct observations of solar activity, such as the sunspot relative number or radio intensity , in which case one speaks of direct indices . Or they are based on effects that are in turn caused by solar activity. In this case one speaks of indirect indices . Direct indices are comparatively accurate, but only go back to the beginning of the 17th century. Indirect indices can be given for the last ten thousand years or so, up to the beginning of the Holocene , with decreasing accuracy.

### Sunspot relative number

The first observations of sunspots were made as early as the 4th century BC. u. Presented by Theophrastus of Eresus . Sunspots have been systematically observed and counted with a telescope since 1610. This makes them one of those astronomical phenomena that have been studied longest using modern scientific methods.

A good and easily determinable measure of solar activity is the sunspot relative number :

${\ displaystyle R = k \, (10 \, g + f)}$

k is a correction factor for the size of the telescope used and the current viewing conditions (A. Wolfer, pilot observer from 1876 to 1928, is the new reference observer), g is the number of groups of spots and f is the number of individual spots.

Around 1970, some solar observatories began measuring the total area of the spots on a daily basis . This elaborate alternative method, however, shows almost the same activity profile as simple counting using a relative number.

### Radio intensity

Another measure of solar activity is the sun's radio intensity at a wavelength of 10.7 cm, corresponding to a frequency of 2.8 GHz. This intensity correlates with the relative number and is determined using radio astronomical methods .

### Indirect determination from radionuclides

Using time series of radionuclides 14 C and 10 Be, the magnetic activity of the sun can be reconstructed over several thousand years.

High-energy cosmic radiation from space is weakened and deflected in the heliosphere by the solar wind and the solar magnetic field. With less solar activity, more and more energetic cosmic radiation can penetrate the earth's magnetic field and get into the earth's atmosphere . There it leads to interactions in which the nuclides 14 C and 10 Be are generated in nuclear reactions . This process is the main source for the production of the two nuclides.

The two radionuclides produced in this way end up in natural climate archives after a complicated transport process : the carbon isotope 14 C enters the biosphere as part of the carbon cycle - there it can be detected in tree rings using the radiocarbon method , for example - or it is deposited in the sea. As soon as it is in the troposphere , the beryllium isotope 10 Be accumulates in aerosols within one to two weeks or is brought to the surface of the earth by precipitation. There it can be detected , for example, in ice cores of the polar ice sheets .

From the nuclide concentrations measured in climate archives, conclusions can be drawn about the solar activity. Transport processes, climatic influences on transport, the strength of the earth's magnetic field, other sources of nuclides and, over very long periods of time, changes in galactic cosmic radiation must be taken into account. The accuracy of the reconstruction is on the order of a decade. Time series of solar activity for the last millennium reconstructed from 14 C agree very well with sunspot indices, for 10 Be a little less well.

## Cycles

### Schwabe cycle (11 years)

The most striking cycle is the approximately 11-year-old Schwabe cycle based on Samuel Heinrich Schwabe . Successive maxima of the sunspot relative number follow one another at this time interval.

### Hale cycle (22 years)

Since the solar magnetic field was found to be the cause of sunspots with the Zeeman effect , its magnetic polarity can also be determined. On a solar hemisphere, the magnetic polarity of the spots changes from one cycle to the next. The 11-year cycle is therefore based on a cycle that is twice as long, the 22-year Hale cycle .

### More guesswork

Research into the history of the climate in particular enabled regularities to be recognized and further solar cycles postulated.

• Gleißberg cycle (85 ± 15 years): The 80 to 90 year old Gleißberg cycle was discovered by Wolfgang Gleißberg . There may be a connection with the year 2008 spotless minimum. Wolfgang Gleißberg has developed a forecasting method based on the comparison of several successive cycles.
• Suess cycle (180–210 years), also called cycle 208a or de Vries cycle .
• 1470-year cycle, associated with the Bond and Dansgaard-Oeschger events . After 1470 years, the 210 cycle has expired seven times and the 86.5 cycle seventeen times.
• Hallstatt or Bray cycle (2400 ± 200 years). The Hallstatt or Bray cycle was postulated by various scientists after analyzing 14 C and 10 Be in rock and ice cores, as well as analyzing various glacier advances in the last millennia. The cause is likely to be fluctuations in solar activity. A possible connection with a 2318-year-old pattern of the orbit of large planets in our solar system was postulated in 2016.

## Radiation spectrum and origin

For several decades, solar research has established that solar activity is even more noticeable in other areas of the spectrum , for example solar radioflux is used as an activity indicator. The northern or northern lights are also related to it.

The sun's radiant energy comes from nuclear fusion of hydrogen to helium in the sun's core and gets to the outside through particles ( neutrinos ), radiation transport and convection . Interactions result in a broad spectrum of radiation from gamma radiation to UV to the radio wave range . There are large and small-scale temperature differences, gas eruptions and isolated radiation storms in the X-ray , UV and radio wave range.

## Hot gas clouds, flares and polar lights

Strong magnetic fields at large sunspots (type E, type F) can throw clouds of hot gas from the outer layers of the sun into space. These gas clouds are electrically charged and therefore disrupt the earth's magnetic field when they arrive at the earth after a few days.

Flares are sudden bursts of radiation in the outer layers that last a few minutes to hours. Increased gamma radiation , UV and radio radiation are observed. High-energy atomic particles (electrons, protons, helium nuclei) can also be emitted.

A geomagnetic storm usually goes unnoticed. However, severe storms can disrupt satellites, electrical systems or radio links, which has happened several times in recent years. While the increased radiation exposure during a magnetic storm on the earth's surface is safe, it can be dangerous in space travel and on some long-haul flights.

According to the Geoforschungszentrum Potsdam , the largest geomagnetic storm in history to date occurred on January 1st and 2nd. September 1859 the newly introduced telegraph lines came to a standstill and generated northern lights, which were still visible in Rome and Havana. In autumn 2003, the northern lights could be observed as far as southern Germany and Austria.

In addition to observing sunspots, every solar observatory is also used to measure flares and structures of the solar corona . There have recently been special satellites that register intensified gas clouds from flares long before they even hit Earth. It is also hoped that NASA's STEREO satellites will provide new information about the physics of the sun and its anomalies.

By Andrew Ellicott Douglass has been suggested that the growth of trees may depend on the solar activity.

## Solar activity in modern times

### Modern maximum

In the 20th century the sun was in an unusually active phase, a grand maximum that peaked in 1957/1958. The number of sunspots in 1950–2000 was on average more than twice as high as in 1750–1900. Reconstructions on the basis of cosmogenic isotopes suggest that phases of such high activity rarely occur. A similar active phase was last probably around 2000 BC. u. At the end of the 20th century this maximum ended, the sun entered a phase of moderate activity again.

### Moderate 24th cycle

24. Solar cycle and its sunspots until early 2019

After the unusually long minimum of 2008/09, where the sun was spotless for months , the maximum of the current 24th solar cycle was initially forecast for the end of 2012. The increase in activity took place in 2011 and early 2012 as expected and reached a flat maximum in February, but the sunspot relative number fell again in summer 2012 and remained atypically low until the end of the year. The maximum forecast was therefore revised to the end of 2013.

Unusual was the 24th cycle, the unequal distribution of the activity centers and the large spot groups of type E and type F . While the northern hemisphere of the sun was somewhat more active in 2012, in 2013 almost all large groups were in the southern hemisphere. Correlated with the solar rotation were the highest relative numbers (about 130 to 160) since May 2013 by the middle of the month, and some groups were even spot freiäugig visible; the lowest values ​​were around 20.

The activity centers have been even more unevenly distributed since spring 2014, where they became visible in the first third of the month due to the rotation of the sun. The previous maximum value occurred on July 5 , 2014 with around 160 spots and a relative number around 250, while 2 weeks earlier it was only 50.

In 2015 the relative number fell to around 50 to 130 and in the first half of 2016 to 20 to 70. At the beginning of July 2016, the sun was completely free of spots for a few days for the first time in this cycle.

## Comparison with other sun-like stars

A comparison with 369 sun-like stars observed with the Kepler and Gaia space telescopes shows that their brightness fluctuations are typically five times stronger. In the sun, they are typically 0.07 percent. Isotope analyzes from ice cores show that solar activity has been similarly low for at least 9,000 years. In the comparison with other stars, age, surface temperature, proportion of heavy elements and rotation period were used. Possible explanations are that the sun is currently in a resting phase and shows similarly high fluctuations in the long term, or that the sun differs from these other stars in a previously unknown way.

## literature

• Helmut Zimmermann, Alfred Weigert: ABC Lexicon Astronomy . Spektrum Akad. Verlag, Heidelberg 1995, ISBN 3-8274-0575-0 .
• J.Bennett, M.Donahue, N.Schneider, M.Voith: Astronomie (Chapter 14) (Ed.): Harald Lesch, 5th edition (1170 pages), Pearson-Studienverlag, Munich-Boston-Harlow-Sydney- Madrid 2010
• Rudolf Kippenhahn : The star we live on . DVA, Stuttgart 1990
• Gordon D. Holman: Explosive sun . Spectrum of Science, June 2006, pp. 41–47.
• Ilya G. Usoskin: A History of Solar Activity over Millennia . In: Living Reviews in Solar Physics . February 2017, doi : 10.1007 / s41116-017-0006-9 (Open Access).

## Web links

Commons : Solar activity category  - collection of images, videos and audio files

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