Suns solar cycle or the solar magnetic activity cycle (SC24)

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The solar cycle, or the solar magnetic activity cycle, is the main source of periodic solar variation (changing the level of irradiation experienced on Earth) which drives variations in space weather and to some degree weather on the ground and possibly climate change.[1] The cycle is observed by counting the frequency and placement of sunspots visible on the Sun. Powered by a hydromagnetic dynamo process driven by 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.


Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots


Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century

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.

Until recently it was thought that there were 28 cycles in the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784-99) seems actually to have been two cycles[2][3], so that the average length is only around 10.66 years. 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 graph below). 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:

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 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 graph below). These various observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.





Solar Maximum

During times of maximum activity there are lots of sunspots and solar flares as well as a small uptick in the total solar irradiance emitted to by the Sun (referred to by scientists as TSI), and of course vice versa for solar minimum. (More on sunspots here. More on solar flares here.)

Variations in solar activity are more than a scientific curiosity. Solar flares, for example, can disrupt or even wipe out radio communications. Variations in TSI can affect global temperatures. In fact it is believed that periods of extreme solar quiescence such as the so-called Maunder Minimum (named after the English scientist Edward W. Maunder, who identified it) may have led to a mini ice age that hit regions of North America and Europe in the late 1600s.

Welcome to Solar Cycle 24

For these reasons scientists have been puzzling over the Sun’s recent behavior.

The last solar maximum, during Cycle 23, peaked in 2000, and probably provided a little oomph to 1998's record-making warmth.

In 2008 the Sun reached Cycle 23’s solar minimum and sort of stayed there. It stayed there so long in fact that some began to wonder if the Sun would ever wake up, and the cries of an impending ice age echoed in the blogosphere (another example here). Finally in December 2008 the Sun awoke and began Cycle 24.

But by most indications, Cycle 24 will be a very weak cycle. While Cycle 23 peaked out in 2000 with an average sunspot number of 120, the current consensus prediction for Cycle 24 is for a maximum sunspot number of only about 90, to peak around May 2013. (Note: This prediction is a bit different from a year ago when the panel issued two conflicting predictions. See my related post from last year.)

With an average daily sunspot number of about 11, December 2009’s sunspot activity aligned well with the current prediction of about 13. (The month's maximum sunspot number was 30 on December 20.) And the month of January was much the same with an average sunspot number of 13.

But there was one surprising aspect of the Sun’s behavior in January — a number of large solar flares. On January 17 and 18, the Soviet TESIS Satellite observed moderate Class C flares, and then on the 19th observed a series of increasingly intense flares, culminating in two Class M flares. (Flares are categorized according to the power they emit: A, B, C, M, and X.) These were the first large flares seen on the Sun since the summer of 2007.

Is this uptick in solar flares an anomaly or a sign that we are headed for a more dynamic and energetic sunspot cycle than what has been predicted? We’ll have to wait and see.


In the meantime, you may be interested to know there are a host of amateur scientists our there following the Sun’s every change. They are a great source of up-to-the-minute information. Check them out:

[Bpeirce2 Profile]

Variations are minimal,...can't see a cycle or even a pattern,...increased cycles, how is that predicted by these charts!