The oldest record of solar sunspot observations dates back to at least 800BC, where a Chinese text from this time records a darkening on the face of the Sun. Galileo made the first telescopic observations of Sun in 1610, and was the first to show sunspots are a feature of the solar surface, rather than a result of planets traversing the Sun.
A plot of the number of sunspots observed on the face of the Sun shows this number varies cyclically with a mean value of about 11 years. This is known as the sunspot cycle. At solar minimum very few sunspots are visible on the face of the Sun.
A typical sunspot cycle shows a sharp rising period of 3-6 years, followed by a more gradual decline lasting 5-8 years. The cycle is believed to be modulated on a longer period of around 88-years (the Gleissberg cycle.)
Sunspots are not the only feature of the magnetic field varying in time. Modulation of solar activity is very small in visible light, and so a variation of solar activity is not immediately apparent to the naked eye. However in the soft X-ray range the modulation is about an order of magnitude, and increases at higher energies.
Examples of other features which vary in time include
Sunspot numbers turn out to be a reasonable measure of solar activity. Many indicators of activity, such as radio emission at a 10.7 cm wavelength, have been found to correlate well with the sunspot number, with most indicators varying in phase. Thus historical sunspot records can tell us how solar activity has varied in time.
Records show, for example, that from 1645-1715 there was very little sunspot activity, with only around 50 sunspots being observed during this time, compared to 40-50,000 sunspots in other similar timespans. This period of reduced solar activity is now known as the Maunder minimum. At the time both the Thames and the Baltic Sea regularly froze over, and sea-ice increased so much that in 1685 no open water was present around Iceland. European artists at the time depict landscapes covered in snow, with canals and rivers frozen over. It is possible these changes in climate were a consequence of the decrease in solar activity.
The amount of cosmic rays that reach the Earth is modulated by the solar magnetic field. These rays are responsible for the production of 10-Be on the Earth, so measurements of this isotope in arctic ice cores are believed to be an indicator of solar activity. The 10-Be data shows an 11-year cycle that is anti-correlated with solar activity. It suggests several previous minima (i.e. times when solar magnetism drops to levels below those found between the 11 year cycles) have occurred. These are spaced irregularly at intervals of 200-300 years, a well known example being the Sporer minimum (1420-1530.)
Sunspots typically occur in pairs. The axis of a bipolar sunspot pair is slightly tilted with respect to the equator (by about 4o), with the leading spot being closest to the equator. This is Joy’s law.
The pairs also obey Hale's law. This law states that in a given hemisphere the leading sunspot has one polarity, and the trailing spot the opposite polarity. The inverse occurs in the other hemisphere, so, for example, if in the northern hemisphere negative polarity leads, then positive polarity will lead in the southern hemisphere. These polarities reverse from one activity cycle to the next, giving an overall magnetic cycle of 22-years.
Sunspots do not appear at random over the surface of the Sun, but are concentrated in two latitude bands on either side of the equator. At the start of each new cycle sunspots appear at high latitudes of about 27 degrees. As the cycle progresses the sunspots migrate close to the equator, reaching about 8 degrees. This is illustrated with the well-known butterfly diagram.
Throughout the Maunder minimum the northern belt of activity was almost absent, and the southern belt did not extend beyond 20 degrees of latitude.
The Sun is unique, being the only star for which we can observe the detailed surface magnetic features. We also have records of its activity dating back for hundreds of years. However the large scale properties of the Sun, such as rotation rate, mass, and age evolve only very slowly.
Perhaps the best way of understanding how solar activity depends on such parameters is by looking at other stars. Nearly all stars with outer convection zones show chromospheric activity.
We can measure the magnetic field of stars in several ways. The most direct method is to use the Zeeman broadening of spectral lines but indirect indicators include
Emission at the centre of singly ionised Ca II H and K spectral lines increases with nonthermal heating of the chromosphere. There is a maximum of H and K emission occurring at the time of sunspot maximum, so that if the Sun itself were a distant star, its 11-year cycle would be observable as a variation of integrated H+K flux with this period.
At the Mt. Wilson Observatory (California) in the mid-1960s the astronomer O.C. Wilson began a long-term study of the magnetic cycles of cool stars, (known as the H-K project.) Although the aim was to observe for one solar cycle, the project continues to this day, now using a 100-inch telescope for observations. Observations suggest that surface magnetic activity is a universal phenomenon in lower main-sequence stars.
111 stars on or near the main sequence, of spectral type F2-M2, have been monitored. Although this measurement is not ideal (non-magnetic phenomena can also affect this measure), the 350,000 or so observations made so far make this that longest running project to monitor stellar activity cycles, and as such the data has been thoroughly analysed.
Despite this short observation time, these H&K emission data, together with other observations, show some patterns of behaviour in stellar activity.
The sample groups naturally into four categories (Baliunas et al. 1995):
There were 31 stars with flat or linear trends, and 29 showing variability with no clear periodicity. 52 stars (including the Sun) showed magnetic cycles.
Of the 52 stars with magnetic cycles, periods range from 2.5 years to at least the 25 year length of the observation span. A few stars were doubly periodic (quasiperiodic)
There have been many attempts to correlate cycle periods and amplitudes with stellar properties such as rotation and age. The Rossby number is defined as
Grouping the stars into those with constant or regular cycles and those with irregular cycles we see they are clearly separated by Rossby number.
Ro>1 (slow rotation) is the domain of constant or regular stars, with Ro<1 (faster rotation) being the domain of irregular or chaotic stars.