Sunspots and the Solar Cycle: From Minimum to Maximum

Sunspots on the Sun's surface — *View of a very large group of sunspots from NASA's Solar Dynamics Observatory in October 2014. Credits: NASA*

The 11-Year Solar Cycle

The Sun follows an approximately 11-year magnetic activity cycle, marked by variations in the number of sunspots. These spots, discovered by Galileo in 1610, appear as dark areas on the solar surface due to their lower temperature (about 3800 K compared to 5800 K for the surrounding photosphere).

Magnetic Origin of Sunspots

Emergence of Magnetic Fluxes

Sunspots appear when intense magnetic fluxes emerge from the Sun's interior and pierce the photosphere, the visible layer of our star. These magnetic fields locally inhibit convection, reducing heat transport to the surface. As a result, the temperature in these regions drops by about 1500 K compared to the surrounding photosphere, making them visibly darker.

The Role of the Solar Dynamo

The mechanism behind this emergence is related to the solar dynamo, a complex process generated by the Sun's differential rotation and the turbulent movements of plasma in its convective zone. Magnetic field lines twist, tangle, and eventually reconnect or rise through the surface, forming coronal loops and sunspots.

A Magnetic Reversal Every 11 Years

Every 11 years or so, the Sun's magnetic field completely reverses: the north pole becomes the south pole and vice versa. This reversal marks the transition between two cycles and explains the gradual increase in solar activity toward a maximum (numerous sunspots, frequent flares, coronal mass ejections), followed by a return to the minimum (few or no sunspots, low activity).

Magnetic Field Intensity

The magnetic field density in a sunspot can reach up to \(\sim 0.3\,\text{Tesla}\), thousands of times stronger than Earth's magnetic field. These magnetic structures are often bipolar, with north and south polarities, illustrating their origin in magnetic flux tubes.

Table of Solar Cycles Since the Beginning of Observations

History of 25 Solar Cycles
Cycle	Period	Maximum Number of Spots	Duration (years)
Cycle 1	1755-1766	86.5	11.3
Cycle 2	1766-1775	115.8	9.0
Cycle 3	1775-1784	158.5	9.3
Cycle 4	1784-1798	141.2	13.6
Cycle 5	1798-1810	49.2	12.2
Cycle 6	1810-1823	48.7	12.8
Cycle 7	1823-1833	71.5	10.5
Cycle 8	1833-1843	146.9	9.8
Cycle 9	1843-1855	131.6	12.4
Cycle 10	1855-1867	97.9	11.3
Cycle 11	1867-1878	140.5	11.8
Cycle 12	1878-1890	74.6	11.3
Cycle 13	1890-1902	87.9	11.8
Cycle 14	1902-1913	64.2	11.5
Cycle 15	1913-1923	105.4	10.0
Cycle 16	1923-1933	78.1	10.1
Cycle 17	1933-1944	119.2	10.4
Cycle 18	1944-1954	151.8	10.2
Cycle 19	1954-1964	201.3	10.5
Cycle 20	1964-1976	110.6	11.4
Cycle 21	1976-1986	164.5	10.3
Cycle 22	1986-1996	158.5	9.7
Cycle 23	1996-2008	120.8	12.3
Cycle 24	2008-2019	116.4	11.0
Cycle 25	2019-2030 (predicted)	115 (estimated)	11 (estimated)

Source: SILSO Sunspot Index and Long-term Solar Observations (new window) and NASA Solar Physics (new window).