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The rise in sunspot activity every 11 years or so is associated with greater magnetic activity, with an increase in the number of active regions, flares, and coronal mass ejections. This cycle was discovered in 1843 by German astronomer Heinrich Schwabe as a by‐product of his search for a planet closer to the Sun than Mercury.

Astronomers give each 11‐year solar cycle a number. For obscure historical reasons, solar cycle 1 was a fairly ordinary cycle which peaked in 1760. The most recent cycle, number 24, began in 2007 and ended in 2019. However, cycle 24 was extremely reluctant to start, with the Sun remaining largely blank well into the autumn of 2009, interrupted by an occasional sunspot with reversed magnetic polarity (indicative of the start of a new cycle). In addition to the deepest solar minimum in nearly a century, there was clear evidence of a decline in sunspot magnetism of about 50 gauss per year since 1992.

This was not the first time that sunspots largely disappeared. The solar minima of 1901 and 1913, for instance, were even longer than the 2008–2009 hiatus. In the 17th century, the Sun experienced a 70‐year sunspot drought, known as the Maunder Minimum, that still baffles scientists. Between 1645 and 1715, the number of observed sunspots plummeted from thousands per year to a few dozen.

Figure 2.27 Plasma flows within and around a sunspot, derived from SOHO data. Red is hot gas, blue is cooler gas. Outflowing plasma at the surface is underlain by material rushing inward, like a giant whirlpool. This inflow is strong enough to pull the magnetic fields together and reduce the amount of heat that normally flows from the interior. The cooled material sinks to a depth of only a few thousand kilometers and then spreads out.

(ESA/NASA)

Figure 2.28 An image of the chromosphere obtained by Hinode on November 20, 2006. Plasma aligned along the solar magnetic field lines is rising vertically from a sunspot (an area of strong magnetic field) toward the corona.

(Hinode, JAXA/NASA/PPARC)

The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The final sunspot number represents the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups contain, on average, about 10 spots, this formula has provided reliable numbers even when observing conditions were less than ideal and small spots were difficult to see.

Each cycle tends to start with a few small spots close to latitudes 40° north and south. As it progresses, the spots and associated active regions move closer to the equator, and larger, more long‐lived spots appear. After approximately 11 years, the final spots of that particular cycle form at latitudes of about 5°, before disappearing altogether (Figure 2.31).

Why do the bands where sunspots form drift equatorward over time and then disappear? Data from space observatories, such as the Solar Dynamics Observatory, suggest that it may be caused by a giant circulation system, known as meridional plasma flow (Figure 2.32).

The meridional flow works something like a conveyor belt. The basic flow pattern shows two main areas of circulation, either side of the Sun's equator. Near the equator, the plasma rises and moves toward the poles, within 32,000 km of the Sun's surface. Flowing through the surface layers, where the plasma is less compressed, the material is able to move quite quickly, reaching 32–64 km/h.

Near the poles, the plasma sinks. Compressed plasma, 100,000 km below the surface, moves from the poles toward the equator at a speed of about 5 km/h – equivalent to a leisurely walking pace. The variable speed means that plasma can take anywhere from 30 to 50 years to complete the full circuit.

However, helioseismic data show that there is actually a double conveyor belt: the equatorward flow occurs in the middle of the convection layer, sandwiched between two streams of material moving toward the poles. This results in a double‐cell system in which two elongated flow systems are stacked on top of each other.

Since the speed of this meridional circulation system changes slightly from one sunspot cycle to the next, it may act like an internal clock that sets the period of the sunspot cycle. The circulation is faster in cycles that are shorter than the average 11‐year period and slower in longer‐than‐average cycles.

The absence of sunspots in 2008–2009 has been attributed to the fact that, from 1996 onwards, the deep, internal plasma flow associated with the next solar cycle was moving more slowly than usual, whilst the top of the conveyor belt was moving at record‐high speed. This seems to have affected the internal dynamo, delaying the rearrangement of the solar magnetic field. This, in turn, prolonged the period of switchover between cycles, leading to an extended solar minimum.

Near‐surface jet streams, which flow from the poles toward the equator at depths of 1,000 to 7,000 km, have also been associated with the sunspot cycle. By using helioseismology to keep track of gas moving below the surface, scientists from the National Solar Observatory, Arizona, found that the Sun generates new jet streams near its poles every 11 years. The streams migrate to the equator and are apparently associated with the production of sunspots once they reach a critical latitude of 22°.

Despite important progress in recent years, predictions of future sunspot cycles are not very accurate. Time will tell if the slow start to cycle 24 may mark the start of an extended quiet period of solar activity. However, exactly how the jet streams are generated, and the precise mechanism that enables them to trigger sunspot production, remains uncertain.

Figure 2.29 Snapshots of the changing solar magnetic field (left) and the soft X‐ray corona (right) from 1991 to 2000 – almost an entire solar cycle. Obtained one year apart between one solar maximum (lower right) and the next, they show the evolution of coronal structure due to changes in the magnetic fields. Note the few magnetic features and lack of X‐ray bright loops in the middle, at solar minimum. The strongest magnetic fields (shown in dark blue and white) occur in the active regions and coincide with the brightest coronal X‐ray emissions. White shows an upward pointing magnetic field and dark blue a downward pointing field. Pale blue shows a weak field of “mixed” polarity.

(Left: National Solar Observatory/NOAO/NSF. Right: Yohkoh/ISAS/Lockheed‐Martin/NAOJ/NASA)

Figure 2.30 This butterfly diagram (top), named for its characteristic appearance, shows the average positions of sunspots for each rotation of the Sun, based on observations obtained by the Royal Greenwich Observatory, London, since May 1874. The bands first form at mid‐latitudes, widen, and then move toward the equator as each cycle progresses. The smallest spots are shown in black, the largest ones in yellow. Below is a plot of the average area covered by sunspots over the same period. Note the decline during cycles 23 and 24.

(David H. Hathaway)

Figure 2.31 Plasma in the Sun's outer regions moves rather like a double conveyor belt. Near the surface, plasma flows slowly toward the poles and sweeping up knots of solar magnetism (decaying sunspots). It then sinks and returns toward the equator at a depth of 100,000 km, transporting magnetic flux along the way. This meridional plasma flow has two branches, north and south, each taking about 40 years to complete one circuit. Deeper still is another stream of material moving toward the poles. The structure and strength of this meridional flow is a major influence on the strength of the Sun's polar magnetic field, which then determines the duration of the sunspot cycle and number of sunspots.

(NASA, Stanford University)