The Solar Cycle

TLDR: For example, the Z urich sunspot index as discussed by the authors, which measures the number of sunspots in the Sun's magnetic field, is the best known index of the solar cycle.

Abstract: Deenition The term solar cycle refers to a quasi-periodic variation with a period of about 11 years, visible in many of the Sun's observables. The solar cycle is most easily observed in indices directly related to the Sun's magnetic eld, such as sunspots. During the last decades, solar-cycle variations have also been found in many other aspects of the Sun (irradiance, surface ows, coronal shape, oscillation frequencies, etc). The modulation amplitudes may vary widely between diierent indices. It is minute in visible light, and this explains why the solar cycle is not visible to a casual naked-eye observer. Elsewhere (in the far ultraviolet, X-rays, radio frequencies) the modulation amplitude is large (Table 1). The prime cause of the solar cycle is a quasi-periodic oscillation of the solar magnetic field. Magnetic-eld indices The oldest and best known index of the solar cycle is provided by sunspots, whose discovery for Europe around 1610 is associated with the names of Fabricius, Galileo and Scheiner. The commonest sunspot index is the Z urich sunspot number R = K(10g + f), where g is the number of sunspot groups, f is the number of individual spots and K is a coeecient that corrects for diierences in the quality of the observations. Usually one derives an annual average R, shown in Fig. 1. The most striking feature of the sunspot record is the Schwabe cycle of about 11 years, discovered by Hein-rich Samuel Schwabe in 1843. By convention, the cycle that began in 1755 is assigned number 1; hence cycle 23 began in 1997. Cycle shape. A typical sunspot cycle in Fig. 1 is characterized by a sharp rise from minimum to maximum, lasting 3-6 years (on average 4.8). The duration of the rise phase is anticorrelated with the height of the maximum (Waldmeier's rule). The maximum is followed by a gradual decline lasting 5-8 years (on average 6.2). Whereas the maxima of the Z urich number are typically well-deened, this is not the case for some other solar activity indices. For example, it has been suggested that the number of large sunspots has a double-peaked maximum. Most indices vary roughly in phase with the sunspot number, and therefore the sunspot extrema are also referred to as the solar minimum and the solar maximum. On the other hand, some coronal indices are clearly out of phase with the sunspot cycle, and they peak at the sunspot …