that consistent and long term data accumulated with daily measurements being taken and the introduction of the Zurich number of sunspots. The Zurich number, or Wolf number, is now known as the International Sunspot Number and is coordinated by the Sunspot Index Data Centre in Belgium. The Zurich number and the group sunspot number, RG, are indicators of solar activity, the latter of which fills in missing sections of the Zurich record by combining data from various sources (Hoyt and Schatten, 1998). Both records are shown in Fig. 1.9, the Zurich number with dashed lines and the group sunspot num-ber as a solid line. The sunspot record provides the longest direct proxy record of solar activity. Two important results emerge from the record: the strong ∼11-year cycle and a long-term trend of the sunspot number amplitude increasing in time. This century-scale trend will be discussed in section 1.8.
1.6 Faculae, plages and the network
Faculae represent magnetic features that have the opposite effect on solar irradiance to that of sunspots and their relatives. Faculae are regions with higher contrasts than the quiet sun, as can be seen in Figs. 1.6 and 1.7. They can be found anywhere on the solar disk, but are always found near sunspots. At solar maximum, their influence increases the SSI at almost all wavelengths leading to the 0.1% amplitude in TSI over the solar cycle.
While observations of the entire disk show large, continuous areas of faculae, high res-olution images reveal they are composed of small brightenings extending up to just 0.600 (the Sun is 32’ across) or less than 500 km across (Lites et al., 2004). Although alter-native models have been proposed to describe faculae, including the hillock and cloud models (Schatten et al., 1986), Spruit (1976) was the first to describe these features and his bright wall model has become the generally accepted model and remained relatively unchanged since. It basically describes small, vertical flux tubes located on the edge of granules, evacuated and thus made optically thin from a strong magnetic field on the order of 1500 G (Rouppe van der Voort et al., 2005). This can be seen in the facular schematic of Fig. 1.10, taken from Keller et al. (2004) and adapted, where the thin layer between the granule wall and magnetic boundary, on the right, is the source of most of the brightness observed (white shading). Radiation flows in from the bright walls and heats the inner tube to temperatures above those of the surrounding quiet sun. The path through the at-mosphere to the observer is optically thinner than on the left and the oblique angle allows a larger area of the facula region to be seen.
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Chapter 1: Introduction 1.6 Faculae, plages and the network
The bright wall model is consistent with observations of facular contrast across the disk.
Facular regions of high magnetic field strength are darker at disk centre, at visible wave-lengths, and brighten towards the limb, while lower-strength faculae remain bright at the disk centre and increase in contrast slightly towards the edge (Ortiz et al., 2002). This fits with the idea that for a stronger magnetic field a larger flux tube base forms. With an increased diameter comes a reduced heating effect from the inflowing radiation, increas-ing the optical thickness and decreasincreas-ing the contrast. In fact, micropores may represent a contrast transition, between darker faculae and pores, beyond a diameter of 300 km (Grossmann-Doerth et al., 1994). At disk centre only the cool, low contrast (if large) base is visible, but as it rotates towards the limb, foreshortening effects reduce the base area while the hotter, brighter walls come into view and the increase in contrast differentiates faculae from the surrounding quiet sun. Therefore, the contrast of faculae are a function of diameter and disk position (Ortiz et al., 2002; Fligge et al., 2000). By comparing the left image of Fig. 1.6 at disk centre with the right image of Fig. 1.7 near to the limb, the faculae appear more prominent and easier to identify near to the limb due to their three-dimensional nature.
Plages are also regions of increased contrast, but are visible only in certain spectral bands or lines, famously the Ca II (Shine and Linsky, 1972) and Hα lines. They are found in the chromosphere co-spatially with small magnetic features, such as faculae and micropores (see Fig. 1.6 and Ortiz et al., 2002). Although their relationship is not fully understood they appear to be associated with magnetic field lines from facular regions extending above the photosphere (Martinez Pillet et al., 1997).
The last magnetic construct considered within this chapter is the network. This feature is defined in relation to the intra-network as as in Keller et al., (1994): there is a network of 0-2.5 kG field flux tubes located (Dom´ınguez Cerde˜na et al., 2006) at supergranular4 boundaries and an intra-network magnetic flux, within the supergranules, of very low magnetic field strength close to zero (Keller et al., 1994). Network elements consist of tiny, unresolvable flux tubes or magnetic elements, similar in contrast to faculae. Unlike faculae, though, they are thought not to be related to the large-scale solar dynamo but to small-scale dynamo action within small-scale convection closer to the surface. The rate of this small-scale low-flux emergence is anti-correlated with the solar cycle, but the
dis-4Supergranular regions are thought to be the highest order of convection that fragment into granules closer to the surface. They are a factor of 100 larger than the small scale granulation (e.g. in Fig. 1.6) and their boundaries can be identified as darker down-flow regions.
1.6 Faculae, plages and the network Chapter 1: Introduction
Figure 1.10: Schematic of magnetic flux tube, modified from Keller et al., (2004). Facu-lar brightening originates at the granule wall (right) where deeper layers can be observed and where most of the brightness contribution comes from (white shading). Looking into the downward convective region at an angle increases the visible area of the bright wall making faculae more prominent when closer to the limb.
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