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Sunspots: an Overview The Astron Astrophys Rev (2003) 11: 153–286 THE Digital Object Identifier (DOI) 10.1007/s00159-003-0018-4 ASTRONOMY AND ASTROPHYSICS REVIEW Sunspots: An overview Sami K. Solanki Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany (e-mail: [email protected]) Received 27 September 2002 / Published online 3 March 2003 – © Springer-Verlag 2003 Abstract. Sunspots are the most readily visible manifestations of solar magnetic field concentrations and of their interaction with the Sun’s plasma. Although sunspots have been extensively studied for almost 400 years and their magnetic nature has been known since 1908, our understanding of a number of their basic properties is still evolving, with the last decades producing considerable advances. In the present review I outline our current empirical knowledge and physical understanding of these fascinating structures. I concentrate on the internal structure of sunspots, in particular their magnetic and thermal properties and on some of their dynamical aspects. Key words: Sunspots – Sun: magnetic field – Sun: active regions – Sun: activity 1. Introduction Sunspots are magnetic structures that appear dark on the solar surface. Each sunspot is characterized by a dark core, the umbra, and a less dark halo, the penumbra. The presence of a penumbra distinguishes sunspots from the usually smaller pores. Images of sunspots made in the g-band around 4305 A˚ are shown in Fig. 1.1. Naked eye observations of sunspots are known from different cultures (Bray & Loughhead 1964). In particular, the ancient Chinese have kept detailed although very incomplete records going back over 2000 years (Wittmann & Wu 1987,Yau & Stephen- son 1988, Eddy et al. 1989). Nevertheless, it was the rediscovery of sunspots by Galilei, Scheiner and others around 1611, with the help of the then newly invented telescope, that marked the beginning of the systematic study of the Sun in the western world and heralded the dawn of research into the Sun’s physical character. Over the ages the pre- vailing view on the nature of sunspots has undergone major revisions. The discovery of the Wilson effect in 1769 (see Sect. 2.5) even completely changed the prevailing picture of the structure of the entire Sun, at least temporarily. Since the dark umbrae appeared 154 S.K. Solanki Fig. 1.1. Images recorded in a roughly 10 A˚ wide band centred on 4306 A˚ (g-band) of a relatively regular sunspot (top) and a more complex sunspot (bottom). The central, dark part of the sunspots is the umbra, the radially striated part is the penumbra. The surrounding bright cells with dark boundaries are granular convection cells. The upper sunspot has a maximum diameter of approx- imately 30000 km, the lower sunspot of roughly 50000 km (upper image courtesy of T. Berger; lower image taken by O. von der Lühe, M. Sailer, T. Rimmele). Sunspots: An overview 155 to lie deeper than the rest of the solar surface, it was for a time widely believed that the entire solar interior is dark and hence cool, compared to the bright photosphere outside sunspots. The breakthrough in our understanding of the nature of sunspots came in 1908 when George Ellery Hale first measured a magnetic field in sunspots (Hale 1908b). Since then the magnetic field has become firmly established as the root cause of the sunspot phenomenon. Overviews of the structure and physics of sunspots are given in the monograph by Bray & Loughhead (1964), in the proceedings edited by Cram & Thomas (1981), Thomas & Weiss (1992a), Schmieder et al. (1997) and Strassmeier (2002), as well as in the review articles by Zwaan (1968, 1981), Moore & Rabin (1985), Schmidt (1991), Soltau (1994), Thomas & Weiss (1992b), Schmidt (2002), Stix (2002) and Sobotka (2003). In addition, specific aspects of sunspots have been reviewed by a host of other authors. References to these are given in the relevant sections of this article. The present article borrows heavily from earlier reviews, specifically those by Solanki (1995, 1997b, 2000a, b, 2002). An introduction to the related, generally smaller sunspot pores, often also simply termed pores, has been given by Keppens (2000). I take the approach of dividing the discussion of sunspots according to their phys- ical parameters, e.g., magnetic field, velocity, thermal structure, etc. Another possible approach would be to discuss sunspots piecewise according to morphological features, i.e., umbra, penumbra, light bridges, δ-spots etc., or according to the physical processes acting on and within them. Each approach has its advantages and disadvantages. Thus, in the present article, e.g., the magnetic structure of light bridges is discussed in the context of that of the rest of the sunspot, at the cost of making the connection between the magnetic and thermal structure of these features more cumbersome. The advantage is that e.g., the magnetic properties of all parts of a sunspot can be easily compared. Basically, this article covers similar ground as chapters 4, 5 and 8 of Bray & Lough- head (1964), with the difference that the current article is more up-to-date and the emphasis is placed on aspects presently more in vogue, which reflects the evolution of the field over the past 40 years. A number of important aspects of sunspots are not or hardly covered here. These in- clude their morphology, oscillations (references to relevant reviews are given in Sect. 7), their spectrum and their relation to starspots (see the volume edited by Strassmeier 2002 for more on this connection). This review is structured as follows: In Sect. 2 general properties of sunspots are summarized, including location on the Sun, size, lifetime, evolution and Wilson effect. This is followed by a description of our empirical knowledge of the sunspots’ magnetic field in Sect. 3. An overview of the physical understanding of the magnetic structure of sunspots is given in Sect. 4. Sections 5 and 6 review the observations and theory of sunspot brightness and temperature. Both observational and theoretical aspects of flows within sunspots are presented in Sect. 7. Finally, I conclude with a list of open questions in Sect. 8. 156 S.K. Solanki 2. General properties of sunspots In this section I briefly summarize the basic observed characteristics of sunspots, i.e. their sizes, lifetimes, brightness, evolution, Wilson depression, as well as their magnetic, thermal and velocity structure. The last three quantities are discussed in far greater detail in the following sections. 2.1. Location and evolution The number of sunspots varies strongly over a solar cycle (Harvey 1992), as was first discovered by Heinrich Schwabe in 1843. There are times at and near solar activity minimum when not a single sunspot is present on the solar disc, while 10 or more sunspots are not uncommon around the maxima of recent solar cycles. Later, systematic daily observations of the solar disc with the specific aim of counting the number of sunspots were started by Rudolf Wolf, who also introduced the sunspot relative number (also called the Zürich number or Wolf number), which is still used as a measure of the coverage of the solar disc by sunspots. Recently, a somewhat more robust variant, the group sunspot number, has been introduced by Hoyt & Schatten (1998). Sunspots are always located within active regions, which, typically, have a bipolar magnetic structure. The sunspots are thus mainly restricted to the activity belts reaching up to 30◦ on each side of the solar equator. The latitudes of the sunspots vary with the solar activity cycle. Early in a cycle they appear at high latitudes, with occasional spots lying up to 40◦ from the equator. In the course of the cycle the new sunspots appear at increasingly lower latitudes, with the last sunspots of a cycle lying close to the equator. This behaviour was first noticed by Carrington (1863) and can be illustrated by a so- called butterfly diagram. Such a diagram is plotted in Fig. 2.1. The fact that sunspots are restricted to low latitudes as well as other observations – such as the somewhat lower latitude of the preceding polarity relative to the following polarity of an active region (Joy’s law, Joy 1919, Brunner 1930) – can be reproduced by a model in which sunspots and their host active regions are formed by the emergence of a large magnetic flux tube through the solar surface. It is proposed that near the solar surface this flux tube breaks up into many smaller tubes, the larger of which constitute sunspots (e.g. Zwaan 1978). Sunspots are thus only the most prominent examples of magnetic flux tubes on the Sun. The visible sunspot constitutes the intersection of the solar surface with such a flux tube. The sub-surface footpoints of this flux tube are thought to be anchored in the overshoot layer below the convection zone, where its field strength is an order of magnitude above the equipartition value of approximately 104 G (D’Silva & Choudhuri 1993, D’Silva & Howard 1994, Schüssler et al. 1994, Caligari et al. 1995). An initial field strength of roughly 105 G is also supported by studies of the stability of this flux tube in the overshoot layer (Moreno Insertis et al. 1992, Moreno Insertis 1992, 1997). A review is given by Schüssler (2002). Sunspots form the heart of an active region. There is, however, often an asymmetry between the leading and following polarities, with the leading polarity often harbouring the dominant sunspot although in some cases the following polarity may contain an equally massive spot. Sunspots and in particular sunspot groups are classified according Sunspots: An overview 157 Fig. 2.1. Butterfly diagram (upper panel) and record of relative solar surface area covered by sunspots (lower panel). Upper panel: the vertical axis indicates solar latitude, the horizon- tal axis time.
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