Magnetic fields in the solar photosphere

Paul J Bushby * Articles on similar topics can be found in the following collections Receive free email alerts when new articles cite this article - sign up in the box at the top right-hand corner of the article or click here - Email alerting service Magnetic fields in the solar photosphere BY PAUL J. BUSHBY* School of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK Recent high-resolution observations of the surface of the Sun have revealed the fine structure of a vast array of complex photospheric magnetic features. Observations of these magnetic field structures have already greatly enhanced our theoretical understanding of the interactions between magnetic fields and turbulent convection, and future photospheric observations will inevitably present new theoretical challenges. In this review, I discuss recent progress that has been made in the modelling of photospheric magnetic fields. In particular, I focus upon the complex field structures that are observed within the umbrae and the penumbrae of sunspots. On a much smaller scale, I also discuss models of the highly localized magnetic field structures that are observed in less magnetically active regions of the photosphere. As the spatial resolution of telescopes has improved over the last few years, it has now become possible to observe these features in detail, and theoretical models can now describe much of this behaviour. In the last section of this review, I discuss some of the remaining unanswered questions. 1. Introduction Modern ground-based and space-borne telescopes enable us to make detailed observations of magnetic fields at the solar surface (also known as the solar photosphere). For example, the Swedish 1 m Solar Telescope (SST) on La Palma can resolve features down to 70 km at the surface of the Sun. Such high spatial resolution is essential in order to capture the fine structure of magnetic fields in sunspots, which are the largest and therefore the most obvious photospheric magnetic features. On a much smaller scale, in less magnetically active regions of the photosphere it is also now possible to observe highly localized concentrations of intense vertical magnetic flux (with a diameter of the order of 100 km). Over the last couple of years, our understanding of all these magnetic field structures has been further enhanced by observations that have been carried out using the Solar Optical Telescope (SOT) on board the recently launched Hinode satellite. These observations have already presented theorists with new challenges, and further questions are likely to emerge in the near future. One contribution of 12 to a Triennial Issue Astronomy. It is clear that the vigorous convective motions that are observed at the solar surface play a crucial role in the evolution of photospheric magnetic fields. For this reason, the Sun is an excellent laboratory for the study of magnetoconvection, a branch of magnetohydrodynamics, which describes the complex interactions between magnetic fields and convection in an electrically conducting fluid (e.g. Proctor & Weiss 1982). A proper description of this process is essential in order to understand photospheric magnetic fields. Although it is not yet possible to carry out numerical simulations of magnetoconvection in realistic solar parameter regimes, idealized numerical calculations can now reproduce behaviour that can be qualitatively related to magnetic features at the solar surface. In this short paper, I review some of the recent progress that has been made in the theory and observation of photospheric magnetic fields. In 2, I focus upon the properties of sunspots. This is followed by a discussion of magnetic features away from active regions, in the quiet Sun. In 4, I describe some of the remaining issues that have yet to be resolved. 2. Sunspots The presence of dark spots on the surface of the Sun was noted long before the invention of the telescope. Historical accounts of sunspot observations in China and Greece suggest that astronomers have been aware of the existence of these features for well over 2000 years. Telescopic observations in the seventeenth century, carried out by Galileo and others, provided the first hint that sunspots were not uniformly dark features. In fact, these observations indicated that sunspots consist of a dark umbra surrounded by a slightly brighter region called the penumbra. However, it was not until the beginning of the twentieth century that the true nature of sunspots became apparent. Hale (1908) showed that sunspots are the sites of strong magnetic fields. Using the Zeeman effect, Hale measured a magnetic field strength of approximately 3000 G, which is several orders of magnitude larger than the Earths magnetic field. We now know that sunspots are simply the surface manifestations of an underlying large-scale magnetic field within the Sun, which is generated and maintained by a hydromagnetic dynamo (Ossendrijver 2003). The properties of the large-scale solar magnetic field are also discussed by Silvers (2008). In recent years, it has become possible to observe the fine structure of sunspots. Figure 1 shows a high-resolution SST image of part of a large sunspot (Scharmer 2002). The dark umbral region of the spot is clearly visible in the lower part of this image. This umbra is surrounded by a well-defined penumbra, which is characterized by a complex radial pattern of bright and dark filaments. Magnetic field measurements within sunspots indicate that the field lines are predominantly vertical in the umbral region (i.e. perpendicular to the surface of the Sun). However, the magnetic field geometry within the filamentary penumbral region is much more complicated (e.g. Title et al. 1993). Along dark filaments within the penumbra, the magnetic field lines are nearly parallel to the surface of the Sun, dropping just below the surface towards the outer edge of the spot. On the other hand, the magnetic field lines that are associated with the bright filaments are inclined at an angle of approximately 30 degrees to the vertical at the umbralpenumbral boundarythe angle of inclination of these Magnetic fields in the solar photosphere field lines increases with radial distance away from the umbra. The net effect of all this is that the magnetic field in the penumbra forms a remarkable interlocking-comb structure. This is shown schematically in figure 2. Furthermore, even within the filaments themselves, the magnetic geometry is nontrivialrecent observations suggested that the field lines in these regions are often twisted (Ryutova et al. 2008). Much more detailed accounts of sunspot observations are given by Solanki (2003) and Thomas & Weiss (2004, 2008). Theoretical studies of magnetoconvection have shown that a strong vertical magnetic field, such as that found within a sunspot umbra, inhibits convective motions (e.g. Proctor & Weiss 1982). Near the visible surface of the Sun, convection i (...truncated)