The Evershed flow

The horizontal Evershed outflow at photospheric heights in the penumbra is an inherent feature of sunspots. The existence of this flow is inferred from the Evershed effect, 6 As previously suggested by Wentzel (1992). which consists of a wavelength shift and an asymmetry of spectral lines formed in penumbra. The effect is seen in essentially all fully developed sunspots and appears immediately after the penumbra first forms. Beginning with Evershed's (1909a) first report, the cause of the...

A13 Imaging and narrowband filters

In contrast to stellar observations, the availability of sufficient sunlight allows the use of filters with very narrow band passes. Filters of bandwidth around 1 nm are routinely used to isolate certain spectral regions, such as the broad chromospheric Ha and Ca II H and K lines, and for the photosphere most notably the G-band at wavelength 430 1nm which is dominated by many absorption lines of the CH molecule. The intensity in this band is highly temperature sensitive, due to the high...

Sunspots and Starspots

The past two decades have seen remarkable advances in observations of sunspots and their magnetic fields, in imaging of spots and fields on distant stars, and in associated theoretical models and numerical simulations. This volume provides the first comprehensive combined account of the properties of sunspots and starspots. It covers both observations and theory, and describes the intricate fine structure of a sunspot's magnetic field and the prevalence of polar spots on stars. The book...

Light bridges

The umbrae of most sunspots are at some time, especially late in their lives, crossed by narrow, bright features known as light bridges. These features come in a variety of shapes, sizes, and brightnesses, with the largest of them extending all the way across the umbra and covering an appreciable fraction of its area. Most light bridges are segmented, with bright segments separated by narrow dark lanes lying perpendicular to a long, narrow, central dark lane running along the length of the...

Sunspot groups

Although individual, isolated sunspots are not uncommon, most sunspots occur in groups, which are often very large and complex. Studies of the properties and motions of spot groups reveal information about the dynamo-generated magnetic field in the solar interior. Such studies are quite demanding, however, requiring long-term synoptic observations and careful data analysis. Important contributions include the summary by McIntosh (1981) of his 25 years of sunspot observations. Sunspot groups...

Observations of the magnetic field in sunspots

Measurements of magnetic fields on the Sun rely on the Zeeman effect, which produces splitting and polarization of spectral lines. Here we outline how this effect can be used not only to measure strong longitudinal fields directly but also to detect weaker fields by measuring circular polarization and, more recently, to measure the full vector magnetic field by determining the four Stokes parameters across a spectral line. Then we go on to describe the average magnetic structure, first of pores...

The Sun as a star

In this section we first describe the overall properties of the Sun and the structure of its interior and atmosphere, and then go on to summarize the overall properties of other stars to set the stage for our discussions of starspots and stellar activity. Table 1.1 provides a list of important properties of the Sun that will prove useful in our discussions of sunspots and solar magnetism. For a clear discussion of how the values of these various quantities are determined, see the book by Stix...

Stellar dynamos

While it is reasonable to adapt solar dynamo models to describe cyclic magnetic activity in other slowly rotating stars, it is dangerous to assume that younger, rapidly rotating and more active stars can be treated in the same way. Indeed, the observations suggest that the relationships between Pcyc and Ro differ depending on whether stars lie below or above the Vaughan-Preston gap in Figure 8.2 (see Section 10.4.1). Furthermore, the magnetic behaviour of late M stars, which are fully (or...

Basic dynamo theory

Most studies of solar or stellar dynamos have been restricted to axisymmetric models, in which the magnetic field is averaged azimuthally to give a solenoidal mean field B that satisfies the induction equation dB dt V x (U x B) + n0V2B, (11.1) 1 By contrast, although x 7000 yr for the Earth, the geomagnetic field has existed for at least 3 x 109 yr, and therefore has to be maintained by a dynamo. where the magnetic diffusivity no is assumed to be uniform. Here U um + sz)e is the axisymmetric...

Penumbral filaments

At moderate spatial resolution (1 -2 ) the penumbra is seen to consist of alternating bright and dark elongated filaments. These filaments have a predominantly radial alignment, which is most apparent in a single isolated spot, like that in Figure 3.1. In a minority of such spots, the pattern shows a distinct vortical structure (which is much more prominent in Ha spectroheliograms see Section 3.3.3), with a sense that corresponds (statistically) to that of terrestrial cyclones and is...

The intricate structure of the penumbral magnetic field

Early measurements of azimuthally averaged velocities and magnetic fields had already raised a contradiction in a steady state the flow should be parallel to the magnetic field in a highly conducting plasma, yet the persistent Evershed flow was horizontal while the magnetic field reached an inclination of only 70 (with respect to the local vertical) at the edge of the spot (e.g. Adam and Petford 1991), as shown schematically in Figure 5.5a. This paradox could only be resolved by assuming an...

Properties of individual starspots

We now turn to the physical properties of individual starspots, including their temperatures and magnetic fields, and their lifetimes, as deduced from the available observations. Finally, we assess the similarities between starspots and sunspots. Measurements of starspot temperatures are made using several different techniques, the most elementary being simultaneous modelling of photometric brightness and colour variations. Doppler imaging also provides best-fit values of these temperatures....

Starspots differential rotation and dynamo patterns

The Sun's surface differential rotation was discovered by tracking the motion of sunspots across the solar disc, so it is only natural to think that similar tracking of starspots might reveal surface differential rotation on stars. Analysis of the photometric variability of stars such as BY Dra led some authors to suggest that the surface features that caused this variability differed in their rotation rates because of surface differential rotation (Vogt 1975 Oskanyan et al. 1977). Other early...

Spectroscopy and atmospheric models

3.3.1 Spectral analysis of the umbra and penumbra The vertical temperature structure in the photosphere of a sunspot is revealed by measurements of the variation of continuum intensity with wavelength within the spot at a fixed position on the solar disc, or by the variation of intensity at a fixed wavelength as the spot moves from disc centre to the limb (Maltby 1992). Measuring the centre-to-limb variation of continuum intensity ('limb darkening') is a standard technique for determining the...

A22 Kinematic MHD flux freezing flux concentration and flux expulsion

In the limit of perfect electrical conductivity (n 0), the induction equation reduces to dB dt V x (u x B) (B V)u - (u V)B - BV u, (A2.9) whence it follows that the magnetic flux through a circuit moving with the fluid remains constant (Alfven's theorem). Alternatively, combining Equations (A2.9) and (A2.8), we find that This can be interpreted as implying that the magnetic field moves with the fluid, i.e. that magnetic field lines are frozen into the perfectly conducting fluid. It follows that...

Sunspot seismology

Thomas, Cram and Nye (1982) were the first to suggest that sunspot oscillations could be used to probe the subsurface structure of a sunspot, introducing the concept of 'sunspot seismology' based on the interaction of the solar p-modes with the sunspot and the way in which different p-modes sample different depths below the solar surface. They interpreted a temporal power spectrum of 5-minute umbral oscillations in terms of a naive model of the interaction of these oscillations with the...

Modelling the overall magnetic structure of a sunspot

We turn now to a discussion of theoretical models of the gross magnetic structure of a sunspot. Two alternative pictures were originally proposed see Fig. 4 of Thomas and Weiss 1992b . Cowling 1946, 1976a had favoured a single, coherent, monolithic tube of magnetic flux both above and below the solar surface. Ignoring fine structure, this assumption is justified for the visible layers of the spot, where the plasma beta is relatively low and the field must be essentially space-filling. For the...

Thermal properties of sunspots

Here we discuss the distribution of temperature and other thermodynamic variables in a sunspot, beginning with the fundamental question of why sunspots are significantly cooler and darker than the surrounding quiet photosphere. In a typical sunspot the umbra radiates only 20 to 30 of the flux integrated over wavelength of the quiet Sun and the penumbra radiates some 75 to 85 of the quiet Sun flux. This implies that, on average, the umbra is 1000 to 1900 K cooler and the penumbra is 250 to 400 K...

The sunspot cycle

In 1826 the German amateur astronomer Heinrich Schwabe 1789-1875 began his systematic observations of the Sun in search of the transit of a possible planet inside the orbit of Mercury. As part of this search, which continued for 43 years, Schwabe carefully recorded the occurrence of sunspots and in the process discovered the sunspot cycle. His first announcement Schwabe 1843 of a possible 10-year periodicity in the number of sunspots attracted little notice, but in 1851 his table of...

Fine structure in sunspots

Langley Sunspot Drawing

Over the past two centuries the study of sunspots has emphasized more and more the determination of the detailed structure of an individual spot, based on observations with increasing resolution. The invention of achromatic lenses made it possible to construct refracting telescopes with much higher resolution. In 1826 several large spots were observed by Ernesto Capocci at Naples, with a 9-foot Fraunhofer refractor he noted for the first time the characteristic filamentary structure of the...

Early observations of sunspots

Sunspot Rotation

Seasonal changes were all-important to early agrarian societies and so they naturally worshipped the Sun. Indeed, the sun-god headed the pantheon in many cultures, ranging from Egypt to Peru. Since astronomy was also practised in these cultures, it seems likely that sunspots must occasionally have been detected with the naked eye, which is possible when the Sun is low on the horizon and partially obscured by dust storms, volcanic dust or smoke. Thus Needham 1959 conjectured that the traditional...

Morphology of sunspots

Sunspots come in a wide range of sizes. The largest spots have diameters of 60 000 km or more, while the smallest have diameters of only about 3500 km, less than the diameters of the largest pores about 7000 km . The area of a sunspot is typically given in units of millionths of the surface area of a solar hemisphere 1.0 x 10-6 Aq 2 3.044 x 106 km2, where Aq 2 2n rq is the area of the visible hemisphere. The largest recorded sunspot in March 1947 had an area of 4300 x 10-6 Aq 2 Newton 1955...