Other Stellar Surface Features

When observing the Sun in detail, there are not only spots which have been found. Surface-wide features include granulation which are a by-product of the convection originating from beneath the surface. There are also more localised features than spots. Pores, faculae and plages are often seen near spots and, as a collection, they are often calledactive regions. These active regions can be tens of thousands of kilometres wide and all the different constituents are all generated by a common source: magnetic activity. Active regions form from a bundle of magnetic field lines quickly rising buoyantly from the convective zone and out through the photosphere. This process then rapidly generates spots (for the Sun, within a few days) which

Figure 1.6:Images of other common surface features seen on the Sun:

a) Example of granulation (the bright cells and dark channels covering the whole image) andpores(the smaller, dark regions) taken with the Swedish Solar Telescope on 23 May 2010

(Image credit: Vasco Henriques, Institute for Solar Physics, Royal Swedish Academy of Sciences).

b) An active region near limb of the Sun, taken with the Swedish Solar Telescope on 29 June 2003; accompanying the spots in the photosphere are bright regions known as faculaewithin the chromosphere.

(Image credit: Dan Kiselman, Institute for Solar Physics, Royal Swedish Academy of Sciences).

slowly decay away (of the order of weeks and occasionally months) due to the fluid motions in the convective zone and photosphere. This indicates that the initial bundle of magnetic field lines was larger and more compact and coherent before being thrust out of the photosphere (Zwaan 1978,1992), and at that point it splinters into smaller, more intense bundles to form the spots and pores (Keppens & Martinez Pillet 1996;Leka & Skumanich 1998).

1.3.1.1 Granulation

This global feature is generated by the convective motion of plasma beneath the photosphere and creates a cellular appearance across the entire star (each granule can range in size from 700-1500km on the Sun). Similar to a saucepan of boiling water, ‘bubbles’ of plasma are heated from further down in the star and rise to the surface as a cell (Figure1.6). At the surface, the energy is dispersed into the photosphere and the plasma sinks back down through the darker, intergranual lanes surrounding the convection cells. The turnover time of a granule is only a few minutes.

Granulation has a complicated relationship with sunspots. Formed sunspots inhibit the

underlying convection, exposing and cooling the matter below. However, as will be seen in Chapter5, granulation has a significant role in the evolution of spots on a stellar surface.

1.3.1.2 Pores

In essence, pores are tiny spots where there is only an umbra and no penumbra (see Figure1.6).

They are typically a similar size to an individual granule (∼1000km), though pores on the Sun have been observed to reach sizes that rival the smallest sunspot. Often, pores are brighter in their umbrae than standard spots, but are darker than the intergranular lanes. Pores usually form when enough stray magnetic flux comes together through the action of convection, after being generated by convective collapse.

Whilst pores are not necessarily spot precursors, if several come together and coalesce a spot with a penumbra can begin to form. On average, they last for only 10-15 minutes when formed in the quiet photosphere; they can survive for up to a day when part of an active region.

1.3.1.3 Faculae & Plage

Spots appear as dark features, whereas faculae and plages appear close by as bright patches (faculae are only approximately 100K more than the local photosphere). In fact, when observing the Sun in white light, the most discernible features are the spots and faculae, with the faculae seen clearest at the limb of a star (Figure 1.6). Faculae consist of a large group of small facular points in the intergranular lanes, where the facular points are areas of magnetic flux concentration with diameters of roughly 200km on the Sun (Keller & von der Luehe 1992).

Ordinarily faculae are associated with active regions, but it is possible to find small facular points dotted across a stellar surface. They can sometimes also cluster into little lines within the intergranular lanes and formfiligreestructures (Dunn & Zirker 1973). Faculae are difficult to see when not close to the limb of a star. But they can be much more easily detected via line radiation – chromospheric lines like Ca II H and K correspond to the brightening in the chromosphere due to the facular brightening, these chromospheric bright patches are plages.

The pattern of the granulation beneath the plage regions is also altered – the convective cells are much smaller and the intergranular lanes are filled with strong magnetic field lines.

Within a facular region, elements form and decay with a time scale of a few hours; the regions themselves not only live longer, but exist much longer than spots. They also appear as much as several days before spots do, and will remain two or three times longer than the spot.

Spots are rarely seen without neighbouring faculae, though facular regions can appear without accompanying spots. As faculae are generated by strong magnetic fields, their size are a good indicator of the overall active region size. They begin as small, more compact regions but evolve

into larger, broken groups as convection picks away at the magnetic field lines and redistributes them across the quiet stellar surface. The total number of faculae seen on the stellar surface varies with the spot cycle. Given all the the additional emission from large faculae, for the Sun the overall brightness is in fact greatest at sunspot cycle maximum even when the reduction in brightness due to the dark spots is taken into consideration.