An active region on the Sun (one is shown in figure 8.23) includes not only sunspots but related phenomena: plages, prominences, faculae, and flares, to name but a few. One feature common to all these natural phenomena is the strong magnetic field.

PLAGES are highly disturbed zones in the chromospheres. They usually appear before the spots and outlive them. Plages show as regions of intensified emission in photographs taken in Ha or the K-line of calcium. Local concentrations of magnetic field in the chromospheres apparently heat the gas to incandescence.

Brightening around an active region can also be seen in the white light from the photosphere. Such areas are called FACULAE, and they are located in the photosphere. They were first described in the seventeenth century by Hevelius.

FILAMENTS show on chromospheric photographs as dark streaks above a region of high magnetic field. They are actually pro¬minences seen in absorption against the bright disc.

PROMINENCES are among the most beautiful astronomical phenomena. Best seen when they extend beyond the Sun’s limb in an eclipse (or coronograph picture), they are luminous clouds of gas that appear in the corona. Spectra indicate a temperature of 10 000-20 000 K for prominence material. QUIESCENT PROMINENCES last for several solar rotations; they are regions of locally enhanced density that are falling out of the corona back to the photosphere . LOOP PROMINENCES are those in which the condensing material returns to the photosphere along a loop of magnetic flux similar to those hi figure 8.22. Early solar physicists thought that prominences were pyrotechnic displays hi which matter is violently spewed out of the photosphere, but time-lapse photography shows that the material is usually descending from the corona to the photosphere. Rarely, energetic ERUPTIVE PROMINENCES, which are spectacular surges in the vicinity of sunspots, do throw material in a burst that may extend 3 X 105km above the photosphere .

The appearance and extent of the corona are modulated by the solar cycle. The density increases twofold and the temperature by 20 per cent between solar minimum and maximum. Structural asymmetries are pronounced in the inner corona, where the details of plasma flow are under the control of the magnetic field. At sun-spot maximum the corona attains its greatest extent and is generally at its most symmetrical. Condensations, loops, and local enhancements hover over the active regions. Figure 8.26, taken at solar maximum, shows the coronal streamers extending to several solar radii. POLAR PLUMES have sprouted at the magnetic north and south poles. Large streamers, known as HELMET STREAMERS, overlie prominences in the lower corona. The corona of the active Sun evidently incorporates much structure which is controlled by com¬plex magnetic fields.

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