5. A Magnetic Star
The solar chromosphere
Just above the photosphere lies a relatively thin layer, about 2.5 million meters thick, called the chromosphere, from chromos, the Greek word for “color”. The chromosphere is so faint, and the underlying photosphere so bright, that the chromosphere was first observed during a total eclipse of the Sun. It became visible a few seconds before and after the eclipse totality, creating a narrow, rose-colored band at the limb of the Sun, punctuated by extended curls of red (Fig. 5.7).
The Sun’s temperature rises to about 10,000 degrees Kelvin in the chromosphere, but the gas density in the chromosphere drops to roughly a million times less than that of the photosphere. Because of its very low density, the chromosphere does not create absorption lines. Instead of absorbing radiation, the tenuous gas is heated to incandescence and emits spectral lines. Whenever the light from the chromosphere is isolated, during a total solar eclipse or by other means, we see bright emission lines shining at precisely the same wavelengths as many dark absorption lines in the photosphere’s light.
Dark Regions And Bright Plage
Sunspots extend from the photosphere into the chromosphere, creating dark regions in hydrogen-alpha photographs (Fig. 5.8). Bright regions, called plage from the French word for “beach”, glow in hydrogen light; they are often located near sunspots in places with intense magnetism. The plages are a chromospheric phenomena detected in monochromatic hydrogen-alpha light; they are associated with, and often confused with, bright patches in the photosphere, called faculae, that are seen near the solar limb in white light.
The chromosphere is jagged, irregular and by no means smooth or homogeneous. When observed in hydrogen alpha, numerous thin, luminous extensions, dubbed spicules by Father Angelo Secchi, may be seen (Fig. 5.9). They rise and fall like chopping waves on the sea or a prairie fire of burning, wind-blown grass. The needle-shaped spicules are about 2 thousand meters in width, and shoot up to heights of 15 million meters at speeds of about 20 thousand meters per second. Individual spicules persist for only five or ten minutes, but new ones continuously arise as old ones fade away. Approximately half a million of the evanescent, flame-like spicules are dancing in the chromosphere at any given moment.
We obtain a completely different view when the chromosphere is pictured in a calcium emission line (Fig. 5.10). Bright regions of calcium light correspond to places where there are strong magnetic fields, both above sunspots and all over the Sun in a network of magnetism (Fig. 5.11).
The calcium, or magnetic, network, coincides with the pattern formed by large-scale convective cells, known as the supergranulation, each about 30 million meters in diameter or 2.5 times the size of the Earth. The giant cells move horizontally across the photosphere, carrying the magnetic fields with them. Each supergranulation cell sweeps the magnetic fields to its outer edges, where the field collects and strengthens (Fig. 5.12). Chromospheric heating is produced above these field concentrations, resulting in the bright calcium emission that outlines the magnetic network (Fig. 5.10).
Filaments And Prominences
The hydrogen-alpha photographs of the chromosphere reveal massive loops of cool dense gas that arch up over the photosphere. Seen head on, they are elongated, dark features, called filaments, that stretch up to halfway across the face of the Sun (Fig. 5.13). The cool gas looks dark against the brightness of the hot Sun beneath it. When seen from the side at the edge of the solar disk, where the chromosphere extends beyond the lowest layers of the Sun’s atmosphere, these same features light up as bright loops, called prominences, against the dark background. They can be detected during a solar eclipse as large curling pink protuberances that extend beyond the Moon’s edge, hence the name prominence from the French word for “protuberance”.
Active prominences lie along the polarity inversion line of strong magnetic fields connected to sunspots within active regions. Active prominences are dynamic structures with violent motions and have life-times of only minutes or hours. There are various types, such as surges, sprays and loop prominences (Fig. 5.14).
Quiescent prominences are long, thin, vertical sheets of dense plasma, with a characteristic width of about 5 million meters and length of 100 million meters. They can extend tens and even hundreds of millions of meters above the edge of the Sun (Fig. 5.17). Some of them are big enough to girdle the Earth or even to stretch from the Earth to the Moon. Quiescent prominences are exceedingly stable structures that can last for many months. They lie along the magnetic neural lines of weak bipolar magnetic regions of the solar photosphere, and can form within the cavity below a coronal streamer.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University