Sunspots and faculae

The most prominent features on the solar disk are sunspots - dark blots that range from Earth-size motes to large complex regions covering billions of square kilometers.

Sunspots are essentially areas where the forces of concentrated magnetic fields inhibit currents of rising hot gas from deep within the Sun. With less hot gas reaching the surface, that part of the Sun is cooler (by about 1,500 kelvins) and thus appears darker. Despite their dark color, sunspots are actually hotter than the surfaces of a great many cooler stars. The spots only appear dark because they are cooler and contrast with their hotter surroundings. If we could somehow place a sunspot in nearby space, it would shine brighter than the Moon.

Sunspots consist of two sections: the penumbra and the umbra. The penumbra forms the outer halo of the sunspot with its feathery brushstroke appearance. Within the penumbra lies the darker umbra, where the magnetic field forces are strongest. Under high magnification, each sunspot has a character all its own. Some are remarkably symmetrical, resembling the deep yellow petals and black conical center of the North American wild flower, the black-eyed Susan, while others are peanut shaped or completely amorphous. Sunspots can form singly, but most cluster in sunspot 'archipelagoes' with multiple umbrae linked by sinuous dark threads running from one spot to the next. The most complex spots are tell-tale indicators of an intense magnetic field that might spawn a solar flare - a short-lived, intensely bright region caused

Sunspots and faculae. (Illustration by Jeff Kanipe.)

by the explosive release of energy from within the Sun. Flares are more prevalent during peaks in solar activity and, unfortunately, none except the very brightest are visible without a special narrow-band filter called a hydrogen-alpha filter.

Sunspots develop and decay over a period of two to three weeks, although in some cases, they can last as long as a month or more. In the 12 or so days it takes for sunspots to cross from the eastern limb to the western limb of the Sun, a sunspot group can change shape rapidly. In their development phase, they may swell in size and alter their structure or blossom into complex sunspot groups before dwindling back into the solar granules (the 1,000-kilometer wide convection cells that give the Sun its mottled appearance). If you follow sunspot activity for very long, you'll notice that spots near the Sun's poles take a little longer to transit from limb to limb than those nearer the equator. Though not completely understood by astronomers, this differential rotation is thought to be caused by the Sun's rapidly rotating core.

In addition to sunspots, bright, sinuous filaments called faculae may give the Sun a veined appearance. Faculae (from the diminutive Latin word facula, meaning 'little torch') are clouds of tenuous hot gas floating several hundred kilometers above the photosphere. They often precede the formation of sunspots and are commonly observed near a sunspot group. The brightest faculae (only slightly brighter than the photosphere) are harbingers of flares.

Faculae are best seen near the Sun's limb because of a phenomenon called limb darkening. Light from the Sun's limb must pass obliquely through a greater amount of the Sun's atmosphere lying above the photosphere, a region called the chromosphere, to reach the observer. This absorbs photospheric light from the limb. But since faculae actually float above the photosphere, their brightness isn't as suppressed by the chromosphere, making them stand out against the darker limb.

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