Sunspots

Sunspots are temporary phenomena on the Sun’s photosphere that appear as spots darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection. Sunspots usually appear in pairs of opposite magnetic polarity. Their number varies according to the approximately 11-year solar cycle.

The earliest record of sunspots is found in the Chinese I Ching, completed before 800 BC. The text describes that a dou and mei were observed in the sun, where both words refer to a small obscuration. The earliest record of a deliberate sunspot observation also comes from China, and dates to 364 BC, based on comments by the astronomer Gan De in a star catalogue. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.

Individual sunspots or groups of sunspots may last anywhere from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from 16 km (10 mi) to 160,000 km (100,000 mi). 

Larger sunspots can be visible from Earth without the aid of a telescope. They may travel at relative speeds, or proper motions, of a few hundred meters per second when they first emerge.

Sunspots have two parts: its center umbra, the darkest part, where the magnetic field is approximately vertical (normal to the Sun’s surface) and the surrounding penumbra, which is lighter, where the magnetic field is more inclined.

The temperature of the umbra is roughly 3,000–4,500 K (2,700–4,200 °C), in contrast to the penumbra at about 5,780 K (5,500 °C) leaving sunspots clearly visible as dark spots, occasionally visible even to the naked eye.

This is because the luminance (which is essentially “brightness” in visible light) of a heated black body (closely approximated by the photosphere) at these temperatures varies greatly with temperature. Isolated from the surrounding photosphere, a single sunspot would shine brighter than the full moon, with a crimson-orange color.

Sunspots are observed with land-based and Earth-orbiting solar telescopes. These telescopes use filtration and projection techniques for direct observation, in addition to various types of filtered cameras. Specialized tools such as spectroscopes and spectrohelioscopes are used to examine sunspots and sunspot areas. Artificial eclipses allow viewing of the circumference of the Sun as sunspots rotate through the horizon.

Due to its link to other kinds of solar activity, sunspot occurrence can be used to help predict space weather, the state of the ionosphere, and hence the conditions of short-wave radio propagation or satellite communications.

High sunspot activity is celebrated by members of the amateur radio community as a harbinger of excellent ionospheric propagation conditions that greatly increase radio range in the HF bands. During sunspot peaks, worldwide radio communication can be possible on frequencies as high as the 6-meter VHF band.

Solar activity (and the solar cycle) have been implicated in global warming, originally the role of the Maunder Minimum of sunspot occurrence in the Little Ice Age in European winter climate.

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