A comet tail and coma are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[1]
While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 3.8 astronomical units (570 million km).[2] The observation of antitails contributed significantly to the discovery of solar wind.[3] The ion tail is the result of ultraviolet radiation blowing electrons off particles in the coma. Once the particles have been ionised, they form a plasma which in turn induces a magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the solar wind, so a bow shock is formed upstream of the comet (i.e., facing the sun), in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma. The field lines "drape" around the comet forming the ion tail.[4] (This is similar to the formation of planetary magnetospheres.)
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In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[5][6] but these detections have been questioned,[7][8] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.
If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[4] This has been observed on a number of occasions, notable among which was on the 20th. April 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection.[9] This event was observed by the STEREO spacecraft.[10] A disconnection event was also seen with C/2009 R1 (McNaught) on May 26, 2010.[11]
Comets were found to emit X-rays in 1996.[12] This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping of" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.[13]
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