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The heliosphere is the bubble-like region of space dominated by the Sun, which extends far beyond the orbit of Pluto. Plasma "blown" out from the Sun, known as the solar wind, creates and maintains this bubble against the outside pressure of the interstellar medium, the hydrogen and helium gas that permeates the Milky Way Galaxy. The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly. The Voyager spacecraft have actively explored the outer reaches of the heliosphere, passing through the shock and entering the heliosheath, a transitional region which is in turn bounded by the outermost edge of the heliosphere, called the heliopause. The overall shape of the heliosphere is controlled by the interstellar medium, through which it is traveling, as well as the Sun, and does not appear to be perfectly spherical.〔(Voyager 2 Proves Solar System Is Squashed NASA.gov #2007-12-10 )〕 The limited data available and unexplored nature〔(J. Matson - Voyager 1′s Whereabouts: No News, but Plenty of Noise (2013) - Scientific American )〕 of these structures have resulted in many theories. On September 12, 2013, NASA announced that Voyager 1 had exited the heliosphere on August 25, 2012, when it measured a sudden increase in plasma density of about forty times.〔(NASA Spacecraft Embarks on Historic Journey Into Interstellar Space (Sept. 2013) )〕 Because the heliopause marks one boundary between the Sun's solar wind and the rest of the galaxy, a spacecraft such as Voyager 1 which has departed the heliosphere can be said to have reached interstellar space. ==Summary== Except for localized regions near obstacles such as planets or comets, the heliosphere is dominated by material emanating from the Sun, although cosmic rays and fast-moving neutral atoms can penetrate the heliosphere from the outside. Originating at the extremely hot surface of the corona, solar wind particles reach escape velocity, streaming outwards at 300 to 800 km/s (671 thousand to 1.79 million mph or 1 to 2.9 million km/h).〔(The Solar Wind )〕 As it begins to interact with the interstellar medium, its velocity slows before finally stopping altogether. The point where the solar wind becomes slower than the speed of sound is called the termination shock; the solar wind continues to slow as it passes through the heliosheath leading to a boundary called the heliopause, where the interstellar medium and solar wind pressures balance. The termination shock was traversed by Voyager 1 in 2004,〔 and Voyager 2 in 2007.〔 It was thought that beyond the heliopause there was a bow shock, but data from Interstellar Boundary Explorer suggested the velocity of the Sun through the interstellar medium is too low for it to form. It may be a more gentle "bow wave".〔(G. P. Zank, et al. - HELIOSPHERIC STRUCTURE: THE BOW WAVE AND THE HYDROGEN WALL (2013) )〕 Voyager data led to a new theory that the heliosheath has "magnetic bubbles" and a stagnation zone.〔〔(NASA 2011 )〕 The 'stagnation region' within the heliosheath, starting around 113 AU, was detected by Voyager 1 in 2010.〔(NASA's Voyager Hits New Region at Solar System Edge 12.05.11 )〕 There the solar wind velocity drops to zero, the magnetic field intensity doubles, and high-energy electrons from the galaxy increase 100-fold.〔 Starting in May 2012 at 120 AU, Voyager 1 detected a sudden increase in cosmic rays, an apparent signature of approach to the heliopause.〔(NASA = Data From NASA's Voyager 1 Point to Interstellar Future 06.14.12 )〕 In December 2012 NASA announced that in late August 2012 Voyager 1, at about 122 AU from the Sun, entered a new region they called the "magnetic highway", an area still under the influence of the Sun, but with some dramatic differences.〔(NASA Voyager 1 Encounters New Region in Deep Space )〕 In the summer of 2013 NASA announced that Voyager 1 had reached interstellar space as of August 25, 2012.〔(NASA Spacecraft Embarks on Historic Journey Into Interstellar Space )〕 Cassini and IBEX data challenged the "heliotail" theory in 2009.〔〔 In July 2013, IBEX results revealed a 4-lobed tail on the Solar System's heliosphere.〔(NASA’s IBEX Provides First View Of the Solar System’s Tail' )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heliosphere」の詳細全文を読む スポンサード リンク
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