<P> Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K . The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma . The transition region does not occur at a well - defined altitude . Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion . The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum . </P> <P> The corona is the next layer of the Sun . The low corona, near the surface of the Sun, has a particle density around 10 m to 10 m . The average temperature of the corona and solar wind is about 1,000,000--2,000,000 K; however, in the hottest regions it is 8,000,000--20,000,000 K . Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection . The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere . A flow of plasma outward from the Sun into interplanetary space is the solar wind . </P> <P> The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with the solar wind plasma . This outermost layer of the Sun is defined to begin at the distance where the flow of the solar wind becomes superalfvénic--that is, where the flow becomes faster than the speed of Alfvén waves, at approximately 20 solar radii (0.1 AU). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves . The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun . In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause . In late 2012 Voyager 1 recorded a marked increase in cosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered the interstellar medium . </P> <P> High - energy gamma - ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only a few millimeters . Re-emission happens in a random direction and usually at a slightly lower energy . With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface . Estimates of the photon travel time range between 10,000 and 170,000 years . In contrast, it takes only 2.3 seconds for the neutrinos, which account for about 2% of the total energy production of the Sun, to reach the surface . Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years . This is the time it would take the Sun to return to a stable state, if the rate of energy generation in its core were suddenly changed . </P>

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