The Heliosphere (from the Greek word "helios" meaning sun) is a huge magnetic bubble created by the solar wind emanating from our Sun, consisting almost entirely of charged material (electrons and ions) ejected from the Sun's surface. Discovered three decades ago by Eugene N. Parker of the University of Chicago, the heliosphere sits above the chromosphere and extends out past the far reaches of our solar system. At the point at which this solar wind passes the Earth, it's moving at around 1,500,000 km/h.

At some point outside our solar system this supersonic wind slows down to meet the gases already existent in the interstellar medium. The gases first undergo a termination shock, where the gases are "backed up" behind the heliopause, drop to a subsonic speed and slowly start to turn in the direction of the the interstellar flow, giving the effect of a comet-like tail spiralling out behind the Sun (this is also caused by the Suns 27 day rotational period). This subsonic wind exists in an area known as the heliosheath. The outer skin of this heliosheath, which where the heliosphere meets the interstellar medium, is called the heliopause.

The precise locations of the heliopause, heliosheath and termination shock are unknown to scientists and are also thought to vary dependant upon solar activity. More information will become available soon when Voyager 1 and Voyager 2 pass through the termination shock and out into the heliosheath. They are expected to reach the interstellar gap sometime around 2008

Information taken from:
http://wwwssl.msfc.nasa.gov/ssl/pad/solar/heliosph.htm
http://helios.gsfc.nasa.gov/heliosph.html
http://www.windows.ucar.edu/cgi-bin/tour_def/glossary/heliosphere.html
http://sci.esa.int/content/doc/10/2576_.htm

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