<P> The observation of antitails contributed significantly to the discovery of solar wind . The ion tail is the result of ultraviolet radiation ejecting 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 . (This is similar to the formation of planetary magnetospheres .) </P> <P> 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". 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 . This event was observed by the STEREO spacecraft . A disconnection event was also seen with C / 2009 R1 (McNaught) on May 26, 2010 . </P> <P> On January 29, 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions ." </P>

Comet tails have been observed to extend to a distance of