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The Wikipedia article on solar sails mentions that they could have a use in modyfing a spacecraft's orbit:

Robert L. Forward pointed out that a solar sail could be used to modify the orbit of a satellite around the Earth. In the limit, a sail could be used to "hover" a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits about the Sun that are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a statite. This is possible because the propulsion provided by the sail offsets the gravitational potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations.

Such a spacecraft could conceivably be placed directly over a pole of the Sun, and remain at that station for lengthy durations. Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity.

In his book The Case for Mars, Robert Zubrin points out that the reflected sunlight from a large statite placed near the polar terminator of the planet Mars could be focussed on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material.

Have solar sails ever been used for such a purpose, outside of proof-of-concept testing?

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A few years ago the Japanese spacecraft Ikaros traveled from Earth to Venus using a solar sail. See http://en.wikipedia.org/wiki/IKAROS. From the article:

The IKAROS probe is the world's first spacecraft to use solar sailing as the main propulsion.

Other spacecraft (noteably Hayabusa) have used solar pressure for attitude control. Again, quoting wikipedia (http://en.wikipedia.org/wiki/Solar_sail) :

Both the Mariner 10 mission, which flew by the planets Mercury and Venus, and the MESSENGER mission to Mercury demonstrated the use of solar pressure as a method of attitude control in order to conserve attitude-control propellant. Hayabusa also used solar pressure as a method of attitude control to compensate for broken reaction wheels and chemical thruster

However, no spacecraft has so far used solar sailing as a technique for hovering outside traditional orbital paths, as you have described in your question.

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  • $\begingroup$ All spacecraft have to account for solar pressure on the panels in their budgeting for attitude control fuel, and can position the panels advantageously to minimize that. However that's not what I'd call solar sailing. MESSENGER went beyond that, and used solar pressure to change its trajectory to the desired one for Mercury flybys. That's solar sailing. $\endgroup$
    – Mark Adler
    Jan 14, 2014 at 17:51
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    $\begingroup$ IKAROS was a technology demonstration mission. $\endgroup$
    – Mark Adler
    Jan 14, 2014 at 17:53
  • $\begingroup$ The secondary Kepler mission will also use solar pressure for attitude control, if I understood that correctly. $\endgroup$
    – E.P.
    Jan 23, 2014 at 0:58
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As far as I know, no solar sail has been used for any purpose, other than demonstration.

Well, except for MESSENGER. It was not designed as a solar sail, but since it flies so close to the Sun, they ended up using it like one. They were able to use solar sailing instead of firing rockets for trajectory correction maneuvers for Mercury flybys.

Sunjammer plans to demonstrate pole-sitting at Earth, as well as maintaining position at L1, which is unstable otherwise.

Update a few years later ...

Alas, Sunjammer was cancelled due to development issues. However the Planetary Society's citizen-funded LightSail successfully tested a solar sail deployment in May. That was only a deployment test, since the planned orbit was too low to permit solar sailing (too much drag). Another test is planned for next year in a higher orbit that will permit solar sailing.

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