Arien was the maiden whom the Valar chose from among the Maiar to guide the vessel of the Sun. In the days of the trees in Valinor, Arien had been the one to tend the tree of Laurelin. It is said that she was mightier than Tilion, the Maia chosen to guide the Moon, because she was a spirit of fire, and able to withstand the heats of Laurelin.
"Too bright were the eyes of Arien for even the Eldar to look on, and leaving Valinor she forsook the form and raiment which like the Valar she had worn there, and she was as a naked flame, terrible in the fullness of her splendor."
In other writings, Morgoth wanted to claim Arien as a wife, and ravished her, upon which she abandoned her body and 'died', leaving the Sun to travel through the skies uncontrollably and burning parts of Arda the world. As well, it is stated in other abandoned writings that she was a Maia of Varda. It is not clear if it was Tolkien's intent to keep these elements in The Silmarillion tradition, had he lived long enough to publish it.