In J. R. R. Tolkien's high fantasy legendarium, Arien [ˈari.en] is a beautiful maiden who guides 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 is mightier than Tilion, the Maia chosen to guide the Moon, because she is 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 splendour." (The Silmarillion
In writings by Tolkien, Tilion is said to have been in love with Arien, and because he sought her out the Moon was scarred by the sun's heat.
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. It is also stated in other abandoned writings that she is a Maia of Varda. It is not clear whether it was Tolkien's intent to keep these elements in The Silmarillion, had he lived long enough to publish it.
It is said that the most powerful of Morgoth's servants, Balrogs, were the spirits of the same kind as Arien, yet they had been seduced by the Dark Lord. The Maia Olórin is also frequently associated with fire. Nothing is known about the other fiery spirits.