Intrinsic authority is authority that comes from the rhetor herself. It might come from her work experience or college degrees or generally good morality, or it might come from how well she demonstrates that she can speak or write about her topic.
Aristotle, who coined the term "ethos," said that "persuasion is achieved by the speaker's personal character when the speech is so spoken as to make us think him credible." This is true, he said, because an audience will "believe good men more fully and more readily than others." For Aristotle, though, this kind of persuasion shouldn't derive from who the person is exactly; rather, it "should be achieved by what the speaker says, not by what people think of his character before he begins to speak."  For us, however, in an age in which Google is a click away, the speaker's character and achievements have an impact on the way we think about what she says. It's unavoidable. But the person's ability to speak authoritatively on her topic is just as important.
Remember, though, that a rhetorical situation, in which a speaker or writer seeks to purposefully persuade her audience, is an artificial situation: an author shares only a part of herself with the audience. To put it another way, as M. Jimmie Killingsworth, a scholar of rhetoric from Texas A&M University, does in a 2005 article in Rhetoric Review: "The author's position is not simply a personal account of himself or herself. The author is a complex individual who selectively reveals (or invents) aspects of character pertinent to the rhetorical work required at the moment" (251–52). 
 Aristotle. On Rhetoric. Trans. W. Rhys Roberts. The Internet Classics Archive. Web Atomic and Massachusetts Institute of Technology. 13 Sept. 2007. Web. 4 July 2010.
 Killingsworth, M. Jimmie. "Rhetorical Appeals: A Revision." Rhetoric Review 24.3 (2005): 249–63. Academic Search Premier. Web. 11 July 2010.
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