To show one's acknowledgement to something by raising an eyebrow. -_^ can be used in various contexts; it can mean one is excited, aroused, inquisitive, curious, or just stimulated in general.
~Someone in the neighborhood sparks a bowl~
The neighborhood: -_^
Unlike other similar faces (like ^_^), this is supposed to look like eyebrows, with one raised.
Girl: Yeah, I would so do Amanda Palmer.
Girl's Boyfriend: -_^