I'd guess I. violacea, but I'm not sure. Wikiposedly there are some 500 Ipomoeas, and I don't have any idea how many of those are common enough that they might have wound up in the cornfield behind the house. I. violacea seems like a reasonable guess, from what I see while googling.
The cornfield is also home to a couple Helianthus sp. (possibly H. tuberosus?), though those are a lot harder to photograph, being tall, and so far I've been unsuccessful. Though it's possible that I've only tried once.