the US Indices are traded 24 hours a day(excluding Saturday and Sunday)…What you see in the London hours(before US hours) of trading are mostly UK institutions adjusting their US positions.
However, as majority of orders hit the exchange at 09:30 EST. 9:30-10:00 EST will always have the highest volatility as orders from around the world are flooding in and a price is trying to settle.
I was part of a team that built an algo for a bank that wanted to trade the first 5 minutes before open…I can walk you through the process and some of the rules they used
If you want high vol I would recommend US100 at the open