adin musa JV
https://www.reddit.com/r/space/comments/3jm77s/picture_of_the_atlas_v_launch/
https://www.reddit.com/r/space/comments/3jmkvg/an_atlas_v_rocket_launches_a_new_tactical/
What you're seeing here is the exhaust of the rocket. But why is it spread out in this weird bell shape?
Well basically, rocket nozzles are 'tuned' to a certain pressure, which means they're tuned to a certain altitude (because the air gets thinner as you go up). When it's at that certain altitude, all the exhaust leaves the nozzle in a straight line. When it's lower than that altitude, air pressure pushes in on the exhaust and you get shock diamonds. When you're above that altitude, the exhaust expands outwards.
OP's picture is great, because you can see exactly where this transition happens! Where the exhaust is thick is below the nozzle's tuned altitude, and where it blossoms out into the cool bell shape is above it. Right between those is the point where the rocket is working most efficiently.
