When an atomic bomb goes off, what are the lines of smoke that shoot straight up next to the mushroom cloud?

Answer 1

I think what you're thinking of are are vortices - some of the gases at the edge of the "stem" cool off and begin to move back downward towards the earth (since cold gases are less dense than hot gases). They curve downward and outward, away from the stem.

Answer 2

Those pictures of A-bomb explosions showing those vertical lines of smoke are the trails of small rockets that were once launched during A-bomb tests to help scientists measure differences in the explosion's force at different altitudes.

Answer 3

Souls winging heavenward.