ELI5: What causes the 'space jellyfish' effect during rocket launches?

A "space jellyfish" is created when the sun shines on the expanding gas coming out of a rocket flying high in the dark sky.

To understand how this happens, imagine playing outside on a summer evening:

• The Flashlight Effect: Think about standing in a dark room. You cannot see the dust in the air until you shine a bright flashlight through it, making the floating specks suddenly glow. The rising rocket leaves behind a huge trail of exhaust gas—like the smoke from a car, but much bigger. The ground is dark because it is dusk or dawn, but the rocket is so high up that the sun can still see it. The sun acts like a giant flashlight, lighting up that gas cloud against the dark sky.
• The Balloon Effect: Down on the ground, thick air presses against everything. If you blow a bubble outside, it stays small. But way up high, where the rocket travels, the air is extremely thin and has almost no strength to push back. Because of this, the rocket's exhaust puffs out into a massive, wide dome instead of a straight line, just like watching a drop of food coloring bloom and spread through a bowl of water.

• The rocket's engine sprays out gas into space.
• The gas expands into a giant bell shape with long trails behind it, looking just like a swimming jellyfish.
• The hidden sun lights it up like a giant, glowing neon lamp in the night sky.