Sonic rainboom (event)

A sonic rainboom is an aerial stunt, depicted in the show twice, both times performed by Rainbow Dash. "Sonic rainboom" is a portmanteau of "sonic boom" and "rainbow". It appears in the episodes Sonic Rainboom and The Cutie Mark Chronicles. It is characterized by a radially rainbow-colored ring that expands from the point the sonic rainboom occurs, accompanied by a shockwave and a rainbow wake spanning behind the flier performing the stunt.

Sonic Rainboom
Sonic rainbooms are first mentioned in the episode Sonic Rainboom, where Dash practices her routine for the Best Young Flyer competition. Fluttershy tells her friends that Rainbow Dash is unable to perform the trick despite her dedicated practicing. Later in the episode, Dash performs a sonic rainboom as she speeds downwards to save Rarity and three Wonderbolts who are plummeting down to Earth. The sonic rainboom impresses the competition's spectators, and Rainbow Dash manages to save Rarity and the Wonderbolts, and wins the competition. This event is mentioned by Spitfire in the episode The Best Night Ever.

The Cutie Mark Chronicles
The second time sonic rainbooms are mentioned is in the episode The Cutie Mark Chronicles. Rainbow Dash, in a flashback, races against two other Pegasus ponies, and unwittingly performs a sonic rainboom while she speeds along the race's obstacle course. She earns her cutie mark almost immediately afterwards. Both this sonic rainboom and the one in Sonic Rainboom are performed when Rainbow Dash is leading with both front legs forward, while in her failed practice attempts she is leading with a single leg.

The very same sonic rainboom that earned Dash her cutie mark was involved in the other leading characters earning theirs. Applejack, who was visiting relatives in Manehattan, sees its rainbow wake which seemed to point back to Ponyville, and discovers she wants to work on her family's farm. Fluttershy calms animals that have been frightened by the sonic rainboom, and discovers her affinity to animals. Rarity witnesses a giant rock getting cracked open by the sonic rainboom, and discovers it contains gems. She affirms her love for fashion and gems and gets her cutie mark. Twilight Sparkle is personally affected by the sonic rainboom; it takes place during her entrance exam for Celestia's School for Gifted Unicorns, and sends her into a powerful magical state which allows her to hatch Spike. After a few accidents and Princess Celestia's intervention, Twilight discovers her cutie mark. Pinkie Pie draws inspiration from the sonic rainboom's rainbow wake and holds her very first party, which earns her her cutie mark. Pinkie Pie mentions in Sonic Rainboom that she has never seen a sonic rainboom before in her life. This contradicts her flashback in The Cutie Mark Chronicles where she witnesses one, however she only realizes it's a sonic rainboom at the end of the episode.

Reuse of animation
In the Equestria Girls song, a clip from The Cutie Mark Chronicles of Rainbow Dash performing a sonic rainboom is played while Pinkie Pie sings about "seeing rainbooms".

Similar effects
The "rainbow colored shockwave ring" effect is used again in different episodes. When the ponies use the Elements of Harmony to defeat Discord in The Return of Harmony Part 2, a similar ring emanates from them when they activate the Elements' magic. In Lesson Zero, Rainbow Dash demolishes a broken-down barn for Applejack in by flying into it. Dash crashes into the barn, creating a colorful mushroom cloud and a ring-shaped shockwave much like that of the sonic rainboom.