Idol Aura

An Idol Aura (アイドルオーラ) is the "trademark" or "scent" of an idol. Each idol has a different aura depending on their type, theme, and personality. In the anime, Raichi Hoshimiya is known to be an observer of idol auras and is capable of sensing the qualities of each aura to the point of giving a brief but accurate description.

Circumstances
There are three different circumstances regarding idol auras: changing auras, weak/strong auras, and unit auras.

Changing Auras
At some point, idol auras tend to change in different situations. Most often, wearing Premium Rare dresses will add a new element to an idol's aura, which eventually becomes an official part of their aura in later episodes. In other cases, performing in a group will often result in an aura becoming smaller and more compressed or depending on what type of performance (i.e. live, fashion show), normally in addition to a group, the entire aura will change positioning. This is more noticeable in idols who have a large number of elements in their aura like Yurika Todo or idols who have more than one positioning for different elements like Otome Arisugawa.

Weak and Strong Auras
Some girls, like Akari Ōzora, can have auras that are noticeably weaker than others during their first few performances. This is due to the fact that they have not yet "discovered themselves" as idols. Other idols, like Ichigo Hoshimiya and Sakura Kitaōji have exhibited especially strong auras that usually appear when they perform at important events (e.g. the guerrilla live announcing the Great Starmiya Ichigo Festival).

Unit Auras
As of Season 2, when idols perform in a unit they will sometimes acquire a unit aura. This is only possible when the idols are in complete harmony, and perfectly compliment each other's differences. This was originally only present with duo units, but as of Season 3, it applies to larger units as well.

Trivia

 * Before Episode 16 idol auras didn't always appear.
 * There are instances where an aura will appear as faint glows rather than the actual aura.
 * On rare occasions, an idol's aura can appear outside of the stage.
 * Although Soleil has performed as a unit many times, they didn't have a group aura until Episode 125.
 * The unit Skips♪ is the only known unit to have their own auras appear in a unit performance first, and then into a group aura that changes form.
 * However in Episode 138, the unit no longer used their solo auras, but used their group aura for the whole performance.
 * The first generation of Starlight girls are the first to have a group aura despite not being an official unit.
 * Ichigo, Mizuki and Akari are the second group of people to have a group aura despite not being a unit.
 * The second generation of Starlight girls are the third group of people to have a group aura despite not being an official unit.