Optical and particle observations of type B red aurora

Aurora occasionally exhibits short-lived appearance of a bright red border at the bottom of an auroral arc, band, or curtain. This is called a type B red aurora. Based on simultaneous measurements of auroral emissions from N ₂ 1PG ("red" aurora) and O (¹S) ("green" aurora) as well as incident electrons by the Reimei satellite, we show the following two individual observations. Energy flux and average energy of incident electrons (1) were not always higher in the red-dominated aurora than in the green-dominated aurora, and (2) were not correlated with the intensity of "green" auroras, but with the intensity of "red" auroras. These observational facts suggest that for the reddening of auroras, intense electron precipitation is unnecessary. A rapid movement, or appearance of electron precipitation is sufficient for the reddening because of the difference in lifetimes of N² 1PG and O (¹S).