first light vs aurora

first light and aurora both are nouns.

Word	Noun	Adjective	Verb	Adverb
first light	Yes	No	No	No
aurora	Yes	No	No	No

As nouns, first light and aurora are synonyms defined as:

Other synonyms of first light include break of day, break of the day, cockcrow, dawn, dawning, daybreak, dayspring, morning, sunrise, sunup.

first light (noun)	aurora (noun)
the first light of day	an atmospheric phenomenon consisting of bands of light caused by charged solar particles following the earth's magnetic lines of force
	the first light of day