偶现流星 [天] sporadic meteor

偶现日珥 [天] incidental prominence

偶现种 casual species

偶然发现 happen on ; come across

偶尔发现 come across ; Run across

偶然发现的 serendipitous

偶然出现 come along ; Turn up

偶然发现或遇到 come upon ; To discover or meet by accident

偶然倒错现象 occasional inversion

偶然的发现 Accidental discovery ; a chance discovery ; occasionally found in