In Japanese Universities, there is no term that directly corresponds to the 'tenure' system that describes the permanent employment status of professors in American academia.

I am at a Japanese university as this very moment, and my (rather well-versed in English) professor just said that tenure would apply to both the 教授 (professor) and　助教授 (associate professor) of a Japanese university because both are the equivalent of a 正社員 (regular, salaried employee) of a Japanese company. He also said both 教授 and 助教授 are considered 教員 by the university, which refers to all regularly salaried academic professionals at a university.

For the sentence, I would say something like,

即時に地理学部の（正式・在職・在任）教員となり、
地理学部の教員資格を即時に得、

something like this...

Hope this helps.