Complex sentence: After completing his plasma physics Ph.D. and upon the advice of his thesis advisor, Soon did post-doctoral research at the Harvard-Smithsonian Center for Astrophysics and has been doing research in astrophysics and earth science there as an externally funded non-tenured employee since 1991.
Split into simpler sentences: After completing his plasma physics Ph.D. and upon the advice of his thesis advisor, Soon took up a post-doctoral research position at the Harvard-Smithsonian Center for Astrophysics. He has been doing research in astrophysics and earth science there as an externally funded non-tenured employee since 1991.