"aaaa aa aaaaa_aaaaa aaaa a_aaaaaaa_aaaaaaaaa_aaa_aaaaa_a_aaaaaa"
Then you need TIDs!:
set long_string to "aaaa aa aaaaa_aaaaa aaaa a_aaaaaaa_aaaaaaaaa_aaa_aaaaa_a_aaaaaa"
set AppleScript's text item delimiters to "_"
set one_string to long_string's text items
set AppleScript's text item delimiters to {""}
one_string --> {"aaaa aa aaaaa", "aaaaa aaaa a", "aaaaaaa", "aaaaaaaaa", "aaa", "aaaaa", "a", "aaaaaa"}