truncate table sys.aud$ failed

 originally posted by Sister DBA(but as a document)

 Hi All:

need advice  - it is a simple test and auditing has been turn-off already; however, one is able to desc,select from aud$, but NOT delete!

SQL> connect / as sysdba
Connected.

SQL> select count(*) from sys.aud$;

  COUNT(*)
----------
  15053519

SQL> truncate table aud$;
truncate table aud$
               *
ERROR at line 1:
ORA-00942: table or view does not exist

SQL> truncate table sys.aud$;
truncate table sys.aud$
                   *
ERROR at line 1:
ORA-00942: table or view does not exist

I also tried 'execute immediate 'truncate table sys.aud$' but failed again.  What to do next??