dba_audit_trail does not monitor sysdba

I want to be able to monitor sysdba activity in dba_audit_trail view.

I have set audit_sys_operations to TRUE

SQL> show parameter audit_sys_operations

NAME                                 TYPE                           VALUE
------------------------------------ ------------------------------ ------------------------------
audit_sys_operations          boolean                        TRUE

I connect as SYSTEM and run

SQL> create user tt identified by tt;

User created.

SQL> drop user tt;

User dropped.

This commands are logged in the view.

The I connect as SYS and run the same commands. These commands are not logged in the view. Why. What I'm I missing?

Also I have set

SQL> show parameter audit_trail

NAME                                 TYPE                           VALUE