How to purge .aud files correctly.

HelloI found a huge documentation in Google but I'm a little confused to choose the right one.

Here my problem since 2 months I run DB instances 11.2.0.3.0 on Linux  6.4 x86_64.
Just now I notested that I have a huge files .aud into /opt/oracle/admin/ORA1/adump
 $ ls *.aud | wc
 71892   71892 1774123    (71892 files).

sqlplus> SQL> show parameter audit;

NAME                     TYPE     VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest              string     /opt/oracle/admin/ORA1/adump
audit_syslog_level             string
audit_sys_operations             boolean     FALSE
audit_trail                 string     DB

What should do exactely to reduce these files on /adump directory and
in the DB itself . Could you please give an example or tutorial about this?