I have made the folowing settings in my initkdb.ora file. 

log_archive_format = _%s.arc

I have also started the database in the archive log mode , still the database is showing me the auto archival is disabled. What is the parameter I have to set to start the automatic archival. The ARCHIVE LOG LIST command gives following results.

Database log mode Archive Mode
Automatic archival Disabled
Archive destination /opt/oracle/product/9.0.1/dbs/arch
Oldest online log sequence 4
Next log sequence to archive 5
Current log sequence 6