There are three ways of specifying an immediate dump
Immediate dumps can be specified using the ALTER SESSION command
ALTER SESSION SET EVENTS 'immediate trace name dump level level';
Immediate dumps can be specified in ORADEBUG
ORADEBUG DUMP dump level
Immediate dumps can also be specified using the DBMS_SYSTEM.SET_EV procedure
EXECUTE dbms_system.set_ev (sid,serial#,65535,level,'dump');
Immediate dumps cannot be invoked from init.ora