I've not set the parameter UNDO_MANAGEMENT in init.ora file,so it takes the default value of MANUAL,

SQL> show parameter undo_management;

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string MANUAL


When i queries to know the status of undo segments, it shows all are in OFFLINE,


SQL> select segment_name,tablespace_name,status from dba_rollback_segs;

SEGMENT_NAME TABLESPACE_NAME STATUS
------------------------------ ------------------------------ ----------------
SYSTEM SYSTEM ONLINE
_SYSSMU1$ UNDOTBS1 OFFLINE
_SYSSMU2$ UNDOTBS1 OFFLINE
_SYSSMU3$ UNDOTBS1 OFFLINE
_SYSSMU4$ UNDOTBS1 OFFLINE
_SYSSMU5$ UNDOTBS1 OFFLINE
_SYSSMU6$ UNDOTBS1 OFFLINE
_SYSSMU7$ UNDOTBS1 OFFLINE
_SYSSMU8$ UNDOTBS1 OFFLINE
_SYSSMU9$ UNDOTBS1 OFFLINE
_SYSSMU10$ UNDOTBS1 OFFLINE

Then i tried,to set UNDO_TABLESPACE,so that all undo segments can be brought ONLINE,but it shows the following the error,

SQL> alter system set undo_tablespace=UNDOTBS1
2 /
alter system set undo_tablespace=UNDOTBS1
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-30014: operation only supported in Automatic Undo Management mode

Can anyone explain me,

1) what could be the reason for all the segments to be in OFFLINE,eventhough the undo tablespace is in ONLINE?

2) What i've to do to make all undo segments ONLINE?

3) Since UNDO_MANAGEMENT parameter is static,how can i change its value dynamically?