设置自动归档参数时出现问题不懂

SQL> alter system set log_archive_start = true scope = spfile ;

System altered.

SQL> alter system set log_archive_start = true scope = both;
alter system set log_archive_start = true scope = both
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified


SQL> archive log list
Database log mode              Archive Mode
Automatic archival             Disabled
Archive destination            D:\ORA_ARCH\ARCHIVE2
Oldest online log sequence     25
Next log sequence to archive   25
Current log sequence           26

SQL> alter system set log_archive_start = true scope = spfile ;

System altered.

SQL> archive log list
Database log mode              Archive Mode
Automatic archival             Disabled
Archive destination            D:\ORA_ARCH\ARCHIVE2
Oldest online log sequence     25
Next log sequence to archive   25
Current log sequence           26
SQL> alter system set log_archive_start = true scope = memory;
alter system set log_archive_start = true scope = memory
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified


SQL>

设置scope = spfile 就可以both和memory就不行，WHY？