How to capture the name of a user that logon on the database as sysdba?

In sql*plus, when a user connects to database as sysdba, no matter what his logon nam is, the value of sys_context(¡®userenv¡¯,¡¯session_user¡¯) is always ¡®sys¡¯.

Then, how I get the real name of the user who logons on database as sysdba? That is:  

SQL>conn system/manager as sydba;

SQL>create user wxy identified by wxy;

SQL>grant sysdba to wxy;

SQL>conn wxy/wxy as sysdba  

How can I capture the name of the user ¡®wxy¡¯?

Thanks in advance!  

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