no password for CoreOS to login as username: core (on AWS EC2)

ming➜~/.ec2» ssh -i turboteam.pem root@54.64.18.203                                                              [16:45:05]
root@54.64.18.203's password:
Permission denied, please try again.
root@54.64.18.203's password:
Permission denied, please try again.
root@54.64.18.203's password:
Connection closed by 54.64.18.203
ming➜~/.ec2» ssh -i turboteam.pem root@54.64.18.203                                                              [16:54:17]
root@54.64.18.203's password:
Permission denied, please try again.
root@54.64.18.203's password:
Permission denied, please try again.
root@54.64.18.203's password:
Permission denied (publickey,password,keyboard-interactive).
ming➜~/.ec2»                                                                                                     [16:54:27]
ming➜~/.ec2» ssh -i turboteam.pem root@54.64.18.203                                                              [16:54:28]
root@54.64.18.203's password:

ming➜~/.ec2» ssh -i turboteam.pem core@54.64.18.203                                                              [16:54:45]
CoreOS (beta)