How do I find out which username an Amazon AMI expects?

I just launched a new AMI. Specifically it was the one called "Amazon Linux AMI 2014.09.2"

I try to log into it using the commandline:

ssh -i my_keypair.pem ec2-user@<my_servers_ip>

But I get:

Permission denied (publickey).

So obviously ec-user is not the correct username. Wouldn't it make sense for Amazon to make it clear what the login username is supposed to be? Does anybody know how I can find this information out?