I think you mean earth's protective shield. The ozone gas forms it.

The earth's protective shield is formed by one of the molecules of the oxygen. It is ozone.

She doesn't have any powers in twilight but in breaking dawn when she becomes a vampire, she is really strong and can put a protective shield over who she wants to protect them.

Nuclear transmission is that measurement of amount of radiation found inside of a shield versus the outside. The transmission is the amount of radiation that is received by passing through the protective shield.

Aegis