Authors: Holly Gilbert (HAO), CMEx team (various affiliations)

We discuss a new mission that uses ultraviolet spectropolarimetry to diagnose magnetism from the solar photosphere to the transition region. CMEx explores how the magnetic field evolves from the β >> 1 photosphere to the β << 1 corona to form twisted non-potential flux ropes in the corona that are the energy storage devices for magnetic eruptions. This key β transition happens in the chromosphere where UV spectropolarimetry can give us the best access to the highly complex dynamics where plasma properties change rapidly.