The Chromospheric Magnetism Explorer (CMEx) is a NASA Heliophysics Small Explorers (SMEX) mission concept in an extended Phase A. CMEx uses ultraviolet spectropolarimetry to diagnose magnetism from the solar photosphere to the transition region, exploring how the magnetic field evolves from the dynamically-driven photosphere to the magnetically-dominated corona to form twisted non-potential flux ropes in the corona. This key transition happens in the chromosphere where only UV spectropolarimetry can give access to the highly complex dynamics where plasma properties change rapidly. Recent advances in the understanding of the polarization of UV chromospheric lines, which are not observable from the ground, have primed a space-based mission like CMEx for success. CMEx observes many near-UV spectral lines, including the well-known Mg II h and k lines as well as a series of Fe II lines, with full-Stokes polarimetry to provide quantitative diagnostics of plasma parameters densely sampling the 9 pressure scale heights in the chromosphere. This enables studies of how the magnetic field in and around active regions gets reconfigured leading up to the eruption process, how flares lead to persistent changes in photospheric and chromospheric magnetic fields, and what the large-scale magnetic structure of prominences is in their stable phase, and how it changes in the transition from equilibrium to eruption. Here we summarize the planned science and its implementation enabled by the mission concept.
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