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.

Gaia has recently revealed a population of over 20 compact objects in wide astrometric binaries, while LIGO-Virgo-KAGRA (LVK) have observed around 100 compact object binaries as gravitational-wave (GW) mergers. Despite belonging to different systems, the compact objects discovered by both Gaia and the LVK follow a multimodal mass distribution, with a global maximum at neutron star (NS) masses ( ∼ 1 $\sim 1$ – 2 M ⊙ $2\,M_{\odot }$ ) and a secondary local maximum at black hole (BH) masses ∼ 10 M ⊙ $\sim 10\,M_{\odot }$ . However, the relative dearth of objects, or “mass gap,” between these modes is more pronounced among the wide binaries observed by Gaia compared to the GW population, with 9 − 6 + 10 % $9^{+10}_{-6}\%$ of GW component masses falling between 2.5– 5 M ⊙ $5\,M_{\odot }$ compared to ≲ 5 % $\lesssim 5\%$ of Gaia compact objects. We explore whether this discrepancy can be explained by the natal kicks received by low-mass BHs. GW progenitor binaries may be more likely to survive natal kicks, because the newborn BH has a more massive companion and/or is in a tighter binary than Gaia progenitor binaries. We compare the survival probabilities of Gaia and GW progenitor binaries as a function of natal kick strength and pre-supernova binary parameters, and map out the parameter space and kick strength required to disrupt the progenitor binaries leading to low-mass BHs in Gaia systems more frequently than those in GW systems.