The Chromospheric Magnetism Explorer (CMEx): Mission Concept Overview

The Chromospheric Magnetism Explorer (CMEx) seeks to conduct unprecedented measurements of the Sun's magnetic field between the photosphere and the base of the corona. This mission contributes to the critical problems documented in the 2013 Solar and Space Physics Decadal Survey, namely “Determine How Magnetic Energy is Stored and Explosively Released.” CMEx does so by returning magnetic field strength and direction information of active regions prior to, and following eruptions. CMEx is also poised to provide insight into heliospheric magnetic flux, adding unique observational data to answer the so-called “open flux problem.” The CMEx mission collects spectropolarimetry data and generates magnetic field information utilizing inversion codes and other techniques that interpret Zeeman- and Hanleeffect changes to spectral line polarization. The CMEx instrument consists of a two-band ultraviolet spectropolarimeter with a single band ultraviolet imager. The instrument performs repeated raster scans of active regions, prominences, filaments, and coronal holes at a cadence allowing direct observation of evolving and changing solar magnetic structures. Launched into a 6 A.M. sun-synchronous orbit, CMEx will have continuous visibility of the sun outside of its 3-month eclipse season, allowing near constant monitoring of solar features of interest. Image stacking and subsequent spectrum demodulation onboard the observatory provides for downlink of full Stokes vector information for the observed spectral lines. CMEx also utilizes the instrument raster scan mirror to provide line-of-sight stability by compensating for spacecraft motion and attenuating system jitter. Observation plans developed by the Science Operations Center (SOC) are transferred to the Mission Operations Center (MOC) for conversion into command sequences subsequently uplinked to the observatory via KSAT ground stations. After launch in 2029, CMEx will complete a two-year science mission following a short period of combined on-orbit spacecraft and instrument commissioning. CMEx provides a high-performance space observatory by combining heritage instrument and spacecraft element designs, as well as commercial-off-the-shelf (COTS) technologies into a low-cost solution appropriate for a cost-capped small explorer class NASA mission. This paper provides an overview of the CMEx mission concept and of key observatory and ground system conceptual designs. CMEx is a candidate Heliophysics Small Explorer (SMEX) mission led by the Principal Investigator, Dr. Holly Gilbert, at the High Altitude Observatory (HAO) at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR). The CMEx mission partners include BAE Systems Space and Mission Systems (BAES), and the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder (CU/LASP). As of the publication date (March 2025), the CMEx project has completed its Phase A Concept Study Report and awaits the results of the Heliophysics SMEX mission down selection process expected to complete in the second quarter of 2025.