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.

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 Hanle-effect 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.

Steven Tomczyk, Joan Burkepile, Roberto Casini, Marcel Corchado-Albelo, Ed DeLuca, Giuliana de Toma, Alfred de Wijn, Mausumi Dikpati, Yuhong Fan, Samaiyah Farid, Sarah E. Gibson, Holly Gilbert, Philip G. Judge, Therese Kucera, Enrico Landi, Haosheng Lin, Valentin Martinez Pillet, Richard J. Morton, Alin Paraschiv, Katharine K. Reeves, Thomas A. Schad, Daniel B. Seaton, Jie Zhang National Center for Atmospheric Research, University of Colorado, Harvard-Smithsonian Center for Astrophysics, NASA GSFC, University of Michigan, University of Hawaii, National Solar Observatory, Northumbria University, Southwest Research Institute, George Mason University

COMPLETE is a flagship mission concept combining broadband spectroscopic imaging and comprehensive magnetography from multiple viewpoints around the Sun to enable tomographic reconstruction of 3D coronal magnetic fields and associated dynamic plasma properties, which provide direct diagnostics of energy release. COMPLETE re-imagines the paradigm for solar remote-sensing observations through purposefully co-optimized detectors distributed on multiple spacecraft that operate as a single observatory, linked by a comprehensive data/model assimilation strategy to unify individual observations into a single physical framework. We describe COMPLETE's science goals, instruments, and mission implementation. With targeted investment by NASA, COMPLETE is feasible for launch in 2032 to observe around the maximum of Solar Cycle 26.

Heliophysics image data largely relies on a forty-year-old ecosystem built on the venerable Flexible Image Transport System (FITS) data standard. While many in situ measurements use newer standards, they are difficult to integrate with multiple data streams required to develop global understanding. Additionally, most data users still engage with data in much the same way as they did decades ago. However, contemporary missions and models require much more complex support for 3D multi-parameter data, robust data assimilation strategies, and integration of multiple individual data streams required to derive complete physical characterizations of the Sun and Heliospheric plasma environment. In this white paper we highlight some of the 21$^\mathsf{st}$ century challenges for data frameworks in heliophysics, consider an illustrative case study, and make recommendations for important steps the field can take to modernize its data products and data usage models. Our specific recommendations include: (1) Investing in data assimilation capability to drive advanced data-constrained models, (2) Investing in new strategies for integrating data across multiple instruments to realize measurements that cannot be produced from single observations, (3) Rethinking old data use paradigms to improve user access, develop deep understanding, and decrease barrier to entry for new datasets, and (4) Investing in research on data formats better suited for multi-dimensional data and cloud-based computing.

Ground-based synoptic solar observations provide critical contextual data used to model the large-scale state of the heliosphere. The next decade will see a combination of ground-based telescopes and space missions that will study our Sun's atmosphere microscopic processes with unprecedented detail. This white paper describes contextual observations from a ground-based network needed to fully exploit this new knowledge of the underlying physics that leads to the magnetic linkages between the heliosphere and the Sun. This combination of a better understanding of small-scale processes and the appropriate global context will enable a physics-based approach to Space Weather comparable to Terrestrial Weather forecasting.

The coronal magnetic field is the prime driver behind many as-yet unsolved mysteries: solar eruptions, coronal heating, and the solar wind, to name a few. It is, however, still poorly observed and understood. We highlight key questions related to magnetic energy storage, release, and transport in the solar corona, and their relationship to these important problems. We advocate for new and multi-point co-optimized measurements, sensitive to magnetic field and other plasma parameters, spanning from optical to $\gamma$-ray wavelengths, to bring closure to these long-standing and fundamental questions. We discuss how our approach can fully describe the 3D magnetic field, embedded plasma, particle energization, and their joint evolution to achieve these objectives.

Much of solar activity within a sunspot cycle occurs as bursts, or 'seasons' of strong activity over several months, separated by periods of much less activity.The most important space weather effects occur during these bursty periods.Previous modeling and forecasting efforts have focused on time-scales of hours-to-days and decades-to-centuries.The recent discovery of Rossby waves in the Sun, together with recently developed global models of solar MHD Rossby waves and their interactions with differential rotation and spot-producing magnetic fields, reveal the opportunity to simulate and predict the occurrence, strength and location of enhanced activity bursts a few weeks up to several months in advance.We now have a golden opportunity to fill in this gap in forecasting space weather in these time scales.This requires a) continuous observations of solar Rossby waves by various techniques; b) development of coupled nonlinear MHD models that simulate both global Rossby waves and the much smaller spatial scale emergence of new active regions; c) application of advanced data assimilation techniques to couple surface observations to update the model-system to integrate forward in time for creating forecasts months ahead.Then it will be possible to build operational space weather prediction models to meet the needs of customers and stakeholders, including support of future NASA missions, on a time scale of a few weeks to several months ahead.

Even as our understanding of the Sun has grown, many fundamental questions remain—some of which have big implications for life on Earth.

We take a broad look at the problem of identifying the magnetic solar causes of space weather. With the lackluster performance of extrapolations based upon magnetic field measurements in the photosphere, we identify a region in the near-UV (NUV) part of the spectrum as optimal for studying the development of magnetic free energy over active regions. Using data from SORCE, the Hubble Space Telescope, and SKYLAB, along with 1D computations of the NUV spectrum and numerical experiments based on the MURaM radiation-magnetohydrodynamic and HanleRT radiative transfer codes, we address multiple challenges. These challenges are best met through a combination of NUV lines of bright Mg ii, and lines of Fe ii and Fe i (mostly within the 4s-4p transition array) which form in the chromosphere up to 2 x 10(4) K. Both Hanle and Zeeman effects can in principle be used to derive vector magnetic fields. However, for any given spectral line the tau = 1 surfaces are generally geometrically corrugated owing to fine structure such as fibrils and spicules. By using multiple spectral lines spanning different optical depths, magnetic fields across nearly horizontal surfaces can be inferred in regions of low plasma beta, from which free energies, magnetic topology, and other quantities can be derived. Based upon the recently reported successful sub-orbital space measurements of magnetic fields with the CLASP2 instrument, we argue that a modest space-borne telescope will be able to make significant advances in the attempts to predict solar eruptions. Difficulties associated with blended lines are shown to be minor in an Appendix.

We have cataloged 196 filament oscillations from the Global Oscillation Network Group Ha network data during several months near the maximum of solar cycle 24 (2014 January-June). Selected examples from the catalog are described in detail, along with our statistical analyses of all events. Oscillations were classified according to their velocity amplitude: 106 small-amplitude oscillations (SAOs), with velocities < 10 km s(-1), and 90 large-amplitude oscillations (LAOs), with velocities > 10 km s(-1). Both SAOs and LAOs are common, with one event of each class every two days on the visible side of the Sun. For nearly half of the events, we identified their apparent trigger. The period distribution has a mean value of 58. +/-. 15 minutes for both types of oscillations. The distribution of the damping time per period peaks at tau/P = 1.75 and 1.25 for SAOs and LAOs, respectively. We confirmed that LAO damping rates depend nonlinearly on the oscillation velocity. The angle between the direction of motion and the filament spine has a distribution centered at 27 degrees for all filament types. This angle agrees with the observed direction of filament-channel magnetic fields, indicating that most of the cataloged events are longitudinal (i.e., undergo field-aligned motions). We applied seismology to determine the average radius of curvature in the magnetic dips, R approximate to 89 Mm, and the average minimum magnetic field strength, B approximate to 16 G. The catalog is available to the community online and is intended to be expanded to cover at least 1 solar cycle.

Partially ionized plasmas are found across the Universe in many different astrophysical environments. They constitute an essential ingredient of the solar atmosphere, molecular clouds, planetary ionospheres and protoplanetary disks, among other environments, and display a richness of physical effects which are not present in fully ionized plasmas. This review provides an overview of the physics of partially ionized plasmas, including recent advances in different astrophysical areas in which partial ionization plays a fundamental role. We outline outstanding observational and theoretical questions and discuss possible directions for future progress.

In recent years high-resolution and high-cadence EUV imaging has revealed a new phenomenon, impacting prominence debris, where prominence material from failed or partial eruptions can impact the lower atmosphere and release energy. We report a clear example of energy release and EUV brightening due to infalling prominence debris that occurred on 2011 September 7-8. The initial eruption of prominence material was associated with an X1.8-class flare from AR11283, occurring at 22:30 UT on 2011 September 7. Subsequently, a semi-continuous stream of this material was observed to return to the solar surface with a velocity v u003e 150 km/s, impacting a region remote from the original active region between 00:20 - 00:40 UT on 2011 September 8. Using SDO/AIA, the differential emission measure of the plasma was estimated throughout this brightening event. We found that the radiated energy of the impacted plasma was L ~ 10^27 ergs, while the thermal energy peaked at ~ 10^28 ergs. From this we were able to determine the mass content of the debris to be in the range 2x10^14 u003c m u003c 2x10^15 g. Given typical prominence masses, the likely debris mass is towards the lower end of this range. This clear example of a prominence debris event shows that significant energy release takes place during these events, and that such impacts may be used as a novel diagnostic tool for investigating prominence material properties.

AbstractThe Coronal Solar Magnetism Observatory (CoSMO) is a proposed new facility led by the High Altitude Observatory and a consortium of partners to measure magnetic field and plasma properties in a large (one degree) field of view extending down to the inner parts of the solar corona. CoSMO is intended as a research facility that will advance the understanding and prediction of space weather. The instrumentation elements of CoSMO are: a white-light coronagraph (KCor), already operational at the Mauna Loa Solar Observatory (MLSO); the Chromosphere and Prominence Magnetometer (ChroMag), due for deployment to MLSO next year; and the CoSMO Large Coronagraph (LC) which has completed Preliminary Design Review.

On 2010 August 20, an energetic disturbance triggered large-amplitude longitudinal oscillations in a nearby filament. The triggering mechanism appears to be episodic jets connecting the energetic event with the filament threads. In the present work, we analyze this periodic motion in a large fraction of the filament to characterize the underlying physics of the oscillation as well as the filament properties. The results support our previous theoretical conclusions that the restoring force of large-amplitude longitudinal oscillations is solar gravity, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Based on our previous work, we used the fitted parameters to determine the magnitude and radius of curvature of the dipped magnetic field along the filament, as well as the mass accretion rate onto the filament threads. These derived properties are nearly uniform along the filament, indicating a remarkable degree of cohesiveness throughout the filament channel. Moreover, the estimated mass accretion rate implies that the footpoint heating responsible for the thread formation, according to the thermal nonequilibrium model, agrees with previous coronal heating estimates. We estimate the magnitude of the energy released in the nearby event by studying the dynamic response of the filament threads, and discuss the implications of our study for filament structure and heating.

The complexity of prominence formation and structure is intimately related to energy balance. Fundamental properties of these structures are still being investigated and understanding the processes involved with heating and cooling of prominence material, which is partially ionized, is a critical piece of the puzzle. It is important to understand the nature of the chromosphere–corona transition region (CCTR) and, more specifically, the interplay among mechanical heating, radiative cooling, radiative heating, and thermal conduction that determines the location and structure of this transition region. For prominences to exist they need mechanical equilibrium (which is described by the equations of magneto-hydrostatics) and detailed energy balance, in which steady radiative cooling is balanced by heating mechanisms. Aspects of mechanical and energy balance have been thoroughly studied in the past, but models have difficulty accounting for both of these equilibria self-consistently on scales ranging from the central cool parts of the prominence into the corona.