Coronal dimmings and what they tell us about solar and stellar coronal mass ejections

Solar coronal dimmings have been observed extensively in the past two decades and are believed to have close association with coronal mass ejections (CMEs). Recent study found that coronal dimming is the only signature that could differentiate powerful flares that have CMEs from those that do not. Therefore, dimming might be one of the best candidates to observe the stellar CMEs on distant Sun-like stars. In this study, we investigate the possibility of using coronal dimming as a proxy to diagnose stellar CMEs. By simulating a realistic solar CME event and corresponding coronal dimming using a global magnetohydrodynamics model (AWSoM: Alfvén-wave Solar Model), we first demonstrate the capability of the model to reproduce solar observations. We then extend the model for simulating stellar CMEs by modifying the input magnetic flux density as well as the initial magnetic energy of the CME flux rope. Our result suggests that with improved instrument sensitivity, it is possible to detect the coronal dimming signals induced by the stellar CMEs.

Coronal mass ejections (CMEs) are huge expulsions of magnetized matter from the Sun and stars, traversing space with speeds of millions of kilometers per hour. Solar CMEs can cause severe space weather disturbances and consumer power outages on Earth, whereas stellar CMEs may even pose a hazard to the habitability of exoplanets. While CMEs ejected by our Sun can be directly imaged by white-light coronagraphs, for stars this is not possible. So far, only a few candidates for stellar CME detections are reported. Here we demonstrate a different approach, based on sudden dimmings in the extreme-ultraviolet (EUV) and X-ray emission caused by the CME mass loss. We report dimming detections associated with flares on cool stars, indicative of stellar CMEs and benchmarked by Sun-as-a-star EUV measurements. This study paves the way for comprehensive detections and characterizations of CMEs on stars, important for planetary habitability and stellar evolution.

Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma from the Sun's lower atmosphere, significantly influencing space weather and planetary environments. To improve predictions of CME arrival and their impacts on Earth and its surroundings, a deeper understanding of their origins, initiation, and complex early evolution is crucial. While coronagraphic observations have been essential for studying the dynamics of CMEs, they cannot capture the initial, critical phase of CME development. Consequently, investigating indirect phenomena in the lower solar atmosphere has become essential. One of the most prominent indirect indicators associated with CMEs is coronal dimming. These are localized, sudden decreases in coronal emission observed at extreme ultraviolet and soft X-ray wavelengths, which evolve rapidly during the lift-off and early expansion phases of CMEs. Coronal dimmings have been interpreted both as “footprints” of the erupting magnetic structure and as indicators of coronal mass loss in the lower corona.I will review recent advancements in using coronal dimmings to diagnose CMEs. Topics covered will include statistical studies linking dimming characteristics to CME mass and speed, the use of dimmings as early indicators of CME propagation direction, and insights into the magnetic topology and reconfiguration of the early CME stages based on dimming locations and fine structure. Additionally, the potential role of dimmings in the pre-event phase preceding CME onset will be discussed. Finally, I will highlight future research directions and underexplored areas in CME science, emphasizing the untapped potential of coronal dimmings in advancing our understanding of these dynamic solar events.

Context. Coronal dimmings are regions of transiently reduced brightness in extreme ultraviolet (EUV) and soft X-ray (SXR) emissions associated with coronal mass ejections (CMEs), providing key insights into CME initiation and early evolution. During May 2024, AR 13664 was among the most flare-productive regions in recent decades, generating 55 M-class and 12 X-class flares along with multiple Earth-directed CMEs. The rapid succession of these CMEs triggered the most intense geomagnetic storm in two decades. Aims. We study coronal dimmings from a single active region (AR 13664) and compare them with statistical dimming properties. We investigate how coronal dimming parameters – such as area, brightness, and magnetic flux – relate to key flare and CME properties. Methods. We performed coronal dimming detection on observations from the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO). We used a logarithmic base-ratio thresholding technique to identify dimming regions, selecting pixels where log(I/I0) ≤ −0.19. Due to the high activity of the AR, we propose a quantitative threshold for distinguishing real mass depletion dimmings from unrelated intensity reductions by setting a threshold on the dimming area reached within the first hour (A ≥ 6.48 × 109 km2). We systematically identified all flares ≥M1.0, all coronal dimmings and all CMEs (from the CDAW SOHO/LASCO catalogue) produced by AR 13664 during 2024 May 1–15, and studied the associations between the different phenomena and their characteristic parameters. Results. We detect coronal dimmings in 22 events, with 16 occurring on-disc and six off-limb. Approximately 83% of X-class flares and 23% of M-class flares are associated with CMEs, with 13 out of 16 on-disc dimmings linked to CME activity. The dimmings in AR 13664 exhibit total unsigned magnetic fluxes exceeding 5.5 × 1021 Mx, reflecting the region’s high magnetic flux density; and dimming areas greater than 1.16 × 1010 km2. Previous statistical studies had shown a correlation between dimming parameters and flare parameters. We find that dimming parameters for the May 2024 events, particularly total dimming area and area growth rate, have a stronger correlation with GOES soft X-ray peak flux and fluence than anticipated, highlighting the connection between energy release in flares and the accompanying dimming. We find correlations between dimming properties and CME maximum velocities, which indicate that coronal dimmings serve as proxies for CME speeds. Conclusions. Our results support the strong interplay between coronal dimmings and flares, as we find increased correlations between flare and dimming parameters in this single-AR study compared to the general dimming population. Furthermore, we confirm that coronagraphic observations, unable to observe the lower corona, underestimate correlations between CME velocities and dimming parameters, as they fail to capture the early CME acceleration phase. This highlights the critical role of dimming observations in providing a more comprehensive understanding of CME dynamics.

Solar coronal dimmings have been observed extensively in recent years. Due to their close association with coronal mass ejections (CMEs), there is a critical need to improve our understanding of the physical processes that cause dimmings as well as their relationship with CMEs. In this study, we investigate coronal dimmings by combining simulation and observational efforts. By utilizing a data-constrained global magnetohydrodynamics model (Alfvén-wave solar model), we simulate coronal dimmings resulting from different CME energetics and flux rope configurations. We synthesize the emissions of different EUV spectral bands/lines and compare with SDO/AIA and EVE observations. A detailed analysis of the simulation and observation data suggests that the transient dimming/brightening are related to plasma heating processes, while the long-lasting core and remote dimmings are caused by mass-loss process induced by the CME. Moreover, the interaction between the erupting flux rope with different orientations and the global solar corona could significantly influence the coronal dimming patterns. Using metrics such as dimming depth and dimming slope, we investigate the relationship between dimmings and CME properties (e.g., CME mass, CME speed) in the simulation. Our result suggests that coronal dimmings encode important information about the associated CMEs, which provides a physical basis for detecting stellar CMEs from distant solar-like stars.

Context. Coronal dimmings are transient depletions of coronal plasma observed in extreme ultraviolet and soft X-ray wavelengths interpreted as low-corona signatures of coronal mass ejections (CMEs). Their evolution is closely linked to CME dynamics, flare reconnection, and the large-scale reconfiguration of the coronal magnetic field. During May 2024, the active region (AR) 13664 produced 66 ≥M-class flares alongside a sequence of fast CMEs that caused the largest geomagnetic storm since 2003. AR 13664 was also among the largest active regions and contained one of the largest amounts of magnetic flux ever recorded. It provided an exceptional opportunity to study the magnetic coupling between dimmings, flares, and CMEs within a single highly active AR. Aims. We investigate the morphology, magnetic properties, and temporal evolution of coronal dimmings produced by AR 13664. We expand on a previous paper where we identified all coronal dimmings from AR 13664 and performed a statistical analysis of their characteristic parameters in relation to the associated flares and CMEs to determine how the spatial development of the dimmings relates to flare ribbon locations and to the magnetic field configuration of the AR. We aim to identify the magnetic flux systems involved in the eruptions and assess how the observed dimming evolution reflects large-scale coronal restructuring and CME initiation. Methods. We analysed 16 on-disc coronal dimmings from AR 13664 detected in SDO/AIA 211 Å observations between May 1 and 14, 2024. We extracted coronal dimmings using logarithmic base-ratio thresholding, and their magnetic properties were derived from SDO/HMI line-of-sight magnetograms. We detected flare ribbons in AIA 1600 Å data using an adaptive thresholding technique and computed their reconnection fluxes from radial magnetic field maps. To explore the magnetic environment, we used high-resolution potential field source surface (PFSS) and non-linear force-free (NLFF) extrapolations of the coronal magnetic field and traced the magnetic flux systems connecting dimming regions, flare ribbons, and coronal holes. Results. We found strong correlations between flare ribbon and dimming parameters. The magnetic dimming area and flare ribbon area correlate with c = 0.65 ± 0.13, and the unsigned dimming and reconnection fluxes correlate with c = 0.60 ± 0.18, consistent with earlier statistical studies. The morphology of the dimmings changed systematically over the evolution of the AR, with southwards expanding dimmings occurring before May 9 and northwards expanding ones thereafter. This transition coincides with a change in the flare ribbon locations. AR 13664 contains two long, strong, and almost horizontal, i.e. east-west, polarity inversion lines (PILs), and the flare ribbon locations shift from the southern to the northern PIL. Together with the dimming location, these changes imply the presence of two distinct magnetic domains. The PFSS extrapolations showed that southward (northward) dimmings are mainly strapping flux dimmings with magnetic field lines vaulting above the southern (northern) PIL. The final extent of the dimmings was then given by the exterior flux involved in the eruption via stretching and reconnection. In one event, we found an extended quiet Sun dimming potentially triggered by field line opening due to the passage of an extreme ultraviolet wave.

As one of the most violent phenomena known from the Sun, coronal mass ejections (CMEs), associated with escaping plasma energized by magnetic reconnection events, may lead to mass and angular momentum loss and cause very severe space weather disturbances around the Earth. CMEs are also thought to play key roles in stellar evolution, star-planet interaction and planetary habitability. On the Sun, CMEs have been studied intensively using the direct-imaging technique. For active stars, CMEs are expected to occur more frequently and strongly because of their higher activity levels reflecting by various statistics, yet the detection of stellar CMEs is quite challenging. So far only a handful of candidates have been reported, but all of them were identified by single indicator in a rather ambiguous fashion (Moschou et al, 2019; Argiroffi et al, 2019; Leitzinger and Odert, 2022). Here we report the high confidence discovery and characterization of a superflare-associated CME event on the rapidly rotating active star AB Dor, cross-identified by several diagnostics from solar CMEs. Using time-resolved high-resolution X-ray spectroscopy of a superflare observed with the onboard Reflection Grating Spectrometer (RGS) of the X-ray Multi-Mirror Mission (XMM-Newton) telescope (Jansen et al, 2001), we have detected coronal dimming by a decrease of electron density (ne) at the flare onset and decreases of flux, temperature (T) and emission measure (EM) at the end of decay phase, along with plasma motion indicating a high speed escape at $v_b sim 725 \pm 200\,\mathrm{km\,s}^{−1}$ . These indicators robustly suggest the occurrence of a stellar CME event in analogy to the well known solar counterparts. We highlight the feasibility of searching for stellar CMEs in large X-ray spectroscopic archives of the XMM-Newton, Chandra, etc. by more detailed analyses.

Context. On May 8, 2024, the solar active region 13664 produced an X-class flare, several M-class flares, and multiple coronal mass ejections (CMEs) directed towards Earth. The initial CME resulted in coronal dimmings, which are characterized by localized reductions in extreme-ultraviolet (EUV) emissions and are indicative of mass loss and expansion during the eruption. On June 8, 2024, after one solar rotation, the same active region produced another eruptive M-class flare that was followed by coronal dimmings that were observed by the Solar Dynamics Observatory (SDO) and the Solar Terrestrial Relations Observatory (STEREO) spacecraft. Aims. We analyzed the early CME evolution and propagation direction from the expansion of the coronal dimming observed low in the corona using the method called dimming inferred estimation of the CME direction (DIRECD). Methods. DIRECD derived the key parameters of the early CME propagation from the expansion behavior of the associated coronal dimming at the end of its impulsive phase by generating a 3D CME cone model whose orthogonal projection on the solar sphere matches the dimming geometry. To validate the resulting 3D CME cone, we compared the CME properties derived in the low corona with white-light coronagraph data. Results. Using DIRECD, we find that the CME on May 8, 2024 expands close to radially, with an inclination angle of 7.7°, an angular width of 70°, and a cone height of 0.81 Rsun, which was derived at the end of the impulsive dimming phase, and for which the CME showed connections to the dimming and still left footprints in the low corona. It was inclined 7.6° north in the meridional plane and 1.1° east in the equatorial plane. The CME on June 8, 2024, after one solar rotation, was inclined by 15.7° from the radial direction, had an angular width of 81°, and had a cone height of 0.89 Rsun. The CME was inclined 6.9° south in the meridional plane and 14.9° west in the equatorial plane. A validation with white-light coronagraph data confirmed the accuracy of the 3D cone by matching the CME characteristics and projections with STEREO-A COR2 observations. Conclusions. Our study demonstrates that by tracking low coronal signatures such as the coronal dimming expansion in 2D for the May and June 2024 CMEs, we can estimate the 3D CME direction early in the CME evolution. This provides early lead times for mitigating adverse space weather impacts.

Context. Coronal mass ejections (CMEs) are large-scale eruptions of plasma and magnetic field from the Sun that can cause severe disturbances in space weather. Earth-directed CMEs are responsible for the disruption of technological systems and damaging power grids. However, the early evolution of CMEs, especially Earth-directed ones, is poorly tracked using traditional coronagraphs along the Sun-Earth line. Aims. The most distinct phenomena associated with CMEs in the low corona are coronal dimmings, which are localized regions of reduced emission in the extreme-ultraviolet (EUV) and soft X-rays formed due to mass loss and expansion during a CME. We present a new approach to estimating the early CME propagation direction based on the expansion of coronal dimmings. Methods. We developed the Dimming InfeRred Estimate of CME Direction (DIRECD) method. First, we performed simulations of CMEs in 3D using a geometric CME cone model and varying parameters such as width, height, source location, and deflection from the radial direction to study their influence on the CME projection onto the solar sphere. Second, we estimated the dominant direction of the dimming extent based on the evolution of the dimming area. Third, using the derived dominant direction of the dimming evolution on the solar sphere, we solved an inverse problem to reconstruct an ensemble of CME cones at different heights, widths, and deflections from the radial propagation. Finally, we searched for which CME parameter combinations the CME orthogonal projections onto the solar sphere would match the geometry of the dimming at the end of its impulsive phase best; we did so to derive the CME direction in 3D. We tested our approach on two case studies on 1 October, 2011 and 6 September, 2011. We also validated our results with 3D tie-pointing of the CME bubble in an EUV low corona and with 3D reconstructions by graduated cylindrical shell modeling (GCS) of white-light CMEs higher up in the corona. Results. Using DIRECD, we found that the CME on 1 October, 2011 expanded dominantly toward the south-east, while the CME on 6 September, 2011 was inclined toward the north-west. This is in agreement with the CME direction estimates from previous studies using multi-viewpoint coronagraphic observations. Conclusions. Our study demonstrates that coronal dimming information can be used to estimate the CME’s direction early in its evolution. This allows us to provide information on the CME direction before it is observed in the coronograph’s field of view, which is of practical importance for space weather forecasting and the mitigation of potential adverse impacts on Earth.

Stellar flares and coronal mass ejections (CMEs) can strip planetary atmospheres, reducing the potential habitability of terrestrial planets. While flares have been observed for decades, stellar CMEs remain elusive. Extreme-ultraviolet (EUV) emissions are sensitive to both flares and CME-induced coronal dimming. We assess the detectability of stellar CME-induced EUV dimming events by adapting a known “Sun-as-a-star” dimming technique—validated by the Solar Dynamics Observatory’s EUV Variability Experiment (EVE)—to stellar conditions. We adapt the solar data to reflect a range of stellar intensities, accounting for intrinsic brightness, distance, and interstellar medium (ISM) attenuation. We generate synthetic light curves for two different missions: the legacy EUV Explorer (EUVE) and the proposed ESCAPE mission. Our results indicate that dimming detections are well within reach. EUVE’s broadband imager was capable of detecting stellar CMEs—albeit with limited spectral (temperature) resolution—but that was not part of the observing plan. EUVE’s spectroscopic survey lacked sufficient sensitivity for CME detections. Optimizing modern instrument design for this task would make the observation fully feasible. In this work, we present a tool to explore the stellar-CME detection parameter space. Our tool shows that with an instrument with performance similar to ESCAPE, setting a 600 s integration period, and integrating the spectra into bands, any star with a X-ray flux ≥2.51 × 10−12 erg s−1 cm−2 should have a ≥3σ detection even for a modest few-percent dimming profile, regardless of ISM attenuation. Such measurements would be crucial for understanding the space weather environments of exoplanet host stars and, ultimately, for evaluating planetary habitability.

Stellar eruptive events, such as flares and coronal mass ejections (CMEs), can affect planetary habitability by disturbing the stability of their atmospheres. For instance, strong stellar flares and CMEs can trigger atmospheric escape and, in extreme cases, may strip away the atmosphere completely. While stellar flares have been observed and explored at a wide range of wavelengths, the physical properties of stellar CMEs remain unconstrained due to the difficulty in observing them. In this context, the Sun provides our only window on the potential characteristics of CMEs on Sun-like stars. A correlation between solar X-ray flare peak flux and the mass of flare-associated solar CMEs has been reported using solar data collected during Solar Cycle 23. Here, we build upon that work. We extend the correlation into the far-UV (FUV), where stellar flares are, and will continue to be, routinely detected with existing and future FUV observatories by incorporating data spanning two entire Solar Cycles (23 and 24; 1996–2019). Using three different space missions (CMEs from Large Angle Spectrometric Coronagraph/Solar and Heliospheric Observatory, X-ray flare events from X-ray Spectrometer/GOES, and FUV flares from Atmospheric Imaging Assembly/Solar Dynamics Observatory), we report a correlation between FUV flare peak flux and energy centered at 1600 Å and mass, kinetic energy, and linear speed of flare-associated CMEs. These empirical relations enable estimates of CME masses and kinetic energies from FUV flares on Sun-like stars. While direct stellar CME detections remain elusive, the correlations derived here are likely applicable to Sun-like stars and provide a working framework for evaluating exoplanet atmospheric erosion.

We present a statistical study of 62 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) during the quasi-quadrature period of STEREO and the Solar Dynamics Observatory (SDO). This unique setting allows us to study both phenomena in great detail and compare characteristic quantities statistically. Coronal dimmings are observed on-disk by the SDO/Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager, while the CME kinematics during the impulsive acceleration phase is studied close to the limb with STEREO/EUVI and COR, minimizing projection effects. The dimming area, its total unsigned magnetic flux, and its total brightness, reflecting properties of the total dimming region at its final extent, show the highest correlations with the CME mass (c ∼ 0.6–0.7). Their corresponding time derivatives, describing the dynamics of the dimming evolution, show the strongest correlations with the CME peak velocity (c ∼ 0.6). The highest correlation of c = 0.68 ± 0.08 is found with the mean intensity of dimmings, indicating that the lower the CME starts in the corona, the faster it propagates. No significant correlation between dimming parameters and the CME acceleration was found. However, for events where high-cadence STEREO observations were available, the mean unsigned magnetic field density in the dimming regions tends to be positively correlated with the CME peak acceleration (c = 0.42 ± 0.20). This suggests that stronger magnetic fields result in higher Lorentz forces providing stronger driving force for the CME acceleration. Specific coronal dimming parameters correlate with both CME and flare quantities providing further evidence for the flare-CME feedback relationship. For events in which the CME occurs together with a flare, coronal dimmings statistically reflect the properties of both phenomena.

We present a statistical analysis of 43 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) that occurred during the period of quasi-quadrature of the Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory (STEREO) satellites. We studied coronal dimmings that were observed above the limb by STEREO Extreme Ultraviolet Imager and compared their properties with the mass and speed of the associated CMEs. The unique position of the satellites allowed us to compare our findings with the results from Dissauer et al., who studied the same events observed against the solar disk by the SDO Atmospheric Imaging Assembly. Such statistics is done for the first time and confirms the relation of coronal dimmings and CME parameters for the off-limb viewpoint. The observations of dimming regions from different lines of sight reveal a similar decrease in the total extreme ultraviolet intensity (c = 0.60 ± 0.14). We find that the (projected) dimming areas are typically larger for off-limb observations (mean value of 1.24 ± 1.23 × 1011 km2 against 3.51 ± 0.71 × 1010 km2 for on-disk), with a correlation of c = 0.63 ± 0.10. This systematic difference can be explained by the (weaker) contributions to the dimming regions higher up in the corona that cannot be detected in the on-disk observations. The off-limb dimming areas and brightnesses show very strong correlations with the CME mass (c = 0.82 ± 0.06 and 0.75 ± 0.08), whereas the dimming area and brightness change rate correlate with the CME speed (c ∼ 0.6). Our findings suggest that coronal dimmings have the potential to provide early estimates of the mass and speed of Earth-directed CMEs, relevant for space weather forecasts, for satellite locations at both L1 and L5.

Stellar coronal mass ejections (CMEs) may play an important role in mass- and\nangular momentum loss of young Sun-like stars. If occurring frequently, they\nmay also have a strong effect on planetary evolution by increasing atmospheric\nerosion. So far it has not been possible to infer the occurrence frequency of\nstellar CMEs from observations. Based on their close relation with flares on\nthe Sun, we develop an empirical model combining solar flare-CME relationships\nwith stellar flare rates to estimate the CME activity of young Sun-like and\nlate-type main-sequence stars. By comparison of the obtained CME mass-loss\nrates with observations of total mass-loss rates, we find that our modeled\nrates may exceed those from observations by orders of magnitude for the most\nactive stars. This reveals a possible limit to the extrapolation of such models\nto the youngest stars. We find that the most uncertain component in the model\nis the flare-CME association rate adopted from the Sun, which does not properly\naccount for the likely stronger coronal confinement in active stars. Simple\nestimates of this effect reveal a possible suppression of CME rates by several\norders of magnitude for young stars, indicating that this issue should be\naddressed in more detail in the future.\n

Coronal dimming can be considered to be a disk signature of front-side coronal mass ejections (CMEs) (Thompson et al.: 2000, Geophys. Res. Lett.27, 1431). The study of the magnetic connectivity associated with coronal dimming can shed new light on the magnetic nature of CMEs. In this study, four major flare-CME events on 14 July 2000, 28 October 2003, 7 November 2004, and 15 January 2005 are analyzed. They were all halo CMEs associated with major flare activity in complex active regions (ARs) and produced severe space weather consequences. To explore the magnetic connectivity of these CMEs, global potential-field extrapolations based on the composite synoptic magnetograms from the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory are constructed, and their association with coronal dimming is revealed by the Extreme ultraviolet Imaging Telescope. It is found that each flare-CME event involved interaction of more than ten sets of magnetic-loop systems. These loop systems occupied over 50% of all identified loop systems in the visible hemisphere and covered a wide range of solar longitudes and latitudes. We categorize the loop systems as active-region loops (ARLs), AR-interconnecting loops (ARILs) including transequatorial loops (TLs), and long arcades (LAs) straddling filament channels. A recurring pattern, the saddle-field configuration (SFC), consisting of ARILs, is found to be present in all four major flare-CME events. The magnetic connectivity revealed by this work implies that intercoupling and interaction of multiple flux-loop systems are required for a major CME. For comparison, a simple flare-CME event of 12 May 1997 with a relatively simple magnetic configuration is chosen. Even for this simple flare-CME event, we find that multiple flux-loop systems are also present.