Karin Dissauer

Context. Coronal mass ejections (CMEs) are driven by the eruption of magnetic flux ropes (MFRs), whose footpoints are often observed indirectly as coronal dimming regions. However, the relationship between observed dimmings and true MFR footpoint locations remains uncertain due to limitations and uncertainties in both detection methods as well as coronal magnetic field modelling caused by the complexity of eruptive events. Aims. We aim to investigate the M4 flare and CME event on 28 March 2022 in AR 12975 to determine how well observed coronal dimmings trace the footpoints of an erupting MFR. Furthermore, we intend to assess how these signatures compare with the MFR evolution from a data-driven magnetofrictional simulation. Methods. We combined multiwavelength SDO/AIA observations with a time-dependent data-driven magnetofrictional simulation of the active region (AR). We identified coronal dimmings using intensity-thresholding and minimum-intensity maps. For challenging regions we used a tailored region growth algorithm from strong gradients. We extracted MFR structures in the simulation using a combined twist and squashing factor metric. We projected MFR footpoints onto photospheric magnetograms for direct comparison and derived magnetic fluxes from both dimmings and the modelled MFR. Results. The simulation reproduces the formation, evolution, and eruption of the observed MFR whose footpoints migrate during the eruption. Moreover, the footpoints match well with the dimming location. The southern dimming shows characteristics of a moving flux rope dimming, while the northern dimming behaves as a shrinking flux rope dimming that is rapidly closed by the flare ribbons. Quantitative comparisons of area and magnetic flux show good overall agreement between simulated footpoints and observed dimmings, with discrepancies mainly arising from limitations of the detection methods and the inherent complexity of the event. The dimmings at the northern footpoint region is particularly challenging in observations due to pre-existing low intensities and its fast shrinkage.

Coronal mass ejections (CMEs) are among the primary drivers of space weather disturbances at Earth, yet their early propagation in the low corona remains poorly constrained owing to occultation and projection effects inherent to coronagraph observations. Coronal dimmings offer an alternative diagnostic to infer CME propagation direction directly from the low corona. We present the Dimming Inferred Estimation of CME Direction (DIRECD) CME Direction Catalog, a unified data set of 3D CME propagation directions derived via the DIRECD method, which reconstructs CME cone geometry from Solar Dynamics Observatory/Atmospheric Imaging Assembly 211 Å coronal dimming observations. The catalog comprises 64 events spanning 2010–2026, combining 31 Solar Cycle 25 (2021–2026) events with 33 events from prior studies. For each event we report the 3D propagation direction, 2D inclination angles, angular width, and cone height. Statistical analysis reveals a systematic asymmetry in low-corona CME propagation: meridional inclinations exhibit a systematic tendency toward latitude-dependent deflection, with poleward deflections more pronounced at higher source latitudes, while equatorial inclinations show no significant dependence on source longitude. This is consistent with low-corona CME trajectories being governed primarily by local active-region magnetic topology, with the large-scale coronal field and heliospheric current sheet assuming a progressively dominant role at greater heliocentric distances. Incorporating DIRECD-derived propagation directions into CME arrival forecasts improves predictions of geomagnetic storm intensity, raising the correlation with observed storm strength from r = 0.58 to 0.70 when combined with CME speed. The DIRECD software, catalog, and all associated data products are publicly available to support space weather research.

We investigate the two-dimensional velocity field of the fast and complex large-scale coronal wave observed on September 6, 2011. We use both a classical perturbation profile approach and the newly developed multi-sector method of the SOLERwave tool, using a Huygens-plotting-based approach. The multi-sector method utilizes perturbation profiles derived in multiple directions (sectors) to determine the location of the wavefront at a given time. The two-dimensional velocity vector at each point along the wavefront is derived by identifying the point closest to it along the wavefront observed one time step earlier and dividing the distance between the two points along the solar surface by the time difference between the observations. For the event under study the resulting two-dimensional velocity field shows a significant difference between the northward traveling and the northwestward traveling part of the wave front of over 40%, in the range of 750 to 1500 km/s. To determine the cause of this difference in speed, we investigate the coronal structures, the photospheric magnetic field distribution and the Alfvén speed derived from a steady-state coronal magnetohydro dynamic (MHD) solution along different propagation directions of the wave. We find the large difference in magnetosonic speed determined in the investigated sector as the most likely explanation for the velocity difference.

Large-scale coronal waves are globally propagating intensity enhancements in extreme-ultraviolet (EUV) and soft X-ray (SXR) observations, in association with solar flares and coronal mass ejections (CMEs). They are interpreted as low-coronal signatures of a large-amplitude fast magnetosonic wave. On 6 September 2011, a fast (v≈ 1000 km s -1 ) large-scale coronal wave accompanied an eruptive X2.1 class flare. A notable feature of this event was the temporary disappearance of a segment of the wave front in EUV channels sensitive to quiet-Sun plasma, while the same structure remained visible in higher temperature channels. We analyse the plasma properties associated with this large-scale coronal wave and quantify its impact on the local plasma as it passed through the corona. We aim to determine whether the temperature increase at the wave front can be explained solely by a compressive wave, and how the observed spatial variability relates to the plasma temperature distribution. We evaluate plasma parameters at multiple locations along the wave front and across its propagation path. We apply differential emission measure (DEM) diagnostics to SDO/AIA EUV observations to derive local density, temperature, emission measure (EM), and DEM distributions, and examine their temporal evolution during the wave passage. The wave passage causes increases of 6-8% in density and 10-18% in temperature. While the density increase is comparable to earlier reports, the temperature increase exceeds expectations. This indicates that the temperature enhancement cannot be explained by compressional adiabatic heating alone, and instead suggests the presence of additional heating mechanisms, such as magnetic reconnection or wave mode conversion. During the temporary disappearance of the wave, the plasma parameters at the wave front increase, but with a strong spatial variability, with density increases as low as 1% and as high as 10%. The DEM distribution shows that the initial temperature in the affected area is notably higher than typical quiet-Sun regions (bar T MK), which allows plasma to be heated beyond the peak response of the AIA 193 and 211 Å channels. We conclude that the apparent temporary disappearance of the wave front is primarily due to the combined effects in the intensity of the CME-associated coronal dimming following the wave and the wave itself, with heating further reducing its detectability in channels sensitive to quiet-Sun temperatures.

Solar flare ribbons, manifesting as transient brightenings in the chromosphere, are believed to trace the footpoints of magnetic field lines reconnecting higher in the solar atmosphere. These field lines lie within separatrix or quasi-separatrix layers that separate domains of different magnetic connectivity and hence form natural locations for reconnection. Solar flares are typically characterized as circular-ribbon flares, two-ribbon flares, or complex-ribbon flares based on the number and shape of the ribbons. There are relatively well-developed models for the first two types of flares based on the location of the reconnection powering the flare. Complex ribbons are less well understood, but are often posited as the result of reconnection at multiple locations. Here, we demonstrate that reconnection at a magnetic separator connecting two coronal null points can naturally explain the complex ribbons observed in two events: an M2.9-class flare from NOAA AR 11112 (SOL2010-10-16T19:12), and an X2.2-class flare from NOAA AR 11158 (SOL2011-02-15T01:56).

A coronal hole formed as a result of a quiet-Sun filament eruption close to the solar disk center on 2014 June 25. We studied this formation using images from the Atmospheric Imaging Assembly (AIA), magnetograms from the Helioseismic and Magnetic Imager, and a differential emission measure analysis derived from the AIA images. The coronal hole developed in three stages: (1) formation, (2) migration, and (3) stabilization. In the formation phase, the emission measure (EM) and temperature started to decrease 6 hr before the filament erupted. Then, the filament erupted and a large coronal dimming formed over the following 3 hr. Subsequently, in a phase lasting 15.5 hr, the coronal dimming migrated by ≈150 ″ from its formation site to a location where potential field source surface extrapolations indicate the presence of open magnetic field lines, marking the transition into a coronal hole. During this migration, the coronal hole drifted across quasi-stationary magnetic elements in the photosphere, implying the occurrence of magnetic interchange reconnection at the boundaries of the coronal hole. In the stabilization phase, the magnetic properties and area of the coronal hole became constant. The EM of the coronal hole decreased, which we interpret as a reduction in plasma density due to the onset of plasma outflow into interplanetary space. As the coronal hole rotated toward the solar limb, it merged with a nearby preexisting coronal hole. At the next solar rotation, the coronal hole was still apparent, indicating a lifetime of >1 solar rotation.

Coronal mass ejections (CMEs) are among the most energetic phenomena in our solar system, with significant implications for space weather. Understanding their early dynamics remains challenging due to observational limitations in the low corona. We present a statistical evaluation of the Dimming InfeRred Estimation of CME Direction (DIRECD) method, which provides a novel approach to determining initial CME propagation directions using coronal dimmings. We analyze 33 coronal-dimming events well observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly and validate our DIRECD results with 3D reconstructions from the graduated cylindrical shell (GCS) model. We find generally good agreement between the DIRECD-derived inclinations and the GCS model. In the meridional plane (north–south direction), the mean difference in inclinations is 0 . ° 3 ± 7 . ° 8. In the equatorial plane (east–west direction), the mean difference is −2 . ° 9 ± 18 . ° 9. In 3D, the inclinations show a mean difference of 1 . ° 2 ± 10 . ° 4. We further visually compare our method by projecting the DIRECD cones onto Large Angle and Spectrometric Coronagraph/C2 observations, and verify the model’s ability to capture both the primary CME structure and associated secondary dimming regions. This work establishes DIRECD as a powerful, observationally grounded technique for determining the initial CME direction, offering new insights that complement existing reconstruction methods. The technique’s unique capability to determine early CME direction in the low corona using coronal dimmings observed in extreme ultraviolet images makes it particularly valuable for improving space weather forecasting models.

A coronal hole formed as a result of a quiet-Sun filament eruption close to the solar disk center on 2014 June 25. We studied this formation using images from the Atmospheric Imaging Assembly (AIA), magnetograms from the Helioseismic and Magnetic Imager (HMI), and a differential emission measure (DEM) analysis derived from the AIA images. The coronal hole developed in three stages: (1) formation, (2) migration, and (3) stabilization. In the formation phase, the emission measure (EM) and temperature started to decrease six hours before the filament erupted. Then, the filament erupted and a large coronal dimming formed over the following three hours. Subsequently, in a phase lasting $15.5$~hours, the coronal dimming migrated by 150" from its formation site to a location where potential field source surface extrapolations indicate the presence of open magnetic field lines, marking the transition into a coronal hole. During this migration, the coronal hole drifted across quasi-stationary magnetic elements in the photosphere, implying the occurrence of magnetic interchange reconnection at the boundaries of the coronal hole. In the stabilization phase, the magnetic properties and area of the coronal hole became constant. The EM of the coronal hole decreased, which we interpret as a reduction in plasma density due to the onset of plasma outflow into interplanetary space. As the coronal hole rotated towards the solar limb, it merged with a nearby pre-existing coronal hole. At the next solar rotation, the coronal hole was still apparent, indicating a lifetime of >1 solar rotation.

Coronal dimmings associated with coronal mass ejections (CMEs) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as “footprints” of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and how to use them as a diagnostics for stellar coronal mass ejections and their properties.