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Project titleCoronal dimmings and their relevance to the directivity of coronal mass ejections and recovery of solar corona

AffiliationAutonomous Non-Profit Organization for Higher Education "Skolkovo Institute of Science and Technology",

KeywordsSun, coronal mass ejections, coronal dimmings, solar activity, space weather

Annotation
Coronal mass ejections (CMEs) are large-scale structures of magnetized plasma that are expelled from the Sun with speeds of some hundred up to >3500 km/s. They are the main source of strong geomagnetic storms and space weather effects on Earth, having a significant impact on space and ground-based infrastructure and biological objects. However, the early evolution of CMEs, especially Earth-directed ones, is poorly traced using traditional coronagraphs, covering the solar disk by several radii, and, in general, determining the properties of CMEs is challenging due to projection errors. In addition, only two coronagraphs are currently available onboard the STEREO-A and SOHO satellites, and the launch of new missions carrying coronagraphs are expected no earlier than in a few years. The most distinct phenomena associated with Earth-directed CMEs are coronal dimmings, localized regions of reduced emission in the extreme-ultraviolet (EUV) and soft X-rays, formed due to mass loss and CME expansion low in the corona. The observations of dimmings are unique and of high scientific value, because they appear during the entire evolution of a solar CME, from the pre-eruption phase to the post-event recovery of the corona. Thus, the project has the following main aims: 1. To study the relations between coronal dimmings and CMEs: how is the early evolution and morphology of dimmings connected to the propagation direction of CME? 2. To study dimming lifetime: how and when does the solar corona recover after a CME eruption, and how long are CMEs/flux ropes still connected to the Sun? The study of coronal dimmings is of particular interest for Earth-directed CMEs, as they provide information on the occurrence and early evolution of the CME before the event is observed by coronagraphs along the Sun-Earth line. Scientific novelty of the proposed project: 1. The methods of using dimmings as an early indicator of the dominant CME direction will be proposed for the first time. Approaches to study the relationship between the evolution and morphology of dimmings and CMEs, and methods to determine the direction of CME propagation at an early stage of its evolution based on the results of dimming identification will be developed. 2. Long-term observations of coronal dimmings (2-3 days from the start of the CME) from different viewpoints: on the solar disk using SDO and on the limb using STEREO will be used for the first time to assess the processes of coronal reconfiguration and recovery after a CME, as studies on how long CMEs have been connected to the Sun. For the first time, the evolution of the complete region of coronal dimmings and their individual regions (core vs secondary dimmings) with a different nature of formation will be considered.

Expected results
1. Development of methods for estimating the dominant direction of the dimming development based on the assessment of the evolution of the dimming intensity and area. 2. Development of methods for comparing the CME propagation direction with the dominant direction of the dimming development based on 3D reconstruction of CMEs. 3. Development of methods for analyzing the relationship between the geometry of the CME projection onto the solar sphere and the geometry of the dimming and its evolution. Development of methods for early prediction of the CME direction based on dimming observations. Performance of simulated CME/dimming observations (using a geometric CME cone model) as well as detailed case studies of well-observed dimmings and observed CMEs to establish the relationship between them. 4. Development of methods for analyzing the dimming lifetime for estimating coronal reconfiguration processes based on the use of multipoint satellite observations of dimmings both on the solar disk and on the solar limb. This allows us to estimating how and when the solar corona recovers and replenishes after a CME, as well as how long the CME/flux rope footpoints are still connected to the Sun. These results on the usage of the unique potential of CME-initiated dimmings are of great scientific importance and make significant contribution in astrophysics (see the recent discovery of CME-associated dimmings on solar like-stars, Veronig et. al., 2021), including solar physics, physics of stellar atmospheres, and solar-terrestrial physics, and are also of great practical importance and relevance for providing early space weather forecasts. These results can be attributed to the problem of preventing natural hazards.

Annotation of the results obtained in 2023
1. Methods have been developed for estimating the dominant direction of the dimming development using automatic detection and sector analysis of dimmings based on the assessment of the evolution of the dimming area. For a more accurate estimation of the dimming area on a sphere, and in particular the area of a dimming located closer to the solar limb, a method was developed to calculate the surface area of a sphere for each pixel based on solving the double contour integral using a combination of analytical and numerical methods. For the October 1, 2011 event, dimming was shown to extend to the southeast. For the event of September 6, 2011, it was found that the dimming evolution is dominant mainly in the northwest direction. For the October 28, 2021 event, it was demonstrated that the dimming dominates in the southern sectors. 2. Methods have been developed to compare the directions of CME/magnetic flux and dimming using 3D reconstruction of eruptive prominences and 3D CME reconstruction, and the relationship between dimming, prominence and CMEs has been established. A 3D reconstruction of the eruptive prominence was performed using epipolar geometry methods in the lower layers of the EUV solar corona, as well as a 3D reconstruction of the white-light CME using the graduated cylindrical shell (GCS) model in the higher layers of the corona in order to determine the propagation direction of CME/magnetic flux in 3D. For the October 28, 2021 event, it was shown that the dominant direction of dimming evolution coincides with the orthogonal projections of the 3D vertices of the prominence, and thus reflects the direction of its propagation in the low layers of the solar corona, although the filament evolution is not directly related to the direction of the global CME expansion. At the same time, the overall dimming morphology was found to closely resemble the inner part of a GCS croissant, indicating a clear spatial relationship between dimming and CME, which supports the possibility of using dimming observations to obtain information on the CME propagation direction. 3. A 3D CME simulation was carried out based on the geometric model of the CME cone for different parameters (width, height, radial direction, deflection from the radial direction, source location on the solar disk) and the influence of the CME on the shape of the CME orthogonal projections onto the solar sphere was established to analyze how the plasma evacuation manifests itself in the form of coronal dimming. The simulation was performed for nine different source region locations, where one is in the disk center, and the other eight are in four quadrants (2 points per quadrant: one near the disk center and the other one near the limb). The quadrants considered included the upper right (northwest), upper left (northeast), lower left (southeast), and lower right (northwest) parts of the solar sphere.It was shown that with increasing height and width of the cone, the area of the orthogonal projection of the CME onto the sphere and its shape changes from a circle to an ellipse, depending on the deflection angle from the radial direction and the position of the eruptive center on the sphere. 4. The DIRECD (Dimming InfeRred Estimate of CME Direction) method was developed to estimate the early propagation direction of CMEs based on the expansion of coronal dimmings. The method includes the following steps. First, 3D CME simulations were carried out based on the geometric model of the CME cone for different CME parameters, and the influence of CMEs on the shape of CME orthogonal projections onto the solar sphere was analyzed. Second, the dominant direction of dimming evolution is determined, and an inverse problem is solved to reconstruct an ensemble of CME cones at various heights, widths, and deflections from the radial direction, and third, the resulting CME direction in 3D was estimated by searching for CME parameters for which the orthogonal projection of the cone on the sphere was match the dimming geometry at the end of its impulsive phase. Validated through case studies on October 1, 2011, and September 6, 2011, the DIRECD method reveals the early propagation directions of CMEs. Specifically, the CME on October 1, 2011, predominantly expands towards the South-East, while the CME on September 6, 2011, inclines towards the North-West. These findings align with previous studies utilizing multi-viewpoint coronagraphic observations. The study underscores the utility of coronal dimming information for early CME direction estimation, offering valuable insights for space weather forecasting and the mitigation of potential adverse impacts on Earth before observation in the field-of-view of coronographs. The practical feasibility of the method was demonstrated using examples of two events on October 1, 2011 and September 6, 2011. Using the DIRECD method, it was found that for the event on October 1, 2011, the projections of the CME cones onto the sphere match the dimming geometry at the end of its impulsive phase for the following CME parameters: height - 1.04 RSun (height at which the CME is still connected to the dimming), width - 48.7 degrees and inclination angle - 24.3 degrees. Additionally, by projecting the resulting 3D direction onto the meridional and equatorial planes, it was found that the CME is directed to the southeast (21 degrees south and 12 degrees east) from the radial direction. For the event of September 6, 2011, it was found that the projections of the CME cones onto the sphere match the dimming geometry for the following CME parameters: height - 1.394 RSun, width - 50.2 degrees and deflection angle - 25.1 degrees. It was also further shown that the CME is directed northwest (22 degrees north and 15 degrees west) from the radial direction. 5. Validation of the DIRECD method was performed based on 3D reconstruction of extended CME loops using STEREO-A and STEREO-B EUV images in the lower layers of the solar corona, where it is assumed that the CME is still connected to the dimming, as well as based on 3D reconstruction of CME using the GCS model according to images from the STEREO-A/STEREO-B COR 2 coronagraphs in white light in higher layers of the corona. The propagation direction of the CME using the DIRECD method was shown to be generally consistent with the direction of the CME determined from the 3D reconstruction in the lower EUV corona, as well as from the 3D reconstruction of the CME using the GCS model in the higher corona. For the September 6, 2011 event, it was shown that the height of the CME cone obtained using the DIRECD method (1.39 RSun) is close to the height of the inner part of the GCS croissant (1.45 RSun). As both the DIRECD CME cone and the GCS croissant are reconstructed at the same time at the end of the dimming impulsive phase (the CME bubble is already seen in coronagraphs at this time), this supports the argument that the CME propagation is connected to the dimming and only leaves footprints in the low corona up to a limited height. 6. Thus, for the first time, the methods have been proposed to use dimming as an early indicator of the CME propagation direction, which is of great practical importance and relevance for providing early space weather forecasts and understanding the early evolution of CMEs.

Publications

1. Chikunova G., Podladchikova T., Dissauer K., Veronig A.M., Dumbović M., Temmer M., Dickson E.C.M. Three-dimensional relation between coronal dimming, filament eruption, and CME Astronomy and Astrophysics, том 678, номер статьи A166, 14 стр. (year - 2023) https://doi.org/10.1051/0004-6361/202347011

2. Jain Ш., Podladchikova T., Chikunova G., Dissauer K., Veronig A.M. Coronal dimmings as indicators of early CME propagation direction Astronomy and Astrophysics, - (year - 2023)

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