Investigation of the two-dimensional velocity field of the fast large-scale coronal wave observed on September 6, 2011

Large-scale coronal waves are large-amplitude or shocked fast magnetosonic waves, most probably caused by the rapid lateral expansion of the flanks of a coronal mass ejection (CME).  These waves can be observed in soft X-ray and EUV images as bright fronts crossing large areas of the solar disk, with the strongest signals observed in filters that image the solar corona at temperatures of around 1-2 MK. While some large-scale coronal waves are observed to be quasi-circular, most exhibit non-isotropic propagation in terms of direction and speed. As expected of magnetosonic (shock) waves, they exhibit wavelike behavior, such as reflection, refraction, and transmission, in regions with different magnetosonic speeds, such as coronal holes and active regions, due to variations in magnetic flux density and plasma density. However, the resulting non-isotropic wavefront behavior is rarely investigated in detail.

Here, we investigate the two-dimensional velocity field of the fast and complex large-scale coronal wave observed on September 6, 2011. We use 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 northwest ward 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 and the photospheric magnetic field distribution along different propagation directions of the wave, and set the findings in context with alternative interpretations like potential misidentification of the expansion/opening of CME loops as wave front.

This project has received funding from the European Union's Horizon Europe research and innovation program under grant agreement No 101134999.