Parker Solar Probe reveals a coronal mass ejections deformation around 0.1 au

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Many coronal mass ejections (CMEs) follow a regular shape and evolve self-similarly, but observations and simulations suggest deformations in some events. We identified a coronal mass ejection that resembles the flux rope models and expands self-similarly when observed by all coronagraphs, but Parker Solar Probe suggests a different picture. According to our reconstruction, the CME follows the shape expected by the “croissant” model up to around 0.1 au, when it becomes clearly deformed and a dimple forms in its front (Figure 1).

Figure 1: We calculate the position and speed of three different CME front portions on WISPR-O observations: the farthest points in the northern and southern parts of the CME front (red and blue plus signs), and the latitude where the front looks closer to the Sun (green plus signs). As we repeat the front identification by eye six times for each portion to estimate errors in the kinematics, we have six plus signs for each color.

The CME expands more at higher latitudes, where the background solar wind is faster. We consider other possible causes for deformations, such as loss of coherence and slow-mode shocks. Our interpretation is that the CME becomes deformed at ∼0.1 au owing to differences in the background solar wind speeds.
We estimate that this would result in a CME time-of-arrival error of up to 16 hours for observers close to 1 au. As the deformation is not clear in any observations except on Parker Solar Probe heliospheric imager, we would have missed it if we did not have this spacecraft. Such deformations may help explain the time-of-arrival errors in events where only coronagraph observations are available. 

Movie 1. CME observations by the heliospheric imager onboard Parker Solar Probe on 2021 January 20. This movie shows observations from WISPR-I, the camera that observes closer to the Sun. The CME moves from left to right in the images while it advances. The Sun is located toward the left, at the coordinate system origin. The horizontal grid line at 0° indicates Parker Solar Probe's orbit plane. The grids are given in degrees in both dimensions. 

Movie 2. CME observations by the heliospheric imager onboard Parker Solar Probe on 2021 January 21. This movie shows observations from WISPR-O, the camera that observes closer to the Sun. The CME moves from left to right in the images while it advances. The Sun is located toward the left, at the coordinate system origin. The horizontal grid line at 0° indicates Parker Solar Probe's orbit plane. The grids are given in degrees in both dimensions. 
See the full journal publication at Carlos R. Braga et al 2022 ApJ 938 13

Author: Carlos R Braga (The Johns Hopkins University Applied Physics Laboratory, George Mason University)

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