Coronal Mass Ejection Deformation at 0.1 au Observed by WISPR

Although coronal mass ejections (CMEs) resembling flux ropes generally expand self-similarly, deformations along their fronts have been reported in observations and simulations. We present evidence of one CME becoming deformed after a period of self-similar expansion in the corona. The event was observed by multiple white-light imagers on 2021 January 20-22. The change in shape is evident in observations from the heliospheric imagers from the Wide-Field Imager for Solar Probe Plus (WISPR), which observed this CME for similar to 44 hr. We reconstruct the CME using forward-fitting models. In the first hours, observations are consistent with a self-similar expansion, but later on the front flattens, forming a dimple. Our interpretation is that the CME becomes deformed at similar to 0.1 au owing to differences in the background solar wind speeds. 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. The CME deformation seems to cause a time-of-arrival error of 16 hr at similar to 0.5 au. The deformation is clear only in the WISPR observations; thus, it would have been missed by 1 au coronagraphs. Such deformations may help explain the time-of-arrival errors in events where only coronagraph observations are available.

Automated summary

Researchers have found that a coronal mass ejection (CME) resembling a flux rope can become deformed during its expansion. The deformation is likely caused by differences in the background solar wind speeds. This finding may have implications for the accuracy of arrival time predictions.