Huge clouds of plasma periodically erupt from the Sun in coronal mass ejections. The magnetic field structure of each blast can help determine whether it might endanger spacecraft, power grids, and other human infrastructure. New research by Palmerio et al. highlights the importance of detecting any changes in the magnetic field structure of a coronal mass ejection as it races toward Earth.
Coronal mass ejections often erupt in the form of a flux rope—a twisted, helical magnetic field structure that extends outward from the Sun. A flux rope can come in a variety of types that depend on the direction of the magnetic field axis and whether its helical component curves to the left or right. While the direction of the helical curve remains unchanged, the axis can alter direction after eruption from the Sun.
In the new study, the researchers analyzed observations of 20 different coronal mass ejections, comparing their flux rope structure at eruption to their structure once they reached satellites near Earth. They used a variety of satellite and ground-based observations to reconstruct the eruption structures, and they directly observed structures close to Earth as the plasma blasts washed over NASA’s Wind spacecraft.
The analysis showed that between Sun eruption and Earth arrival, flux rope structure changed axis direction by more than 90° for 7 of the 20 coronal mass ejections. The rest of the blasts had an axis rotation of less than 90°, with five changing by less than 30° after eruption.
These results highlight the importance of capturing posteruption changes in flux rope magnetic field structures of coronal mass ejections to refine space weather predictions. Such rotations can result from a variety of causes, including deformations in the Sun’s corona and interaction with other coronal mass ejections.
However, capturing these changes remains a challenge. Reconstructions of flux rope structure from direct spacecraft observations may vary depending on which reconstruction technique is used. In addition, such observations depend on the spacecraft’s particular path through a coronal mass ejection, which might not give an accurate picture of the overall structure.
And although posteruption structural changes are important, the researchers emphasize that the flux rope structure of a coronal mass ejection at eruption is still a good approximation for its structure upon Earth arrival and serves as a key input for space weather forecasting models. (Space Weather, https://doi.org/10.1002/2017SW001767, 2018)
—Sarah Stanley, Freelance Writer