PUNCHing Through the Alfvén Surface: Using An Earth-Scale Imager to Detect Inbound Waves

The Alfvén surface is a useful boundary separating the Sun’s corona from the solar wind. It is defined as the region for which inbound motion past a certain radius from the Sun is incapable of reaching the rest of the corona, as the solar wind speed exceeds the Alfvén speed. Previous work has focused on using Fourier filtering to separate inbound and outbound motion in the corona to locate the radius at which inbound motion ceases. With the launch of the Polarimeter to Unify the Corona and Heliosphere (PUNCH), examining the Alfvén surface at a wider range of radii is now possible. Possible inbound features with speeds ranging from 200 to 300 km/s were detected by Fourier filtering methods. These features are long-lived and exist at speeds significantly different from the filter speeds. The efficacy of the Fourier filter was also demonstrated in isolating outbound motion from noise and velocity cutoffs.